Slow inward tail currents in rabbit cardiac cells

Cholesterol depletion does not alter the capacitance or Ca handling of the surface or t-tubule membranes in mouse ventricular myocytes

Cholesterol is a key component of the cell plasma membrane. It has been suggested that the t‐tubule membrane of cardiac ventricular myocytes is enriched in cholesterol and that this plays a role in determining t‐tubule structure and function. We have used methyl‐β‐cyclodextrin (MβCD) to deplete cholesterol in intact and detubulated mouse ventricular myocytes to investigate the contribution of cholesterol to t‐tubule structure, membrane capacitance, and the distribution of Ca flux pathways. Depletion of membrane cholesterol was confirmed using filipin; however, di‐8‐ANEPPS staining showed no differences in t‐tubule structure following MβCD treatment. MβCD treatment had no significant effect on the capacitance:volume relationship of intact myocytes or on the decrease in capacitance:volume caused by detubulation. Similarly, Ca influx and efflux were not altered by MβCD treatment and were reduced by a similar amount following detubulation in untreated and MβCD‐treated cells. These data show that cholesterol depletion has similar effects on the surface and t‐tubule membranes and suggest that cholesterol plays no acute role in determining t‐tubule structure and function.

Sarcolemmal distribution of ICa and INCX and Ca2+ autoregulation in mouse ventricular myocytes

This study shows that in contrast to the rat, mouse ventricular Na⁺/Ca²⁺ exchange current density is lower in the t-tubules than in the surface sarcolemma and Ca²⁺ current is predominantly located in the t-tubules. As a consequence, the t-tubules play a role in recovery (autoregulation) from reduced, but not increased, sarcoplasmic reticulum Ca²⁺ release.

The balance of Ca²⁺ influx and efflux regulates the Ca²⁺ load of cardiac myocytes, a process known as autoregulation. Previous work has shown that Ca²⁺ influx, via L-type Ca²⁺ current (I_Ca), and efflux, via the Na⁺/Ca²⁺ exchanger (NCX), occur predominantly at t-tubules; however, the role of t-tubules in autoregulation is unknown. Therefore, we investigated the sarcolemmal distribution of I_Ca and NCX current (I_NCX), and autoregulation, in mouse ventricular myocytes using whole cell voltage-clamp and simultaneous Ca²⁺ measurements in intact and detubulated (DT) cells. In contrast to the rat, I_NCX was located predominantly at the surface membrane, and the hysteresis between I_NCX and Ca²⁺ observed in intact myocytes was preserved after detubulation. Immunostaining showed both NCX and ryanodine receptors (RyRs) at the t-tubules and surface membrane, consistent with colocalization of NCX and RyRs at both sites. Unlike I_NCX, I_Ca was found predominantly in the t-tubules. Recovery of the Ca²⁺ transient amplitude to steady state (autoregulation) after application of 200 µM or 10 mM caffeine was slower in DT cells than in intact cells. However, during application of 200 µM caffeine to increase sarcoplasmic reticulum (SR) Ca²⁺ release, DT and intact cells recovered at the same rate. It appears likely that this asymmetric response to changes in SR Ca²⁺ release is a consequence of the distribution of I_Ca, which is reduced in DT cells and is required to refill the SR after depletion, and NCX, which is little affected by detubulation, remaining available to remove Ca²⁺ when SR Ca²⁺ release is increased.

NEW & NOTEWORTHY This study shows that in contrast to the rat, mouse ventricular Na⁺/Ca²⁺ exchange current density is lower in the t-tubules than in the surface sarcolemma and Ca²⁺ current is predominantly located in the t-tubules. As a consequence, the t-tubules play a role in recovery (autoregulation) from reduced, but not increased, sarcoplasmic reticulum Ca²⁺ release.

Systematic characterization of the ionic basis of rabbit cellular electrophysiology using two ventricular models

Several mathematical models of rabbit ventricular action potential (AP) have been proposed to investigate mechanisms of arrhythmias and excitation-contraction coupling. Our study aims at systematically characterizing how ionic current properties modulate the main cellular biomarkers of arrhythmic risk using two widely-used rabbit ventricular models, and comparing simulation results using the two models with experimental data available for rabbit. A sensitivity analysis of AP properties, Ca²⁺ and Na⁺ dynamics, and their rate dependence to variations (±15% and ±30%) in the main transmembrane current conductances and kinetics was performed using the Shannon et al. (2004) and the Mahajan et al. (2008a,b) AP rabbit models. The effects of severe transmembrane current blocks (up to 100%) on steady-state AP and calcium transients, and AP duration (APD) restitution curves were also simulated using both models. Our simulations show that, in both virtual rabbit cardiomyocytes, APD is significantly modified by most repolarization currents, AP triangulation is regulated mostly by the inward rectifier K⁺ current (I(K1)) whereas APD rate adaptation as well as [Na⁺](i) rate dependence is influenced by the Na⁺/K⁺ pump current (I(NaK)). In addition, steady-state [Ca²⁺](i) levels, APD restitution properties and [Ca²⁺](i) rate dependence are strongly dependent on I(NaK), the L-Type Ca²⁺ current (I(CaL)) and the Na⁺/Ca²⁺ exchanger current (I(NaCa)), although the relative role of these currents is markedly model dependent. Furthermore, our results show that simulations using both models agree with many experimentally-reported electrophysiological characteristics. However, our study shows that the Shannon et al. model mimics rabbit electrophysiology more accurately at normal pacing rates, whereas Mahajan et al. model behaves more appropriately at faster rates. Our results reinforce the usefulness of sensitivity analysis for further understanding of cellular electrophysiology and validation of cardiac AP models.

