Effects of nystatin-mediated intracellular ion substitution on membrane currents in calf purkinje fibres

EAD and DAD mechanisms analyzed by developing a new human ventricular cell model

It has long been suggested that the Ca(2+)-mechanisms are largely involved in generating the early afterdepolarization (EAD) as well as the delayed afterdepolarization (DAD). This view was examined in a quantitative manner by applying the lead potential analysis to a new human ventricular cell model. In this ventricular cell model, the tight coupled LCC-RyR model (CaRU) based on local control theory (Hinch et al. 2004) and ion channel models mostly based on human electrophysiological data were included to reproduce realistic Ca(2+) dynamics as well as the membrane excitation. Simultaneously, the Ca(2+) accumulation near the Ca(2+) releasing site was incorporated as observed in real cardiac myocytes. The maximum rate of ventricular repolarization (-1.02 mV/ms) is due to IK1 (-0.55 mV/ms) and the rest is provided nearly equally by INCX (-0.20 mV/ms), INaL (-0.16 mV/ms) and INaT (-0.13 mV/ms). These INaL and INaT components are due to closure of the voltage gate, which remains partially open during the plateau potential. DADs could be evoked by applying high-frequency stimulations supplemented by a partial Na(+)/K(+) pump inhibition, or by a microinjection of Ca(2+). EADs was evoked by retarding the inactivation of INaL. The lead potential (VL) analysis revealed that IK1 and IKr played the primary role to reverse the AP repolarization to depolarizing limb of EAD. ICaL and INCX amplified EAD, while the remaining currents partially antagonized dVL/dt. The maximum rate of rise of EAD was attributable to the rapid activation of both ICaL (45.5%) and INCX (54.5%).

Repolarization abnormalities, arrhythmia and sudden death in canine tachycardia-induced cardiomyopathy

OBJECTIVES

This study sought to determine whether the canine model of tachycardia-induced heart failure (HF) is an effective model for sudden cardiac death (SCD) in HF.

BACKGROUND

Such a well established HF model that also exhibits arrhythmias and SCD, along with repolarization abnormalities that could trigger them, may facilitate the study of SCD in HF, which still eludes effective treatment.

METHODS

Twenty-five dogs were VVI-paced at 250 beats/min for 3 to 5 weeks. Electrocardiograms were obtained, and left ventricular endocardial monophasic action potentials (MAPs) were recorded at six sites at baseline and after HF. Weekly Holter recordings were made with pacing suspended for 24 h.

RESULTS

Six animals (24%) died suddenly, one with Holter-documented polymorphic ventricular tachycardia (VT). Holter recordings revealed an increased incidence of VT as HF progressed. Repolarization was significantly (p < 0.05) prolonged, as indexed by a corrected QT interval (mean [+/-SD] 311 +/- 25 to 338 +/- 25 ms) and MAP duration measured at 90% repolarization (MAPD90) (181 +/- 19 to 209 +/- 28 ms), and spatial MAPD90 dispersion rose by 40%. We further tested whether CsCl inhibition of repolarizing K+ currents, which are reportedly downregulated in HF, might preferentially prolong the MAPD90 in HF. With 1 mEq/kg body weight of CsCl, MAPD90 rose by 86 +/- 100 ms in dogs with HF versus only 28 +/- 16 ms in control animals (p = 0.002). Similar disparities in CsCl sensitivity were observed in myocytes isolated from normal and failing hearts.

CONCLUSIONS

Tachycardia-induced HF exhibits malignant arrhythmia and SCD, along with prolonged, heterogeneous repolarization and heightened sensitivity to CsCl at chamber and cellular levels. Thus, it appears to be a useful model for studying mechanisms and therapy of SCD in HF.

Sodium current block caused by group IIb cations in calf Purkinje fibres and in guinea-pig ventricular myocytes

The action of group IIb cations [Cadmium (Cd2+), Zinc (Zn2+), Mercury (Hg2+)] on the cardiac fast sodium current (INa) was investigated in calf Purkinje fibres and in ventricular cells isolated from guinea-pig hearts. In calf Purkinje fibres, INa was depressed by submillimolar concentrations of Zn2+ and Hg2+. With both cations, the current reduction occurred at all voltages in the range of current activation and the voltage dependence of peak current was unchanged. The degree of peak current inhibition depended on the cation concentration but not on voltage. The position of the inactivation curve on the voltage axis was unaltered at cation concentrations giving substantial current inhibition, and moved to the right only with concentration exceeding 1-1.5 mM. These effects can be interpreted as due to INa channel blockade. The action of Zn2+ and Hg2+ was similar to that described earlier of Cd2+ on Purkinje fibres (DiFrancesco et al. 1985b). INa was also inhibited by group IIb cations in isolated guinea-pig ventricular cells. Depression of INa by Cd2+, Zn2+ and Hg2+ was essentially voltage-independent, in agreement with its being caused by channel block. The dependence of INa block by Cd2+ upon external Na concentration [Na+0] was investigated in ventricular myocytes. The fraction of INa block by 0.1 mM CdCl2 was 0.50 at 140 mM, 0.81 at 70 mM and 0.83 at 35 mM [Na+]0. A similar increase of block efficiency at low [Na+0] was observed with 0.05 mM CdCl2. In both the Purkinje fibre and the ventricular cell, the order of potency of INa block by group IIb cations was Hg2+ greater than Zn2+ greater than Cd2+. Manganese (Mn2+, 2-5 mM), an ion of group VIIa, also depressed the INa in Purkinje fibres and ventricular myocytes. This effect was however due mainly to a positive shift on the voltage dependence of current kinetics rather than to a reduction of the conductance of the channel (GNa), and can be accounted for by an ion-screening action of Mn2+ on the external membrane surface. The block by group IIb cations is a typical property of cardiac Na+ channels and characterizes the cardiac as opposed to other types of Na+ channel.

