bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ca2+ and Na+ permeability of high‐threshold Ca2+ channels and their volt age‐dependent block by Mg2+ ions in chick sensory neurones
Authors: Carbone E. , Lux H. D. , Carabelli V. , Aicardi G. , Zucker H. 
Published: September 2004

The Mg2+ block of Na+ and Ca2+ currents through high‐voltage activated (HVA; L‐ and N‐type) Ca2+ channels was studied in chick dorsal root ganglion neurones.
In low extracellular [Ca2+] (< 10−8 M) and with Nao+ and Csi+ as the main charge carriers (120 mm), HVA Na+ currents started to activate at −40 mV, reached inward peak values near 0 mV and reversed at about +40 mV.
Addition of 30–500 μm Mg2+ to the bath caused a strong depression of inward Na+ currents that was voltage and dose dependent (KD=39 μm in 120 mm Na+ at −10 mV). The block was maximal at negative potentials (< −70 mV) and decreased with increasing positive potentials, suggesting that Mg2+ cannot escape to the cell interior.
Block of Ca2+ currents by Mg2+ was also voltage dependent, but by three orders of magnitude less potent than with Na+ currents (KD= 24 mm in 2 mm Ca2+ at −30 mV). The high concentration of Mg2+ caused a prominent voltage shift of channel gating kinetics induced by surface charge screening effects. To compensate for this, Mg2+ block of inward Ca2+ currents was estimated from the instantaneous I–V relationships on return from very positive potentials (+100 mV).
Inward Na+ and Ca2+ tail currents following depolarization to +90 mV were markedly depressed, suggesting that channels cleared of Mg2+ ions during strong depolarization are quickly re‐blocked on return to negative potentials. The kinetics of re‐block by Mg2+ was too fast (< 100 μs) to be resolved by our recording apparatus. This implies a rate of entry for Mg2+ > 1.45 × 108 M− s−1 when Na+ is the permeating ion and a rate approximately 3 orders of magnitude smaller for Ca2+.
Mg2+ unblock of HVA Na+ currents at +100 mV was independent of the size of outward currents, whether Na+, Cs+ or NMG+ were the main internal cations.
Consistent with the idea of a high‐affinity binding site for Ca2+ inside the channel, micromolar amounts of Ca2+ caused a strong depression of Na+ currents between −40 and 0 mV, which was effectively relieved with more positive as well as with negative potentials (KD= 0.7μm in 120 mm Na+ at −20 mV). In this case, the kinetics of re‐block could be resolved and gave rates of entry and exit for Ca2+ of 1.4 × 108 M−1 s−1 and 2.95 × 102 s−1, respectively.
The strong voltage dependence and weak current dependence of HVA channel block by divalent cations and the markedly different KD values of Na+ and Ca2+ current block by Mg2+ can be well described by a previously proposed model for Ca2+ channel permeation based on interactions between the permeating ion and the negative charges forming the high‐affinity binding site for Ca2+ inside the pore (Lux, Carbone & Zucker, 1990).
Measurement of sarcoplasmic reticulum Ca2+ content and sarcolemmal Ca2+ fluxes in isolated rat ventricular myocytes during spontaneous Ca2+ release
Authors: Díaz M.E. , Trafford A.W. , O'Neill S.C. , Eisner D.A. 
Published: September 2004

Intracellular calcium concentration ([Ca2+]i) and Na+–Ca2+ exchange currents were measured in calcium‐overloaded voltage‐clamped rat ventricular myocytes loaded with the Ca2+sensitive fluorescent indicator indo‐1. Sarcoplasmic reticulum (SR) Ca2+ content was measured from the integral of the caffeineevoked current. In cells that had spontaneous SR Ca2+ release in 1 mm external Ca2+ concentration ([Ca2+]o), raising [Ca2+]o increased the frequency of release with no effect on SR Ca2+ content. In quiescent cells, increased [Ca2+]o produced spontaneous Ca2+ release associated with increased SR Ca2+ content. Further increase of [Ca2+]o had no effect on SR Ca2+ content. The amount of Ca2+ leaving the cell during each release was constant over a wide range of frequencies and [Ca2+]o values. It appears there is a maximum level of SR Ca2+ content, perhaps because spontaneous Ca2+ release results when the content reaches a threshold.
From the relationship between [Ca2+]i and Na+–Ca2+ exchange current during a caffeine response, it is possible to estimate the changes in Na+–Ca2+ exchange current expected from a change of [Ca2+]i. The data show that the calcium oscillations contribute a significant fraction of the total extra Ca2+ efflux induced by increasing [Ca2+]o. Raising [Ca2+]o decreased the rate of calcium removal from the cell as measured from the rate of decay of the caffeine response, suggesting that both inhibition of Ca2+ efflux and increased Ca2+ entry account for the Ca2+ overload at elevated [Ca2+]o.
Inhibiting spontaneous SR Ca2+ release increases resting [Ca2+]i. The Ca2+ efflux is identical to that in the presence of release. It is concluded that spontaneous release of calcium, although potentially arrhythmogenic, is an effective way to activate Ca2+ efflux in overloaded conditions and minimizes any increase of diastolic tension.
A C‐terminal peptide of the GIRK1 subunit directly blocks the G protein‐activated K+ channel (GIRK) expressed in Xenopus oocytes
Authors: Luchian Tudor , Dascal Nathan , Dessauer Carmen , Platzer Dieter , Davidson Norman , Lester Henry A. , Schreibmayer Wolfgang 
Published: September 2004

