Cesium abolishes the barium-induced pacemaker potential and current in guinea pig ventricular myocytes

The vicissitudes of the pacemaker current I Kdd of cardiac purkinje fibers

The mechanisms underlying the pacemaker current in cardiac tissues is not agreed upon. The pacemaker potential in Purkinje fibers has been attributed to the decay of the potassium current I (Kdd). An alternative proposal is that the hyperpolarization-activated current I (f) underlies the pacemaker potential in all cardiac pacemakers. The aim of this review is to retrace the experimental development related to the pacemaker mechanism in Purkinje fibers with reference to findings about the pacemaker mechanism in the SAN as warranted. Experimental data and their interpretation are critically reviewed. Major findings were attributed to K(+) depletion in narrow extracellular spaces which would result in a time dependent decay of the inward rectifier current I (K1). In turn, this decay would be responsible for a "fake" reversal of the pacemaker current. In order to avoid such a postulated depletion, Ba(2+) was used to block the decay of I (K1). In the presence of Ba(2+) the time-dependent current no longer reversed and instead increased with time and more so at potentials as negative as -120 mV. In this regard, the distinct possibility needs to be considered that Ba(2+) had blocked I (Kdd) (and not only I (K1)). That indeed this was the case was demonstrated by studying single Purkinje cells in the absence and in the presence of Ba(2+). In the absence of Ba(2+), I (Kdd) was present in the pacemaker potential range and reversed at E (K). In the presence of Ba(2+), I (Kdd) was blocked and I (f) appeared at potentials negative to the pacemaker range. The pacemaker potential behaves in a manner consistent with the underlying I (Kdd) but not with I (f). The fact that I (f) is activated on hyperpolarization at potential negative to the pacemaker range makes it suitable as a safety factor to prevent the inhibitory action of more negative potentials on pacemaker discharge. It is concluded that the large body of evidence reviewed proves the pacemaker role of I (Kdd) (but not of I (f)) in Purkinje fibers.

Mechanisms of adrenergic control of sino-atrial node discharge

Among the mechanisms proposed for the increase in discharge of sino-atrial node (SAN) by norepinephrine (NE) are an increase in the hyperpolarization-activated current I(f) and in the slow inward current I(Ca,L). If I(f) is the primary mechanism, cesium (a blocker of I(f)) should eliminate the positive chronotropic effect of NE. If I(Ca,L), is involved, [Ca(2+)](o) should condition NE effects. We studied the electrophysiological changes induced by NE in isolated guinea pig SAN superfused in vitro with Tyrode solution (both SAN dominant and subsidiary pacemaker mechanisms are present) as well as with high [K(+)](o), higher Cs(+) or Ba(2+) (only the dominant pacemaker mechanism is present). In Tyrode solution, NE (0.5-1microM) increased the SAN rate and adding Cs(+) (approximately 12 mM) caused a decaying voltage tail during diastole in subsidiary pacemakers. NE enhanced the Cs(+)-induced tail, and increased the rate but less than in Tyrode solution. In higher [Cs(+)](o) (15- 18 mM), Ba(2+) (1 mM) or Ba(2+) plus Cs(+) (10 mM) dominant action potentials (not followed by a tail) were present and NE accelerated them as in Tyrode solution. In high [K(+)](o), NE increased the rate in the absence and presence of Cs(+), Ba(2+) or Ba(2+) plus Cs(+). In these solutions, NE increased the overshoot and maximum diastolic potential of dominant action potentials (APs) and increased the rate by steepening diastolic depolarization and shifting the threshold for upstroke to more negative values. High [Ca(2+)](o) alone increased the rate and NE enhanced this action, whereas low [Ca(2+)](o) reduced or abolished the increase in rate by NE. In SAN quiescent in high [K(+)](o) plus indapamide, NE induced spontaneous discharge by decreasing the resting potential and initiating progressively larger voltage oscillations. Thus, NE increases the SAN rate by acting primarily on dominant APs in a manner consistent with an increase of I(Ca,L) and I(K) and under conditions where I(f) is either blocked or not activated. NE INITIATES spontaneous discharge by inducing voltage oscillations unrelated to I(f).