Inhibition of Ca2+ and K+ channels in sympathetic neurons by neuropeptides and other ganglionic transmitters

Neuropeptides are known to modulate the excitability of frog sympathetic neurons by inhibiting the M-current and increasing the leak current, but their effects on Ca2+ channels are poorly understood. We compared effects of LHRH, substance P, epinephrine, and muscarine on Ca2+, K+, and leak currents in dissociated frog sympathetic neurons. At concentrations that inhibit M-current, LHRH and substance P strongly reduced N-type Ca2+ current and induced a leak conductance that may contribute to slow EPSPs. In contrast, muscarine produced little reduction of Ca2+ current, even in cells in which it strongly suppressed the M-current. We find that peptidergic inhibition of Ca2+ channels involves G proteins, but does not require protein kinases. In addition, it leads to reductions in Ca2(+)-activated K+ current and catecholamine release.

Actions of pinacidil on membrane currents in canine ventricular myocytes and their modulation by intracellular ATP and cAMP

We studied the effects of pinacidil (3-50 microM) on the membrane currents of canine ventricular myocytes, using the whole-cell variant of the patch-clamp technique, and the modulation of these effects by intracellular environment, using the pipette perfusion technique. The following observations were obtained: (1) pinacidil induced a dose-dependent outward shift in current at voltages positive to -70 mV; (2) the pinacidil-induced current was largely time-independent at voltages positive to -50 mV and displayed an increase in current fluctuations at more positive voltages, resembling the kinetic properties of current through the ATP-regulated K+ channels; (3) elevating the extracellular potassium concentration [( K+]o) caused a positive shift in the voltage where the pinacidil-induced current crossed the voltage axis and increased the slope conductance of this current; (4) the pinacidil-induced current was reduced by Ba2+ (0.5-1.5 mM) and abolished by intracellular Cs+ (125 mM); (5) glibenclamide reversibly reduced or abolished the pinacidil-induced current; (6) the action of pinacidil was decreased by elevating [ATP] in the pipette solution (from 1 to 10 mM); (7) the action of pinacidil was augmented by adding isoproterenol (1 microM) to the superfusate or adding cAMP (0.1 mM) to the pipette solution; (8) elevating temperature augmented, and accelerated the onset of pinacidil's action; (9) pinacidil reversibly decreased the Ca2(+)-independent transient outward current (Ito1) but augmented the Ca2(+)-dependent transient outward current (Ito2). Based on these observations, we reached the following conclusions: (1) the main effect of pinacidil is to increase an outward current through the ATP-regulated K+ channels; (2) pinacidil's action is modulated by an enzymatic reaction.

Effects of general anesthetics on current flow across membranes in guinea pig myocytes

Myocytes were isolated from adult guinea pig ventricles. Whole cell, tight-seal recording was employed to investigate the electrical properties of the junctional (nexal membrane) and nonjunctional membrane (sarcolemma) under the influence of n-alkanols (heptanol, octanol) and halothane. Studies of cell pairs with a double voltage-clamp approach showed that these agents give rise to a reversible electrical uncoupling. Examination of single myocytes with a single voltage-clamp method showed that these substances modify several sarcolemmal current systems. The slope conductance was reduced over the entire voltage range examined (-90 to +50 mV). The Ca2+ inward current (Isi) showed a decreased amplitude and an accelerated inactivation. The repriming of Isi remained unchanged. The steady-state inactivation of Isi was shifted by 2-3 mV toward more negative potentials. Optical measurements demonstrated an increase in sarcomere spacing at rest and a decrease during peak systolic shortening. The results suggest that n-alkanols and halothane exert their effects on membrane currents via incorporation into the lipid bilayer.

Effects of AN-132, a novel antiarrhythmic lidocaine analogue, and of lidocaine on membrane ionic currents of guinea-pig ventricular myocytes

We studied the effects of AN-132 (10, 30 and 100 mumol/l), an analogue of lidocaine, on membrane currents and action potentials of single guinea-pig ventricular cells using whole-cell clamp techniques. The effects of lidocaine, an authentic class I antiarrhythmic agent were used for comparative purposes. (1) AN-132 decreased the Na current (INa) in a concentration-dependent manner, with a greater efficacy than seen with lidocaine. The concentration of the half maximal inhibition on INa (Kd) was 31.7 mumol/l for AN-132 and 94.9 mumol/l for lidocaine. (2) AN-132 also decreased the Ca current (ICa), concentration-dependently, while lidocaine had only a minor effect on ICa. The half maximal inhibition on ICa (Kd) was 23.1 mumol/l and 27.4 mumol/l for AN-132 and lidocaine, respectively. (3) AN-132 decreased the IK1, in a concentration-dependent manner; lidocaine was without effect. (4) AN-132 increased the unspecified steady state outward current, at positive potentials and depressed the time- and voltage-dependent outward K current (IK). Lidocaine had no effect on either current. (5) AN-132 shortened the action potential duration (APD), in a concentration-dependent manner, without altering the resting potential. From these findings, we conclude that apart from a potent inhibitory effect on INa, AN-132 had a variety of effects on other currents, properties not shared by lidocaine. Such multiple blocking effects on the membrane currents may relate to the alleged potent antiarrhythmic effect of AN-132.

