bluuebunny/crossref_metadata_2025 huggingface dataset:
Spinothalamic lumbosacral lamina I cells responsive to skin and muscle stimulation in the cat.
Authors: Craig A D , Kniffki K D
Published: December 2014

The response characteristics of lamina I neurones recorded extracellularly in the lumbosacral enlargement of chloralose‐anaesthetized cats were examined with peripheral nerve electrical stimulation, adequate mechanical and thermal stimulation of hind‐limb skin, and algesic mechanical and chemical stimulation of musculotendinous structures, particularly the gastrocnemius‐soleus (g.s.) muscle. Antidromic activation from an electrode array that spanned the contralateral thalamus was used to identify lamina I spinothalamic tract (lam.I‐s.t.t.) neurones. Recordings were made from a total of 218 lumbosacral lam.I‐s.t.t. neurones. Their mean central conduction latency was 90.1 ms (range 20‐300 ms), corresponding to a mean conduction velocity of 3.7 m/s (range 1.1‐16.7). Neurones responsive only to peripheral A delta fibre stimulation had significantly shorter central conduction latencies (mean = 62.8 ms) than those with both A delta and C fibre input (mean = 81.9 ms) and those with only C fibre input (mean = 134.6 ms). Of these 218 neurones, 103 (47%) projected only to medial thalamus, 41 (19%) only to lateral thalamus, and 56 (26%) to both; 18 (8%) were classified as mid‐thalamic projecting cells. About 10% of all cells had ongoing activity when first isolated. Ninety‐three lam.I‐s.t.t. neurones responded to stimulation of the sciatic nerve. The response characteristics of forty‐seven of these were examined with the complete set of stimuli used. Twenty‐four non‐s.t.t. lamina I neurones were also characterized for comparison. Twenty‐eight of the lam.I‐s.t.t. neurones tested with the complete set of stimuli responded specifically to either cutaneous noxious (n = 19), cutaneous innocuous cold (n = 6) or algesic musculo‐tendinous (n = 3) stimulation. Thirteen neurones responded to cutaneous noxious stimulation, and, in addition, to cold stimulation (n = 6), to deep stimulation (n = 4), or to both (n = 3). Six cells did not respond to any of the natural stimuli employed. All of the cold‐specific and many of the multireceptive cold‐sensitive neurones had ongoing discharge. The average central conduction latencies of cold‐sensitive neurones (65.5 ms) and unresponsive neurones (48.7 ms) were shorter than that of nociceptive neurones (91.2 ms). Two response categories had distinct thalamic projection patterns. The majority of cold‐specific neurones projected only to medial thalamus. Almost all multireceptive cold‐sensitive neurones projected to both medial and lateral thalamus.(ABSTRACT TRUNCATED AT 400 WORDS)
Activity‐dependent excitability changes in normal and demyelinated rat spinal root axons.
Authors: Bostock H , Grafe P
Published: December 2014

Myelinated nerve fibres with a reduced safety factor for conduction due to demyelination are easily blocked by trains of impulses. To find out why, in vivo recordings from rat ventral root fibres demyelinated with diphtheria toxin have been supplemented with in vivo and in vitro recordings from normal fibres. Despite a small rise in extracellular potassium activity, normal fibres were invariably hyperpolarized by intermittent trains of impulses. This hyperpolarization resulted in an increase in threshold and also in an enhancement of the depolarizing after‐potential and the superexcitable period. Replacement of NaCl in the extracellular solution by LiCl completely blocked both the membrane hyperpolarization and the threshold increase which were normally observed during intermittent trains of impulses. At demyelinated nodes which were blocked by trains of impulses (10‐50 Hz), conduction block was preceded by a rise in threshold current and in an increase in internodal conduction time, but by no detectable reduction in the outward current generated by the preceding node. It was found possible to prevent the threshold from changing during a train by automatic adjustment of a d.c. polarizing current. This 'threshold clamp' prevented the conduction failure and virtually abolished the changes in internodal conduction time. The threshold changes were attributed to hyperpolarization, as in normal fibres, since (a) the polarizing current required to prevent them was always a depolarizing current, and (b) they were accompanied by an increase in superexcitability. The post‐tetanic depression that can follow continuous trains of impulses was attributed to the combination of increased threshold and enhanced superexcitable period due to hyperpolarization. It is concluded that the susceptibility of these demyelinated fibres to impulse trains is not due to a membrane depolarization induced by extracellular potassium accumulation but to a membrane hyperpolarization as a consequence of electrogenic sodium pumping.
Voltage‐dependent activation of the inward‐rectifier potassium channel in the ventricular cell membrane of guinea‐pig heart.
Authors: Kurachi Y
Published: December 2014

