1J Physiol 00.0 (2020) pp 1–21

WHITE PAPER

Mechano-electric and mechano-chemo-transduction
in cardiomyocytes
Leighton T. Izu1 , Peter Kohl2 , Penelope A. Boyden3 , Masahito Miura4 , Tamas Banyasz5 ,Nipavan Chiamvimonvat6 , Natalia Trayanova7 , Donald M. Bers1 and Ye Chen-Izu1,6,8
1Department of Pharmacology, University of California, Davis, CA, 95616, USA
Institute for Experimental Cardiovascular Medicine, University Heart Centre, and Faculty of Medicine, University of Freiburg, D-79110, Germany
3
Department of Pharmacology, Columbia University, NY, 10032, USA
4
Department of Clinical Physiology, Health Science, Tohoku University Graduate School of Medicine, Sendai, 980–8574, Japan
5
Department of Physiology, University of Debrecen, Debrecen, Hungary
6
Department of Internal Medicine, Cardiovascular Medicine, University of California, Davis, USA
7
Department of Department of Medicine, Johns Hopkins University, Baltimore, MD, 21205, USA
8
Department of Biomedical Engineering, University of California, Davis, CA, 95616, USA

The Journal of Physiology

2

Edited by: Harold Schultz & Jolanda Van der Velden

Mechano-Chemo
Transduction

Ca2+ - Electro
Transduction

Electrical
Excitation

Ca2+
Signalling

MechanicalContraction

Mechano-Electro-Transduction

Abstract Cardiac excitation-contraction (E-C) coupling is influenced by (at least) three
dynamic systems that couple and feedback to one another (see Abstract Figure). Here we
review the mechanical effects on cardiomyocytes that include mechano-electro-transduction(commonly referred to as mechano-electric coupling, MEC) and mechano-chemo-transduction(MCT) mechanisms at cell and molecular levels which couple to Ca2+ -electro and E-C coupling

Leighton Izu studied mathematics, psychology andbiophysics. His research interests include developing
mathematical methods for modelling complex cellularbehavior, understanding how the heart adapts tochanging mechanical loads, and studying how variation
is beneficial to the population. Peter Kohl studied
medicine and biophysics. His research interests include
experimental and modelling studies of cardiac electrophysiology and heart disease. Don Bers studied physiology, biophysics and cellular and molecular cardiology. His research interests include cardiac excitation-contraction coupling and electrophysiology inheart diseases, using both experimental and modelling approaches. Ye Chen-Izu studied physics, bioengineering, cellular and molecular cardiology. Her
research interests include electrophysiology, Ca2+ signalling, muscle contraction and mechano-transduction in the heart.

C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

DOI: 10.1113/JP276494

2

L. T. Izu and others

J Physiol 00.0

reviewed elsewhere. These feedback loops from muscle contraction and mechano-transduction
to the Ca2+ homeodynamics and to the electrical excitation are essential for understanding theE-C coupling dynamic system and arrhythmogenesis in mechanically loaded hearts. This white
paper comprises two parts, each reflecting key aspects from the 2018 UC Davis symposium: MEC(how mechanical load influences electrical dynamics) and MCT (how mechanical load alters
cell signalling and Ca2+ dynamics). Of course, such separation is artificial since Ca2+ dynamics
profoundly affect ion channels and electrogenic transporters and vice versa. In time, these dynamic
systems and their interactions must become fully integrated, and that should be a goal for acomprehensive understanding of how mechanical load influences cell signalling, Ca2+ homeodynamics and electrical dynamics. In this white paper we emphasize current understanding,consensus, controversies and the pressing issues for future investigations. Space constraints makeit impossible to cover all relevant articles in the field, so we will focus on the topics discussed at
the symposium.
(Received 29 September 2019; accepted after revision 21 November 2019; first published online 2 December 2019)
Corresponding authors Leighton T. Izu: Department of Pharmacology, University of California, Davis, 451 Health
Science Drive, Tupper Hall, Rm 2221, Davis, CA 95616, USA. Email: ltizu@ucdavis.edu
Peter Kohl: Institute for Experimental Cardiovascular Medicine, University Heart Centre Freiburg – Bad Krozingen,
Faculty of Medicine, University of Freiburg, D-79110, Freiburg, Germany.
Email: peter.kohl@universitaets-herzzentrum.de
Abstract figure legend Cardiac excitation-contraction coupling and feedback pathways by mechano-electro- andmechano-chemo-transduction.

Preamble
Recognizing the critical need to combine experimental
methods and mathematical models in studying complex
heart disease mechanisms, the UC Davis Cardiovascular Symposia series provides an on-going forum
for experimentalists, modellers, physicians and scientists
to exchange ideas and build collaborations. The goal ofthis meeting series is to develop an in-depth quantitative
understanding of the interplay of cardiac electrical, Ca2+and mechanical signalling, and how each of these systems
feed back on each other dynamically (Abstract Figure).
Such feedback systems can both stabilize physiological
function and cause pathological dysfunction. Understanding these feedback systems is critically importantfor identification of causes of cardiac arrhythmias andcontractile dysfunction (e.g. in heart failure) and thedevelopment of effective therapies.
The UC Davis Cardiovascular Symposium series
has been organized to integrate such experimental andmodelling studies in a series of meetings. The first
symposium (2010) took an integrative view of E-C
coupling and arrhythmia mechanisms that set the stage
for a series of more focused topics: on Ca+ channels andCa2+ signalling system (2012), Na+ channels and Na+
regulation (2014), and K+ channels and K+ regulation(2016) and mechanics and energetics (2018). Each of theseare essential to our integrated understanding and benefit
greatly from combining experimental investigations withmathematical modelling studies and clinical perspectives.
We shall come full circle from the first 2010 meeting in
2020, when we return to focus on integration among

these areas. (See https://basicscience.ucdmc.ucdavis.
edu)
Mechano-electric coupling (MEC)
Consensus, controversies and open issues

There is consensus about the following:(1) Many ion channels (including sarcolemmal andnon-sarcolemmal) are mechanically modulated.

r Ion channels that change their open probability
primarily in response to mechanical stimulation(mechano-sensitive channels) can be divided intocell-stretch and cell-volume activated ion channels(SAC, VAC, respectively).
r VAC increase their open probability in responseto cell swelling; they are constitutively active in
hypertrophic myocardium – but they are notbelieved to be major determinant of beat-by-beatmechano-sensory behaviour or drivers of responses
to acute mechanical stimulation.
r SAC can be divided into cation-non-selective(SACNS ) and potassium-selective channels (SACK ).
r Non-myocytes (fibroblasts, macrophages) aremechano-sensitive; they express SAC and cancontribute to cardiac electrophysiology.

(2) Mechano-sensitive channels affect cardiac electrophysiology

r SACNS appear to contribute to acute electricalresponses

tostretch,

so

much

so

that


C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

J Physiol 00.0

Mechano-transduction in cardiomyocytes

electrophysiological responses to mechanicalstimuli can generally be accounted for inquantitative models by simply invoking SACNS .
r SACK cause re/hyperpolarization; they may
contribute to action potential (AP) shortening,in particular in pathological settings such as
ischaemia.
r SACNS activation can increase intracellular Ca2+ ,either directly or indirectly (via Na+ /Ca2+
exchange, NCX). The resulting intracellular Ca2+
signals can be highly compartmentalized.
r Drug actions, including drug effects on ion
channels, can be mechano-modulated.

3

Theophile, 1967). Since then, pioneering work by Max
Lab, Frederick Sachs, Michael Franz and others has moved
MEC from a footnote in cardiac electrophysiology to its
appreciation as a fundamental part of cardiac functional
integration (for a comprehensive collection of international research on the topic, see the multi-author
textbook, edited by Kohl, Franz and Sachs (Kohl et al.
2011)).
MEC is based on at least two interacting molecularmechanisms: mechano-sensitive ion channels (see next
section) and mechanical modulation of intracellular
calcium handling (see section on MEC effects via changes
in intracellular Ca2+ handling).

Controversies and open issues are:r Amplitudes and kinetics of SAC and VAC in physiological and pathological states, and how they affect thedelicate balance of currents during the cardiac AP, areill-explored.
r We can track strain, but not stress, at the celllevel in native cardiac tissue. As a result, for manymechano-electric coupling processes we do not know
key biophysical input parameters.
r The explicit in vivo molecular mechanisms ofactivation, and how best to mimic them experimentally,
are unclear.
r Our ability to interpret dynamic mechanical effects onelectrophysiology is equally limited by difficulties in
identifying ‘differential’ and ‘proportional’ signalling
pathways.
r The dynamics of 3-D deformation of subcellular nano-structures, and their relevance formicro-to-macro cardiac function are ill-explored andform a key frontier for further research.
r Mechanistic links between acute (beat-by-beat timescale or faster), intermediate (second to minutes)
and chronic effects (e.g. involving cell and tissue
remodelling) of stretch-induced changes in cardiac
electrophysiology are unclear.
r In general, we know more about MEC effects inpathological settings (arrhythmogenic effects) thanabout the role of VAC and SAC in normal homeostasis.
MEC conceptual orgins

Clinical awareness of mechanical modulation of heart rate
and rhythm can be traced back by at least a century
in the Western medical literature, with examples bothfor mechanical induction (Meola, 1879) and mechanicaltermination (Schott, 1920) of heart rhythm disturbances.
Following early experimental work that identified theintra-cardiac nature of mechanically induced changes in
cardiac electrophysiology (Schlomka & Hinrichs, 1932),
mechano-electric coupling (MEC) was conceptualized
as part of cardiac electro-mechanical auto-regulation by
Ursula Ravens (née Thophile) in 1967 (Kaufmann &
C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

MEC effects via mechano-sensitive ion channels
General aspects. Mechano-sensitive ion channels can be

split into channels that require a change in cell volume(cell volume-activated ion channels: VAC) and those that
activate directly in response to membrane deformation(stretch-activated ion channels: SAC). Since acute effectsof mechanical stimulation on cardiac electrophysiology,
including beat-by-beat responses, are thought not to
involve cell volume changes, SAC are usually considereda primary target for MEC research as it pertains to acuteelectrophysiological responses to mechanical stimulation.
It is important, however, to note that VAC contributeto cardiac MEC responses in cells subjected to swelling,for example during ischaemia or upon reperfusion. Interestingly, VAC are constitutively activated in the enlarged
cells of failing hearts (Clemo et al. 1999), suggesting thepresence of a ‘set point’ for cardiomyocyte cell volume
by an as-yet-unidentified mechanism. Like many otherchanges in cardiac structure and function, causal chainsof events occurring during chronic tissue remodelling are
difficult to explore. This represents an important area forfurther study.
SAC can be sub-divided by their ion selectivity into
cation non-selective (SACNS ) and potassium-selective(SACK ) channels. While somewhat arbitrary, this division
is helpful when considering effects on cell electrophysiology, as SACNS will depolarize resting cardiomyocytes, while SACK will have a re- or hyperpolarizing
effect. Given that resting cardiomyocytes respond tomechanical stimulation (if they show any response at all)by depolarization, it is thought that SACNS dominate MEC
effects in healthy cells under normal conditions (Kohl et al.
2006).
In addition to ion channels whose open probability is
primarily graded by mechanical events, many ligand- andvoltage-gated ion channels are mechanically modulated –both in terms of their open probability (Morris, 2011)
and in terms of their functional integration into thesurface sarcolemma (Boycott et al. 2013). Figure 1 provides
a recent overview of mechano-sensitive ion channels