Extracellular Ba(2+) blocks the cardiac transient outward K(+) current

Ba(2+) is widely used as a tool in patch-clamp studies because of its ability to block a variety of K(+) channels and to pass Ca(2+) channels. Its potential ability to block the cardiac transient outward K(+) current (I(to)) has not been clearly documented. We performed whole cell patch-clamp studies in canine ventricular and atrial myocytes. Extracellular application of Ba(2+) produced potent inhibition of I(to) with an IC(50) of approximately 40 microM. The effects were voltage independent, and the inactivation kinetics were not altered by Ba(2+). The potency of Ba(2+) was approximately 10 times higher than that of 4-aminopyridine (a selective I(to) blocker with an IC(50) of 430 microM) under identical conditions. By comparison, Ba(2+) blockade of the inward rectifier K(+) current was voltage dependent; the IC(50) was approximately 20 times lower (2.5 microM) than that for I(to) when determined at -100 mV and was comparable to I(to) as determined at -60 mV (IC(50) = 26 microM). Ba(2+) concentrations of </=1 mM or higher failed to block ultrarapid delayed rectifier K(+) current. Our data suggest that Ba(2+) can be considered a potent blocker of I(to).

On the mechanism of cesium-induced voltage and current tails in single ventricular myocytes

The mechanisms by which different concentrations of cesium modify membrane potentials and currents were investigated in guinea pig single ventricular myocytes. In a dose-dependent manner, cesium reversibly decreases the resting potential and action potential amplitude and duration, and induces a diastolic decaying voltage tail (Vex), which increases at more negative and reverses at less negative potentials. In voltage-clamped myocytes, Cs+ increases the holding current, increases the outward current at plateau levels while decreasing it at potentials closer to resting potential, induces an inward tail current (Iex) on return to resting potential and causes a negative shift of the threshold for the inward current. During depolarizing ramps, Cs+ decreases the outward current negative to inward rectification range, whereas it increases the current past that range. During repolarizing ramps, Cs+ shifts the threshold for removal of inward rectification negative slope to less negative values. Cs+-induced voltage and current tails are increased by repetitive activity, caffeine (5 mM) and high [Ca2+]O (8.1 mM), and are reduced by low Ca2+ (0.45 mM), Cd2+ (0.2 mM) and Ni2+ (2 mM). Ni2+ also abolishes the tail current that follows steps more positive than ECa. We conclude that Cs+ (1) decreases the resting potential by decreasing the outward current at more negative potentials, (2) shortens the action potential by increasing the outward current at potentials positive to the negative slope of inward rectification, and (3) induces diastolic tails through a Ca2+-dependent mechanism, which apparently is an enhanced electrogenic Na-Ca exchange.

Effects of ionic channel antagonists barium, cesium, and UL-FS-49 on vagal slowing of atrial rate in dogs

In response to a brief vagal stimulus, the atrial rate initially slows, then transiently accelerates, and slows a second time. We determined the effects of three antagonists to two ionic channels on this characteristic triphasic pacemaker response. Brief bursts of vagal stimulation were delivered to anesthetized dogs, and atrial cycle lengths were recorded. Either barium, cesium, or UL-FS-49 was administered. Barium, which primarily blocks the acetylcholine-sensitive potassium current (IK,ACh), attenuated the initial vagally induced bradycardia by > 50% without affecting the subsequent acceleration or the secondary slowing. Cesium and UL-FS-49 [both of which primarily block the pacemaker current (If)] did not affect the initial vagal slowing of atrial rate but abolished the acceleratory portion of the response. The secondary slowing was abolished by cesium but not by UL-FS-49. We conclude that the initial rapid atrial response to acetylcholine is mediated mainly by the IK,ACh, with little contribution from the If. The subsequent acceleration is mediated by activation of the If.

Mechanisms for depolarization by l-palmitoylcarnitine in single guinea pig ventricular myocytes

INTRODUCTION

The changes of the resting potential (RP) and of the current-voltage (I-V) relationship induced by l-palmitoylcarnitine (l-PC) in the presence of the IKI blocker, cesium, or in the presence of the INa/K blocker, ouabain, were tested in guinea pig ventricular myocytes to ascertain the relative contributions of IKI and INa/K suppression to the membrane depolarization caused by this amphiphile.