The activation kinetics of the inward‐rectifier K+ channel were studied by single‐channel recording in isolated single cells of the guinea‐pig ventricle with two different extracellular concentrations of K+ ([K+]o 150 and 75 mM). When voltage pulses were applied from a holding potential more positive than the potassium equilibrium potential (EK), to potentials more negative than EK, the probability of the channel being in the open state (Po) increased with time after the onset of the command pulse. The ensemble averaged current increased in its initial phase (activation). When the command potential was more negative than EK‐40 mV, the current decreased after rapid activation due to the inactivation of the channel. The averaged current could be divided into an instantaneous and a time‐dependent activation component; the latter was fitted by a single exponential function. The time constant of the time‐dependent component became shorter, at more negative command potentials. When compared at the same command potential, the instantaneous component became smaller, as the patch membrane was held at more depolarized potential. This indicates that the steady‐state Po of the channel decreases with depolarization at potentials more positive than EK. The Po of the activation gate of the channel was estimated by dividing the steady‐state Po of the channel by the Po of the inactivation gate at each potential. It was about 0.1 at EK + 20 mV and increased sigmoidally with hyperpolarization. At potentials more negative than EK‐40 mV, the Po of the activation gate saturated and was almost 1. The single‐channel analysis and the noise analysis of the steady‐state current fluctuations revealed that the activation gate of the channel follows first‐order kinetics between the open and closed states. The activation kinetics shifted along the voltage axis in a similar way as EK when different [K+]o were used. Thus, the activation of the channel depends not only on the membrane potential but also on EK, when [K+]o is changed. The macroscopic current flowing through the inward‐rectifier K+ channel during the activation process was calculated, assuming that the elementary conductance of the channel is not voltage dependent. The calculated current showed a prominent inward‐rectifying property in the steady state and formed a negative conductance region at potentials positive to EK. It was, therefore, concluded that the properties of the inward‐rectifier time‐independent background K+ current (iK1) in the whole‐cell current records (Noble, 1979) mainly depend on the activation kinetics of the inward‐rectifier K+ channel in the cardiac myocyte membrane.
Calcium regulation of potassium fluxes in rabbit aorta during activation by noradrenaline or high potassium medium.
Authors: Aaronson P I , Jones A W
Published: December 2014

The dependency of noradrenaline‐ and depolarization‐mediated increases in K permeability on cellular Ca was investigated by measuring the effect of diltiazem and Ca‐free medium on stimulated 42K efflux and contracture. The increase in the rate constant (k) for 42K efflux induced by cellular depolarization with 80 mM‐K was inhibited by 70% in the presence of 10(‐5) M‐diltiazem. The noradrenaline (NA)‐mediated increase in k was only slightly suppressed by diltiazem at 10(‐5) M‐NA, but was inhibited by diltiazem in a dose‐dependent fashion for a submaximal concentration of NA 10(‐7) M. Similar inhibitory effects were observed on contractile responses. Basal 42K efflux progressively increased in a 0 Ca physiological salt solution (PSS) containing 2 mM‐EGTA. This process was suppressed in a concentration‐dependent manner as [Mg] was increased. In experiments utilizing 0 Ca PSS, [Mg] was therefore raised to 15 mM to maintain stable basal effluxes. Ca removal for 30 min reduced the 80 mM‐K‐mediated increase in k by 64%. The NA‐induced increase in k became more transient in 0 Ca PSS with a progressive diminution in the magnitude of this response as the duration of Ca depletion was increased. The diminution of the 42K efflux response in 0 Ca PSS was well fitted by a mono‐exponential function (half‐time, t1/2 = 46 min). The NA‐induced contracture in 0 Ca solution decreased with a biphasic time course subsequent to Ca removal. Intracellular Ca release by NA, measured by means of a 45Ca efflux protocol, decreased as a mono‐exponential function of time, with a t 1/2 of 21 min. We conclude that both alpha‐receptor activation and membrane depolarization increase K permeability largely as a result of increased cellular free [Ca]. The effect of depolarization appears to depend mainly on influx of extracellular Ca. NA increases both influx and intracellular release of Ca which serve to open K channels. The apparent release of cellular Ca as measured by 45Ca efflux into Ca‐free solution decreased more rapidly, however, than did the NA stimulation of 42K efflux and tension. These observations may result from the presence of a slowly depleted Ca store which cannot be detected directly by measuring NA‐induced release of 45Ca.
The neural and non‐neural mechanisms involved in urethral activity in rabbits.
Authors: Ito Y , Kimoto Y
Published: December 2014

The effects of electrical and chemical stimulation on the mechanical or electrical properties of the circular smooth muscle cells of the bladder neck, and proximal urethra of the male rabbit were investigated by means of micro‐electrode, double‐sucrose‐gap and tension‐recording methods. In the bladder neck, application of short current pulses (50 microseconds) produced an initial excitatory junction potential (e.j.p.) with a superimposed spike, followed by a late depolarization, and these electrical events evoked contraction. The initial e.j.p. was unaffected by guanethidine, phentolamine, methysergide or mepyramine, indicating the initial e.j.p. is not mediated by activation of adrenergic, tryptaminergic or histaminergic receptors. The late depolarization was enhanced by pre‐treatment with neostigmine (10(‐7) M) and abolished by atropine (10(‐6) M). In the proximal urethra, electrical‐field stimulation evoked phasic contraction which was followed by relaxation, associated with initial e.j.p.s, late depolarization and inhibitory junction potentials (i.j.p.s). Guanethidine (10(‐5) M) or phentolamine (10(‐6) M) reduced the size of the initial e.j.p. to 40‐50% of the control value and combined application of guanethidine and atropine further reduced the amplitude of the e.j.p. to 20‐30%. There was a parallel reduction in the mechanical response. The late depolarization was enhanced by neostigmine and abolished by atropine. The i.j.p. and muscle relaxation were not affected by propranolol, phentolamine, guanethidine or atropine. These results indicate that the proximal urethral smooth muscle cells are innervated by adrenergic and cholinergic excitatory, and by non‐cholinergic non‐adrenergic inhibitory nerve fibres. In the prostatic urethra, field stimulations also evoked twitch contractions with or without following phasic contraction and relaxation. The twitch contractions were abolished by d‐tubocurarine (10(‐6) M), suggesting that they arise from striated muscle. Exogenously applied prostaglandin (PG) E1, PGE2 or PGF2 alpha (greater than 10(‐10) M) evoked sustained increase in the muscle tone in the presence or absence of indomethacin, and enhanced the amplitude of muscle relaxation evoked by the field stimulation without affecting the resting membrane potential. Indomethacin (10(‐6)‐10(‐5) M) gradually reduced the muscle tone of the proximal urethra with no change in the resting membrane potential. At the reduced muscle tone, electrical‐field stimulation did not evoke muscle relaxation. Thus, the amplitude of muscle relaxation evoked by field stimulation was dependent on the level of muscle tone of the circular muscle strips.(ABSTRACT TRUNCATED AT 400 WORDS)
Membrane voltage, resistance, and channel switching in isolated mouse fibroblasts (L cells): a patch‐electrode analysis.
Authors: Hosoi S , Slayman C L
Published: December 2014