4

L. T. Izu and others

that highlights both their ubiquitous presence acrossliving kingdoms and the breadth of proteins identified(from Peyronnet et al. 2016). This listing is in constantflux, and the relatively recently identified Piezo channels(Coste et al. 2012), which appear to be gated by bilayer
tension (Cox et al. 2016), have now been found in theheart as well (Liang et al. 2017). In addition, we now
appreciate that there are individual mutations that may
convey increased mechano-sensitivity to ion channels
such as TREK-1, which may be associated with anincreased risk for arrhythmogenesis in patients (Decher
et al. 2017). These findings could open up pathways
to ‘mechano-transduction targeting’ for personalized
therapeutic strategies.
That said, in general it has remained challenging to
identify biologically relevant biophysical input signals
for mechano-sensitive ion channel gating. Part of thechallenge is that, while we are getting better at tracking
cell strain in native tissue (Botcherby et al. 2013), weare still far from true readouts of cell stress in the myocardium. While suitable fluorescent probes for measuring
cytoskeletal stress have been developed (Rahimzadeh et al.
2011), challenges remain in terms of their calibration anduse within native tissue, such as the heart.
Given our current inability to accurately identify
biologically relevant input parameters, can we at least
unequivocally identify the respondents to mechanicalstimulation? Well, to some extent yes. As underlying molecular targets are increasingly being identified,both up- and down-regulation approaches have beenused to substantiate the players involved in stretch

Healthy myocardium. The physiological response of

cardiac myocytes to acute mechanical stimulation

SACK

Piezo 1, 2
TRPA-1
TRPC-1, 3, 5, 6
TRPV-1, 2, 4
TRPM-3, 4, 7
Asic
TRPP-2
Nav1.4, 1.5, 1.6, 1.7

Cav 1.2 (L-type)N-typeT-typeRYR?

effects on cardiac electrophysiology. Of course, genetic
modification is burdened by the possibility (probability)of compensatory changes in the presence and/or activityof other pathway elements.
This can, in part, be compensated using acutepharmacological interventions. A problem here is that
we are lacking target-specific blockers and activators (asreviewed in detail elsewhere: Peyronnet et al. 2016). To
assess SACNS contributions, GsMTx-4, a peptide from thevenom of the Chilean tarantula Grammostola spatulata(Suchyna et al. 2000), is the intervention tool of choice.
GsMTx-4 shows good selectivity for SACNS and is useful
not only in cardiac cells, but also in native tissue (Bode et al.
2001). This is not ‘a given’ and has led to false-negative
observations with other, less suitable pharmacologicalagents such as gadolinium and streptomycin (Cooper& Kohl, 2005; Zhang et al. 2018). A note of caution is
warranted here for all cell culture studies: streptomycin, an
obligatory component of most commercial culture media,
is a formidable blocker of SACNS and stretch-inducedchanges in Ca2+ handling (Gannier et al. 1994; Belus &White, 2003).
The experimental difficulties in linking biophysicalinputs to molecular players, and cellular responses
to organ-level mechano-sensitivity, have contributed tothe high relevance of computational models in thequantitative assessment of potential pathways of cardiac
MEC (see section on Modelling MEC at the organ level).

C. elegans Zebrafish Drosophila

Mammals
SACNS

Ca2+

J Physiol 00.0

KATP
TREK-1
TREK-2
TRAAK

CI–
CFTR
CLC-3?
Connexin46

NOMPC

KCNQ

BK
BK

BK

Golgi

Mito

Kir 2.1
Kir 2.2

NOMPC
Piezo
Fungi

TRP4
Mid1

LOV-1

Alamethicin

KvAP

Yeast

Kv 1.5NMDA receptor
AChR

Archaea

TRPN1
Piezo1

OSM9
Kir 2.3

KATP
BK
SR

MscMJ

MEC-4
MEC-10

Mys1

Mys2

Plants

MCA1
MscL
MscS
MscM
MscK
Gramicidin
Bacteria

TRPY1

TPK
i9MscS
MSC1

MSL10
MSL9
MSL8

Algae

Figure 1. Mechanically gated and modulated ion channels across living organisms
Proteins highlighted in red have been identified in the mammalian heart, channels confirmed as stretch-activatedare underlined. From Peyronnet et al. (2016).

C 2019 The Authors. The Journal of Physiology C 2019 The Physiological SocietyMechano-transduction in cardiomyocytes

J Physiol 00.0

is diastolic depolarization, which facilitates diastolicdepolarization in pacemaker (Cooper et al. 2000)
and conduction cells (Kaufmann & Theophile, 1967).
This can trigger excitation in resting atrial (Bodeet al. 2001) and ventricular myocardium (Franz et al.
1992). This is in line with a major contribution
by SACNS , whose pharmacological block can abolishectopy. During the action potential (AP), acute stretch
responses depend on the difference between current
cellular membrane potential and reversal potential of
stretch-activated channels (for SACNS between 0 and−15 mV), causing abbreviation of early and prolongationof late AP time courses, including early and delayed
afterdepolarization-like potential changes (EAD, DAD; see
Fig. 2 (Kohl et al. 2001)).
It has been difficult to identify an evolutionary
advantage for cardiac MEC. One line of thought isthat mechano-regulation of the ionic milieu would
have been an early requirement during the emergenceof cellular life forms, whose presence/preservation
did not exert a negative evolutionary pressure – somechano-modulation of ion channels is conserved in
all cell types that have been assessed. Alternatively, theelectrophysiological consequences of mechano-sensitiveion channel activation could be a side-effect of a
QRS

T

ECG

0.2s
1mV

Vulnerable
Window forStretch-Induced
VF
Changed Repolarisation

Ventricular Action Potentials

0
mV

A

-90
Ectopic Action Potential
0mV

B

EAD-like
Depolarisation-90
Ectopic Action Potential
0mV

C

DAD-like
Depolarisation5

different, more important role in the heart: the local
matching of cellular Ca2+ load (and, hence, contractile
performance) to global circulatory demand (Markhasin
et al. 2003). This would be relevant for an organ that
relies on synchronized and force-balanced contractionof millions of muscle cells that lack targeted external
control mechanisms (such as individual neuro-muscularjunctions), in particular as each of these cells experiences
slightly different stress-strain dynamics during normal
cardiac activity. Relevant mechanisms would include both(i) mechanical modulation of trans-sarcolemmal Ca2+
flux (whether directly by permeation of Ca2+ through
SAC, or indirectly, e.g. by SAC-medicated Na+ influxthat alters NCX (Gannier et al. 1996; Jin et al. 2017;
Zhao et al. 2017)) and (ii) mechanical modulation of
Ca2+ flux between intracellular stores and cytosol (forexample via a stretch-induced increase in ryanodine
receptor (RyR) open probability (Iribe et al. 2009a)or mitochondrial Ca2+ release (Belmonte & Morad,2008)). Whatever the explanation, the physiologicalbenefit of cardiac mechano-electric cross-talk – beyond
potential contributions to preload-adjusted sino-atrial
node pacemaking (Quinn & Kohl, 2012) – remains amatter of debate.
What is well-established is that the arrhythmogenic
potential of mechanical stimulation, if ill-timed, may
trigger sustained ventricular tachyarrhythmias (Schlomka& Hinrichs, 1932; Kohl et al. 2001; Link, 2012). Interestingly, this is an example of cardiac MEC researchwhere conceptual (Kohl et al. 2001) and computational
prediction of quantitatively plausible mechanisms in
2-D (Garny & Kohl, 2004) and 3-D (Li et al. 2004)
computational models of ventricular myocardium preceded experimental confirmation by many years (Quinn
et al. 2017). We now know that focal electrical excitation,for example triggered by a precordial impact in myocardium just underneath the impact site (Link, 2012), may
give rise to reentrant excitation if this ectopic focus occurs
just on the trailing edge of the preceding repolarization
wave, which acts as a functional block zone preventing
uniform spread of the ectopic wave (Quinn et al. 2017).
While this may give rise to sustained tachyarrhythmia, thevulnerable window for this is impact location-dependentand short (Fig. 2), so that most mechanically induced
ectopic beats are benign in healthy myocardium.

-90

Figure 2. Schematic representation of cardiomyocyte action
potential (AP) responses to stretch-activated ion channel(SAC) opening at different timings during the AP
Within a short critical time-window during early ventricularrepolarization (just prior to the peak of the T wave; see schematic
ECG trace), these can combine to render both trigger (red) andsustaining mechanisms (blue) for ventricular rhythm disturbances(Commotio cordis). Reproduced from Kohl et al. (2001).
C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

Diseased myocardium. Overall, mechanistic insight into
MEC in diseased tissue is limited. Reasons include theaforementioned inability to determine local stress levels
in native tissue and the often significant structural andfunctional remodelling in chronic disease settings.
Functional remodelling includes alterations in ion
channel presence and activity, such as an upregulation
of VAC in hypertrophied cells (Clemo et al. 1999),

6

L. T. Izu and others

or metabolic and mechanical co-activation of ATPK
channels (Van Wagoner, 1993) in ischaemic tissue (Huang
et al. 2013), with demonstrable knock-on effects on
local intra- and extracellular ion concentrations andarrhythmogenesis (Bollensdorff & Lab, 2011). In addition,
mutations that alter ion channel mechano-sensitivity arestarting to emerge as clinically relevant contributors toarrhythmogenesis (Decher et al. 2017).
Structural remodelling gives rise to significant
heterogeneity in active and passive mechanical properties,
which will affect MEC responses. Examples include
inhomogeneous stress-strain behaviour in and aroundischaemic foci (where paradoxical segment lengthening
can be observed), scar border zones (BZ; where stiff
scar tissue gives rise to varying stress gradients as thesurrounding tissue changes from resting to contracted
and back), or asynchronous activation of different cardiac
tissue regions (for example as a result of bundle branch
block or during cardiac resynchronization therapy). Someof the scenarios in which non-uniform contraction
contributes to arrhythmogenesis are explained the next
two sections: MEC effects via changes in intracellular Ca2+
handling and Modelling MEC at the organ level.
MEC effects via changes in intracellular Ca2+ handling
General aspects. E-C coupling is traditionally seen asa sequence of events, where electrical activity (e.g. anAP) results in the rise and decay of an intracellular
Ca2+ transient, with the consequential rise and fall ofcontraction. Yet there is feedback from mechanics toCa2+ and electrophysiology, as well as from Ca2+ toelectrophysiology. This could be called reverse mode
E-C coupling. A key signalling molecule involved is
intracellular Ca2+ . A typical example of Ca2+ -related
arrhythmogenesis is when Ca2+ is released from thesarcoplasmic reticulum (SR) stores by ‘leaky’ RyR during
diastole, or during AP repolarization. This Ca2+ caninitiate a Ca2+ wave, followed by DAD or EAD (Bers,
2014). Below we will focus on an example that depends
on Ca2+ release from myofilaments, rather than the SR.
Ectopy secondary to non-uniform EC coupling. In failinghearts, wall stress increases as a result of dilatationof the left ventricle (LV). This increased wall stressaggravates regional differences in contractile strengtharound the impaired muscle. Regional differences incontractile strength may cause paradoxical stretching of
impaired muscle by contractions of neighbouring more
viable muscle. This forms a BZ between regions of stretch
and shortening.
In 2005, Wakayama et al. (2005) describedspatially non-uniform muscle contractions that lead to
DAD-induced arrhythmias. Quite different from Ca2+J Physiol 00.0