A novel cardiac potassium channel that is active and conductive at depolarized potentials

We report the existence of a novel potassium channel revealed in single-channel recordings from guinea-pig ventricular heart cells. The channel, observed in approximately 10% of patches, demonstrates a 14 pS conductance at physiological potassium concentrations, does not rectify over the voltage range of the action potential, and is quite selective for K ions. The channel activates with depolarization, but does not require intracellular Ca2+ ions to open. Open channel probability increases rapidly (less than 10 ms) to a plateau in response to depolarizing voltage steps, and demonstrates no detectable inactivation (greater than 600 ms). These features clearly distinguish this channel from other known K channels in cardiac muscle. Because of its high activity at plateau potentials, we propose the name iKp.

Influence of halothane on electrical coupling in cell pairs isolated from guinea-pig ventricle

1. The actions of halothane on electrical coupling between cells were investigated in cell pairs isolated from guinea-pig ventricular muscle. 2. Under voltage-clamp conditions a step depolarization applied to one cell caused a similar change in potential in the second. Application of halothane led to the appearance of double peaks in inward current evoked by step depolarizations. These observations were interpreted in terms of uncoupling of the cells leading to escape of the second cell from the influence of the voltage-clamp in the first cell. 3. This suggestion that uncoupling in the presence of halothane led to differences in electrical activity in the two cells was confirmed in experiments in which independent electrodes were used to measure membrane potential in the two cells. 4. The voltage responses of both cells of the pair were recorded in response to constant current pulses. Administration of halothane led to abolition of the response recorded from the second cell while that of the first was enhanced. The actions are consistent with an action of halothane on gap junctions to block electrical coupling. 5. Qualitatively similar observations, consistent with electrical uncoupling, were observed with isoflurane. 6. These findings may be significant in relation to the arrhythmogenic actions of halothane.

Adrenergic modulation of the transient outward current in isolated canine Purkinje cells

Single canine Purkinje cells were voltage clamped under Ca2+-free conditions using the patch pipette. Depolarizing pulses from a holding potential of -42 mV induced a time-dependent rapidly activating-slowly inactivating outward current, which was identified as the transient outward current. The current showed two exponential time constants of inactivation (48,352 msec at +58 mV and 53,325 msec at +78 mV). Norepinephrine in concentrations exceeding 10(-9) M modified the inactivation kinetics of this current without affecting the activation kinetics. The half-maximum dose for norepinephrine effect was 1.9 X 10(-8) M, and the effect was saturated at 10(-6) M. Norepinephrine reduced the amplitude of the fast time constant component of inactivation, while increasing the amplitude of the slow component, without changing their time constants. Norepinephrine also increased the amplitude of a time-independent current component. The beta-antagonist sotalol blocked the norepinephrine effect on the transient outward current. On the other hand, both activation of adenyl cyclase by forskolin and increase of intracellular cAMP concentration produced the same effect as exposure to norepinephrine. These results suggest a role for neurotransmitter regulation of the transient outward current in cardiac cells, perhaps by channel phosphorylation.

Potassium currents in cardiac cells

The kinetic properties of the inwardly rectifying K current and the transient outward current in cardiac cells were investigated. In sheep Purkinje fibers superfused with Na-free K-free solution, time-dependent changes in the conductance of the inward rectifier are described. In patch clamp experiments the inward rectifier inactivates during hyperpolarization, as can be seen by a decrease in the open state probability. Using whole cell clamp on ventricular myocytes it is demonstrated that the inactivation during hyperpolarization is due to blocking of the channel by external Na, Mg and Ca. The channels responsible for the transient outward current in cow, sheep and rabbit Purkinje fibers are identified using single channel recording. It is demonstrated that in all three preparations the channels are K-selective. The channel in cow Purkinje cells has a large conductance and is regulated by voltage and internal Ca concentration. The channels identified in the sheep and rabbit cells have a much smaller conductance.

Cardiac calcium currents at the level of single channels

Properties of cardiac Ca channels have come into sharper focus with the advent of single cell preparations and suction pipette recording methods. We briefly summarize our present picture of the gating and permeation properties of the conventional, dihydropyridine-sensitive type of Ca channel (L-type). Distinctive features of a second type of voltage-gated Ca channel (T-type) are discussed.

Tension activation and relaxation in frog atrial fibres. Evidence for direct effects of divalent cations (Ca2+, Sr2+, Ba2+) on contractile proteins and Na-Ca exchange

The effect of alkali-earth cations (Ca2+, Sr2+, Ba2+) on the excitation-contraction coupling events of the frog atrial fibres were studied using a double mannitol gap voltage clamp technique coupled with a mechano-electric transducer. Photoremoval of the suppressive effect of nifedipine on the calcium channels allowed to obtain rapid transient Ca2+, Sr2+ or Ba2+ ions current jumps. The effect on the amplitude of the associated contraction was proportional to the current jumps. These results together with the correlation established between the estimated increase in the internal concentration of divalent cations and the amplitude of the phasic tension suggest that the essential source of divalent cations for activation of contraction is the extracellular space. Also Ba2+ ions reduced the tonic tension and strongly slowed the relaxation of the phasic component whereas Sr2+ exhibited smaller effects. Sr2+ ions could be more efficient than Ba2+ ions in substituting for Ca2+ ions in the Na+-Ca2+ exchange mechanism known to regulate these two mechanical events. The conclusions are that the order of effectiveness of these ions (Ca2+ greater than Sr2+ greater than Ba2+) is the same with regard to transarcolemmal exchange for Na+ ions, presumed uptake by a "second relaxing system", activation of contraction, and inactivation of the slow inward current.