The whole‐cell patch‐electrode technique of Fenwick, Marty & Neher (1982) has been applied to single suspension‐cultured mouse fibroblasts. Seals in the range of 10‐50 G omega were obtained without special cleaning of the cell membranes. Rupture of the membrane patch inside the electrode was accompanied by a shift of measured potential into the range ‐10 to ‐25 mV, but in most cases with little change in the recorded resistance. The latter fact implied that the absolute resistance of the cell membrane must be in the same range as the seal resistance and the recorded potential is a poor measure of actual cell membrane potential. Steady‐state current‐voltage curves (range ‐160 mV to +80 mV) were generated before and after rupture of the membrane patch, and the difference between these gave (zero‐current) membrane potentials of ‐50 to ‐75 mV, which represents a leak‐corrected estimate of the true cell‐membrane potential. The associated slope conductivity of the cell membrane was 5‐15 microS/cm2 (assumed smooth‐sphere geometry, cells 13‐15 microns in diameter) and was K+‐dominated. With 0.1 mM (or more) free Ca2+ filling the patch electrode, membrane potentials in the range ‐60 to ‐85 mV were observed following patch rupture, with associated slope conductivities of 200‐400 microS/cm2, also K+‐dominated. Similar voltages and conductivities were observed at the peak of pulse‐induced 'hyperpolarizing activation' (Nelson, Peacock, & Minna, 1972), and the two phenomena probably reflect the behaviour of Ca2+‐activated K+ channels. Both the pulse‐induced conductance and the Ca2+‐activated conductance spontaneously decayed, the latter over periods of 5‐15 min following patch rupture. Sr2+, Ba2+, and Co2+ could also activate the putative K+ channels, but only Sr2+ really mimicked Ca2+. Co2+ and Ba2+ activated with a delay of several minutes following patch rupture, and deactivated quickly with a small decrease of conductance and a large decrease of membrane potential. Evidently, Co2+ and Ba2+ affect channel specificity as well as channel opening and closing kinetics.
Fluctuations in intracellular calcium concentration and their effect on tonic tension in canine cardiac Purkinje fibres.
Authors: Kort A A , Lakatta E G , Marban E , Stern M D , Wier W G
Published: December 2014

Ca2+‐activated aequorin luminescence and tension were measured in dog Purkinje fibres during twitches and during the increase in resting force produced by exposure of the fibres to a low‐Na+ solution after 3 min without external K+. Over the restricted range which could be examined, the relation between tension and 'mean' aequorin luminescence (luminescence filtered at 0.2 Hz) was approximately linear during the development and maintenance of contracture. For a given level of force, the mean aequorin luminescence during contracture was up to 20 times greater than the peak luminescence during the twitch. Noise analysis of aequorin luminescence and tension during contracture indicated the presence of periodic fluctuations, with a predominant frequency in the range 1‐4 Hz. Ryanodine (1 microM) or caffeine (10 mM) abolished the fluctuations in luminescence and tension and made the relation between tension and mean aequorin luminescence much steeper. A mathematical model, the key feature of which is periodicity in the asynchronous occurrence of spatially localized regions of relatively high [Ca2+], reproduces the experimental data derived from contractures. From the model analysis, we infer that tonic tension is produced by recruitment of increasing numbers of regions of high [Ca2+], rather than by homogeneous graded activation. These results indicate that during contracture or 'tonic tension', intracellular [Ca2+] is not at steady state, but rather undergoes large, asynchronous spatio‐temporal fluctuations. Thus the assumptions that intracellular [Ca2+] is at steady state or homogeneous during tonic tension are not valid.
Selective cardiovascular and neuroendocrine effects of a kappa‐opioid agonist in the nucleus tractus solitarii of rats.
Authors: Carter D A , Lightman S L
Published: December 2014