leak-induced Ca2+ waves (Ter Keurs & Boyden, 2007),the arrhythmogenic Ca2+ waves initiating these triggered
propagated contractions in this multicellular cardiac preparation arise from acute reduction of myofilament Ca2+
binding in regions near damaged areas. In this case, Ca2+
waves are dependent on mechanical stretch and release at
the border between the damaged and intact regions.
For these experiments, ter Keurs et al. createda controlled local reduction of myofilament force
development in a length of normal multicellular rat
trabecula. This was accomplished by exposing the preparation to a small jet of a solution that contained either
2,3-butanedione monoxime (BDM), or low external Ca2+
concentration ([Ca2+ ]o ; see Fig. 3B, inset). They measured
sarcomere length (SL) and with the jet off there was SL
shortening in this region. However, when the jet was on,
the weakened region near the centre of the jet experienced
stretch by the strong regions on either side of the jet(increased SL, Fig. 3A, right, and B).
Here the initial Ca2+ rise during the wave initiation
was shown to be caused by Ca2+ dissociation from myofilaments as a result of quick release and shortening ofactive sarcomeres (Wakayama et al. 2001). This is because
the Ca2+ binding affinity is positively correlated with
the number of Ca2+ -activated force-bearing cross-bridges(Allen & Kentish, 1985). Rapid shortening of sarcomeresin the BZ during relaxation causes Ca2+ release fromtroponin C on the thin filaments. This release can initiate
the Ca2+ waves. Elimination of SL shortening will inhibit
the initiation of Ca2+ waves, while SR Ca2+ load andrelease will enhance such waves. Thus the initiation is
not caused by Ca2+ leak from SR, but by an intracellular Ca2+ surge from myofilaments that are a majorsite of Ca2+ buffering in myocytes (Bers, 2001). Sucha Ca2+ surge can then induce SR Ca2+ release waves
which lead to DAD that, under certain conditions, trigger
electrical activity (Fig. 4). Indeed, the amount of Ca2+
dissociated from myofilaments affects the frequency of
the arrhythmias. For example, the maximum rate ofmuscle relaxation was inversely correlated with the cycle
length of the arrhythmia. An unresolved issue is thatthese waves propagate in multicellular preparations at
0.2–2.8 mm s−1 , too fast for SR-mediated Ca2+ waves
travelling via Ca-induced Ca-release (0.1 mm s−1 ; Bretteet al. 2005), which normally do not cross gap junctions
between myocytes, but is much slower than expected forfull-blown AP propagation in tissue (>200 mm s−1 ; Myles
et al. 2012). While the mechanism may be a hybrid, somemechanistic details about this potentially important MEC
mechanism in arrhythmogenesis need further testing.
EAD generation secondary to myofilament protein
dynamics: computational model. Recent computationalmodelling has concluded that myofilament protein
C 2019 The Authors. The Journal of Physiology C 2019 The Physiological SocietyMechano-transduction in cardiomyocytes

J Physiol 00.0

dynamics in a hypertropic cardiomyopathy model could
affect EAD formation and possibly arrhythmias in thisheritable substrate (Zile & Trayanova, 2017). The paper
concluded that incorporation of troponin C buffering ofcytosolic Ca2+ alters the AP repolarization reserve, whichleads to EAD emergence and triggers for arrhythmia. The
utility of computational modelling at the organ level isdiscussed in more detail in the next section.

Modelling MEC at the organ level

We focus on organ-level electro-mechanical modellingstudies, as these have advanced over recent years,
progressing through different scopes of mechanistic
research and corresponding model complexity. MEC
effects at the level of individual cells have, in contrast, beenmodelled before the turn of the Millennium (Landesberg& Sideman, 1994; Noble et al. 1998; Kohl et al. 1998; Riceet al. 1999) and they are thus not discussed in detail here. In
short, experimentally observed responses of single cardiac
cells to stretch have generally lent themselves to being
reproduced using computational models (for reviews, see
Kohl et al. 1999; Trayanova & Rice, 2011). Below we present the main areas of MEC exploration by computationalmodelling at the whole-heart level.
Role of mechanical impact in arrhythmogenesis andarrhythmia termination in the heart. In early whole-heartstudies, mechanical stimulation was incorporated throughits impact on the electrophysiology of the heart,A
SL (μm)2.4

Jet OFF7

namely through uniform regional SAC activation. In themodelling study by Li et al. (2004), both SACNS and
SACK are recruited upon mechanical impact. The impact
is administered at various coupling intervals following
pacing at the apex. The results demonstrated that the
impact induces sustained reentrant excitation only when
(1) a new activation is elicited by mechanical stimulation(caused by activation of SACNS ), and (2) upon returnto the original region of impact, this activation does
not encounter an extension of AP duration (prevented
by activation of SACK , such as KATP ). Furthermore,
a subsequent study by Li et al. (2006) elucidated themechanisms involved in termination of ventricular tachycardia by precordial thump and its decreased rate ofsuccess in ischaemia. Results demonstrate that precordial
thump-induced SACNS opening in normoxia reduces
heterogeneity in transmembrane potential by partiallyrepolarizing excited tissue and depolarizing resting myocardium, causing foci of excitation that eradicate theexcitable gap, thus facilitating tachycardia termination.
Decreased precordial thump efficacy in ischaemia wascaused by recruitment of KATP , which diminishes the
depolarizing effect of SACNS on resting tissue and caused
pronounced AP shortening, thus facilitating establishmentof reentry.
Electromechanical transduction and arrhythmogenesis in
the diseased heart. In these studies, model complexity

has increased, with representation of both electrical andmechanical function and their coupling. Past research
explored the role of acute stretch as a trigger of arrhythmia

Jet ON

Jet ON

Jet ON

2.2
2.0
1.8

Weak
Shortening

Biphasic

Stretch

1.6
0.5 s

B

C
2.4
SL (μm)SL (μm)2.2

Outside Jet
Inside Jet

2.0
1.8

2.2

Stretch (4)Biphasic (2)Weak Shortening (1)* P<0.05

2.0
1.8
1.6

1.6

Peak SL in HC Jet(n=7)

Jet OFF

Jet ON

0.5 s
Figure 3. Experimental model to mimic for non-uniform EC coupling in rat trabeculae
Sarcomere length (SL) changes measured from different regions during experiments in a single trabecula when
jet of butane BDM was ON or OFF. Note: in some regions there was weak shortening while in others stretch waspresent. This set up conditions of non-uniform EC coupling. From ter Keurs et al. (2006).

C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

8

L. T. Izu and others

under the conditions of acute ischaemia, as well as theeffect of acute stretch on the substrate for ventricularfibrillation (VF; Fig. 5; Jie et al. 2010). Jie et al. showedthat the ventricles had a central ischaemic zone (CIZ) andan intermediate BZ, and this was used to represent theelectrophysiological and mechanical milieu in the heart atseveral stages post occlusion. The study found that large
muscle strain developed in the ischaemic region duringcontraction. In both BZ and CIZ, cell membranes underwent mechanically induced subthreshold depolarizations,or DAD-like events, whereas such depolarizations wereabsent in the normal zone. The DAD-like events resulted inmechanically induced ventricular premature beats (VPB),
originating from the ischaemic border (particularly in the
LV endocardium where muscle strain and strain rate werelargest), but not from CIZ, since in the latter ischaemic
injury suppressed excitability. VPB beats then travelled
intramurally until emerging from the ischaemic borderon the epicardium, initiating reentry.
A subsequent study by Hu et al. (2013) examined therole of SAC recruitment on scroll wave stability in VF
using an even more complex model, and image-based
human ventricular electromechanics model. In VF,
SAC recruitment is heterogeneous, and this affects
A

J Physiol 00.0

electrophysiology and VF complexity. The study useda wide variety of SAC reversal potentials and channel
conductances. Opening of SAC with a reversal potentialof −60 mV diminished scroll wave breakup for all valuesof conductances by flattening heterogeneously the AP
duration restitution curve. Opening of SAC with areversal potential of −10 mV inhibited partially scroll
wave break-up at low conductance values by flattening
heterogeneously the conduction velocity restitution
relation. For large conductance values recruitment of SAC
with a reversal potential of −10 mV did not reduce thelikelihood of scroll wave break-up, because Na+ channel
inactivation in regions of large stretch led to conduction
block, which counteracted the increased scroll wave
stability due to an overall flatter conduction velocity
restitution.
The role of mechano-electric coupling in defibrillation.

Finally, mechano-electric transduction effects indefibrillation were explored through a bidomain
electromechanics model suitable for the delivery of
external electrical shocks (Li et al. 2008). This is acomputationally complex model of the heart, particularly
its electrophysiology component. The results of these

OFF

(b)

BDM JET(mN/mm2)

Force

(a)

–10
–0
ST

5s

(b)
–10
–0
(mN/mm2)–0.4–0(μmol/L)

ST ST

0.8

–0
– –50
(mV)
–10
–0
(mN/mm2)–0.4–0(μmol/L[Ca2+]iPosition

Membrane[Ca2+]i Force Potential(a)

0.2 mm
ST ST

Time

1s

0
(μmol/L)

0.8[Ca2+]i2+
BDM [Ca ]i Force

B

Time

0.2 mmTime1s

0(μmol/L)

Figure 4. Effect of regional application of BDM via a jet on a sustained arrhythmiaA, continuous chart recording of force during a sustained arrhythmia induced by electric stimulation (ST). Arrow
with OFF indicates when the BDM jet was turned off. After the jet was turned off, the contractions became larger
and slower. B, expanded recordings of force (black lines), membrane potential (red line), [Ca2+ ]i at the positionindicated by the dotted white lines on the bottom (blue lines), and regional changes in [Ca2+ ]i (bottom) during
the periods indicated by lines a and b in A. Note initiation of arrhythmia by Ca2+ wave seen in Ba. This Ca2+ wave
is dependent on the amount of Ca2+ released from troponin C upon quick release. From Miura et al. (2010).

C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

J Physiol 00.0

Mechano-transduction in cardiomyocytes

whole-heart studies demonstrated that volume overloadin the ventricles could lead to an up to 20% increase in theupper limit of vulnerability, a surrogate for a defibrillationthreshold.
The theoretical studies discussed above demonstratevery nicely the quantitative feasibility that SAC and MEC
effects can contribute significantly to cardiac arrhythmias(for rationally designed models and properties of myofilaments and SAC). What would be especially nice to

9

see going forward is direct experimental validation thatthese mechanisms suffice to mediate arrhythmogenesis in
the functioning mammalian heart, and for example canbe prevented by the inhibition of these implicated MEC
perpetrators.
Mechano-chemo-transduction (MCT)
Consensus, controversies and open issues

There is consensus on the following:(1) The Frank-Starling (F-S) mechanism and Anrep
effect.

r The heart has the intrinsic ability to adapt
to changes in preload and afterload. Intrinsic
adaptation has two components: the Frank-Starlingmechanism (heterometric) and the Anrep effect(homeometric or inotropic).
r The Frank-Starling mechanism is activated byelevated end-diastolic volume (and sarcomere
length) and is thus under heterometric regulation.
The F-S mechanism is instantaneous (beat to beat)without requiring Ca2+ changes.
r The Anrep effect occurs gradually over many beats(minutes timescale) and increasing contractility
by increasing the cytosolic Ca2+ transients and/or
myofilament Ca sensitivity. Multiple biochemicalsignalling pathways have been implicated. Anrep
can restore EDV and sarcomere length towards theinitial state (and is thus homeometric).

(2) Mechano-chemo-transduction (MCT) pathways.

r MCT activates various chemo-transducers that lead

Figure 5. Mechanically induced arrhythmia in the regionallyischaemic rabbit heartA, distribution of muscle strain (Eff ) in the normal and ischaemic
heart during systole. B, evolution of a mechanically induced
ventricular premature beats (VPB). C, Vm traces at sites 1 and 2
marked in the 191-ms inset in B. Black arrow and dashed circle
denote mechanically induced depolarization at these sites. Red
arrow indicates activation at site 2 by propagation of themechanically induced VPB. D, activation maps for the full model,
model ‘No SAC’, and model ‘No Ischemia’ during VPB. No SAC
incorporates all the electrophysiological changes in ischaemia, but
without involvement of SAC, and, thus, without mechanicallyinduced DAD; external stimuli were applied at the same time andlocations where the mechanically induced VPB originated in the fullmodel (arrows in 191-ms inset in B). No Ischemia included SAC andthus the spatial distribution of mechanically induced DAD, but
without ischaemia-induced electrophysiological changes. Black lines
denote conduction block. Asterisk: reentry exit site. Snapshots oftransmembrane potentials are also presented for models No SAC
and No Ischaemia. Modified from Jie et al. (2010).