Calcium-activated inward current and contraction in rat and guinea-pig ventricular myocytes

1. Single ventricular cells from rat and guinea-pig hearts were voltage clamped, and contraction was monitored with an optical method. 2. In rat cells, short (2-10 ms) depolarizing pulses to 0 mV from a holding potential of -40 mV evoked current carried by calcium, and on repolarization to -40 mV there was a slow 'tail' current which decayed much more slowly than the expected deactivation of calcium current at this potential. 3. When rat cells were loaded with EGTA diffusing into the cytosol from an intracellular electrode, contraction and the tail current were both abolished, whereas the peak calcium current was not reduced. 4. Exposure of rat cells to ryanodine (1-2 microM) suppressed both contraction and the tail current, but not peak calcium current. 5. The tail current was unaffected by tetrodotoxin (10 microM), but was reduced by lowering extracellular sodium to 10% by replacement with lithium or choline. 6. In rat cells, exposure to nifedipine (1-5 microM) initially caused a marked reduction of calcium current while substantial contraction and tail current remained; longer exposure to nifedipine suppressed both contraction and the tail current. Isoprenaline (50-100 nM) caused a marked increase in peak calcium current, while under these conditions there was little or no increase in either contraction or tail current. 7. The amplitude of the tail current in rat cells varied with the duration of the depolarization at 0 mV; the tail current evoked by repolarization to -40 mV reached a peak just as contraction was beginning to develop and was back to undetectable levels just as relaxation became significant, as might be expected if the tail current were determined by the cytosolic calcium transient which triggered contraction. 8. In guinea-pig cells, a tail current was also recorded on repolarization to a holding potential of -40 mV, and, as in rat cells, the tail was suppressed by cytosolic EGTA and reduced by exposure of the cells to low-sodium solution. 9. It is concluded that the tail currents recorded in both rat and guinea-pig cells represent current activated by a rise in cytosolic calcium; in rat cells this is markedly dependent on ryanodine-sensitive release of calcium from internal stores. The origin of this current, and its possible role during the plateaux of action potentials are discussed.

Calcium- and voltage-activated plateau currents of cardiac Purkinje fibers

We have used the two-microelectrode voltage-clamp technique to investigate the components of membrane current that contribute to the formation of the early part of the plateau phase of the action potential of calf cardiac Purkinje fibers. 3,4-Diaminopyridine (50 microM) reduced the net transient outward current elicited by depolarizations to potentials positive to -30 mV but had no consistent effect on contraction. We attribute this effect to the blockade of a voltage-activated transient potassium current component. Ryanodine (1 microM), an inhibitor of sarcoplasmic reticulum calcium release and intracellular calcium oscillations in Purkinje fibers (Sutko, J.L., and J.L. Kenyon. 1983. Journal of General Physiology. 82:385-404), had complex effects on membrane currents as it abolished phasic contractions. At early times during a depolarization (5-30 ms), ryanodine reduced the net outward current. We attribute this effect to the loss of a component of calcium-activated potassium current caused by the inhibition of sarcoplasmic reticulum calcium release and the intracellular calcium transient. At later times during a depolarization (50-200 ms), ryanodine increased the net outward current. This effect was not seen in low-sodium solutions and we could not observe a reversal potential over a voltage range of -100 to +75 mV. These data suggest that the effect of ryanodine on the late membrane current is attributable to the loss of sodium-calcium exchange current caused by the inhibition of sarcoplasmic reticulum calcium release and the intracellular calcium transient. Neither effect of ryanodine was dependent on chloride ions, which suggests that chloride ions do not carry the ryanodine-sensitive current components. Strontium (2.7 mM replacing calcium) and caffeine (10 mM), two other treatments that interfere with sarcoplasmic reticulum function, had effects in common with ryanodine. This supports the hypothesis that the effects of ryanodine may be attributed to the inhibition of sarcoplasmic reticulum calcium release.

Mechanisms of arrhythmogenic delayed and early afterdepolarizations in ferret ventricular muscle

Drug-induced triggered arrhythmias in heart muscle involve oscillations of membrane potential known as delayed or early afterdepolarizations (DADs or EADs). We examined the mechanism of DADs and EADs in ferret ventricular muscle. Membrane potential, tension and aequorin luminescence were measured during exposure to elevated [Ca2+]0, strophanthidin and/or isoproterenol (to induce DADs), or cesium chloride (to induce EADs). Ryanodine (10(-9)-10(-6) M), an inhibitor of Ca2+ release from the sarcoplasmic reticulum, rapidly suppressed DADs and triggered arrhythmias. When cytoplasmic Ca2+-buffering capacity was enhanced by loading cells with the Ca2+ chelators BAPTA or quin2, DADs were similarly inhibited, as were contractile force and aequorin luminescence. In contrast to DADs, EADs induced by Cs were not suppressed by ryanodine or by loading with intracellular Ca2+ chelators. The possibility that transsarcolemmal Ca2+ entry might produce EADs was evaluated with highly specific dihydropyridine Ca channel agonists and antagonists. Bay K8644 (100-300 nM) potentiated EADs, whereas nitrendipine (3-20 microM) abolished EADs. We conclude that DADs and DAD-related triggered arrhythmias are activated by an increase in intracellular free Ca2+ concentration, whereas EADs do not require elevated [Ca2+]i but rather arise as a direct consequence of Ca2+ entry through sarcolemmal slow Ca channels.