The cardiovascular and neuroendocrine effects of a selective kappa‐opiate receptor agonist (U50488H) microinjected into the nucleus tractus solitarii have been investigated in urethane‐anaesthetized rats. Comparative experiments were conducted using 8‐arginine vasopressin (AVP)‐deficient Brattleboro rats and an opiate agonist selective for delta receptors. Unilateral injection of U50488H elicited a significant dose‐dependent increase in mean arterial pressure and a small decrease in heart rate in Sprague‐Dawley rats. The pressor effect was blocked preferentially by the relatively selective kappa‐receptor antagonist MR2266BS compared to naloxone. Bilateral injections of U50488H elicited a relatively greater increase in mean arterial pressure than unilateral injections and a significant decrease in heart rate. U50488H did not elicit a pressor effect in Brattleboro rats, whereas a marked response (associated with a significant increase in AVP secretion) was found in parent strain Long‐Evans rats. In contrast, no such differential effects in the response of Brattleboro and Long‐Evans rats were observed in parallel experiments using equimolar doses of the selective delta‐opiate agonist Tyr‐D‐Ser‐Gly‐Phe‐Leu‐Thr which elicited a transient pressor response. An antagonist [1‐(beta‐mercapto‐beta, beta‐cyclopentamethylene‐propionic acid)2‐(0‐methyl) tyrosine] arginine vasopressin (1,d(CH2)5Tyr(ME)AVP) specific for the vasopressor action of AVP blocked the U50488H‐induced pressor response in a dose‐dependent manner when administered intravenously 10 min prior to the kappa agonist, but did not significantly attenuate the response to the delta agonist. Conversely, the U50488H‐induced response was not modified by pre‐treatment with phenoxybenzamine whereas the delta‐agonist pressor response was completely blocked by it. The results provide evidence for specific kappa‐opiate cardiovascular and neuroendocrine responses in the nucleus tractus solitarii and suggest that a kappa‐receptor mechanism, possibly involving a peptide of the dynorphin group as the endogenous ligand, may operate in the central control of blood pressure and AVP secretion.
Gadolinium ions inhibit exocytotic vasopressin release from the rat neurohypophysis.
Authors: Muscholl E , Racké K , Traut A
Published: December 2014

Single rat neurointermediate lobes (n.i.l.s) were fixed by their stalks to a platinum wire clip electrode and incubated in oxygenated Krebs‐HEPES medium. Vasopressin release int the medium was determined by radioimmunoassay. Vasopressin secretion was increased by different stimuli and the effects of gadolinium (Gd3+) were tested. Electrical stimulation (15 Hz, three times 1 min with 1 min intervals) increased vasopressin release in a calcium‐dependent manner. Gd3+ (10 microM to 3 mM) inhibited the evoked release of vasopressin in a concentration‐dependent fashion; at 3 mM the inhibition was 98%. The inhibitory effect of Gd3+ up to 300 microM was antagonized by increasing the calcium concentration in the medium up to 6 mM. The effects of 1 and 3 mM‐Gd3+ were unaffected by increasing the calcium concentration. Exposure of n.i.l.s to depolarizing concentrations of potassium (high K+, 60 mM, 30 min) increased the vasopressin release more than 33‐fold. The elevated vasopressin release remained constant during six consecutive 5 min periods. In the initial 5 min period 300 microM‐Gd3+ reduced the evoked vasopressin release by 80% but during the last 5 min period only by 30%. At 3 mM‐Gd3+ vasopressin release was completely blocked during the whole time of incubation with high K+. Vasopressin release induced by exposure of n.i.l.s to cold (4 degrees C, 20 min) was completely inhibited by 3 mM‐Gd3+, but reduced by only 25% in the presence of 300 microM‐Gd3+. Vasopressin release induced by incubation of n.i.l.s with the ionophore X‐537A (lasalocid) (10 microM, 30 min) was reduced by 90% in the presence of 300 microM‐Gd3+ and completely prevented by 3 mM‐Gd3+. 300 microM‐Gd3+, added to the incubation medium, had no significant effect on the vasopressin release from crude synaptosomal preparations evoked by high K+. However, when 300 microM‐Gd3+ was already present during the tissue homogenization, the evoked vasopressin release from the synaptosomes was completely blocked. It is concluded that Gd3+ inhibits exocytotic vasopressin release at two different sites. First, Gd3+ may block voltage‐regulated calcium channels. Secondly, Gd3+ may inhibit the exocytotic release mechanism by an intracellular site of action. It is speculated that contractile proteins may be the intracellular target for Gd3+.
Responses to adenosine diphosphate in human platelets loaded with the fluorescent calcium indicator quin2.
Authors: Hallam T J , Rink T J
Published: December 2014

ADP produces a rapid elevation in the concentration of cytoplasmic free calcium, [Ca2+]i, in quin2‐loaded human platelets which begins within 1 s of stimulation and peaks after 10 s. In the presence of 1 mM‐extracellular calcium, [Ca2+]i peaks at 670 +/‐ 50 nM in the absence and 610 +/‐ 30 nM in the presence of a cyclo‐oxygenase inhibitor. The production of prostaglandin endoperoxides and thromboxane A2 are not required for stimulation of Ca2+ fluxes by ADP but appear to have a supportive role. In the absence of extracellular calcium ions and with 1 mM‐extracellular EGTA, stimulation with ADP caused [Ca2+]i to peak at 160 +/‐ 20 nM in the absence and 150 +/‐ 10 nM in the presence of a cyclo‐oxygenase inhibitor. ADP can cause the discharge of calcium ions from internal stores and does not require the prior formation of prostaglandin endoperoxides or thromboxane A2. The rise in [Ca2+]i in the presence of extracellular Ca2+ is sixfold larger than in the absence of extracellular Ca2+. This suggests that the major component of the ADP‐stimulated rise in [Ca2+]i is caused by the influx of Ca2+ ions across the plasma membrane. Diltiazem, D600, nimodipine and nifedipine had little or no effect on resting or ADP‐stimulated [Ca2+]i levels. Depolarization with potassium‐rich media alone or in conjunction with valinomycin had no effect on basal [Ca2+]i and only a partial inhibitory effect on ADP‐stimulated increases in [Ca2+]i. Depolarization had no effect on the ADP‐stimulated rise in [Ca2+]i in Ca2+‐free media. Hyperpolarization had no marked effect on the rise in [Ca2+]i produced by ADP in the presence of extracellular calcium. These results are consistent with there being no voltage‐dependent channels in the platelet plasma membrane. Using ionomycin, a selective Ca2+ ionophore, and measuring both quin2 fluorescence and optical density of the suspension simultaneously, the threshold [Ca2+]i for shape change was determined to be 300 nM with half‐maximal effect at 500 nM and maximal shape change at 800 nM. ADP produced maximal shape change confirmed by scanning electron microscopy with corresponding [Ca2+]i at below 200 nM. The level of [Ca2+]i required to produce aggregation using ionomycin was approximately 1 microM. ADP alone, or following a smaller rise in [Ca2+]i produced by ionomycin to disguise the effect of ADP, produced an aggregatory response at concentrations below 1 microM. These data indicate that excitatory mechanisms are involved producing shape change and aggregation to ADP other than a stimulated rise in [Ca2+]i.
Voltage clamp of bull‐frog cardiac pace‐maker cells: a quantitative analysis of potassium currents.
Authors: Giles W R , Shibata E F
Published: December 2014