C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

to modulation of the Ca2+ handling system in the
cardiomyocyte.
r MCT pathways involve cardiac mechanosensors,
chemotransducers, and effectors;
r The mechanical sensors may include myofilament proteins (e.g. titin and crossbridges)
and cytoskeletal and transmembrane protein
complexes that interact with extracellular matrix
and neighbouring cells (e.g. at focal adhesions,dystrophin, integrins and desmosomes).
Controversies and open issues are:r Different labs have found different roles for nitric oxide(NO) signalling in mechano-chemo-transduction:some found that NO plays no role in stretch-induced
Ca2+ sparks, but others found that NO is criticallyimportant for MCT.
r The apparent controversy above might be explainedby hypothesizing that different ‘dimensionality’ of themechanical strain or stress imposed on surface versus

10

L. T. Izu and others

internal mechanosensors may activate different MCT
pathways.
r How do cardiomyocytes distinguish mechanical strain(e.g. stretch) and mechanical stress (e.g. active force or
passive stiffness)?
r What are the mechanosensors activated by celllongitudinal strain and stress?
r What are the mechanosensors activated by celltransverse compression?
r What are the mechanosensors activated by surface
traction and shear stress?
r What are the mechano-chemo-transduction pathways
downstream from each mechanosensor to chemotransducers and to effectors: which, where and how?
r How does mechano-chemo-transduction orchestrateconcerted modulations of multiple Ca2+ handlingmolecules to autoregulate the Ca2+ transient andcontractility?

Frank-Starling mechanism and Anrep effect

The heart has the remarkable ability to adapt its output totremendous variations in the haemodynamic demand of
the body, with the cardiac output ranging from 3 L min−1
at resting level to 20 L min−1 during vigorous exercise.
The heart must not only be able to increase the output
but also do so in the face of afterloads (arterial pressure)that range from < 100 to > 300 mmHg (MacDougall et al.
1985). As Ernest Starling writes in his Linacre Lecture, thisadaptation to changing demands “occurs equally well after
total destruction of the nerves connecting the heart withthe central nervous system” (p.5, Starling, 1915). In otherwords, the adaptive ability of the heart lies largely within
the heart itself. Otto Frank (Frank, 1895) is equally credited
for what is usually called the F-S mechanism. The quest
for molecular understanding has occupied physiologists
since that time.
In this section we will not recapitulate the excellent
recent review on MCT (Schonleitner et al. 2017). Instead,we will recount some old and new research that brings into
focus fundamental questions that remain unanswered andsuggest modern experiments that may cast new light onto
the intrinsic mechanisms that underlie the adaptation of
the heart to changing mechanical loads.
Starling’s promulgation of the ‘law of the heart’(Patterson et al. 1914) and Anrep’s work on the heart’s
adaptation to afterload (von Anrep, 1912) mark themodern period of investigating the heart’s adaptive
response to mechanical load. Starling (Patterson et al.
1914, Starling, 1915) described the law of the heart as“the energy of contraction . . . is a function of the lengthof the muscle fibre.” The Frank-Starling mechanism andAnrep effect are both illustrated by the pressure-volume(P-V) loops obtained from the LV in a mouse in Fig. 6A.

J Physiol 00.0

Curve a is the P-V loop before the outflow resistance wasincreased by transaortic constriction (TAC). Immediately
after TAC, the LV end-diastolic volume (EDV) expands(note the rightward shift of P-V loop b). This expansion
engages the F-S mechanism resulting in more forcefulcontraction that manifests itself as an increase in developed
pressure. The relationship between increasing contraction
force and increasing muscle length stands as one of
the great foundation stones of cardiac physiology. This
fundamental relationship at the sarcomeric level is shownby the monotonically increasing F-S curve (F-S) in Fig. 6B,
based on experiments using trabeculae (Allen & Kurihara,
1982), which shows developed force (F) as a function of
sarcomere length (L).
The F-S law of the heart has stood the test of time, buteven as it was being established, evidence emerged thatmuscle length was not the sole determinant of contractile force. Gleb von Anrep (von Anrep, 1912) found that
increasing the outflow resistance caused an initial increase
in LV EDV (the rightward shift in the P-V loop a to b
in Fig. 6A) but after 10–30 min EDV recovered almost
completely (P-V loop shift from b to c), exhibiting thenormal stroke volume at much higher pressure and stroke
work (SW). Indeed, compared to P-V loop a, the LV wasproducing much higher pressure at the same EDV (or SL).
This is the eponymous Anrep effect.
Figure 6C and D shows the time course of P-V loopchanges (from panel A) in finer detail for EDV, ESV,ejection fraction (EF) and dP/dtmax upon TAC at time
0. These data show that immediately following TAC
there is an expansion of the EDV and ESV (panel C),
a decrease in ejection fraction, and an increase incontraction force as indexed by a greater dP/dt (panel D).
The increase in contraction force is expected from
Starling’s law as a consequence of the increased EDV.
After 30 min, both the EDV and ESV have returned totheir pre-TAC values. Despite the heart’s return to theinitial LV chamber volume, the contractile force remains
sufficiently large (measured by dP/dt) for the ejection
fraction – and therefore the stroke volume (indicated by
the equal widths of the P-V loops before and 30 min after
TAC in panel A) – to recover completely. At the 30 min
time point the increased force cannot be due to the F-S
mechanism because the EDV is at its initial value so themuscle lengths are the same.

Load-tuning property of cardiac muscle: generationof different forces under different mechanical loads
at the same muscle length

We can use Figure 6B to interpret the results shown inpanels A and C. Let Lo be the muscle length at end-diastole(EDV1/3 ) before outflow resistance increase. At this timethe muscle develops force F(a). Let F∗ be the force neededC 2019 The Authors. The Journal of Physiology C 2019 The Physiological SocietyMechano-transduction in cardiomyocytes

J Physiol 00.0

to eject the blood at the higher outflow resistance. The
initial increase in EDV engages the F-S mechanism andafter a few beats the LV expands to the point where
the force of contraction is sufficient to eject the volumeof blood equal to the venous return. This chamber
volume corresponds to length L∗ and the muscle generates
force F(b). During this phase, the muscle length travels
along path A of the Allen-Kurihara F-S curve. As the
EDV decreases (Fig. 6A and C) the force of contraction
must be maintained to eject blood against the higher
output resistance, so the trajectory of the muscle length ishorizontal along path B. After about 15 min the EDV stops
decreasing to a value equal to its initial value Lo . At thesteady state the muscle length equals Lo but must generate
force F(c) = F(b). Therefore, the muscle of length Lo cangenerate different forces, as seen in F(a) and F(c). The
A

Baseline
TAC 1 minute
TAC 30 minutes

Tsp4+/+

150

b

F (c)

Peak tension F

LV Pressure (mmHg)

F-S

a
60

C

10

20

30

40

F (b)

a
LO

50

LV Volume (μL)

b

A
F (a)

0

B

ΔF

30

Fiber length

L*

D 1.3

2.2
2.0

ESV

1.8

1.2

EDV

Relative change

Relative change

B

90

0

C

difference between these two forces, F = F(c) − F(a)reflects the increase in contractility. We call this abilityof the muscle to generate different forces under different
loads at a given muscle length the load-tuning property.
In fact, such load tuning is an inherent propertyof the heart, whose contractions have to be ‘internally heterogeneous’ to achieve ‘externally homogeneous’
pump function (Katz & Katz, 1989). The auto-regulatory
ability of the heart to adjust its regional contractility
to spatio-temporally varying alterations that are driven
by haemodynamic demand has been studied extensively,
including ‘duplex models’ of isolated interacting muscle
segments. These are kept in defined mechanical andbiochemical conditions (interacting mechanically, while
positioned in two separate baths), including more
recent approaches where biological muscle segments

C

120

11

1.6
1.4
1.2

1.1
1.0
dP/dt

0.9

EF

1.0
0.8
0.8
0

5

10

15

20

25

30

Time after TAC (minutes)

–5

0

5

10

15

20

25

Time after TAC (minutes)

Figure 6. Frank-Starling law and Anrep effectA, pressure-volume (P-V) loops before TAC (light black), 1 min after TAC (dashed curve), and 30 min after TAC(heavy black). From Cingolani et al. (2011). B, black curve shows the relationship between fibre length and force
generated by Frank-Starling (F-S) mechanism. Lo is the initial muscle length and steady state muscle length after thechamber volume has decreased due to stress-stimulated contractility (SSC) increase. F (green arrow) is the force
SSC needs to generate. F-S curve redrawn from Allen & Kurihara (1982). C and D, time evolution of left ventricular(LV) haemodynamic parameters following transaortic constriction (TAC). Values are normalized to initial values,
when TAC imposed. EDV, end-diastolic volume; ESV, end-systolic volume; EF, ejection fraction; dP/dt, maximum
rate of LV pressure change. Data redrawn from Cingolani et al. (2011).

C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

30

35

12

L. T. Izu and others‘interacted’ mechanically and in real time with actuatorsthat were controlled by computational models of cardiac
electro-mechanical activity (Tyberg et al. 1969; Wiegner
et al. 1978; Markhasin et al. 2003). These studies highlightthat MCT is the basis on which individual myocardial
segments match their contractility to actual demand –in the absence of neuro-muscular junctions that grade
skeletal muscle performance (for review, see Solovyova
et al. 2014).
Given the muscle’s load-tuning force output, it is clearthat muscle length cannot be the sole determinant ofcontractile force. Other than strain (fractional lengthchange), what mechanical stimulus could also be involved?
A logical candidate is the mechanical stress (force/area).
Laplace’s law tells us that the wall stress is proportional
to the product of the chamber pressure and chamberradius. Anrep’s observation was the first indication thatthe heart responded not only to mechanical strain as
described by Starling’s law but also to mechanical stress.
The response of the heart to an increase in the outflow
resistance shown in Fig. 6 actually has four names: (1) the
Anrep effect, which honors its discoverer; (2) homeometricautoregulation (Sarnoff et al. 1960), which emphasizes theincrease of contractility at the same muscle length; (3) slow
force response (Parmley & Chuck, 1973; Donald et al. 1976;
Allen & Kurihara, 1982; Kentish & Wrzosek, 1998), whichemphasizes the slow time-dependent signalling (up to tensof minutes) to increase inotropy; and (4) stress-stimulated
contractility (Seo et al. 2014), which highlights the role ofstress instead of strain in increasing contraction force. We
think that the attachment of four different names to thesame phenomenon attests to the long history of trying tounderstand this puzzling phenomenon. The Anrep effectpresents two fundamental challenges that were addressedin this symposium: (1) How does the cardiomyocyte
distinguish mechanical stress from strain? (2) What are
the cellular and molecular mechanisms underlying thestress-stimulated contractility?