Electrophysiological effects of tetracaine in single guinea-pig ventricular myocytes

The effect of tetracaine on the ionic current in enzymatically dissociated single guinea-pig ventricular cells was studied using a two micro-electrode voltage-clamp technique. The myocytes were pre-incubated with Cs+ and the experiments were performed at room temperature in order to reduce the contribution of the delayed outward current. Tetracaine decreased the maximum rate of rise of the action potential with a dissociation constant (KD) strongly dependent on the holding potential (0.77 microM at -80 mV, and 6.2 microM at -95 mV). Application of 20 microM-tetracaine resulted in about a 50% reduction of the inwardly rectifying K+ current, while ten times higher concentrations were required to suppress the delayed K+ current. The inactivation time course of the Ca2+ current could be fitted with two exponentials, with time constants tau f = 15 ms and tau s = 150 ms at around 0 mV. Tetracaine decreased the amplitude of the Ca2+ current and speeded its decay. This effect was found to be primarily due to a marked inhibition of the amplitude of the slowly inactivating component (apparent KD = 80 microM, nH = 2). The drug had little effect on the time constants of the two components of Ca2+ channel inactivation. When Sr2+ or Ba2+ were the charge carriers, inactivation of the Ca2+ channel was again fitted with a fast and a slow exponential. In addition, a maintained (or very slowly inactivating) component was present. Tetracaine not only suppressed the amplitudes of the slowly inactivating and the maintained components, but also decreased the time constant of the slowly inactivating component. The results are consistent with a direct effect of tetracaine on the high threshold Ca2+ channel and do not support indirect effects of the drug secondary to suppression of Ca2+ release from internal stores.

Inactivation of calcium channels in mammalian heart cells: joint dependence on membrane potential and intracellular calcium

Ca channel currents were recorded in Cs-loaded calf cardiac Purkinje fibres and Cs-dialysed myocytes from guinea-pig ventricle to evaluate the dependence of Ca channel inactivation on membrane depolarization and intracellular free Ca concentration ([Ca]i). The decay of Ca channel current during a maintained depolarization was slowed when external Ca was replaced by Sr or Ba. The decay reflected a genuine inactivation of Ca channel conductance, as assessed by the decreased amplitude of inward tail currents following progressively longer depolarizing pulses in ventricular cells. Increasing depolarization slowed inward current inactivation in the presence of extracellular Ca concentration ([Ca]o), but speeded inactivation in the presence of extracellular Ba concentration ([Ba]o), suggesting the participation of fundamentally different mechanisms. Ca channel currents were recorded in Ca-free external solutions to study 'voltage-dependent inactivation'. Inactivation of outward Ca channel current due to Cs efflux was seen with external Ba or in the absence of any permeant divalent cation. With Ca as the charge carrier, increasing [Ca]o speeded the rate of inactivation as expected for [Ca]i-dependent inactivation. The relationship between inactivation and the intracellular Ca transient was assessed by double-pulse experiments. Conditioning pulses that produced maximal inward Ca current and contractile tension left behind more inactivation than either stronger or weaker depolarizations. The agreement between maximal inward current and maximal inactivation remained close when their voltage dependence was shifted along the voltage axis by elevation of [Ca]o. We conclude that inactivation of cardiac Ca channels is both [Ca]i dependent and voltage dependent. The [Ca]i-dependent process may serve as a negative feed-back mechanism for regulating Ca entry into heart cells; the voltage-dependent mechanism may prevent a secondary rise in Ca channel current when intracellular Ca falls during maintained depolarization of cardiac cells.

Removal of Ca current inactivation in dialysed guinea-pig atrial cardioballs by Ca chelators

Ca currents flowing during voltage clamp depolarizations were studied in cultured guinea-pig atrial cardioballs by means of single low resistance patch clamp pipettes. The pipettes were filled with solutions containing Cs+ as major cation in order to block K+ currents and high concentrations of various Ca chelating agents (EGTA, nitrilotriacetic acid, citrate, dipicolinic acid) to prevent rises of the intracellular Ca-activity by Ca-entry. Ca currents of myocytes loaded with 20 mM of either EGTA [(ethylenedioxy)-diethylenedinitrilo)tetra-acetic acid] or NTA (nitrilotriacetic acid) display a biphasic time course of inactivation at membrane potentials between -25 and +45 mV. The fast phase is reduced with increasingly positive membrane potentials. In cells loaded with either citrate or DPA (dipicolinic acid, pyridine-2,6-dicarboxylic acid) inactivation is negligible or absent for small depolarizations. In the range of membrane potentials where maximum current flows (0-+10 mV) a monophasic slow time course of inactivation is observed. At more positive membrane potentials inactivation is slowed. The amount of inactivation under this condition is related to the current density of the cell. Conditions, which for a given membrane potential reduce the amplitude of ICa such as extracellular application of blocking ions (Co2+, Cd2+), a conditioning depolarization, or 'rundown' of Ca-channels lead to a slowing or a complete removal of inactivation in cells dialysed with citrate or DPA respectively. Cells loaded with these Ca chelators did not show any symptom of voltage dependent inactivation of ICa. Under the conditions described action potentials were recorded in the current clamp mode. Upon dialysis with EGTA the typical 'triangular shaped' atrial action potential develops a plateau of 500 to 800 ms in duration. With citrate-containing pipette solutions the action potential duration usually is several seconds. The results for the first time demonstrate that inactivation of cardiac ICa can be considerably slowed or even removed. They provide further strong support for the hypothesis that inactivation of this current depends on Ca entry rather than membrane potential. The fast phase of inactivation observed with EGTA (NTA) possibly reflects the slow kinetics of the binding reaction of this type of Ca chelators.