Spontaneously active single cells have been obtained from the sinus venosus region of the bull‐frog, Rana catesbeiana, using an enzymic dispersion procedure involving serial applications of trypsin, collagenase and elastase in nominally 0 Ca2+ Ringer solution. These cells have normal action potentials and fire spontaneously at a rate very similar to the intact sinus venosus. A single suction micro‐electrode technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981; Hume & Giles, 1983) has been used to record the spontaneous diastolic depolarizations or pace‐maker activity as well as the regenerative action potentials in these cells. This electrophysiological activity is completely insensitive to tetrodotoxin (TTX; 3 X 10(‐6) M) and is very similar to that recorded from an in vitro sinus venosus preparation. The present experiments were aimed at identifying the transmembrane potassium currents, and analysing their role(s) in the development of the pace‐maker potential and the repolarization of the action potential. Depolarizing voltage‐clamp steps from the normal maximum diastolic potential (‐75 mV) elicit a time‐ and voltage‐dependent activation of an outward current. The reversal potential of this current in normal Ringer solution [( K+]0 2.5 mM) is near ‐95 mV; and it shifts by 51 mV per tenfold increase in [K+]0, which strongly suggests that this current is carried by K+. We therefore labelled it IK. The reversal potential of IK did not shift in the positive direction following very long (20 s) depolarizing clamp steps to +20 mV, indicating that 'extracellular' accumulation of [K+]0 does not produce any significant artifacts. The fully activated instantaneous current‐voltage (I‐V) relationship for IK is approximately linear over the range of potentials ‐130 to ‐30 mV. Thus, the ion transfer mechanism of IK may be described as a simple ohmic conductance in this range of potentials. Positive relative to ‐30 mV, however, the I‐V exhibits significant inward rectification. A Hodgkin‐Huxley analysis of the kinetics of IK, including a demonstration that the envelope of tails quantitatively matches the time course of the onset of IK during a prolonged depolarizing clamp step has been completed. The steady‐state activation variable (n infinity) of IK spans the voltage range approximately ‐40 to +10 mV. It is well‐fitted by a Boltzmann distribution function with half‐activation at ‐20 mV. The time course of decay of IK is a single exponential. However, the activation or onset of IK shows clear sigmoidicity in the range of potentials from the activation threshold (‐40 mV) to 0 mV.(ABSTRACT TRUNCATED AT 400 WORDS)
Thixotropic behaviour of human finger flexor muscles with accompanying changes in spindle and reflex responses to stretch.
Authors: Hagbarth K E , Hägglund J V , Nordin M , Wallin E U
Published: December 2014

Prompted by previous reports on muscle thixotropy, we have investigated changes in inherent and reflex stiffness of the finger flexor muscles of human subjects at rest, following transient conditioning manoeuvres involving contractions and/or length changes of the finger flexors. The stiffness measurements were combined with electromyographic recordings from forearm and hand muscles and with microneurographic recordings of afferent stretch responses in finger flexor nerve fascicles. Finger flexor stiffness was evaluated by measuring (a) the flexion angle of the metacarpo‐phalangeal joints at which the system during rest balanced the force of gravity and (b) the speed and amplitude of angular finger extensions induced by recurrent extension torque pulses of constant strength delivered by a torque motor. In the latter case, extension drifts in the resting position of the fingers were prevented by a weak flexion bias torque holding the fingers in a pre‐determined, semiflexed position against a stop‐bar. Stiffness changes following passive large amplitude finger flexions and extensions were studied in subjects with nerve blocks or nerve lesions preventing neurally mediated contractions in the forearm and hand muscles. Inherent stiffness was enhanced following transient finger flexions and reduced following transient finger extensions. The after‐effects gradually declined during observation periods of several minutes. Similar results were obtained in subjects with intact innervation who succeeded during the pre‐ and post‐conditioning periods in keeping the arm and hand muscles relaxed (i.e. showed no electromyographic activity). In these subjects it was also found that the after‐effects were similar for active and passive finger movements and that isometric voluntary finger flexor contractions loosened the system in a way similar to finger extensions. In some subjects electromyographic reflex discharges appeared in the finger flexors in response to the extension test pulses. When elicited by small ramp stretch stimuli of constant amplitude, the stretch reflex responses were found to vary in strength in parallel with the changes in inherent stiffness following the various conditioning manoeuvres. The strength of the multi‐unit afferent stretch discharges in the muscle nerve, used as index of muscle spindle stretch sensitivity, varied in parallel with the changes in inherent stiffness. Post‐manoeuvre changes in muscle spindle stretch sensitivity were seen also when the spindles were de‐efferented by a nerve block proximal to the recording site. The results can be explained in terms of thixotropic behaviour of extra‐ and intrafusal muscle fibres.(ABSTRACT TRUNCATED AT 400 WORDS)
Long‐term changes in hippocampal physiology and learning ability of rats after intrahippocampal tetanus toxin.
Authors: Brace H M , Jefferys J G , Mellanby J
Published: December 2014