Stress-stimulated contractility is caused by increased
Ca2+ transient

The ability of cardiac muscle to generate two different
forces at the same muscle length suggests that the cardiomyocyte can distinguish stress from strain. Around the1980s, researchers used multicellular preparations suchas trabeculae and papillary muscle to measure strain
and stress, as well as cytosolic Ca2+ level. In a typicalexperiment, the muscle was stretched to a fixed lengthand isometric tension was measured. The consensus
emerging from these studies is that stretch of the muscle
caused an immediate increase in the contractile force(Frank-Starling), followed by a gradual further increaseof force over minutes, called the slow force or Anrep

J Physiol 00.0

response (Parmley & Chuck, 1973; Donald et al. 1976;
Allen & Kurihara, 1982; Kentish & Wrzosek, 1998)
Importantly, the immediate force increase did not involveany change in the Ca2+ transient, which is caused by
the strain-dependent myofilament-based F-S mechanism(Moss & Fitzsimons, 2002; Solaro, 2007), whereas the slow
force response involves increased cytosolic Ca2+ transientsand in some cases enhanced myofilament Ca2+ sensitivity
to enhance contractility (Allen & Kurihara, 1982; Kentish& Wrzosek, 1998).
Since the 1990s, researchers further developed
techniques to study the strain and stress effects on single
cardiomyocytes that allow in-depth exploration of thecellular and molecular mechanisms. A classical technique
is to attach a pair of micro-cantilevers (i.e. carbon fibres,
glass rods) to the surface of cardiomyocyte and then stretch
the cell along the longitudinal direction (Gannier, et al.
1994).
This one-dimensional (1-D) stretch technique applies
longitudinal strain and stress on the cell but not transverse
compression, because the cell is free to expand transversely
in bath solution. The Kass group (Seo et al. 2014) foundthat myocyte stretch during auxotonic contraction caused
an immediate increase of the force, and then a gradualincrease of contractility accompanied by increased Ca2+transient; this single cell response recapitulates the F-S
and Anrep effects. Kohl’s group (Iribe et al. 2009b) and
Lederer’s group (Prosser et al. 2011) found that stretch of
the resting cardiomyocyte caused a transient rise of Ca2+
sparks and waves, and repetitive stretches caused sustained
rise of Ca2+ sparks. (Prosser et al. 2013).
The Chen-Izu group developed a ‘cell-in-gel’ systemallowing them to embed single cardiomyocytes in a 3-D
hydrogel made of polymer matrix in solution (Jian et al.
2014). When the cell is paced to contract in the gel,the viscoelastic gel resists both the cell shortening andbroadening to exert 3-D stress (i.e. longitudinal tension,transverse compression, and surface traction with normal
and shear stresses; Shaw et al. 2013) The stiffness ofthe gel reduces shortening, but increases Ca2+ transient
amplitude and Ca2+ spark frequency (Fig. 7). A distinct
novelty of the ‘cell-in-gel’ approach is to apply stress on the
myocyte during auxotonic contraction against a 3-D viscoelastic matrix, without pre-stretching the cell, and thereby
separating the stress from strain. How mechanical stresses
affect the electrical dynamics in the cell-in-gel system is
yet to be studied.

Mechano-chemo-transduction pathways linking
mechanical forces to Ca2+ dynamics

Research from the 1990s to the present has focused ondelineating the molecular pathways, or MCT, that link
cellular mechanics to changing Ca2+ dynamics. TheseC 2019 The Authors. The Journal of Physiology C 2019 The Physiological SocietyMechano-transduction in cardiomyocytes

E

t

2.0

-2
-4

0

-6

el

x

Time (s)
1.0
1.5

0.5

0

ee

B

-fr

A

-G

is an matricellular protein that increases in hypertension
and heart failure. Cingolani et al. (2011) found that in micelacking TSP4 the P-V loops did not have the evolutionary

compliant carbon fibres attached on the cardiomyocyte,
Seo et al. (2014) tested the hypothesis that the transient receptor potential canonical channel 6 (TRPC6)
participates in stress-stimulated contractility. Inhibiting
TRPC6 using cGMP to activate protein kinase G (PKG) or
using a TRPC6 knockout eliminated the increase in Ca2+transient amplitude and slow force response of papillarymuscle to stretch. In whole-animal studies, where outflow
resistance was increased by TAC, the pressure-volume
loops in control animals exhibited the kind of response
shown in Fig. 6A. Immediately following TAC, the P-V
loop shifted to the right and narrowed but after 15 min
the end-diastolic volume of the P-V loop shifted backto the left (in fact below the control volume in theirfigure) and the stroke volume increased, indicating an
increase of stress-stimulated contractility. The TRPC6
knockout lacked the decrease in the end-diastolic volume
after 15 min. These elegant studies reveal the importanceof TRPC6 in stress-stimulated contractility increase but
leaves open the identity of the mechanosensor, a possible
G-coupled protein receptor.

ad

Thrombospondin pathway. Thrombospondin-4 (TSP4)

TRPC6/3-PKG pathway. Using the 1-D stretch system within

Integrin-angiotensin-NHE pathway. Mechanical stretchof β1 integrin at the cardiomyocyte surface initiates
a number of different signalling pathways by causing
release of angiotensin II (Ang II) from myocytes(Browe & Baumgarten, 2004). The Ang II then activatesangiotensin II type 1 receptor (AT1 receptor), which begins
a cascade that ultimately activates chloride-permeant
stretch-activated channels (Browe & Baumgarten, 2004)or the Na+ -H+ exchanger (NHE) and NCX, leading
to a secondary increase in the Ca2+ transient (Alvarez
et al. 1999; Calaghan & White, 2004; Cingolani et al.
2013).

hallmark characteristics of the Anrep effect shown in
Fig. 6A following acute TAC. Instead, 30 min post TAC the
P-V loop remained right-shifted and the stroke volume
was not restored. This paper shows the interaction of theextracellular matrix and the myocyte in mechano-sensing.

Lopathways have been the subject of excellent reviews by
Cingolani et al. (2013) and Schonleitner et al. (2017). Here
we briefly catalogue the known pathways and highlightsome differences that might reveal different mechanosensing modalities. We also consider that different
experimental settings involve different strains and/or
stresses on the cell (Chen-Izu & Izu, 2017), which need tobe taken into account in order to understand and compare
the data obtained using different experimental settings.

13

Shortening (%)

J Physiol 00.0

-4

-8

-8

***


C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

2

3

-fr

in
-G

ee

el

0
Lo

ad

2
1
0.5

12

1.0
1.5
Time (s)

2.0

***

***

8
NS

4

le
+B

5%
7.

el
G

bb

5%
7.
el

el

ee

5%

0
G

G

***

4

4

G

F

-fr

D

CaT peak (F/F0)

C

Lo
ad

Ca2+ sparksA, schematic diagram of a myocyte embedded in
3-D hydrogel matrix containing red fluorescencebeads. B, confocal imaging of the myocyte andbeads demonstrating cell contraction and gel
deformation as seen in the movement of the cell’sedge. C, cell contracting in normal Tyrode’s solution
provides load-free control. D, cell contracting in gel
under load. E, fractional shortening of cell
contraction in gel (n = 17 cells) compared withload-free control (n = 17). F, systolic Ca2+ transient(CaT) peak in cell-in-gel (n = 17) compared withload-free control (n = 17). G, diastolic Ca2+ spark
frequency in the cells for load-free control (n = 18),
in soft gel made of 5% crosslinker (Gel5%, n = 9),
in gel with 7.5% crosslinker (Gel7.5%, n = 18),
and after blebbistatin treatment (n = 5). One-way
ANOVA with Bonferroni post hoc test was used forpair-wise comparison: ∗∗∗ P < 0.001. Reproduced
from Jian et al. (2014).

Spark rate(/100μm*s)

Figure 7. Stress-stimulated Ca2+ transient and14

L. T. Izu and others

NOX-ROS pathway. Using the 1-D stretch system withcarbon fibres attached to the cardiomyocyte, Iribe et al.
(2009b) showed that stretch of resting cardiomyocytes
elicited a transient increase in the Ca2+ spark rate.
Subsequently, Prosser et al. (2011) showed that reactive
oxygen species (ROS) production linked to isoform 2of NADPH oxidase (NOX2), which they call X-ROS
signalling, was responsible for the stretch-induced Ca2+
spark rate increase. The link between cellular mechanics
and X-ROS signalling is compelling because an intact
microtubular network is essential for the stretch-induced
Ca2+ spark rate increase (Iribe et al. 2009b; Prosser et al.
2011; Khairallah et al. 2012; Robison et al. 2016). In
support of this possibility, modelling studies by Limbu
et al. (2015) show that X-ROS activation during thediastolic filling phase of the heart could increase themagnitude of the Ca2+ transient.

J Physiol 00.0

ient outward current (Ito ) and a late low plateaulevel dominated by inward NCX current. In contrast,
larger mammals have long APDs with positive plateaus,
little Ito and repolarization that is driven by delayed
rectifier K+ currents (IKr and IKs ), which are absent inrodents. Furthermore, in rodents, Ca2+ handling is overwhelmingly dominated by SR Ca2+ release and reuptake,with little Ca2+ influx via Ca2+ current, and SR Ca2+
stores are higher at baseline and are maintained in
resting cardiomyocytes. In larger mammals, a greater
fraction of E-C coupling-related Ca2+ enters with eachbeat via Ca2+ current and leaves via NCX, and SR Ca2+
content increases with heart rate are critical to inotropic
reserve. Another complicating factor is that experiments
are often performed at room temperature, which canalter electrophysiological, Ca2+ handling and contractile
properties. Hence, caution needs to be taken in translating
the experimental findings in rodent models to humanpathophysiology.

NOS-NO pathway. Using a cell-in-agar technique, Petroff

et al. (2001) found that stretching the tubing encasing
the cell-in-agar increased Ca2+ transient amplitude andthe Ca2+ spark rate in cardiomyocytes. This MCT
pathway involves nitric oxide (NO) signalling because
both the Ca2+ sparks and the Ca2+ transient increase
were abolished by inhibiting nitric oxide synthases(NOSs) using L-NAME (NG -nitro-L-arginine methyl
ester) or by genetic knockout of eNOS (eNOS−/− ).
Using the 3-D cell-in-gel system, Jian et al. (2014)
found that stress-stimulated Ca2+ sparks were abolishedby pharmacological inhibition of nNOS or by genetic
knockout of nNOS (nNOS−/− ). Furthermore, bothconstitutive nNOS and eNOS are involved in a
stress-stimulated increase in the Ca2+ transient, and thestress effect on the Ca2+ transient was abolished byinhibiting NOS (Jian et al. 2014). These data strongly
support the hypothesis that NOSs and NO signalling
serve as a critically important chemo-transducer in the
stress-stimulated MCT pathway.
The above studies show that mechanical strain and stresson cardiomyocytes activate various chemotransducers
that lead to modulation of the intracellular Na+ andCa2+ handling system. The effects of mechanical strain
and stress on the Ca2+ homeodynamics have several
functional consequences. For example, a stress-induced
increase of systolic Ca2+ transient through the TRPC6/3
and NOS-NO pathways is expected to enhance thecardiomyocyte contractility. Strain- and stress-induced
spontaneous Ca2+ sparks and waves are expected topromote arrhythmogenic activities. However, the electrophysiological and Ca2+ handling systems in rodent myocytes differ significantly from that of larger mammalian(e.g. rabbit, dog, pig, human; Bassani & Bers, 1994;
Bers, 2001). Rodent (mouse and rat) ventricular myocytes exhibit very brief APD driven by robust trans-

Different mechano-sensing modalities

Interestingly, the involvement of NOS-NO signalling in the
MCT pathway differs drastically in different experimental
settings. As mentioned before, the NOS-NO signalling has
been found to be critically involved in stress-stimulated
MCT in the 3-D cell-in-gel system (Jian et al. 2014) andin the 3-D cell-in-agar experiment (Petroff et al. 2001).
In contrast, inhibiting NOS with L-NAME did not affectstrain-stimulated MCT in the 1-D stretch system, as foundin the studies by Calaghan & White (2004), Seo et al.
(2014), Iribe et al. (2009b) and Prosser et al. (2011) To
resolve this apparent controversy, Chen-Izu & Izu (2017)
proposed a hypothesis on different mechano-sensing
modalities based on the ‘dimensionality’ of the mechanosensors (i.e. 1-D longitudinal strain vs. 3-D stresses).
The apparent controversy on the NOS-NO involvement
in MCT pathways opens a window into the different
mechano-sensing modalities. One critical difference
between the 1-D stretch experiments and the 3-D
cell-in-gel experiments is the engagement of cell-surface
mechanosensors (Chen-Izu & Izu, 2017). Figure 8
illustrates a conceptual model of how different mechanosensors may respond to different strains and stresses.
The surface mechanosensors are illustrated as the yellow
circles (or ellipses if under strain/stress), and the internal mechanosensors are seen as the red or green circles(or ellipses if under strain/stress). When the myocyte
lengthens (or contracts) along the longitudinal axis,
the cell also narrows (or widens) in the transverse
direction because cell volume is conserved. The internal mechanosensors experience the same strain (relative
length change) for a given cell strain during stretch
or contraction, regardless of the external environment.
By contrast, the surface mechanosensors are profoundlyC 2019 The Authors. The Journal of Physiology C 2019 The Physiological SocietyMechano-transduction in cardiomyocytes

J Physiol 00.0

affected by the stiffness of the external environment.
Specifically, in the 1-D stretch experiments, myocytes
were immersed in bath solution and stretched along thelong axis, so the myocyte is largely under longitudinalstrain and stress. The bath solution imposes little transverse compression or shear stress on the cardiomyocyte;
hence, the cell-surface mechanosensors are not understress during stretch or contraction. However, in thecell-in-gel system, the myocyte embedded in a 3-D viscoelastic gel must pull the gel along the long axis as it
contracts and push the gel along the transverse axis as
it widens, and the cell surface pulls against shear force(Shaw et al. 2013); hence, the surface mechanosensors are
subject to transverse and shear stresses during stretch orcontraction (as in the myocardium, extracellular matrix,
or hydrogel polymer matrix). Therefore, we propose
a simple hypothesis, according to Occam’s Razor, thatthe cell-surface mechanosensors activate the NOS-NO
signalling pathway.