Simulated calcium current can both cause calcium loading in and trigger calcium release from the sarcoplasmic reticulum of a skinned canine cardiac Purkinje cell

Skinned canine cardiac Purkinje cells were stimulated by regularly repeated microinjection-aspiration sequences that were programmed to simulate the fast initial component of the transsarcolemmal Ca2+ current and the subsequent slow component corresponding to noninactivating Ca2+ channels. The simulated fast component triggered a tension transient through Ca2+-induced release of Ca2+ from the sarcoplasmic reticulum (SR). The simulated slow component did not affect the tension transient during which it was first introduced but it potentiated the subsequent transients. The potentiation was not observed when the SR function had been destroyed by detergent. The potentiation decreased progressively when the slow component was separated by an increasing time interval from the fast component. The potentiation was progressive over several beats under conditions that decreased the rate of Ca2+ accumulation into the SR (deletion of calmodulin from the solutions; a decrease of the temperature from 22 to 12 degrees C). In the presence of a slow component, an increase of frequency caused a positive staircase, and the introduction of an extrasystole caused a postextrasystolic potentiation. There was a negative staircase and no postextrasystolic potentiation in the absence of a slow component. These results can be explained by a time- and Ca2+-dependent functional separation of the release and accumulation processes of the SR, rather than by Ca2+ circulation between anatomically distinct loading and release compartments. The fast initial component of transsarcolemmal Ca2+ current would trigger Ca2+ release, whereas the slow component would load the SR with an amount of Ca2+ available for release during the subsequent tension transients.

Ryanodine as a tool to determine the contributions of calcium entry and calcium release to the calcium transient and contraction of cardiac Purkinje fibers

Our object was to assess the relative roles of transsarcolemmal calcium entry and intracellular calcium release in the contraction of cardiac Purkinje fibers. We observed intracellular calcium transients, membrane potential, and contraction in aequorin-injected canine cardiac Purkinje fibers exposed to highly selective pharmacological modifiers of excitation-contraction coupling. To influence selectively the release of calcium from the sarcoplasmic reticulum, we used the plant alkaloid, ryanodine. To influence calcium entry, selectively, we used either the calcium channel antagonist, nitrendipine, or the calcium channel agonist, Bay k 8644. Ryanodine alone (1 microM) reduced both components of the intracellular aequorin luminescence signal (L1 and L2). In three muscles, the luminescence signals were 3% of control in amplitude (standard error of the mean, 2%) without two distinct components and the twitch tension was 2% of control (standard error of the mean, 3%), whereas the action potential was prolonged. The aequorin signal and twitch remaining in ryanodine were abolished by the calcium antagonist nitrendipine (10 microM), which also lowered the action potential plateau, consistent with the block of functional calcium channels. In two experiments, the calcium-channel agonist, Bay k 8644, in the presence of ryanodine, increased the aequorin luminescence and the contraction, but only to a very small fraction of their control values. Sodium withdrawal in potassium-free, ryanodine-containing solution produced large slow increases in calcium and tension, showing that tension could still be produced, that aequorin remained functional, and that sodium/calcium exchange was not inhibited by ryanodine. Caffeine increased intracellular calcium, showing that calcium stores were not depleted.(ABSTRACT TRUNCATED AT 250 WORDS)

High selectivity of calcium channels in single dialysed heart cells of the guinea-pig

Membrane currents and action potentials were recorded in single ventricular cells obtained from guinea-pig hearts by enzymatic dissociation. Ca2+ channel currents carried by Ba2+ or Ca2+ were recorded with a suction pipette (5-10 microns diameter) for voltage clamp and internal dialysis. Currents through Na+, K+ and non-selective monovalent cation channels were suppressed by suitable holding potentials and external and internal solutions. The dialysis method allowed exchange within minutes of alkali metal cations (e.g. Cs+) and small molecules (e.g. quaternary derivatives of lidocaine and verapamil). Nevertheless, Ca2+ channels remained functional for considerable periods, typically 20 min and sometimes more than 1 h. With Ba2+ outside and Cs+ inside, current flow through Ca2+ channels changed from inward to outward at strongly positive levels beyond a clear-cut reversal potential Erev. Several methods for defining Erev were in close agreement: (1) zero-crossing of leak-subtracted peak current, (2) inversion of time-dependent current changes during channel activation or inactivation, (3) inversion of drug-sensitive current as defined by channel blockers such as Cd2+ or D-600. Erev varied with external Ba2+ or internal Cs+. Erev increased by 29 mV per 10-fold increase in Ba2+. Interpreted with constant-field theory, Erev values correspond to PBa/PCs of approximately 1360. With 5 mM-Ca2+ outside and 151 mM-Cs+ inside, Ca2+ channel current reversed near + 75 mV, corresponding to PCa/PCs approximately 6000. Earlier measurements of Erev (Lee & Tsien, 1982) suggest that PCa/PK greater than 1000. At strongly positive membrane potentials where channel activation is maximal, the Ca2+ channel current-voltage relationship is strongly non-linear, with conductance increasing on either side of an inflexion point near Erev. Activation of inward or outward currents through Ca2+ channels follows a sigmoid time course, as expected if activation were a multi-step process.

Effects of a new antiarrhythmic compound SUN 1165 [N-(2,6-dimethylphenyl)-8-pyrrolizidineacetamide hydrochloride] on the sodium currents in isolated single rat ventricular cells

The effects of a new antiarrhythmic compound, SUN 1165 (which resembles lidocaine in chemical structure) on sodium currents (INa) of enzymatically isolated, single rat ventricle cells were studied under current or voltage clamp conditions. A suction pipette technique was used for current injection and internal perfusion. Potassium currents were blocked by replacing K+ with Cs+ in the internal and external solutions, and calcium current by replacing Ca2+ with Co2+ in the external solution. When cells were stimulated infrequently (less than 1 HZ), SUN 1165 decreased INa without changing the position of the current-voltage curve. The inactivation curve of INa shifted to negative potentials along the voltage axis. The drug also produced an additional use-dependent block of INa, which depended on the frequency of the voltage clamp pulse. The effects of SUN 1165 on INa resemble those of lidocaine, although the relative importance of use-dependent action versus tonic blockade differs from that of lidocaine.