A chronic epileptic syndrome can be induced by injecting minute doses of tetanus toxin into rat hippocampi. This causes intermittent epileptic fits over a period of 2‐4 weeks, after which the fits cease, and the electroencephalogram (e.e.g.) appears to return to normal over the following 2‐3 weeks. However, once they have recovered from the seizures, the rats exhibit a remarkably persistent impairment of learning and memory, which is the subject of the present study. Learning ability was assessed using a radial arm maze task, in which the rats had to visit each of eight arms for a food reward. The toxin‐injected rats learnt this task more slowly than control‐injected. Evoked potentials from the CA3 pyramidal cells were recorded in terminal experiments under halothane anaesthesia. Long term potentiation of the post‐synaptic response to the commissural pathway from the contralateral hippocampus appeared to be unaffected by the previous toxin treatment, at least over periods of up to 5 h. The toxin‐injected group differed from the control in having consistently smaller post‐synaptic population spikes in their evoked responses, so that stimuli were less effective in exciting the post‐synaptic neurones. This applied both to the contralateral commissural input, and to the ipsilateral mossy fibre input. No differences were found between the toxin and control groups in the size of the antidromic population spike in the commissural response, or in the population excitatory post‐synaptic potential (e.p.s.p.) for either input. Thus the depressed output from CA3 pyramidal cells cannot be explained either by a loss of these neurones (confirming earlier neuropathological observations), or by a loss of excitatory afferents. While its precise cause remains unknown, the depressed output from the CA3 region was statistically correlated with the learning impairment, and we believe provides a reasonable explanation of this behavioural deficit.
Ionic basis of the different action potential configurations of single guinea‐pig atrial and ventricular myocytes.
Authors: Hume J R , Uehara A
Published: December 2014

Single myocardial cells were enzymatically dispersed from guinea‐pig atria and ventricles. At 25 degrees C, atrial cell action potentials differed significantly from ventricular cell action potentials in duration (atrial = 141 ms, ventricular = 497 ms) and over‐shoot (atrial = +36 mV, ventricular = +42 mV). Action potentials of atrial and ventricular cells responded differently to changes in external K+ concentration ([K+]o). Elevation of [K+]o from 6 to 11 mM depolarized atrial cells but produced no significant change in action potential duration; similar changes in [K+]o depolarized ventricular cells and produced a significant shortening of the action potential duration. Voltage‐clamp experiments were performed to investigate the ionic basis underlying the different action potential configurations of single atrial and ventricular myocytes. A single‐micropipette voltage‐clamp technique was used, employing either extremely small‐tip diameter pipettes, without internal cell dialysis (Hume & Giles, 1983), or larger tip diameter pipettes, with internal dialysis (Hamill, Marty, Neher, Sakmann & Sigworth, 1981). Two significant differences in background K+ conductance in single atrial and ventricular myocytes were observed: (i) the isochronal (5 s) current‐voltage relationship of single ventricular myocytes exhibited a region of prominent negative slope conductance and elevation of [K+]o produced cross‐over; a negative slope conductance region was absent in atrial cells and elevation of [K+]o produced very little cross‐over of isochronal current‐voltage relationships, and (ii) hyperpolarizing voltage pulses applied from holding potentials of ‐50 mV elicited inward current in ventricular cells which decayed with time; similar voltage‐clamp pulses in atrial cells elicited inward currents which fail to decay. Single K+ channel current measurements confirmed the existence of different resting K+ channel properties in single atrial and ventricular myocytes. Resting K+ channels in both cell types had similar single channel conductances (30‐32 pS with [K+]o = 145 mM) but ventricular K+ channels had significantly slower gating kinetics compared to atrial K+ channels (ventricular K+ channel mean open time = 223 ms; atrial K+ channel mean open time = 1 ms at Vr (resting membrane potential) ‐20 mV). The plateau and duration of the guinea‐pig ventricular action potential was insensitive to high concentrations of tetrodotoxin (3 X 10(‐5) M) but extremely sensitive to external Ca2+ concentration ([Ca2+]o). The second inward Ca2+ current (iCa) density was estimated in small atrial and ventricular myocytes of similar diameter and length.(ABSTRACT TRUNCATED AT 400 WORDS)
The effects of caffeine and ryanodine on the electrical activity of the canine coronary sinus.
Authors: Aronson R S , Cranefield P F , Wit A L
Published: December 2014