Perspectives
Linking modelling and experiments

The study of the heart has enjoyed a stronger interaction between mathematical modelling and experimental
work than many other areas of physiology. Niederer et al.
(2019) give an excellent history of the development ofmathematical models of cardiac mechanics. A special issue
in the JMCC (introduced by Smith and Niederer (Smith &Niederer, 2016)) focuses on cardiac modelling at both thecellular and tissue and whole-organ levels. An important
theme from this special issue is the increased specificity of
STRETCH15

the models, which enables a closer comparison between
experimental data and simulation results.
Here we focus first on what we consider successstories and later on challenges that arise in studying
mechano-electrical coupling and mechano-chemotransduction. Our understanding of the mechanisms
underlying the Anrep effect in the whole heart andits correlates, slow force response and stress-stimulated
contractility in cardiac muscle and single cardiomyocytes,
remains incomplete. Because experimental work in the
1980s showed that the SFR required Ca2+ entry, astretch-sensitive L-type Ca2+ channel would seem to beimportant for the SFR. The importance of modelling ishighlighted by Bluhm et al. (1998), who showed that astretch-sensitive Ca2+ channel would not account for theslow increase of force characteristic of the Anrep effect.
Rather they were able to model both the magnitude andtiming of the SFR by assuming that the Na+ -K+ pumpor the Na+ leak current were stretch dependent. Later
Niederer & Smith (2007) were able to model the SFR
using a non-specific stretch-activated current that allowed
Na+ entry in conjunction with NHE. A surprising andimportant finding was that the stretch-sensitive NHE
could not, by itself, generate the SFR, contrary to the view
arising from extensive experimental studies. These studies
illustrate some of the important contributions modelling
can make to understanding cardiac physiology.
The Niederer & Smith (2007) study showed how
the interaction of SAC and NHE generates the SFR.
Importantly, they explained how the experimental
protocol of preincubation with NHE blockers might have
led to a misinterpretation of the primary role of the NHE
in SFR. The explanation is subtle; SFR depends on the
CONTRACTION

Cell-in-solution

Cell‐in‐gel

Figure 8. Conceptual model of how different mechanosensors may sense different strain and stress
Upper row: when the cell is in solution, the surface mechanosensors (yellow circle with spring) are not undertransverse stress and surface traction. Lower row: when the cell is contracting in 3-D viscoelastic environment,
the surface mechanosensors are under transverse compression and shear stresses. The internal mechanosensors(green and red circles with spring) are under longitudinal tension during stretch or compression during contraction.
C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

16

L. T. Izu and others

balance between Na+ influx and efflux, which can shift
with the resting Na+ level because of the cubic [Na+ ]I
dependence of the NCX flux. Long incubation with an
NHE blocker reduced [Na+ ]i to a point that limits theability of NCX to raise [Ca2+ ]i to support the developmentof the SFR. It would probably be difficult to disentangle
these interactions by experiments alone.
Iribe et al. (2009a) and Prosser et al. (2011, 2013)
showed that stretch immediately increases Ca2+ spark
rate, possibly through X-ROS signalling. In an editorial
Mark Cannell (Cannell, 2009) suggested that thestretch-induced Ca2+ spark rate increase might contributeto Starling’s law of the heart and in particular the SFR.
Limbu et al. (2015) modelled X-ROS signalling undercyclic stretching in a single myocyte and calculated spark
rates and Ca2+ transients. The model faithfully reproduced
the Ca2+ spark data from experiments but the Ca2+transient results diverged from experimental data. In
their simulations the initial stretch caused an immediate
increase in the Ca2+ transient magnitude but its peakmagnitude declined on subsequent stretches. These results
are inconsistent with the body of experimental results
showing no increase in the Ca2+ transient in the initialstretch and slowly increasing Ca2+ transient amplitude.
Such trenchant differences between model results andexperiments should be viewed positively because they
open opportunities for modellers and experimentalists tofocus on how the model got some things right and where
the model needs modification.
We think the current computational models of
cardiac E-C coupling need to be further improved.
Many parameters in the current models were derived
from experimental data obtained in different species
and under various artificial conditions. Hence themodels need to be improved based on new dataobtained under more physiological conditions, such as
using mechanically loaded cardiomyocytes (instead ofload-free cells). Furthermore, adding mechanical load
and mechano-chemo-electro-transduction feedbacks into
the E-C coupling model will be a necessary step
towards developing more realistic models. Recognizing
the importance of the back-and-forth dialogue between
modelling and bench experiments, it is critically important
to bring together modellers and experimentalists to havethis dialogue, to learn from each other, and to developinterdisciplinary collaborations.
Plurality of mechano-chemo-electro-transduction
pathways

The plurality of mechano-chemo-electro-transduction
pathways discovered so far suggest that mechanicalstrain and stress on the cardiomyocyte can be sensed
and transduced by various mechano-sensors located indifferent sub-cellular structures. In this white paper

J Physiol 00.0

we focus only on a few mechano-electro-transduction
and mechano-chemo-transduction pathways discussed
in the symposium and depicted in Figure 9. We
recognize the existence of many other mechanosensors and their mechano-electro-transduction andmechano-chemo-transduction pathways and recommend
excellent reviews on the Anrep effect (Cingolani et al.
2013), SAC (Baumgarten, 2007; Peyronnet et al. 2016),
and mechano-sensitivity of Ca2+ signalling (Schonleitneret al. 2017). Here we wish to emphasize that different
experimental settings involve different mechanical strains
and stresses, which would affect different mechanosensors depending on their sub-cellular localization anddimensionality. It is plausible that the internal mechanosensors are poised to sense longitudinal strain andtension, whereas cell-surface mechanosensors are poised
to sense surface traction and transverse compression.
Therefore, caution is needed in clarifying the precise
mechanical stimuli for analysing molecular mechanisms
and comparing the data from different experimental
settings. It is also important to use different mechanicalstimuli to interrogate different mechanosensors in orderto piece together a more precise and comprehensive mapof the mechano-chemo-electro-transduction pathways.
Why so many pathways? The heart has a plethora ofion channels, electrogenic exchangers and pumps thatare needed to adapt the action potential to the changing
Mechano-Chemo-Electro-Transduction
Extracellular Matrix
Surface-Sensor-X
Internal-Sensor-Y

Nox2
ROS
SAC

AngII

Ca2+

Integrins

PKG
AMPK

TRPC6/3

nNOS
eNOS
NO
CaMKII
VAC

Figure 9. Schematic diagram of themechano-chemo-electro-transduction (MCET) pathways in the
cardiomyocyte embedded in extracellular matrix (ECM)
Stretching the cell applies mechanical strain that can be sensed byinternal mechanosensors (green). Cell contraction against ECM
applies mechanical stress that can be sensed bysurface-mechanosensors (yellow). The molecular identities of manymechanosensors are yet to be discovered (labelled X and Y). Depicted
are several known MCET pathways: Nox2-ROS, integrin-AngII,
TRPC6/3 and nNOS-eNOS-NO-CaMKII pathways; all these pathways
lead to modulation of intracellular Ca2+ homeodynamics (see MCT
section); all these pathways lead to modulation of intracellular Ca2+
homeodynamics (see MCT section). Stretch-activated channels (SAC)and volume activated channels (VAC) have also been linked to Ca2+
permeability that may affect the Ca2+ entry and intracellular Ca2+
homeodynamics (see MEC section).
C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

J Physiol 00.0

Mechano-transduction in cardiomyocytesdemands of the body. One wonders whether the pluralityof mechanosensors and MCT pathways might be neededto adapt the force generation to different stressors anddemands. Different mechanosensors and MCT pathways
might stratify into different time and force hierarchies.
Time hierarchy. The F-S mechanism and the Anrep

effect have distinct time domains. The F-S mechanism
is fully engaged within one or two heartbeats while theAnrep effect develops over many minutes. Teleologicallysuch a separation into rapid and slow time domains
makes sense. Changes in venous return due to postural changes or left-right ventricular flow mismatch aretransient disturbances that the F-S mechanism rapidly
corrects without the expense of increased Ca2+ signalling.
Adjusting to sustained increases in afterload caused by
activities with a large static component such as boxing(Mitchell et al. 2005) or shovelling snow (Franklin et al.
1995), on the other hand, requires a different strategy.
The heart might be trading off the cost of Ca2+ signalling
to reduce the energetic cost of contracting against higher
wall tension resulting from wall stretch. Research discussed
here focused on acute changes in mechanical loadingwhere the response time of the heart is on the order of afew seconds to a few tens of minutes. How chronic hypertension, operating on time scales of months to years, affects
MEC and MEC remains largely unexplored.
Force hierarchy. Systolic blood pressure is around

120 mmHg but can reach 350 mmHg while doing double
leg presses (MacDougall et al. 1985). Just as different tools
are used to measure distance and force it seems reasonable
that different mechanosensors and MCT pathways might
respond to different levels of strain and stress.
To go beyond teleological speculation, future
experiments must pay careful attention to thecharacteristics of mechanical loading (i.e. strain,stress, location, magnitude) imposed on the myocyte and determine which MCET pathway is operativeby pharmacological or genetic dissection. Sequential
application of specific channel blockers enabled electrophysiologists to ‘peel back’ the layers of ionic currents that
underlie the action potential (Banyasz et al. 2011; Hegyi
et al. 2018). Perhaps a similar approach might be useful todetermine how different MCET pathways stratify in timeand force hierarchies.
MCT and MEC feedbacks in cardiac
excitation-contraction coupling

The heart pumps blood against an ever-changing
mechanical load. The heart also has intrinsic mechanisms
to sense the strain and stress and autoregulate contractile force to maintain a constant cardiac output.
Although the heart’s response to mechanical loading
was described by Frank, Starling and Anrep more thanC 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

17

a century ago, the cellular and molecular mechanismsthat transduce mechanical load to influence the Ca2+
handling and electrical activities in the cardiomyocyte
are far from fully understood. The classic description of
excitation-contraction coupling has information flowing
from the electrical system to the Ca2+ control systemand then to the contractile system. MCT and MEC
reverse the flow of information enabling a continuously
bidirectionally information feedforward and feedback
between these three dynamic systems. According to control
theory, such closed-loop feedback is necessary for autoregulation of the system. Continuing research on MCT
and MEC is essential for understanding the heart as asmart pump that senses mechanical strain and stress andautoregulates contractile force to maintain cardiac output.
But as with any control system operating out of its design
space, Ca2+ dysregulation and arrhythmias can emerge.
References
Allen DG & Kentish JC (1985). The cellular basis of thelength-tension relation in cardiac muscle. J Mol Cell Cardiol
17, 821–840.
Allen DG & Kurihara S (1982). The effects of muscle length on
intracellular calcium transients in mammalian cardiac
muscle. J Physiol 327, 79–94.
Alvarez BV, Perez NG, Ennis IL, Camilion de Hurtado MC &Cingolani HE (1999). Mechanisms underlying the increasein force and Ca2+ transient that follow stretch of cardiac
muscle: a possible explanation of the Anrep effect. Circ Res
85, 716–722.
Banyasz T, Horvath B, Jian Z, Izu LT & Chen-Izu Y (2011).
Sequential dissection of multiple ionic currents in single
cardiac myocytes under action potential-clamp. J Mol Cell
Cardiol 50, 578–581.
Bassani RA & Bers DM (1994). Na-Ca exchange is required forrest-decay but not for rest-potentiation of twitches in rabbit
and rat ventricular myocytes. J Mol Cell Cardiol 26,
1335–1347.
Baumgarten CM (2007). Cardiac Mechanotransduction, pp.
8–27. Landes Bioscence.
Belmonte S & Morad M (2008). ‘Pressure-flow’-triggered
intracellular Ca2+ transients in rat cardiac myocytes:
possible mechanisms and role of mitochondria. J Physiol
586, 1379–1397.
Belus A & White E (2003). Streptomycin and intracellular
calcium modulate the response of single guinea-pig
ventricular myocytes to axial stretch. J Physiol 546, 501–509.
Bers DM (2001). Excitation-Contraction Coupling and Cardiac
Contractile Force. Kluwer Academic Press, Dordrecht,
Netherlands.
Bers DM (2014). Cardiac sarcoplasmic reticulum calcium leak:basis and roles in cardiac dysfunction. Annu Rev Physiol 76,
107–127.
Bluhm WF, Lew WY, Garfinkel A & McCulloch AD (1998).
Mechanisms of length history-dependent tension in an ionicmodel of the cardiac myocyte. Am J Physiol Heart Circ
Physiol 274, H1023–H1040.