Inactivation of ICa-L is the major determinant of use-dependent facilitation in rat cardiomyocytes

Two models have been proposed to explain facilitation of the L-type calcium current (ICa-L). A positive feedback model proposes that calcium released during a conditioning pulse (I1) facilitates the subsequent pulse (I2) via calmodulin/calmodulin kinase II (CaMKII) mechanisms. The negative feedback model proposes that the calcium release of each pulse feeds back on itself via calcium-dependent inactivation. The relative physiological roles were evaluated in rat ventricular myocytes. Paired pulses (450 ms interpulse interval) elicited facilitation (I2 of 872 +/- 145 versus I1 of 777 +/- 132 pA, P < 0.01). Inactivation time (T0.37) was prolonged for I2 versus I1 (22 +/- 2 and 16 +/- 2 ms, P > 0.01). Evidence for the negative feedback mechanism includes: (a) ryanodine (0.3 mM ) eliminated facilitation, surprisingly by increasing the amplitude of I1 more than that of I2 (1039 +/- 216 and 977 +/- 186 pA) and eliminated the difference in T0.37 between I2 and I1 (33.1 +/- 4.5 versus 32.5 +/- 4.6 ms); (b) an outward I2, which does not trigger sarcoplasmic reticulum (SR) Ca2+ release, eliminated facilitation even when it was conditioned by an inward I1; (c) facilitation decayed as the I1-I2 interval lengthened (time constant (tau) = 16.9 +/- 1.4 s); (d) thapsigargin (0.1 microM ) slowed this decay (tau = 43.8 +/- 11.7 s) whereas isoproterenol accelerated it (tau = 5.6 +/- 1.4 s, P < 0.01) and T0.37 paralleled this decay; and (e) the magnitude of ICa-L was negatively correlated with the sodium-calcium exchange current (INa/Ca) elicited by the SR-Ca2+ release. In conclusion, Ca2+-dependent inactivation of ICa-L is the major mechanism underlying facilitation.

Ionic basis of ryanodine's negative chronotropic effect on pacemaker cells isolated from the sinoatrial node

Spontaneous electrical activity and indo 1 fluorescence ratios were recorded simultaneously in cultured pacemaker cells isolated from the rabbit sinoatrial node. Ryanodine (10 microM) reduced the amplitude of action potential-induced intracellular Ca2+ (Ca2+i) transients by 19 +/- 3%, increased the time constant for their decay by 51 +/- 5%, and slowed spontaneous firing by 32 +/- 3%. 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-acetoxymethyl ester (AM; 25 microM) inhibited the Ca2+i transients and slowed spontaneous firing by 28 +/- 4%. Ryanodine did not alter hyperpolarization-activated or time-independent inward current, but it reduced the sum of L- and T-type Ca2+ currents (ICa,L and ICa,T) in both the presence and absence of BAPTA-AM. In contrast, ICa,L was unchanged by ryanodine. Slow inward current tails, presumed to be Na/Ca exchange current (INa/Ca), were abolished by BAPTA or ryanodine. The results suggest that a decrement of ICa,T, due to reduction of the intracellular Ca2+ concentration or a direct effect of ryanodine on T-type Ca2+ channels, contributes to the negative chronotropic effect. Another possibility, based primarily on theory and results in other preparations, is that a reduction of INa/Ca, as a consequence of the smaller action potential-induced Ca2+i transients, contributes to the effect of ryanodine.

Neurokinin A and Ca2+ current induce Ca(2+)-activated Cl(-) currents in guinea-pig tracheal myocytes

1. Membrane currents were recorded by a patch clamp technique in guinea-pig tracheal myocytes, using the whole cell mode with Cs(+) internal solution. 2. Both neurokinin A (NKA, 1 mu M) and caffeine (10 mM) evoked Ca(2+)-activated Cl- currents (I[Cl(Ca)]) transiently. In Ca(2+)-free bathing solution, the first application of NKA or caffeine elicited I[Cl(Ca)] but the second application of these substances failed to activate it. In addition, pretreatment with ryanodine in the presence of caffeine abolished the response to both NKA and caffeine whilst heparin (200 mu g ml(-1)) only blocked the NKA-induced response. I[Cl(Ca)] was also elicited by inositol 1,4,5-trisphosphate (IP(3)). 3. Command voltage pulses positive to 0 mV from a holding potential of -60 mV activated the voltage-dependent L-type Ca2+ current (I(Ca,L)) and late outward current. Upon repolarization to the holding potential, slowly decaying inward tail currents were recorded. The outward current during the depolarizing pulses and the inward tail current were enhanced by Bay K 8644, but completely blocked by Cd2+ or nifedipine. Replacement of external Ca2+ with Ba2+, removal of Ca2+ from the bath solution, or inclusion of EGTA (5 mM) in the patch pipette, also led to abolition of these currents, indicating that they were Ca2+ dependent, and that Ca2+ influx due to I(Ca,L) activated the currents. 4. When [Cl(-)](O) or [Cl(-)](i) was changed, the reversal potential (E(rev)) of the Ca2+-activated currents shifted, thus behaving like a Cl(-)-selective ion channel as predicted by the Nernst equation. DIDS (1 mM) completely abolished the currents, also suggesting that they were I[Cl(Ca)]. 5. NKA (1 mu M) and caffeine (30 mM) transiently activated I[Cl(Ca)], and after that both agents markedly reduced I[Cl(Ca)] induced by I(Ca,L). This is probably due to sarcoplasmic reticulum (SR) Ca2+ release induced by NKA or caffeine, followed by inhibition of the Ca(2+)-induced Ca2+ release from the SR. 6. The present results indicate that I[Cl(Ca)] can be activated by SR Ca2+ release due to NKA or caffeine (through IP(3) or ryanodine receptors) as well as by Ca2+ influx due to I(Ca,L). It also suggests that activation of I[Cl(Ca)] by NKA may be mediated by the production of IP(3), which releases Ca2+ from the SR.