In order to find out the functional roles of cytosolic regions of a G protein‐activated, inwardly rectifying potassium channel subunit we studied block of GIRK channels, expressed in Xenopus laevis oocytes, by synthetic peptides in isolated inside‐out membrane patches.
A peptide (DS6) derived from the very end of the C‐terminus of GIRK1 reversibly blocked GIRK activity with IC50 values of 7.9 ± 2.0 or 3.5 ± 0.5 μg ml−1 (corresponding to 3.7 ± 0.9 or 1.7 ± 0.2 μmol l−1) for GIRK1/GIRK5 or GIRK1/GIRK4 channels, respectively.
Dose dependency studies of GIRK activation by purified βγ subunits of the G protein (Gβγ) showed that DS6 block of GIRK channels is not the result of competition of the peptide with functional GIRK channels for the available Gβγ.
Burst duration of GIRK channels was reduced, whereas long closed times between bursts were markedly increased, accounting for the channel block observed.
Block by the DS6 peptide was slightly voltage dependent, being stronger at more negative potentials.
These data support the hypothesis that the distal part of the carboxy‐terminus of GIRK1 is a part of the intrinsic gate that keeps GIRK channels closed in the absence of Gβγ.
Tetrodotoxin‐Sensitive Sodium Current in Sheep Lymphatic Smooth Muscle
Authors: Hollywood M. A. , Cotton K. D. , Thornbury K. D. , McHale N. G. 
Published: September 2004

Fast inward currents were elicited in freshly isolated sheep lymphatic smooth muscle cells by depolarization from a holding potential of −80 mV using the whole‐cell patch‐clamp technique. The currents activated at voltages positive to −40 mV and peaked at 0 mV.
When sodium chloride in the bathing solution was replaced isosmotically with choline chloride inward currents were abolished at all potentials.
These currents were very sensitive to tetrodotoxin (TTX). Peak current was almost abolished at 1 μM with half‐maximal inhibition at 17 nM.
Examination of the voltage dependence of steady state inactivation showed that more than 90 % of the current was available at the normal resting potential of these cells (−60 mV).
The time course of recovery from inactivation was studied using a double‐pulse protocol and showed that recovery was complete within 100 ms with a time constant of recovery of 20 ms.
Under current clamp, action potentials were elicited by depolarizing current pulses. These had a rapid upstroke and a short duration and could be blocked with 1 μM TTX.
Spontaneous contractions of isolated rings of sheep mesenteric lymphatic vessels were abolished or significantly depressed by 1 μM TTX.
The effect of Ca2+–calmodulin‐dependent protein kinase II on cardiac excitation–contraction coupling in ferret ventricular myocytes
Authors: Li Li , Satoh Hiroshi , Ginsburg Kenneth S. , Bers Donald M. 
Published: September 2004