18

L. T. Izu and others

Bode F, Sachs F & Franz MR (2001). Tarantula peptide inhibits
atrial fibrillation. Nature 409, 35–36.
Bollensdorff C & Lab MJ (2011). Stretch effects on potassium
accumulation and alternans in pathological myocardium. In
Cardiac Mechano-Electric Coupling and Arrhythmias, 2ndedn, eds Kohl P, Sachs F & Franz MR. Oxford University
Press, Oxford.
Botcherby EJ, Corbett A, Burton RA, Smith CW, Bollensdorff
C, Booth MJ, Kohl P, Wilson T & Bub G (2013). Fastmeasurement of sarcomere length and cell orientation in
Langendorff-perfused hearts using remote focusing
microscopy. Circ Res 113, 863–870.
Boycott HE, Barbier CSM, Eichel CA, Costa KD, Martins RP,
Louault F, Dilanian G, Coulombe A, Hatem SN & Balse E(2013). Shear stress triggers insertion of voltage-gatedpotassium channels from intracellular compartments inatrial myocytes. Proc Natl Acad Sci U S A 110,
E3955–E3964.
Brette F, Despa S, Bers DM & Orchard CH (2005).
Spatiotemporal characteristics of SR Ca2+ uptake and releasein detubulated rat ventricular myocytes. J Mol Cell Cardiol
39, 804–812.
Browe DM & Baumgarten CM (2004). Angiotensin II (AT1)
receptors and NADPH oxidase regulate Cl− current elicitedby beta1 integrin stretch in rabbit ventricular myocytes.
J Gen Physiol 124, 273–287.
Calaghan S & White E (2004). Activation of Na+ -H+ exchange
and stretch-activated channels underlies the slow inotropic
response to stretch in myocytes and muscle from the rat
heart. J Physiol 559, 205–214.
Cannell MB (2009). Pulling on the heart strings: a newmechanism within Starling’s Law of the Heart? Circ Res 104,715–716.
Chen-Izu Y & Izu LT (2017). Mechano-chemo-transduction in
cardiac myocytes. J Physiol 595, 3949–3958.
Cingolani HE, Pérez NG, Cingolani OH & Ennis IL (2013). The
Anrep effect: 100 years later. Am J Physiol Heart Circ Physiol
304, H175–H182.
Cingolani OH, Kirk JA, Seo K, Koitabashi N, Lee DI,
Ramirez-Correa G, Bedja D, Barth AS, Moens AL & Kass DA(2011). Thrombospondin-4 is required for stretch-mediatedcontractility augmentation in cardiac muscle. Circ Res 109,1410–1414.
Clemo HF, Stambler BS & Baumgarten CM (1999).
Swelling-activated chloride current is persistently activatedin ventricular myocytes from dogs with tachycardia-induced
congestive heart failure. Circ Res 84, 157–165.
Cooper PJ & Kohl P (2005). Species- andpreparation-dependence of stretch effects on sino-atrial
node pacemaking. Ann N Y Acad Sci 1047, 324–335.
Cooper PJ, Lei M, Cheng LX & Kohl P (2000). Selected
contribution: axial stretch increases spontaneous pacemaker
activity in rabbit isolated sinoatrial node cells. J Appl Physiol(1985) 89, 2099–2104.
Coste B, Xiao BL, Santos JS, Syeda R, Grandl J, Spencer KS,
Kim SE, Schmidt M, Mathur J, Dubin AE, Montal M &Patapoutian A (2012). Piezo proteins are pore-forming
subunits of mechanically activated channels. Nature 483,
176–181.

J Physiol 00.0

Cox CD, Bae C, Ziegler L, Hartley S, Nikolova-Krstevski V,
Rohde PR, Ng CA, Sachs F, Gottlieb PA & Martinac B(2016). Removal of the mechanoprotective influence of thecytoskeleton reveals PIEZO1 is gated by bilayer tension. Nat
Commun 7, 10366.
Decher N, Ortiz-Bonnin B, Friedrich C, Schewe M, Kiper AK,
Rinne S, Seemann G, Peyronnet R, Zumhagen S, Bustos D,
Kockskamper J, Kohl P, Just S, Gonzalez W, Baukrowitz T,
Stallmeyer B & Schulze-Bahr E (2017). Sodium permeable
and “hypersensitive” TREK-1 channels cause ventriculartachycardia. EMBO Mol Med 9, 403–414.
Donald TC, Peterson DM, Walker AA & Hefner LL (1976).
Afterload-induced homeometric autoregulation in isolated
cardiac muscle. Am J Physiol 231, 545–550.
Frank O (1895). Zur Dynamik des Herzmuskels. Z Biol 32,
370–447.
Franklin BA, Hogan P, Bonzheim K, Bakalyar D, Terrien E,
Gordon S & Timmis GC (1995). Cardiac demands of heavy
snow shoveling. JAMA 273, 880–882.
Franz MR, Cima R, Wang D, Profitt D & Kurz R (1992).
Electrophysiological effects of myocardial stretch andmechanical determinants of stretch-activated arrhythmias.
Circulation 86, 968–978.
Gannier F, White E, Garnier & Le Guennec JY (1996). A
possible mechanism for large stretch-induced increase in[Ca2+ ]i in isolated guinea-pig ventricular myocytes.
Cardiovasc Res 32, 158–167.
Gannier F, White E, Lacampagne A, Garnier D & Le Guennec
JY (1994). Streptomycin reverses a large stretch induced
increases in [Ca2+ ]i in isolated guinea pig ventricular
myocytes. Cardiovasc Res 28, 1193–1198.
Garny A & Kohl P (2004). Mechanical induction ofarrhythmias during ventricular repolarization: modelingcellular mechanisms and their interaction in two
dimensions. Ann N Y Acad Sci 1015, 133–143.
Hegyi B, Bossuyt J, Griffiths LG, Shimkunas R, Coulibaly Z,
Jian Z, Grimsrud KN, Sondergaard CS, Ginsburg KS,
Chiamvimonvat N, Belardinelli L, Varro A, Papp JG,
Pollesello P, Levijoki J, Izu LT, Boyd WD, Banyasz T, Bers
DM & Chen-Izu Y (2018). Complex electrophysiologicalremodeling in postinfarction ischemic heart failure. Proc
Natl Acad Sci U S A 115, E3036–E3044.
Hu Y, Gurev V, Constantino J, Bayer JD & Trayanova NA(2013). Effects of mechano-electric feedback on scroll wave
stability in human ventricular fibrillation. PLoS One 8,
e60287.
Huang H, Liang L, Liu P, Wei H, Sachs F, Niu W & Wang W(2013). Mechanical effects on KATP channel gating in rat
ventricular myocytes. PLoS One 8, e63337.
Iribe G, Ward CW, Camelliti P, Bollensdorff C, Mason F,
Burton RA, Garny A, Morphew MK, Hoenger A, Lederer WJ& Kohl P (2009a). Axial stretch of rat single ventricularcardiomyocytes causes an acute and transient increase inCa2+ spark rate. Circ Res 104, 787–795.
Iribe G, Ward CW, Camelliti P, Bollensdorff C, Mason F,
Burton RAB, Garny A, Morphew MK, Hoenger A, Lederer
WJ & Kohl P (2009b). Axial stretch of rat single ventricularcardiomyocytes causes an acute and transient increase inCa2+ spark rate. Circ Res 104, 787–795.


C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

J Physiol 00.0

Mechano-transduction in cardiomyocytes

Jian Z, Han H, Zhang T, Puglisi J, Izu LT, Shaw JA, Onofiok E,
Erickson JR, Chen Y-J, Horvath B, Shimkunas R, Xiao W, Li
Y, Pan T, Chan J, Banyasz T, Tardiff JC, Chiamvimonvat N,
Bers DM, Lam KS & Chen-Izu Y (2014).
Mechanochemotransduction during cardiomyocyte
contraction is mediated by localized nitric oxide signaling.
Sci Signal 7, ra27.
Jie X, Gurev V & Trayanova N (2010). Mechanisms of
mechanically induced spontaneous arrhythmias in acuteregional ischemia. Circ Res 106, 185–192.
Jin H, Iribe G & Naruse K (2017). Effects of bepridil on
stretch-activated BKca channels and stretch-induced
extrasystoles in isolated chick hearts. Physiol Res 66, 459–465.
Kaufmann R & Theophile U (1967). Autonomously promoted
extension effect in Purkinje fibers, papillary muscles andtrabeculae carneae of rhesus monkeys. Pflügers Archiv 297,
174.
Katz AM & Katz PB (1989). Homogeneity out of heterogeneity.
Circulation 79, 712-717.
Kentish JC & Wrzosek A (1998). Changes in force and cytosolic
Ca2+ concentration after length changes in isolated rat
ventricular trabeculae. J Physiol 506, 431–444.
Khairallah RJ, Shi G, Sbrana F, Prosser BL, Borroto C, Mazaitis
MJ, Hoffman EP, Mahurkar A, Sachs F, Sun Y, Chen Y-W,
Raiteri R, Lederer WJ, Dorsey SG & Ward CW (2012).
Microtubules underlie dysfunction in Duchenne muscular
dystrophy. Sci Signal 5, ra56.
Kohl P, Bollensdorff C & Garny A (2006). Effects ofmechanosensitive ion channels on ventricularelectrophysiology: experimental and theoretical models. Exp
Physiol 91, 307–321.
Kohl P, Day K & Noble D (1998) Cellular mechanisms ofcardiac mechano-electric feedback in a mathematical model.
Can J Cardiol 14, 111–119.
Kohl P, Hunter P & Noble D (1999) Stretch-induced changes inheart rate and rhythm: clinical observations, experiments
and mathematical models. Prog Biophys Mol Biol 71, 91–138
Kohl P, Nesbitt AD, Cooper PJ & Lei M (2001). Sudden cardiac
death by Commotio cordis: role of mechano-electricfeedback. Cardiovasc Res 50, 280–289.
Kohl P, Sachs F & Franz MR (2011). Cardiac Mechano-Electric
Coupling and Arrhythmias. Oxford University Press.
Landesberg A & Sideman S (1994). Coupling calcium binding
to troponin C and cross-bridge cycling in skinned cardiac
cells. Am J Physiol Heart Circ Physiol 266, H1260–H1271
Li W, Gurev V, McCulloch AD & Trayanova NA (2008). The
role of mechanoelectric feedback in vulnerability to electric
shock. Prog Biophys Mol Biol 97, 461–478.
Li W, Kohl P & Trayanova N (2004). Induction of ventriculararrhythmias following mechanical impact: a simulation
study in 3D. J Mol Histol 35, 679–686.
Li W, Kohl P & Trayanova N (2006). Myocardial ischemia
lowers precordial thump efficacy: an inquiry intomechanisms using three-dimensional simulations. Heart
Rhythm 3, 179–186.
Liang JL, Huang BS, Yuan GY, Chen Y, Liang FS, Zeng HY,
Zheng SX, Cao L, Geng DF & Zhou SX (2017).
Stretch-activated channel Piezo1 is up-regulated in failure
heart and cardiomyocyte stimulated by AngII. Am J Transl
Res 9, 2945–2955.
C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