Characteristics of the delayed rectifier K current compared in myocytes isolated from the atrioventricular node and ventricle of the rabbit heart

The delayed rectifier potassium current (IK) is known to be important in action potential repolarisation and may contribute to the diastolic pacemaker depolarisation in pacemaker cells from the heart. In this study, using whole-cell patch clamp, we investigated the characteristics of IK in morphologically normal cells from the atrioventricular node (AVN) and ventricle of the rabbit heart. Cells were held at -40 mV and 5 microM external nifedipine was used to block L-type calcium current (ICa,L). Significant IK was observed with pulses to potentials more positive than -30 mV. The steady-state activation curve in both cell types showed maximal activation at between + 10 and + 20 mV. Half-maximal activation of IK occurred at -4.9 and -4.1 mV with slope factors of 8.3 and 12.4 mV in ventricular and AVN cells, respectively. Using pulses of increasing duration, significant IK tails after repolarisation from + 40 mV were observed with pulses of 20 ms and increased with pulses up to 100-120 ms in both cell types. Pulses of longer duration did not activate further IK and this suggested that only the rapid component of IK, called IKr, was present in either cell type. Moreover, IK tails after pulses to all potentials were blocked completely by E-4031, a selective blocker of IKr. The reversal potential of IK varied with the concentration of external K. Superfusion of AVN cells with medium containing 4, 15 and 40 mM [K+]o resulted in reversal potentials of -81, -56 and -32 mV, respectively, which are close to values predicted if the IK channel were highly selective for K. The time constants for deactivation of IK in ventricle and AVN on return to -40 mV after a 500-ms activating pulse to + 60 mV were 480 ms and 230 ms, respectively. The faster deactivation of IK in AVN cells was a distinguishing feature and suggests that there may be differences in the IKr channel protein between ventricular and AVN cells.

Estimate of net calcium fluxes and sarcoplasmic reticulum calcium content during systole in rat ventricular myocytes

1. The experiments were performed on voltage-clamped cells in which intracellular calcium concentration ([Ca2+]i) was measured with the fluorescent indicator indo-1 (acetoxymethyl ester (AM) loading). When cells were stimulated with a short (100 ms) depolarizing pulse, following a rest, the magnitude of the first systolic calcium transient was greater than that in the steady state (rest potentiation) and decayed to its steady level over a few stimuli. If a longer pulse (800 ms) was used then the systolic calcium transient was either unaffected or increased in magnitude following a rest. During constant stimulation, if the length of the pulse is decreased, then the magnitude of the calcium transient decreased reversibly over several beats. 2. The calcium entry into the cell was measured from the integral of the inward calcium current and the efflux from the Na(+)-Ca2+ exchange current on repolarization. During the negative staircase the calcium current was approximately constant whilst the Na(+)-Ca2+ exchange current decayed in parallel with the systolic calcium transient. A net loss of calcium from the cell can be calculated from the extra Na(+)-Ca2+ exchange current following the initial pulses. 3. The application of caffeine produces a transient increase of both [Ca2+]i and an inward Na(+)-Ca2+ exchange current. The integral of this current can be used to estimate the caffeine-releasable calcium content of the sarcoplasmic reticulum (SR), which decreases following stimulation with short compared to long pulses. This difference in SR calcium content is quantitatively similar to that estimated from the sarcolemmal currents. 4. At a given membrane potential, the relationship between [Ca2+]i and current during the caffeine exposure can be used to estimate the Na(+)-Ca2+ exchange flux from the measured [Ca2+]i and thence the Na(+)-Ca2+ exchange flux during depolarization. 5. For a long depolarizing pulse the extrusion of calcium from the cell on Na(+)-Ca2+ exchange is comparable to the entry on the calcium current. In contrast, for short pulses the extrusion of calcium on the Na(+)-Ca2+ exchange immediately after the pulse is greater than the entry during the pulse on the calcium current. 6. These results show that rest potentiation can be correlated with changes in the amount of calcium stored in the SR and this, in turn, can be accounted for by sarcolemmal fluxes.

Na-Ca exchange tail current indicates voltage dependence of the Cai transient in rabbit ventricular myocytes

INTRODUCTION

In mammalian cardiac myocytes, a rise of intracellular calcium (Cai) is well known to activate Ca extrusion via forward Na-Ca exchange, which generates an inward membrane current. This can be observed as an inward "tail" current (INa-Ca) when the membrane is repolarized after a depolarization-activated rise of Cai. If, during a voltage step, the membrane is repolarized at the time of the peak of the Cai transient, the size of the INa-Ca tail might be expected to reflect the magnitude of the Cai transient. Therefore, it might be possible to estimate the amplitude and voltage dependence of the Cai transient without, for instance, using fluorescent indicators that can interfere with Cai regulation. The first aim of this study was to use INa-Ca tails to investigate the voltage dependence of the Cai transient in whole cell patch clamped rabbit ventricular myocytes dialyzed with a "normal" level of internal Na. The second aim was to investigate how the voltage dependence of the INa-Ca tails varied with changes to the dialyzing Na concentration. The third aim was to test the correlation of voltage dependence of INa-Ca tails with the voltage dependence of the Cai transient obtained using a fluorescent Ca indicator.