The effects of membrane potential, extracellular potassium, and tetrodotoxin on the intracellular sodium ion activity of sheep cardiac muscle

The intracellular sodium ion activity was measured using liquid ion-exchange microelectrodes with rapid response times in sheep Purkinje fibers and ventricular muscle under voltage control. The mean sodium ion activity in quiescent Purkinje fibers was 8.5 mM at a holding potential of -80 mV. With maintained hyperpolarizing (-110 mV) or depolarizing (-40 and 0 mV) voltage steps, sodium ion activity increased or decreased, respectively. At 0 mV, the mean steady state value for the sodium ion activity was 3.8 mM. Following a voltage step to 0 mV, or back to -80 mV, the time course of the sodium ion activity change could be fitted by single exponentials, with similar half-times. Increasing the extracellular potassium ion concentration from 5.4 to 15 mM did not alter the steady state value of the sodium ion activity at clamped voltages of -80 or 0 mV, which suggests that the external potassium ion activating site of the Na-K pump was saturated. With the extracellular potassium concentration 0 mM (holding potential -80 mV), the sodium ion activity increased. When maintained depolarizing steps to 0 mV were applied, the sodium ion activity decreased by up to 20 mM. This large fall in sodium ion activity is assumed to represent partial reactivation of the Na-K pump due to potassium ion accumulation in clefts. We also studied the stimulation-dependent change in sodium ion activity. Trains of action potentials or short duration depolarizing voltage clamp steps caused a frequency dependent rise in sodium ion activity. The magnitude of the rise of sodium ion activity was not altered by lengthening the duration of each voltage clamp step, but was inhibited by tetrodotoxin or by holding the membrane potential at -50 mV between depolarizing steps. These results show that sodium ion activity is a complex function of membrane voltage, depolarization frequency, and time. The rise in sodium ion activity with stimulation appears to depend on sodium ion entry regulated by the sodium channel, and may be important in the modulation of intracellular calcium and tension through the Na+-Ca++ exchange mechanism.

Membrane current following activity in canine cardiac Purkinje fibers

Membrane current following prolonged periods of rapid stimulation was examined in short (less than 1.5 mm) canine cardiac Purkinje fibers of radius less than 0.15 mm. The Purkinje fibers were repetitively stimulated by delivering trains of depolarizing voltage clamp pulses at rapid frequencies. The slowly decaying outward current following repetitive stimulation ("post-drive" current) is eliminated by the addition of 10(-5) M dihydro-ouabain. The post-drive current is attributed to enhanced Na/K exchange caused by Na loading during the overdrive. Depolarizing voltage clamp pulses initiated from negative (-80 mV) or depolarized (-50 mV) holding potentials can give rise to post-drive current because of activation of tetrodotoxin-sensitive or D600-sensitive channels. The magnitude of the post-drive current depends on the frequency of voltage clamp pulses, the duration of each pulse, and the duration of the repetitive stimulation. The time constant of decay of the post-drive current depends on extracellular [K] in accordance with Michaelis-Menten kinetics. The Km is 1.2 mM bulk [K], [K]B. The mean time constant in 4 mM [K]B is 83 s. Epinephrine (10(-5) M) decreases the time constant by 20%. The time constant is increased by lowering [Ca]o between 4 and 1 mM. Lowering [Ca]o further, to 0.1 mM, eliminates post-drive current following repetitive stimulation initiated from depolarized potentials. The latter result suggests that slow inward Ca2+ current may increase [Na]i via Na/Ca exchange.

Na and Ca channels in a transformed line of anterior pituitary cells

The ionic conductances of GH3 cells, a transformed line from rat anterior pituitary, have been studied using the whole-cell variant of the patch-clamp technique (Hamill et al., 1981). Pipettes of very low resistance were used, which improved time resolution and made it possible to control the ion content of the cell interior, which equilibrated very rapidly with the pipette contents. Time resolution was further improved by using series resistance compensation and "ballistic charging" of the cell capacitance. We have identified and partially characterized at least three conductances, one carrying only outward current, and the other two normally inward. The outward current is absent when the pipette is filled with Cs+ instead of K+, and has the characteristics of a voltage-dependent potassium conductance. One of the two inward conductances (studied with Cs+ inside) has fast activation, inactivation and deactivation kinetics, is blocked by tetrodotoxin (TTX), and has a reversal potential at the sodium equilibrium potential. The other inward current activates more slowly and deactivates with a quick phase and a very slow phase after a short pulse. Either Ca++ or Ba++ serves as current carrier. During a prolonged pulse, current inactivates fairly completely if there is at least 5 mM Ca++ outside, and the amplitude of the current tails following the pulse diminishes with the time course of inactivation. When Ba++ entirely replaces Ca++ in the external medium, there is no inactivation, but deactivation kinetics of Ca channels vary as pulse duration increases: the slow phase disappears, the fast phase grows in amplitude. Inactivation (Ca++ outside) is unaltered by 50 mM EGTA in the pipette: inactivation cannot be the result of internal accumulation of Ca++.