Cells of the coronary sinus of the canine heart can exhibit triggered activity which each action potential arises from a depolarizing after‐potential that follows the previous action potential; an early after‐hyperpolarization commonly precedes the delayed after‐depolarization and both are increased in amplitude by the addition of noradrenaline. The delayed after‐depolarization is thought to be caused by an inward current activated by a rise in intracellular Ca2+ that is, in turn, caused by Ca2+‐induced release of Ca2+ from the sarcoplasmic reticulum (s.r.). The effects of caffeine and of ryanodine on the electrical activity of the coronary sinus were investigated because each of those agents is thought to affect the handling of intracellular Ca2+ by the s.r. The steady‐state effect of exposure to 5 mM‐caffeine is to cause the delayed after‐depolarization to move much earlier in the cycle, and become too small to give rise to an action potential so that preparations cannot show triggered activity; moreover, if a burst of activity is in progress it is terminated by exposure to 5 mM‐caffeine. Exposure to 0.5 mM‐caffeine causes the delayed after‐depolarization to move earlier in the cycle but to become larger so that triggered activity is more easily induced and longer lasting than in the absence of caffeine. Shortly after the addition (or wash‐out) of 5 mM‐caffeine the after‐depolarization transiently resembles that seen in the presence of 0.5 mM‐caffeine so that bursts of triggered activity may occur just after the addition or removal of 5 mM‐caffeine. Exposure to 5 mM‐caffeine abolishes early rapid repolarization (phase 1), shifts the plateau to a more positive level and retards the completion of repolarization. The effect on phase 1 is mimicked by exposure to solutions low in Cl‐; the effect on the plateau is mimicked by exposure to 20 mM‐tetraethylammonium (TEA); fibres exposed to solutions containing 20 mM‐TEA and 21 mM‐Cl‐ show action potentials very like those of fibres exposed to 5 mM‐caffeine. If a fibre already exposed to a low Cl‐, TEA‐containing solution is then exposed to 5 mM‐caffeine, no further change occurs in the action potential but the characteristic effects of caffeine on the after‐depolarization appear. Exposure to ryanodine prevents the appearance of the delayed after‐depolarization but leads to the appearance of an exceptionally long depolarizing after‐potential that begins very early in diastole and, though waning, persists almost throughout diastole.(ABSTRACT TRUNCATED AT 400 WORDS)
Electrophysiological optometry using Scheiner's principle in the pigeon eye.
Authors: Fitzke F W , Hayes B P , Hodos W , Holden A L
Published: December 2014

A new electrophysiological optometer has been developed, based on Scheiner's principle. Using two light‐emitting diodes driven in counterphase, and grating stimuli in Maxwellian view, the system has been used to refract the photoreceptor plane of the pigeon eye. When a grating is conjugate with the photoreceptors, the electroretinographic response (e.r.g.) is minimal, and the image is stationary. As defocus is introduced, so image shift occurs, and the e.r.g. rises on each side of the refractive minimum. Two experiments were devised to test whether a derived minimum point is primary, by using gratings of bar width 3, 5 and 7 units, and by using random checkerboard stimuli. Ray tracing was used to compute the lobe width of dioptric profiles for gratings of spatial frequency 1.47, 0.88 and 0.63 cycles/deg. The computed lobe widths agree well with experimentally determined e.r.g. profiles. Examination of the lateral visual field, on the horizon, shows that the pigeon eye is emmetropic, and well corrected for astigmatism.
Differential expression of inward and outward potassium currents in the macrophage‐like cell line J774.1.
Authors: Gallin E K , Sheehy P A
Published: December 2014

J774.1 cells, a mouse‐derived macrophage‐like tumour cell line, were voltage clamped using whole‐cell patch‐clamp techniques. Cells were maintained in suspension cultures and plated at varying times before recording. The average zero‐current potential of long‐term adherent (greater than 24 h) cells was ‐77.6 mV. A tenfold increase in [K]o produced a 49 mV shift in zero‐current potential. Freshly plated cells (less than 24 h) expressed two voltage‐dependent currents: an outward current expressed transiently from 1 to 12 h post‐plating and an inward current expressed 2‐4 h post‐plating which persisted in 100% of long‐term adherent cells. Inward current was dependent upon voltage, time and [K]o 1/2, similar to the anomalous rectifier of other tissues. The conductance activated at potentials negative to ‐50 mV and plateaued at potentials negative to ‐110 mV. Inactivation was evident at potentials negative to ‐100 mV. Both the rate and extent of inactivation increased with hyperpolarization. Inward rectification was blocked by external BaCl2 or CsCl. The outward current was time‐ and voltage‐dependent. The instantaneous I/V curves derived from tail experiments reversed at the potassium equilibrium potential (EK). A tenfold change of [K]o shifted the reversal potential 52 mV, indicating that the current was carried by potassium. This conductance activated at potentials positive to ‐50 mV, plateaued at potentials positive to ‐10 mV and inactivated completely with an exponential time course at all potentials. At voltages positive to ‐25 mV the rate of inactivation was independent of voltage. The outward current was blocked by 4‐aminopyridine or D600. During the first 10 min after attaining a whole‐cell recording, the conductance/voltage relation of the outward current shifted to more negative voltages and peak conductance showed a slight increase; recordings then stabilized. The voltage dependence of the inward current did not shift with time but wash‐out of inward current was observed in some cells. The J774.1 cell line can serve as a model for the study of the role of voltage‐dependent ionic conductances in macrophages.
Ventilatory drive during face immersion in man.
Authors: Mukhtar M R , Patrick J M
Published: December 2014