METHODS AND RESULTS

Experiments were performed at 35 degrees to 37 degrees C using whole cell patch clamp, and the holding potential was set at -40 mV. Depolarization elicited a Cai transient that peaked in 40 to 50 msec. We reasoned, therefore, that membrane repolarization after 50 msec would cause the raised level of Cai to activate an inward current on forward Na-Ca exchange. The amplitude of INa-Ca measured shortly (10 msec) after repolarization should reflect the peak amplitude of the Cai transient elicited by the depolarization. In cells dialyzed with 10 mM Na-containing solution and depolarized for 50 msec to differing test potentials, the INa-Ca tail on repolarization increased progressively after pulses to between -40 and +20 mV. The INa-Ca tail was maximal after a +20-mV pulse and showed no decline after depolarizations to more positive potentials, up to +100 mV (P > 0.1; n = 8). This implies that the Cai transient has a similar amplitude for depolarizing pulses between +20 and +100 mV. When Na-free solution dialyzed the cell, the voltage dependence of the INa-Ca tail became bell-shaped, with a maximum at +20 mV (n = 4). Voltage dependence of the INa-Ca tail was little affected by raising dialyzing Na from 10 to 20 mM (n = 4); but the amplitude of the INa-Ca tail increased. Inhibition of the Na-K pump with strophanthidin in cells dialyzed with 10 mM Na had qualitatively similar effects to increasing dialyzing Na. In Fura-2 loaded cells dialyzed with 10 mM Na, the Cai transient exhibited a similar voltage dependence to the INa-Ca tail (n = 6).

CONCLUSION

The results of this study suggest that in cells dialyzed with 10 mM Na, the voltage dependence of the Cai transient is different from the L-type Ca current, since this current declines at potentials > +20 mV. The results obtained using Fura-2 suggest that the INa-Ca tail current measurement tracked the Cai sufficiently well to reflect the voltage dependence of the Cai transient. The data also confirm that the voltage dependence of the Cai transient in rabbit cells can be modulated by altering dialyzing Na concentration.

Brief increases in intracellular Ca2+ activate K+ current and non-selective cation current in rat nucleus basalis neurons

Neurons were acutely dissociated from the rat nucleus basalis, and membrane currents (whole-cell patch-clamp) and intracellular free Ca2+ concentrations (Fura-2) were measured simultaneously from large neurons (approximately 25 microns in diameter). A brief depolarization from -60 to 0 mV for 200 ms evoked an increase in intracellular free calcium and a slow outward tail current (72 +/- 8 pA, n = 30). The outward current reversed polarity at -75.5 +/- 2.7 mV (n = 14). The tail current declined and the intracellular calcium recovered its resting level exponentially with time-constants of 1.0 +/- 0.1 s and 2.5 +/- 0.2 s, respectively (n = 17). In neurons loaded with Cs-gluconate, a similar depolarizing pulse evoked a similar increase in intracellular free calcium, but this was now followed by an inward tail current (118 +/- 8 pA, n = 44). The inward tail current reversed polarity at -27.8 +/- 3.8 mV (n = 7), and was suppressed by removal of external sodium ions. Neither outward nor inward tail currents were observed, when the external solution was calcium-free or when the pipette solution contained EGTA (10 mM). These results indicate that a depolarization causes a calcium entry and that this consequently increases both K+ conductance and non-selective cation conductance.

Voltage-dependent kinetics of Na-Ca exchange current in Ca(2+)-loaded guinea pig heart cells

Voltage-dependent properties of Na-Ca exchange current were revealed with the patch-clamp technique in Ca(2+)-overloaded guinea pig ventricular myocytes in the whole cell configuration. With the assumption that the transient inward current (Iti) is mediated by the Na-Ca exchanger, oscillations of internal Ca2+ concentration ([Ca2+]i) were used to investigate voltage-dependent kinetics of exchange current differences at two [Ca2+]i values. After Iti was elicited by clamping from -45 mV to basic pulses of +10 mV, pairs of equipotential short test pulses were applied during the basic pulse at both the phase of low [Ca2+]i (between two neighboring Iti values) and the phase of high [Ca2+]i (at the peak of Iti). The test pulses were short enough to leave the time course of Iti during the basic pulse approximately unchanged, which allowed study of the voltage dependence of the respective current differences without disturbing the underlying oscillation of [Ca2+]i. The current differences were inward at all potentials between -140 and +70 mV, started from an equal initial value, and obeyed characteristic voltage-dependent time courses: hyperpolarization to potentials negative to -70 mV caused an initial current increase, which was followed by a decay to very small amplitudes or zero with a decay time constant decreasing toward hyperpolarization e-fold per 45.6 mV. Depolarizing pulses caused a decay of the current differences to smaller levels. Respective current differences formed during a slowly decaying current component, following the Ca current spike, showed equal voltage-dependent properties. This indicates that the slowly decaying current component is preferentially also carried by the Na-Ca exchanger.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple effects of caffeine on calcium current in rat ventricular myocytes

Caffeine exerts a number of different effects on L-type calcium current in rat ventricular myocytes. These include: (1) a slowing of inactivation that is comparable to, but not additive to, that produced by prior treatment of the cells with ryanodine (a selective sarcoplasmic reticulum Ca2+ releaser) or high concentrations of intracellular 1,2-bis[2-aminophenoxy]ethane-N,N,N',-N'-tetraacetic acid (BAPTA) (a fast Ca2+ chelator), (2) a stimulation of peak ICa that is comparable to, but not additive to that produced by prior treatment with isobutylmethylxanthine (a selective phosphodiesterase inhibitor), and (3) a dose-dependent decrease of peak ICa that is not prevented by pretreatment with any of these agents. None of the caffeine actions could be mimicked or prevented by administration of 8-phenyltheophylline, a specific adenosine receptor antagonist. We conclude that only the slowing of ICa inactivation is due to caffeine's ability to deplete the sarcoplasmic reticulum of calcium. The stimulatory effect of caffeine on peak ICa is probably due to phosphodiesterase inhibition, while caffeine's inhibitory effect on ICa is independent of these processes and could be a direct effect on the channel. The multiplicity of caffeine actions independent of its effects on the sarcoplasmic reticulum lead to the conclusion that ryanodine, though slower acting and essentially irreversible, is a more selective agent than caffeine for probing sarcoplasmic reticulum function and its effects on other processes.