Calcium-mediated inactivation of the calcium conductance in cesium-loaded frog heart cells

Ca current inactivation was investigated in frog atrial muscle under voltage-clamp conditions. To inhibit the outward currents, experiments were performed on Cs-loaded fibers and in 20 mM Cs (K-free) Ringer with 4-AP added. Inactivation, produced by a conditioning pulse, was measured by reducing the current during a subsequent test pulse. The extent of inactivation increased initially with prepulse amplitude and then decreased as the prepulse potential became progressively positive. Relative inactivation follows a U-shaped curve. When Sr was substituted for Ca, both the degree and the rate of inactivation decreased. Relative inactivation appeared to be linearly related to the amount of divalent cations (Ca and Sr) carried into the cell during the prepulse. Elevating Ca enhanced peak current and accelerated its decline. Elevating Mg decreased peak current and slowed its decline. An application of Na-free (LiCl) solution resulted in a somewhat smaller but faster inactivating current. Adrenaline increased and D600 decreased the maximal Ca conductance with little alteration in the inactivation rate; Co decreased both peak current and the rate of inactivation. Enhancement of the outward currents, reduced driving force, and intracellular surface charge screening do not adequately account for the above results. Evidence was considered that Ca entry mediates most of Ca current inactivation in frog atrial fibers. Removal from inactivation was also investigated in normal-Ca, Ca-rich, and Sr solutions. Recovery after partial inactivation by high depolarization was biphasic. Recovery was slowed by 10 Ca and accelerated by 1.8 Sr, whereas opposite effects have been shown on activation.

Do calcium-activated potassium channels exist in the heart?

Changes of intracellular Ca concentration in cardiac muscle have significant effects on transmembrane currents and many of these effects can be accounted for by postulating the existence of Ca-activated K channels in the heart. However, the evidence that such channels exist is equivocal. This is partly because of technical problems, for example the difficulty of identifying an individual ionic current amongst the many currents that exist in the heart. An additional problem, however, is posed by the fact that other currents may also be modulated by Ca ions. It is important therefore to distinguish between these currents and those caused by Ca-activated, K-specific channels. In this review we consider the evidence for Ca activated currents in the heart and, in particular, we discuss whether or not these currents are carried exclusively by K ions.

Characteristics of the second inward current in cells isolated from rat ventricular muscle

The second inward current (Isi) in single cells isolated from ventricular muscle of adult rat hearts was measured in response to step depolarizations under voltage-clamp conditions. The major ion carrying this current was Ca, and Isi was reduced or abolished by Mn, Ni, Cd, nifedipine, nimodipine and D600. Sr and B could substitute for Ca as charge carriers, and reduced the rate of apparent inactivation of Isi. These effects of Sr and Ba, together with the relation between the steady level of apparent inactivation and membrane potential in Ca containing solution, were taken as evidence that inactivation was at least in part dependent on internal Ca. The reduction of external Na to 11% of normal caused a reduction in peak Isi when Ca was present in the external solution, but did not reduce Isi when Ca was replaced by Sr. It therefore seems unlikely that Na is a major charge carrier for Isi under the conditions of our experiments. The time-to-peak and rate of apparent inactivation of Isi were faster than in previous studies that used multicellular preparations. Both the kinetics and peak amplitude of Isi were markedly dependent on temperature (Q10 close to 3). Contraction of the cells, which was monitored optically, was initiated within 3 ms of the peak Isi, reached a maximum level after approximately 40-50 ms, and was about 100 ms in duration.

Mechanism of calcium channel blockade by verapamil, D600, diltiazem and nitrendipine in single dialysed heart cells

Organic inhibitors of calcium influx prevent outward as well as inward current through cardiac calcium channels but do not slow current activation. Although block is antagonized by raising external calcium or barium concentrations, the competitive effect of permeant cations does not occur at the same cation binding site at which inorganic blockers act. Organic drugs show varying degrees of use-dependent block, due in part to blockade of open channels. Nitrendipine blockade of calcium currents requires doses greater than 100-fold higher than expected from radioligand binding to isolated membranes.

Enhancement of calcium current during digitalis inotropy in mammalian heart: positive feed-back regulation by intracellular calcium?

1. Effects of digitalis compounds on slow inward Ca current I(si)) and contractile force were examined in ferret ventricular muscle (single sucrose-gap voltage clamp) and calf Purkinje fibres (two micro-electrode voltage clamp).2. In ventricular muscle, ouabain increased I(si) and inward current tails associated with I(si) conductance. The enhancement of I(si) followed a time course similar to the development of the positive inotropic effect, and it could be observed in the absence of aftercontractions or other signs of toxicit.3. The response of myocardial I(si) and twitch force to ouabain depended strongly on a previous history of driven action potentials.4. Veratridine, a toxin that promotes Na entry through tetrodotoxin-sensitive channels, also increased I(si) and twitch force in driven ventricular muscle preparations.5. The effects of ouabain, action potential stimulation and veratridine are consistent with reported effects of K-poor solutions in indicating that elevation of intracellular Na can lead to enhancement of I(si). Additional experiments suggest that the link between Na(i) and I(si) involves intracellular Ca.6. When Cs-loaded Purkinje fibres were bathed in solutions containing Sr instead of Ca, enhancement of I(si) by strophanthidin was abolished even though a positive inotropic response persisted.7. After intracellular injection of Purkinje fibres with EGTA, I(si) no longer increased with strophanthidin, although it remained responsive to adrenaline.8. Clear-cut increases in I(si) were seen in Cs-loaded Purkinje fibres even at very low concentrations of strophanthidin (20-50 nM), where the occurence of Na pump inhibition has been questioned.9. Positive regulation of Ca entry by intracellular Ca may act as a facilitory mechanism that amplifies myocardial responsiveness to digitalis and other inotropic interventions. Through changes in I(si), small rises in diastolic free Ca might lead to large increases in the activator Ca transient during contraction.