19

Limbu S, Hoang-Trong TM, Prosser BL, Lederer WJ & Jafri MS(2015). Modeling local X-ROS and calcium signaling in theheart. Biophys J 109, 2037–2050.
Link MS (2012). Commotio cordis: ventricular fibrillation
triggered by chest impact-induced abnormalities in
repolarization. Circ Arrhythm Electrophysiol 5, 425–432.
MacDougall JD, Tuxen D, Sale DG, Moroz JR & Sutton JR(1985). Arterial blood pressure response to heavy resistance
exercise. J Appl Physiol 58, 785–790.
Markhasin VS, Solovyova O, Katsnelson LB, Protsenko Y, Kohl
P & Noble D (2003). Mechano-electric interactions inheterogeneous myocardium: development of fundamental
experimental and theoretical models. Prog Biophys Mol Biol82, 207–220.
Meola F (1879). La commozione toracica. Gior Internaz Sci
Med 1, 923–937.
Mitchell JH, Haskell W, Snell P & Van Camp SP (2005). Task
Force 8: classification of sports. J Am Coll Cardiol 45,
1364–1367.
Miura M, Nishio T, Hattori T, Murai N, Stuyvers BD, Shindoh
C & Boyden PA (2010). Effect of nonuniform muscle
contraction on sustainability and frequency of triggeredarrhythmias in rat cardiac muscle. Circulation 121,
2711–2717.
Morris CE (2011). Voltage-gated channel mechanosensitivity:
fact or friction? Front Physiol 2, 25.
Moss RL & Fitzsimons DP (2002). Frank-Starling relationship:long on importance, short on mechanism. Circ Res 90,
11–13.
Myles RC, Wang L, Kang C, Bers DM & Ripplinger CM (2012).
Local beta-adrenergic stimulation overcomes source-sinkmismatch to generate focal arrhythmia. Circ Res 110,
1454–1464.
Niederer SA, Campbell KS & Campbell SG (2019). A shorthistory of the development of mathematical models of
cardiac mechanics. J Mol Cell Cardiol 127, 11–19.
Niederer SA & Smith NP (2007). A mathematical model of theslow force response to stretch in rat ventricular myocytes.
Biophys J 92, 4030–4044.
Noble D, Varghese A, Kohl P & Noble P (1998). Improved
guinea-pig ventricular cell model incorporating a diadic
space, IKr and IKs , and length- and tension-dependent
processes. Can J Cardiol 14, 123–134.
Parmley WW & Chuck L (1973). Length-dependent changes inmyocardial contractile state. Am J Physiol 224, 1195–1199.
Patterson SW, Piper H & Starling EH (1914). The regulation of
the heart beat. J Physiol 48, 465–513.
Petroff MG, Kim SH, Pepe S, Dessy C, Marban E, Balligand JL& Sollott SJ (2001). Endogenous nitric oxide mechanisms
mediate the stretch dependence of Ca2+ release in
cardiomyocytes. Nat Cell Biol 3, 867–873.
Peyronnet R, Nerbonne JM & Kohl P (2016). Cardiac
mechano-gated ion channels and arrhythmias. Circ Res 118,
311–329.
Prosser BL, Ward CW & Lederer WJ (2011). X-ROS signaling:
rapid mechano-chemo transduction in heart. Science 333,
1440–1445.
Prosser BL, Ward CW & Lederer WJ (2013). X-ROS signallingis enhanced and graded by cyclic cardiomyocyte stretch.
Cardiovasc Res 98, 307–314.

20

L. T. Izu and others

Quinn TA, Jin H, Lee P & Kohl P (2017). Mechanically induced
ectopy via stretch-activated cation-nonselective channels is
caused by local tissue deformation and results in ventricularfibrillation if triggered on the repolarization wave edge
(commotio cordis). Circ Arrhythm Electrophysiol 10,
e004777.
Quinn TA & Kohl P (2012). Mechano-sensitivity of cardiac
pacemaker function: pathophysiological relevance,experimental implications, and conceptual integration withother mechanisms of rhythmicity. Prog Biophys Mol Biol 110,
257–268.
Rahimzadeh J, Meng FJ, Sachs F, Wang JB, Verma D & Hua SZ(2011). Real-time observation of flow-induced cytoskeletal
stress in living cells. Am J Physiol-Cell Physiol 301,
C646–C652.
Rice JJ, Winslow RL & Hunter WC (1999) Comparison of
putative cooperative mechanisms in cardiac muscle: length
dependence and dynamic responses. Am J Physiol Heart Circ
Physiol 276, H1734–H1754.
Robison P, Caporizzo MA, Ahmadzadeh H, Bogush AI, Chen
CY, Margulies KB, Shenoy VB & Prosser BL (2016).
Detyrosinated microtubules buckle and bear load incontracting cardiomyocytes. Science 352,
aaf0659.
Sarnoff SJ, Mitchell JH, Gilmore JP & Remenssnyder JP (1960).
Homeometric autoregulation in the heart. Circ Res 8,1077–1091.
Schlomka G & Hinrichs A (1932). Experimentelle
Untersuchungen über den Einfluß stumpfer
Brustkorbverletzungen auf das Elektrokardiogramm.
Zeitschrift für die Gesamte Experimentelle Medizin 81, 43–61.
Schonleitner P, Schotten U & Antoons G (2017).
Mechanosensitivity of microdomain calcium signalling in
the heart. Prog Biophys Mol Biol 130, 288–301.
Schott E.(1920). Über Ventrikelstillstand (Adam-Stokes’scheAnfälle) nebst Bemerkungen über andersartige Arrhythmien
passagerer Natur. (On ventricular standstill [Stokes-Adams
attacks] and other arrhythmias of temporary nature.). Dt
Arch klin Med, 211–229.
Seo K, Rainer PP, Lee DI, Hao S, Bedja D, Birnbaumer L,
Cingolani OH & Kass DA (2014). Hyperactive adverse
mechanical-stress responses in dystrophic heart are coupled
to TRPC6 and blocked by cGMP-PKG modulation. Circ Res
114, 823–832.
Shaw J, Izu L & Chen-Izu Y (2013). Mechanical analysis ofsingle myocyte contraction in a 3-D elastic matrix. PLoS One
10, e75492.
Smith N & Niederer S (2016). Is computational modelingadding value for understanding the heart? J Mol Cell Cardiol
96, 1.
Solaro RJ (2007). Mechanisms of the Frank-Starling law of theheart: the beat goes on. Biophys J 93, 4095–4096.
Solovyova O, Katsnelson LB, Konovalov PV, Kursanov AG,
Vikulova NA, Kohl P & Markhasina VS (2014). The cardiac
muscle duplex as a method to study myocardial
heterogeneity. Prog Biophys Mol Biol 115, 115–128.
Starling EH (1915). The Linacre Lecture on the Law of the Heart.
Longmans, Green and Co., London.

J Physiol 00.0

Suchyna TM, Johnson JH, Hamer K, Leykam JF, Gage DA,
Clemo HF, Baumgarten CM & Sachs F (2000). Identificationof a peptide toxin from Grammostola spatulata spider venom
that blocks cation-selective stretch-activated channels. J Gen
Physiol 115, 583–598.
ter Keurs HE & Boyden PA (2007). Calcium andarrhythmogenesis. Physiol Rev 87, 457–506.
ter Keurs HE, Wakayama Y, Sugai Y, Price G, Kagaya Y, Boyden
PA, Miura M & Stuyvers BD (2006). Role of sarcomere
mechanics and Ca2+ overload in Ca2+ waves andarrhythmias in rat cardiac muscle. Ann N Y Acad Sci 1080,
248–267.
Trayanova NA & Rice JJ (2011) Cardiac electromechanicalmodels: from cell to organ. Front Physiol 2, 43.
Tyberg JV, Parmley WW & Sonnenblick EH (1969). In-vitro
studies of myocardial asynchrony and regional hypoxia. Circ
Res 25, 569–579.
Van Wagoner DR (1993). Mechanosensitive gating of atrial
ATP-sensitive potassium channels. Circ Res 72, 973–983.
von Anrep G (1912). On the part played by the suprarenals in
the normal vascular reactions of the body. J Physiol 45,
307–317.
Wakayama Y, Miura M, Stuyvers BD, Boyden PA & ter Keurs
HE (2005). Spatial nonuniformity of excitation-contraction
coupling causes arrhythmogenic Ca2+ waves in rat cardiac
muscle. Circ Res 96, 1266–1273.
Wakayama Y, Miura M, Sugai Y, Kagaya Y, Watanabe J, ter
Keurs HE & Shirato K (2001). Stretch and quick release of
rat cardiac trabeculae accelerates Ca2+ waves and triggered
propagated contractions. Am J Physiol Heart Circ Physiol
281, H2133–H2142.
Wiegner AW, Allen GJ & Bing OH (1978). Weak and strong
myocardium in series: implications for segmental
dysfunction. Am J Physiol 235, H776–H783.
Zhang H, Walcott GP & Rogers JM (2018). Effects ofgadolinium on cardiac mechanosensitivity in whole isolated
swine hearts. Sci Rep 8, 10506.
Zhao H, Yu Y, Wu X, Liu S, Liu B, Du J, Li B, Jiang L & Feng X(2017). A role of BK channel in regulation of Ca2+ channel
in ventricular myocytes by substrate stiffness. Biophys J 112,
1406–1416.
Zile MA & Trayanova NA (2017). Myofilament protein
dynamics modulate EAD formation in human hypertrophic
cardiomyopathy. Prog Biophys Mol Biol 130, 418–428.

Additional information
Competing interests
All authors have no conflict of interest.
Author contributions
This symposium review article resulted from the presentations
and discussions in the 2018 UC Davis CardiovascularSymposium. Copyrights have been obtained for all figures

C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

J Physiol 00.0

Mechano-transduction in cardiomyocytes

containing published data. Conception or design of the work:
L.T.I., P.K., D.M.B. and Y.C.I.; drafting the work or revising it
critically for important intellectual content: L.T.I., P.K., P.A.B.,
M.M., T.B., N.C., N.T., D.M.B. and Y.C.I. All authors haveapproved the final version of the manuscript. Lead authors agree
to be accountable for all aspects of the work in ensuring thatquestions related to the accuracy or integrity of any part of thework are appropriately investigated and resolved. All persons
designated as authors qualify for authorship, and all those who
qualify for authorship are listed.

21

National Heart, Lung and Blood Institute (NHLBI) R13H
L147532 to Y.C.I. and D.M.B.
Acknowledgements
We thank Dr Remi Peyronnet and Dr Anthony Baker for
their help in editing the manuscript. We also thank many
participants of the UC Davis Cardiovascular Symposium 2018
for contributing to the scientific presentations and discussions
in the symposium that inspired this paper.
Keywords

Funding
This paper is based on the symposium supported by the grantfrom the United States National Institutes of Health (NIH)C 2019 The Authors. The Journal of Physiology C 2019 The Physiological Society

Anrep effect, cardiac muscle, excitation-contraction coupling,
Frank-Starling, mechano-chemo-transduction, mechanoelectric coupling, myocardial contractility