The effect of Ca2+–calmodulin‐dependent protein kinase II (CaMKII) on excitation–contraction coupling (E–C coupling) was studied in intact ferret cardiac myocytes using the selective inhibitor KN‐93. KN‐93 decreased steady‐state (SS) twitch [Ca2+]i (by 51%), resting Ca2+ spark frequency (by 88%) and SS sarcoplasmic reticulum (SR) Ca2+ content evaluated by caffeine application (by 37.5%).
Increasing extracellular Ca2+ concentration ([Ca2+]o) to 5 mm in KN‐93 restored SR Ca2+ load and Ca2+ spark frequency towards that in control (2 mm Cao2+), but SS twitch [Ca2+]i was still significantly depressed by KN‐93.
KN‐93 decreased Ca2+ transient amplitude of SS twitches much more strongly than the amplitude of post‐rest (PR) twitches. In the control, the time constant (τ) of [Ca2+]i decline of SS twitches was faster than that for PR twitches. This stimulation‐dependent acceleration of [Ca2+]i decline was abolished by KN‐93.
Voltageclamp experiments demonstrated that KN‐93 significantly inhibited sarcolemmal L‐type Ca2+ current (ICa) during repetitive pulses by slowing recovery from inactivation. This may explain the preferential action of KN‐93 to suppress SS vs. PR twitches.
In KN‐93, even when both ICa and SR Ca2+ load were matched to the control levels by manipulation of conditioning voltage‐clamp pulses, contraction and twitch Ca2+ transients were still both significantly depressed (to 39 and 49% of control, respectively).
Since KN‐93 reduced SR Ca2+ release channel (RyR) activity during E–C coupling, even for matched SR Ca2+ load and trigger ICa, we infer that endogenous CaMKII is an important modulator of E–C coupling in intact cardiac myocytes. Effects of KN‐93 on ICa and SS twitch [Ca2+]i decline also indicate that endogenous CaMKII may have stimulatory effects on ICa and SR Ca2+ uptake.
Synergistic activation of guinea‐pig cardiac cystic fibrosis transmembrane conductance regulator by the tyrosine kinase inhibitor genistein and cAMP
Authors: Shuba Lesya M. , McDonald Terence F. 
Published: September 2004

The regulation of cardiac Cl− current (IC1) by tyrosine and serine/threonine phosphorylation was examined in guinea‐pig and rat ventricular myocytes. The protein tyrosine kinase (PTK) inhibitor genistein (GST) and phosphotyrosine phosphatase (PTP) inhibitor sodium orthovanadate (VO4) were used to modify tyrosine phosphorylation, whereas forskolin (FSK), cAMP, and other agents were used to modify cytoplasmic cAMP concentration and protein kinase A (PKA) phosphorylation.
Low concentrations (0.1 μm) of FSK did not activate the PKA‐regulated cystic fibrosis transmembrane regulator (CFTR) ICl in guinea‐pig ventricular myocytes, but strongly potentiated activation of an ICl by 20–100 μm GST. The potentiation did not occur when GST was replaced by PTK‐inactive daidzein, and it was strongly inhibited by 1 mm VO4.
Potentiation by 0.1 μm FSK was linked to a small stimulation of the adenylate cyclase–cAMP–PKA pathway. The potentiation was not mimicked by inactive 1,9‐dideoxyforskolin, and was inhibited by muscarinic stimulation (ACh) and by a PKA inhibitor. Internal application of a cAMP solution that alone was too weak to activate CFTR ICl strongly potentiated the activation of ICl by 50 μm GST and occluded potentiation by 0.1 μm FSK.
The foregoing suggests that potentiated ICl flows through cAMP‐dependent CFTR channels. In agreement with this interpretation, GST did not increase ICl when CFTR was maximally activated by a high concentration (5 μm) of FSK and okadaic acid, and neither GST nor GST plus FSK activated an ICl in CFTR‐deficient rat myocytes. The lack of effect in rat myocytes was not due to the absence of functional, channel‐relevant PKA and PTK–PTP systems, because (as in guinea‐pig myocytes) L‐type Ca2+ current (ICa,L) was stimulated by FSK and inhibited in a VO4‐reversible manner by GST.
The synergistic activation of CFTR by low concentrations of FSK and GST cannot be explained by either a GST‐induced elevation of cAMP concentration or inhibition of serine/threonine phosphatase. Rather, it appears to be due to tyrosine dephosphorylation that facilitates PKA‐mediated phosphorylation of the channels.
Swelling‐induced Cl− current in guinea‐pig atrial myocytes: inhibition by glibenclamide
Authors: Sakaguchi Masayuki , Matsuura Hiroshi , Ehara Tsuguhisa 
Published: September 2004