Four series of experiments have been performed in normal subjects to determine whether face immersion gives rise to a reduction in ventilatory drive. Such a response might be advantageous, like the cardiovascular components of the 'diving response', in prolonging breath‐hold diving. In the first series, ventilatory drive was measured indirectly as the maximal voluntary breath‐holding time, starting each breath‐hold at the same alveolar partial pressures and at the same lung volume. When the face was immersed in cold water, breath‐holding times were increased by 14%. The breaking point occurred at a higher alveolar PCO2 and the rate of rise of PCO2 was not affected. Control immersions in warm water had no effect. In the second and third series, subjects lay prone and breathed either air or 5% CO2 through a valve in the bottom of a bowl. Minute ventilation was measured before, during and after 5 min of face immersion in cold water. Transient hypoventilations of 13% and 10% respectively were seen, accompanied by small rises in alveolar PCO2. In control experiments, immersion of the forearm in cold water produced the opposite responses. In the fourth series, a cold wet pack was applied to the face during moderate steady‐state exercise. A small irregular hypoventilation was seen, but not in control experiments when a warm pack was applied. Face temperatures fell by about 10 degrees C in these experiments. No material changes were found in the temperatures of the inspired air or of the aural canal. It is concluded that face immersion in cold water causes a modest reduction in ventilatory drive in man. This appears to be a component of the 'diving response'.
Electrophysiological analysis of projections of enteric inhibitory motoneurones in the guinea‐pig small intestine.
Authors: Bornstein J C , Costa M , Furness J B , Lang R J
Published: December 2014

The projections of neurones that produce inhibitory junction potentials (i.j.p.s) have been analysed in the small intestine of the guinea‐pig. Intracellular microelectrodes were used to record i.j.p.s in circular muscle cells in response to stimulation with pairs of electrodes placed parallel to or across the length of the intestine (longitudinal and transverse electrodes, respectively). In preparations of circular muscle with the myenteric plexus attached, the mean amplitude of i.j.p.s recorded 9 mm lateral to longitudinal electrodes was about 70% of that seen at 1 mm; however, in the presence of hexamethonium (400 microM) the amplitude fell more rapidly to about 40% at 9 mm. The decline was even steeper in preparations of circular muscle from which the myenteric plexus had been removed, with the amplitude falling to 10% at 9 mm. I.j.p.s were detected up to 30 mm anal to stimuli applied with transverse electrodes in the presence of hexamethonium, but the decline was more rapid on the oral side, with i.j.p.s being detected only up to 12 mm oral to the stimulus. Nerves that travel through or originate in the myenteric plexus were severed or removed in operations performed 2‐7 days before tissue was taken. 2‐3 days after removal of a circumferential strip of myenteric plexus, i.j.p.s could not be recorded from the underlying circular muscle. I.j.p.s evoked by longitudinal electrodes were recorded in an area between two circumferential cuts through the myenteric plexus, 1.5‐8 mm apart, 3‐7 days after an operation. In this region, the i.j.p.s were significantly smaller than normal although amplitudes of up to 95% of control were measured. The consequences of single circumferential cuts through the myenteric plexus were examined 3 and 7 days after operation. Longitudinal electrodes were used to stimulate surviving fibres. Small i.j.p.s were recorded on the anal edge of the lesion; the amplitude recovered to 56% of normal in the first 2 mm on the anal side and the amplitude slowly returned to normal at 20 mm anal. On the oral side there was slight reduction in amplitude at 0.5 and 1 mm, but at greater distances amplitudes were normal. It is concluded that the simplest explanation of these results is that two populations of i.j.p.‐producing neurones have their cell bodies in the myenteric plexus. Many axons supply the circular muscle close to or just anal to their cell bodies, and others run for various distances up to 30 mm in the anal direction.(ABSTRACT TRUNCATED AT 400 WORDS)
Relationship between the transient inward current and slow inward currents in the sino‐atrial node of the rabbit.
Authors: Brown H F , Noble D , Noble S J , Taupignon A I
Published: December 2014

In low K+ (0.3 mM) solutions rabbit sinus node preparations show the oscillatory transient inward current, iTI, already recorded in these conditions in Purkinje and ventricular preparations. The time course of iTI closely resembles that of the slow component of the slow inward current (isi) previously reported by us (Brown, Kimura, Noble, Noble & Taupignon, 1984a) in rabbit sinus node, when recorded near its threshold (‐40 mV). When the duration of voltage‐clamp steps is varied there is a strong correlation between the 'envelope' of isi amplitudes on depolarization and the time course of iTI on hyperpolarization. Although oscillations of iTI become smaller near 0 mV, there is no potential at which the current records are completely flat, suggesting that there is no simple reversal potential. 75% substitution of Na+ by Li+ greatly reduces both iTI and slow isi in about the same proportion. Reducing the activity of the Na‐K exchange pump by the amount expected in 0.3 mM‐K+ solutions is sufficient to induce oscillatory iTI in a computer model of the sino‐atrial node (Noble & Noble, 1984). The model reproduces the current as variations in the Na‐Ca exchange current dependent on intracellular Ca2+ concentration ([ Ca]i). The model was also used to test the alternative hypothesis that the slow inward currents might be generated by [Ca]i‐activated non‐specific cation channels. It is shown that this would distort the shape of the repolarization phase of the action potential. It is concluded that the experiments and computations are consistent with the hypothesis that a large fraction of iTI and the slow component of isi could both be generated by Na‐Ca exchange and that only a relatively small fraction might be generated by non‐specific channels.