Na(+)-Ca2+ exchange current in latent pacemaker cells isolated from cat right atrium

1. Single latent pacemaker cells were isolated from cat right atrium, and studied in a whole-cell configuration using a nystatin-perforated patch recording method. The nystatin method avoids alterations in intracellular Ca2+, cellular constituents and run-down of ionic currents. 2. Depolarizing voltage clamp pulses from -40 mV elicited L-type Ca2+ current (ICa) that exhibited an initial rapid phase of inactivation followed by a secondary slower inward current component that decayed over about 100 ms. The secondary inward component appeared as a slowly decaying inward tail current following short (10-40 ms) depolarizing clamp steps. 3. Slowly decaying inward currents were abolished by internally dialysing pacemaker cells with 2 mM EGTA using a ruptured patch recording method. Inward tail currents were also abolished by exposure to 1 microM ryanodine and significantly decreased by replacing 85% of external Na+ with lithium, without effect on peak ICa. These findings identify a Na(+)-Ca2+ exchange current (INa-Ca) that is mediated by sarcoplasmic reticulum (SR) Ca2+ release. 4. Properties of INa-Ca and ICa differed significantly: (i) ICa exhibited a bell-shaped voltage dependence that peaked at 0 mV and decreased at more positive voltages. INa-Ca was maximal at -10 mV and remained relatively constant at more positive voltages; (ii) a paired pulse protocol showed that the time course of INa-Ca recovery (5 s) was significantly longer than that of ICa (2 s); (iii) cadmium (50 microM) induced an inhibition of ICa that did not correlate in time with changes in INa-Ca. 5. The duration of depolarizing steps between 10 and 120 ms had no effect on the time course of INa-Ca tail currents. 6. Isoprenaline > or = 5 x 10(-8) M significantly increased peak ICa amplitude, peak INa-Ca amplitude, accelerated INa-Ca rate of decay and decreased the absolute time of INa-Ca decay. 7. Free-running pacemaker action potentials were clamped during diastole at either -40 or -70 mV (maximum diastolic potential) for variable periods of time. At times between 0.2 and 1 s, INa-Ca exhibited a voltage-dependent increase in amplitude over time, i.e. INa-Ca recovered more rapidly from -70 mV than from -40 mV. At times > 2 s, INa-Ca exhibited a voltage-dependent decline in amplitude over time, i.e. from -40 mV INa-Ca decreased by 10% of maximum whereas from -70 mV INa-Ca decreased by 60% of maximum.(ABSTRACT TRUNCATED AT 400 WORDS)

Background potassium current active during the plateau of the action potential in guinea pig ventricular myocytes

Background outward K+ currents in guinea pig ventricular myocytes were characterized over a broad range of membrane potentials, including those corresponding to the plateau of the action potential. The background current that is blocked by 1 mM Ba2+ (IK,p) activates within 5 msec at positive potentials, does not inactivate, and deactivates very rapidly on repolarization. IK,p is insensitive to Cl- channel blockers, internal or external [Cl-], dihydropyridines, and sulfonylureas. In contrast, the delayed rectifier K+ current (IK) was not completely blocked even by 30 mM Ba2+. Ba(2+)-sensitive current density increased progressively from 0.16 +/- 0.04 pA/pF at 0 mV to 0.52 +/- 0.21 pA/pF at +80 mV (n = 13, mean +/- SEM). The background current remains present when [K+]o is reduced to 0 mM, which suppresses the inward rectifier K+ current (IK1). These and other features suggest that IK,p is generated by K+ channels that are distinct from IK1 or IK. The kinetics and voltage dependence of IK,p render it capable of modulating both the height and duration of the cardiac action potential.

Ca2+ and Ca(2+)-activated Cl- currents in rabbit oesophageal smooth muscle

1. An inward current carried by Ca2+ was recorded from single smooth muscle cells of rabbit oesophageal muscularis mucosae using a whole-cell gigaseal technique with physiological (2 mM) external calcium concentration ([Ca2+]o) in the presence of intracellular Cs+ ([Cs+]i 130 mM). Only one type of Ca2+ current could be identified. The threshold for its activation was approximately -30 mV and maximum inward current (approximately 300 pA) was recorded at 0 mV. 2. This inward current was blocked by Co2+ (4 mM), Cd2+ (0.5 mM) and nifedipine (1 microM) and was enhanced by Bay K 8644 (5 microM). We therefore classify it as a L-type Ca2+ current and denote it ICa. 3. Steady-state inactivation data were well-fitted by a Boltzmann distribution, indicating that inactivation of the Ca2+ current is strongly modulated by membrane potential. However, the inactivation of ICa slowed significantly and became less complete when BaCl2 replaced CaCl2 in the Tyrode solution suggesting that the inactivation of ICa may also be dependent on [Ca2+]i. The steady-state activation and inactivation curves for ICa overlap between -40 and 0 mV indicating that there may be a Ca2+ window current in this range of potentials. 4. When EGTA was omitted from the pipette-filling solution, depolarizations positive to -10 mV resulted in a transient as opposed to a maintained inward Ca2+ current which was followed by a relatively large outward current. Under these conditions, slowly decaying inward tail currents were also recorded upon repolarization to the holding potential, -60 mV. However, when EGTA was omitted from the pipette, marked 'run-down' of the Ca2+ current occurred within 10 min after starting the whole-cell recording. 5. This run-down of ICa was reduced significantly when the nystatin perforated patch technique was used. Under these conditions stable ICa records could be obtained for over 1 h. Outward currents and slow decaying inward tail currents similar to those recorded with no EGTA in the pipette were also obtained consistently using the nystatin recording technique. 6. In nystatin perforated patch recordings, CoCl2 (2 mM) completely abolished the Ca2+ current, the outward currents and the slow inward tails. These findings suggest that the outward currents and slow inward tails are activated by a transmembrane influx of Ca2+. 7. Ion replacement and pharmacological tests provided evidence that both the outward currents and the slow inward tails are due to Ca(2+)-activated Cl- current (ICl(Ca)).(ABSTRACT TRUNCATED AT 400 WORDS)