Whole‐cell currents were recorded from guinea‐pig atrial myocytes using the patch‐clamp technique under conditions designed to block K+ channels, Ca2+ channels and electrogenic transporters.
Exposure of atrial myocytes to the hyposmotic external solution (Na+ reduction to about 70% of control) resulted in hyposmotic cell swelling which was associated with activation of an outwardly rectifying Cl− current (ICl,swell).
Whereas the activation of ICl,swell was not significantly affected by replacement of ATP in the pipette solution with the non‐hydrolysable ATP analogue 5′‐adenylyl‐imidodiphosphate (AMP‐PNP), its activation was greatly reduced in cells dialysed with an ATP‐free pipette solution, thus indicating that the activation process of ICl,swell requires the presence of intra‐cellular ATP, but not its hydrolysis.
Bath application of glibenclamide produced a concentration‐dependent block of ICl,swell with a half‐maximal inhibitory concentration (IC50) of 60.0 μm and a Hill coefficient of 2.1. The maximal effect (100% inhibition) was obtained with 500 μm glibenclamide. The steady‐state inhibition showed little voltage dependence, while glibenclamide at concentrations of more than 100 μm inhibited the outward ICl,swell more rapidly than the inward ICl,swell. The glibenclamide inhibition was fully reversible after removal of the drug, even when a maximal effect (full inhibition) was achieved at a high drug concentration (500 μm).
These results show that (i) glibenclamide is one of the most potent inhibitors of guinea‐pig atrial ICl,swell and (ii) atrial ICl,swell and the cystic fibrosis transmembrane conductance regulator (CFTR) Cl− currents are almost equally sensitive to inhibition by glibenclamide.
A quantitative analysis of the activation and inactivation kinetics of HERG expressed in Xenopus oocytes
Authors: Wang Shimin , Liu Shuguang , Morales Michael J. , Strauss Harold C. , Rasmusson Randall L. 
Published: September 2004

The human etherà‐go‐go‐related gene (HERG) encodes a K+o channel that is believed to be the basis of the delayed rectified current, IKr, in cardiac muscle. We studied HERG expressed in Xenopus oocytes using a two‐electrode and cut‐open oocyte clamp technique with [K+]o of 2 and 98 mm.
The time course of activation of the channel was measured using an envelope of tails protocol and demonstrated that activation of the heterologously expressed HERG current (IHERG) was sigmoidal in onset. At least three closed states were required to reproduce the sigmoid time course.
The voltage dependence of the activation process and its saturation at positive voltages suggested the existence of at least one relatively voltage‐insensitive step. A three closed state activation model with a single voltage‐insensitive intermediate closed state was able to reproduce the time and voltage dependence of activation, deactivation and steady‐state activation. Activation was insensitive to changes in [K+]o.
Both inactivation and recovery time constants increased with a change of [K+]o from 2 to 98 mm. Steady‐state inactivation shifted by ∼30 mV in the depolarized direction with a change from 2 to 98 mm K*o
Simulations showed that modulation of inactivation is a minimal component of the increase of this current by [K+]o, and that a large increase in total conductance must also occur.
Differential Effects of Endothelin‐1 on Basal and Isoprenaline‐Enhanced Ca2+ Current in Guinea‐Pig Ventricular Myocytes
Authors: Thomas George P. , Sims Stephen M. , Karmazyn Morris 
Published: September 2004

We examined the effect of endothelin‐1 (ET‐1) on basal and isoprenaline‐enhanced L‐type Ca2+ current (ICa,L) in guinea‐Pig ventricular myocytes under nystatin‐perforated patch configuration.
ET‐1 at concentrations of 1, 5 and 10 nM had little effect on basal ICa,L. However, ICa,L enhanced by isoprenaline (500 nM) was significantly attenuated by 5 nM ET‐1 by more than 50%. This effect was reversed upon washout. ICa,L enhanced by forskolin was also decreased by ET‐1.
The inhibitory effect of ET‐1 against isoprenaline was completely blocked by the ETA receptor antagonist BQ‐123 (1 μm). In myocytes incubated with pertussis toxin (PTX, 2 μg ml−1) for 5 h, ET‐1 did not inhibit isoprenaline‐enhanced ICa,L.
Although ET‐1 has been shown to activate specific protein kinase C (PKC) isoforms, a significant inhibitory effect of ET‐1 was maintained in the presence of the PKC inhibitor bisindolylmaleimide (20 nm). The nitric oxide (NO) donor SIN‐1 (10 μm) attenuated but failed to prevent the ET‐1 effect.
In summary, our results demonstrate that ET‐1 is devoid of any significant effects on basal ICa,L. However, it exerts a potent inhibitory effect against isoprenaline‐enhanced ICa,L. This effect is mediated through ETA receptors coupled to PTX‐sensitive G‐proteins and occurs in the presence of PKC inhibition and NO generation.
Activation of a novel non‐selective cation channel by alloxan and H2O2 in the rat insulin‐secreting cell line CRI‐G1
Authors: Herson P. S. , Ashford M. L. J. 
Published: September 2004

Alloxan and its auto‐oxidation product hydrogen peroxide (H2O2) irreversibly depolarize insulinoma cells by opening a non‐selective cation channel. The channel opened is characterized by a linear current–voltage relation with a conductance of approximately 70 pS and very slow kinetics (of the order of seconds).
Cells are protected against the alloxan‐induced channel opening and consequent cell depolarization by the presence of H2O2 and hydroxyl radical scavengers.
The free radical‐activated non‐selective cation channel is not operative in isolated patches but can be activated by the application of β‐NAD to the cytoplasmic aspect of the membrane.
Outward potassium currents of supraoptic magnocellular neurosecretory cells isolated from the adult guinea‐pig
Authors: Hlubek Michael D. , Cobbett Peter 
Published: September 2004

Several types of whole‐cell outward K+ current recorded from magnocellular neurosecretory cells (MNCs) dissociated from the supraoptic nucleus of the adult guinea‐pig were identified on the basis of their voltage dependence, kinetics, pharmacology and Ca2+ dependence.
The predominant K+ current evoked from a holding potential of −40 mV was slowly activating, long‐lasting, tetraethylammonium (TEA) sensitive and showed little steady‐state inactivation. Also, this current was reduced by extracellular Cd2+. These data suggest that in supraoptic MNCs classical Ca2+‐insensitive, delayed rectifier channels (Kv) and Ca2+‐sensitive, non‐inactivating channels (KCa) both contribute to the sustained current.
A transient, low‐threshold K+ current, which was 4‐aminopyridine (4‐AP) sensitive and showed significant steady‐state inactivation, was evoked along with the sustained current from a holding potential of −90 mV. Based on these characteristics, this current corresponds to the A‐current (IK(A)) described in other neurons.
I
K(A) was activated when Ca2+ influx was blocked or when Ca2+ was absent from the extracellular medium, suggesting that Ca2+ influx is not necessary for activation of the current.
In many recordings, a transient 4‐AP‐insensitive outward current was evoked from a holding potential of −40 mV. This high‐threshold transient K+ current was abolished by extracellular Cd2+ or TEA and was absent when extracellular Ca2+ was replaced by Sr2+, suggesting that it is a transient Ca2+‐dependent K+ current.
We conclude that the presence of multiple types of K+ current may, in part, underlie the complex firing patterns of oxytocinergic and vasopressinergic MNCs.
Characterization of a nicotinamide–adenine dinucleotide‐dependent cation channel in the CRI‐G1 rat insulinoma cell line
Authors: Herson P. S. , Dulock K. A. , Ashford M. L. J. 
Published: September 2004

Cell‐free excised membrane patches were used to examine the properties of a novel nicotinamide–adenine dinucleotide (β‐NAD+)‐activated ion channel in the rat insulin‐secreting cell line, CRI‐G1.
In inside‐out recordings, β‐NAD+ (0.05–1.0 mm) induced the appearance of a channel characterized by extremely slow kinetics, with mean open times in the range of seconds. The estimated EC50 for activation was 114 μm. Channel activity declined with time (run‐down) following activation by β‐NAD+ in excised patches and this was not prevented by intra‐cellular application of trypsin.
The single channel current–voltage relationship was linear with a conductance of 74 pS in symmetrical NaCl. The channel appears equally permeable to Na+, K+ and Cs+, exhibits an appreciable permeability to Ca2+, Mg2+ and Ba2+, but excludes anions.
The channel displays an unusual voltage sensitivity, with an abrupt increase in open‐state probability at depolarized voltages.
Channel opening, in the presence of β‐NAD+, required both Ca2+ and Mg2+ to be present at the internal side of the membrane. Activation by Ca2+ required a concentration of at least 10 μm and was maximal at 0.1 mm. Ba2+ did not substitute for Ca2+ in inducing channel activity nor did it inhibit activation by Ca2+. Increasing the concentration of intracellular Mg2+ stabilized the open state of NAD+‐activated channels.
The non‐selective cation channel reported here differs in its gating and modulatory characteristics from non‐selective cation channels described in other tissues. This channel may play a role in the pathophysiological reponses of β‐cells to oxidative stress.