Contribution of potassium accumulation in narrow extracellular spaces to the genesis of nicorandil-induced large inward tail current in guinea-pig ventricular cells

The mechanism of nicorandil-induced large inward tail current (Itail) in single guinea-pig ventricular cells was investigated using the whole-cell patch-clamp technique. In the presence of 0.5-1.0 mM nicorandil, an activator of adenosine 5'-triphosphate (ATP)-sensitive K+ current (IKATP), a depolarization pulse causing a large outward current was followed by a large inward Itail on the repolarization step to the holding potential at -85 mV. The larger the outward current, the greater the Itail. The amplitude of Itail increased as a single exponential function (tau = 74.9 ms) as the duration of preceding depolarization was prolonged. Both the outward current and Itail were inhibited nearly completely after application of glibenclamide (1 microM), a specific blocker of IKATP. Substitution of K+ with Cs+ in both the external and internal solutions resulted in a virtual elimination of Itail. Itail was well preserved under the condition where Ca2+ entry during the preceding depolarization was largely inhibited or where external Na+ was replaced by Li+. A transient positive shift of reversal potential for the net current was observed at the peak of Itail). At 30 mM external K+ concentration, Itail was almost eliminated. From these findings, its is concluded that the Itail is a K+ current associated with an alteration of the K+ equilibrium potential (EK) following a substantial K+ efflux. This EK change is most likely explained by an accumulation of K+ in transverse tubules (T-tubules) since Itail was not induced in atrial cells in which T-tubules are poorly developed.

A contraction-related component of slow inward current in dog ventricular muscle and its relation to Na(+)-Ca2+ exchange

1. The slow inward current component related to contraction (Isic) was studied in voltage clamp experiments on canine ventricular trabeculae at 30 degrees C with the aims of (a) estimating its relation to electrogenic Na(+)-Ca2+ exchange and (b) comparing it with similar currents as reported in cardiac myocytes. 2. Isic may be recorded under conditions of augmented contractility in response to depolarizing pulses below the threshold of the classic slow inward current (presumably mediated by L-type Ca2+ channels). In responses to identical depolarizing clamp pulses the peak value of Isic is directly related to the amplitude of contraction (Fmax). Isic peaks about 60 ms after the onset of depolarization and declines with a half-time of about 110 ms. 3. The voltage threshold of Isic activation is the same as the threshold of contraction. The positive inotropic clamp preconditions shift both thresholds to more negative values of membrane voltage, i.e. below the threshold of the classic slow inward current. 4. Isic may also be recorded as a slowly decaying inwardly directed current 'tail' after depolarizing pulses. In this representation the peak value of Isic changes with duration of the depolarizing pulses, again in parallel with Fmax. In response to pulses shorter than 100 ms both variables increase with depolarization time. If initial conditions remain constant, further prolongation of the pulse does not significantly influence either one (tail currents follow a common envelope). 5. Isic differs from classic slow inward current by: (a) its direct relation to contraction, (b) the slower decay of the current tail on repolarization, (c) slower restitution corresponding to the mechanical restitution, (d) its relative insensitivity to Ca(2+)-blocking agents (the decrease of Isic is secondary to the negative inotropic of Ca(2+)-blocking agents (the decrease of Isic is secondary to the negative inotropic effect) and (e) its disappearance after Sr2+ substitution for Ca2+. 6. The manifestations of Isic in multicellular preparations do not differ significantly from those reported in isolated myocytes (in contrast to calcium current). 7. The analysis of the correlation between Isic and Fmax transients during trains of identical test depolarizing pulses at variable extra- and intracellular ionic concentrations (changes of [Ca2+]o, 50% Li+ substitution for Na+, strophanthidin) indicate that the observed effects conform to the predictions based on a quantitative model of Na(+)-Ca2+ exchange. 8. It is concluded that Isic is activated by a transient increase of [Ca2+]i, in consequence of the release from the reticular stores.(ABSTRACT TRUNCATED AT 400 WORDS)

Ionic mechanisms of transient inward current in the absence of Na(+)-Ca2+ exchange in rabbit cardiac Purkinje fibres

1. Membrane currents were measured with a two-microelectrode technique in voltage clamped rabbit cardiac Purkinje fibres under conditions known to cause intracellular calcium overload and to eliminate or minimize Na(+)-Ca2+ exchange. 2. Increasing [Ca2+]o from 2.5 to 5 mM or above and substituting external sodium with either sucrose, choline or Li+ induced an oscillatory transient inward current (TI) which peaked 200-300 ms after repolarization from a previous depolarizing pulse. The TI quickly disappeared upon return to normal Tyrode solution. Both the rate and configuration of action potentials of Purkinje fibres also returned to control upon return to Tyrode solution after 30 min of high Ca2+ exposure, if the Ca2+ concentration was 30 mM or less. 3. The TI in Na(+)-free solution was Ca2+ dependent. Either zero or low (2.5 mM) [Ca2+]o, or replacement of [Ca2+]o by BaCl prevented induction of the TI current upon repolarization from a previous depolarizing pulse. 4. In the presence of 30 mM-CaCl2 and with choline chloride as the substitute for NaCl, TI had a distinct reversal potential (Erev) of -25 mV. The time-to-peak TI, either inward or outward, did not shift significantly with change in voltage. Both inward and outward TI were simultaneously abolished by exposure to 1 microM-ryanodine, suggesting they were both activated by transient release of Ca2+ from the sarcoplasmic reticulum. The occurrence of TI in the absence of [Na+]o is not compatible with an electrogenic Na(+)-Ca2+ exchange mechanism. The existence of a clear-cut reversal potential suggests that an ionic channel may be responsible for the TI under these conditions. 5. Both the magnitude of peak TI and the Erev were affected by changes of CaCl2 concentration. (i) Under steady-state conditions, peak inward TI was significantly increased when the [Ca2+]o was elevated from 5 to 15 mM. The peak TI in the outward direction was significantly increased when [Ca2+]o was elevated from 15 to 30 mM; however, the difference in peak inward TI at 15 and 30 mM [Ca2+]o was small. (ii) Clear-cut reversals of TI were found at Ca2+ concentrations of 10 mM (Erev = -19.5 mV) or greater, and elevation of [Ca2+]o to 20, 30, 50 and 105 mM shifted the Erev to more negative potentials. (iii) In the presence of 5 mM [Ca2+]o the inward TI declined to zero at about -30 mV, and test voltages between -55 and +5 mV failed to reveal a distinct outward TI.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of an inwardly rectifying potassium current in rabbit ventricular action potential

1. Whole-cell voltage-clamp measurements were made of the time- and voltage-dependent properties of the inwardly rectifying background potassium current IK1, in single myocytes from rabbit ventricle. The main goal of these experiments was to define the role of IK1 in the plateau and repolarization phases of the action potential (AP). 2. Action potentials from single ventricular myocytes were used as the command signals for voltage-clamp measurements. In these 'action potential voltage-clamp' experiments, IK1 was isolated from other membrane currents by taking the difference between control currents and currents in K(+)-free bathing solution. The results show that IK1 is small during the plateau, but then rapidly increases during repolarization and declines in early diastole. 3. Evidence of an important functional role for IK1 in AP repolarization was obtained by comparing the magnitude of IK1 and the rate of change of membrane potential (dVm/dt) in the same cell during the AP. The time courses of IK1 and dVm/dt during the AP were closely correlated, indicating that IK1 was the principal current responsible for final repolarization. 4. Rectangular voltage-clamp steps were used to study time- and voltage-dependent changes in IK1 at membrane potentials corresponding to the repolarization phase of the AP. 'Slow' relaxations or tail currents, lasting 100-300 ms, were consistently recorded when the cell was repolarized to potentials in the range -30 to -70 mV, following depolarizations between +10 and -10 mV. 5. The close correlation between the magnitude of the steady-state IK1 (in an external K+ concentration of 5.4 mM), which was outward for membrane potentials in the range -30 to -70 mV, and the magnitude of the tail currents, suggests that they resulted from a slow increase, or reactivation, of IK1. 6. The component of the slow tails due to reactivation of IK1 can be separated from a previously described component due to Na(+)-Ca2+ exchange since the IK1 component: (i) does not depend on the presence of the calcium current, ICa; (ii) can be recorded when internal EGTA (5 mM) suppresses large changes in [Ca2+]i; (iii) does not depend on the Na+ electrochemical gradient; (iv) is abolished in K(+)-free external solution; and (v) is not present in rabbit atrial myocytes, in which IK1 is very small. 7. The time- and voltage-dependent properties of IK1 revealed by these tail current experiments suggest that the measured magnitude of IK1 will be dependent on the voltage-clamp protocol.(ABSTRACT TRUNCATED AT 400 WORDS)

Inositol trisphosphate promotes Na-Ca exchange current by releasing calcium from sarcoplasmic reticulum in cardiac myocytes

An early inward tail current evoked by membrane depolarization (from -80 to -40 mV) sufficient to activate sodium but not calcium current was studied in single voltage-clamped ventricular myocytes isolated from guinea pig hearts. Like forward-mode Na-Ca exchange, this early inward tail current required [Na+]o and [Ca2+]i and is thought to follow earlier reverse-mode Na-Ca exchange that triggers Ca2+ release from sarcoplasmic reticulum. The dependence of the early inward tail current on [Ca2+]i was supported by the ability of small (+10 mV) and large (+80 mV) voltage jumps from -40 mV to decrease and increase, respectively, the size of early inward tail currents evoked by subsequent voltage steps from -80 to -40 mV. As expected, tetrodotoxin selectively inhibited the early inward tail current but not the late inward tail current that followed voltage jumps to +40 mV test potentials. Although tetrodotoxin also blocked the fast Na+ current, replacement of extracellular Na+ by Li+ sustained the fast Na+ current. However, Li+, which does not support Na-Ca exchange, reversibly suppressed both the early and late inward tail currents. Inhibitors (ryanodine and caffeine) and promoters (intracellularly dialyzed inositol 1,4,5-trisphosphate) of sarcoplasmic reticulum Ca2+ release decreased and increased, respectively, the magnitude of the early inward tail current. The results substantiate the hypothesis that Ca2+ release from the sarcoplasmic reticulum participates in early Na-Ca exchange current and demonstrate that inositol 1,4,5-trisphosphate, by releasing Ca2+ from the sarcoplasmic reticulum, can promote Na-Ca exchange across the plasma membrane.

Reoxygenation-induced arrhythmogenic transient inward currents in isolated cells of the guinea-pig heart

Transient inward currents (Iti), activated by a rise in intracellular Ca concentration, are believed to trigger cardiac arrhythmias in reperfused hearts. In this report, Iti in isolated cardiocytes from the guinea-pig were evoked by reoxygenation following a period of anoxia of between 4 min and 35 min. Reoxygenation was performed 1 min after the full development of an anoxia-induced time-independent K current. This current disappeared within 2-6 s and in the following 10 s Iti developed to maximum amplitude. Iti were evoked using a constant pulse pattern (holding potential Vh = -45 mV; test potential Vt = +10; pulse duration 350 ms; frequency 1 Hz). In more than 95% of the cells, Iti at the holding potential Iti (-45 mV) declined with a time constant of tau = 670 +/- 240 ms (mean +/- SD, n = 17). In two cells, undamped oscillatory currents were observed. The amplitude of Iti (-45 mV) was proportional to the amplitude and duration of the preceding depolarizing test pulse. Test pulses of long duration (500 ms and 1000 ms, mean +/- SD) to potentials positive to +10 mV produced slowly decaying tail currents (tau = 391 +/- 51 ms, mean +/- SD), which superimposed with Iti (-45 mV). The current/voltage relationship of Iti peaked between -30 mV and -10 mV and approximated zero at the most positive potentials, i.e. no reversal of Iti was found up to +80 mV. Using double-pulse protocols (prepulse potential +40 mV), Iti were enhanced at potentials negative to -30 mV and were also present in the range of the normal resting potential of ventricular heart cells. The instantaneous current-voltage relationship was monotone between -50 mV and +40 mV. Because of the dependence of Iti on the preceding depolarization, the instantaneous current-voltage relationship provides more reliable information on the voltage dependence of Iti. The interval between two subsequent Iti (-45 mV) values was 237 +/- 35 ms (mean +/- SD, n = 27) and depended on the amplitude of Iti (-45 mV) to increase by 5.2 +/- 0.5% (mean +/- SD) per 100 pA decrease in Iti (-45 mV). A simple noise analysis showed that if one assumes that ionic channels are responsible for the generation of Iti (-45 mV), their unitary conductance cannot exceed 0.36 pS. We conclude that reoxygenation-induced Iti are triggered by a cyclic release of Ca from the sarcoplasmic reticulum and provide evidence that they are mediated by the electrogenic Na/Ca exchanger.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of ryanodine and caffeine on Ca-activated current in guinea-pig ventricular myocytes

1. Action potentials from guinea-pig single ventricular myocytes were interrupted by application of a 300 ms voltage clamp to -40 mV in order to evoke the Ca-activated tail current which is thought to be carried by Na:Ca exchange. Stimulation frequency was 1 Hz and temperature 36 degrees C. 2. The actions of ryanodine (1 microM and 10 microM) and caffeine (1 mM and 10 mM) on Ca-activated tail currents were investigated. 3. Exposure to 10 mM caffeine and ryanodine reduced tail currents associated with very abbreviated (12 ms duration) action potentials and greatly reduced the difference between first and steady-state tail currents at this action potential duration. These observations were interpreted in terms of suppression of Ca release from the sarcoplasmic reticulum (SR) stores. 4. Tail current decay during the voltage clamp is thought to reflect the fall in [Ca]i which accompanies muscle relaxation. Current decay is dependent on Ca extrusion via Na:Ca exchange and on Ca accumulation by the SR stores. Time constants of tail current decay were seen to decrease with increasing action potential duration. This relationship was not affected by 1 mM caffeine or 1 microM ryanodine. Ryanodine at 10 microM and 10 mM caffeine abolished this relationship and increased the time constants of current decay. An increase in the time constant of tail current decay was thought to reflect a reduction in the rate of Ca accumulation by the sarcoplasmic reticulum. 5. The actions of caffeine and ryanodine on the Ca-activated tail currents are consistent with a dose-dependent leakage of Ca from the SR Ca stores. The Ca-activated tail current appears to be a useful tool in the study of Ca homeostasis.

Comparison of sodium-calcium exchanger and transient inward currents in single cells from rabbit ventricle

1. Whole-cell voltage-clamp measurements have been made in rabbit ventricular myocytes under conditions in which both Na(+)-Ca2+ exchanger currents (IEX, slow tails) and transient inward currents (ITI or TI) can be recorded. A number of experimental manoeuvres have been used in an attempt to separate or dissociate these two currents. 2. As expected, partial inhibition of the Na(+)-K+ pump by application of 0.54 mM [K+] Tyrode solution or 10(-5) M-strophanthidin induced TI currents which were recorded in the presence of IEX slow tails. 3. Complete inhibition of the Na(+)-K+ pump with zero [K+] Tyrode solution resulted in larger and more frequent TIs but smaller IEX tails. 4. A somewhat similar dissociation between ITI and IEX was observed when NaCl was reduced to 37.5 mM by using LiCl to replace NaCl. This inhibited the Na(+)-Ca2+ exchanger current, but induced ITI. 5. Transient inward currents and IEX tails could also be separated by selected patterns of stimulation (voltage-clamp depolarizations): following the second pulse of a pair of stimuli, IEX was significantly reduced whereas the TIs increased in size and frequency. 6. Additional experimental tests involving changes in external divalent ions could also separate these two currents. Increasing [Ca2+]o 3-fold increased the TIs without changing IEX. Shortly after [Ca2+]o was replaced with either [Ba2+]o or [Sr2+]o the TIs were blocked but IEX was unchanged. Application of MnCl2 (1 mM) and elevation of [K+]o inhibited IEX but did not significantly change the TI currents. 7. Application of caffeine (5-10 mM) or ryanodine (2 x 10(-6) M) blocked the TI currents at times when the IEX tails were not changed. 8. In combination these results suggest that even though both IEX and ITI are triggered (activated) by increases in [Ca2+]i, these two currents are distinct. IEX is generated by electrogenic Na(+)-Ca2+ exchange, while the TI currents may be due to Ca2(+)-activated cation-selective channels in the sarcolemma.