METHODS AND RESULTS

Ramp voltages were applied to myocytes with the whole cell, patch clamp technique. l-PC (10 microM) produced additional membrane depolarization in the presence of either 10 mM Cs+ or 30 microM ouabain. In the presence of Cs+, l-PC, like 3 microM ouabain, shifted current inward at potentials negative to -20 mV as a result of INa/K blockade. In the presence of 30 microM ouabain, l-PC, like Cs+, shifted current inward at potentials between -27 and -88 mV and outward at potentials negative to -88 mV. This is attributed to IKI block because the current was inwardly rectifying, with a reversal potential near EK. When l-PC or ouabain inhibited INa/K, the presence of an Ni(2+)-sensitive component attributed to INa/Ca distorted the membrane I-V relationship, particularly in the presence of Cs+. The relative contributions of IKI and INa/K block by l-PC were voltage dependent. At the RP, l-PC produced a greater block of INa/K than of IKI.

CONCLUSION

l-PC depolarizes the resting membrane by inhibiting both IKI and INa/K. It is concluded that suppression of INa/K by l-PC predominates over block of IKI to depolarize the membrane at the RP.

Barium-induced diastolic depolarization and controlling mechanisms in guinea pig ventricular muscle

Barium-induced diastolic depolarization (Ba(2+)-DD) and its modulation were studied in guinea pig papillary muscle and single ventricular myocytes. In papillary muscles, Cs+ (4 mM) abolished the DD induced by Ba2+ (0.05-0.2 mM) by abolishing the undershoot at the end of the action potential (AP; consistent with a block of an outward current). Acetylcholine (ACh 1 microM) had little effect on Ba(2+)-DD, whereas norepinephrine (NE 1 microM) enhanced it by increasing the undershoot and by inducing an oscillatory potential. Low [Ca2+]o (0.54 mM) decreased the resting potential and increased Ba(2+)-DD amplitude. High [Ca2+]o (8.1 mM) had opposite effects. Cs+ also reduced Ba(2+)-DD in low [Ca2+]o. In isolated myocytes, Ba(2+)-DD and the pacemaker current induced by Ba2+ (Ba(2+)-IKdd) increased on depolarization and reversed on hyperpolarization. Although not significantly, high [Ca2+]o slightly decreased and low [Ca2+]o slightly increased Ba(2+)-IKdd. Cd2+ markedly reduced the slow inward current ICa and the AP duration (APD), but did not affect Ba(2+)-IKdd. We conclude that Ba(2+)-DD (a) is entirely due to a voltage- and time-dependent decrease in gK1, since it is abolished by Cs+ (no contribution of a nonblocked decaying IK) by eliminating the undershoot (no If), (b) is potentiated by NE through an increased undershoot and an oscillatory potential, (c) is modified by high and low [Ca2+]o mostly through changes in the resting potential, and (d) is not affected by the block of the slow channel by Cd2+.

Cesium, Na(+)-K(+) Pump and Pacemaker Potential in Cardiac Purkinje Fibers

The mechanisms of the hyperpolarizing and depolarizing actions of cesium were studied in cardiac Purkinje fibers perfused in vitro by means of a microelectrode technique under conditions that modify either the Na(+)-K(+) pump activity or I(f). Cs(+) (2 mM) inconsistently increased and then decreased the maximum diastolic potential (MDP); and markedly decreased diastolic depolarization (DD). Increase and decrease in MDP persisted in fibers driven at fast rate (no diastolic interval and no activation of I(f)). In quiescent fibers, Cs(+) caused a transient hyperpolarization during which elicited action potentials were followed by a markedly decreased undershoot and a much reduced DD. In fibers depolarized at the plateau in zero [K(+)](o) (no I(f)), Cs(+) induced a persistent hyperpolarization. In 2 mM [K(+)](o), Cs(+) reduced the undershoot and suppressed spontaneous activity by hyperpolarizing and thus preventing the attainment of the threshold. In 7 mM [K(+)](o), DD and undershoot were smaller and Cs(+) reduced them. In 7 and 10 mM [K(+)](o), Cs(+) caused a small inconsistent hyperpolarization and a net depolarization in quiescent fibers; and decreased MDP in driven fibers. In the presence of strophanthidin, Cs(+) hyperpolarized less. Increasing [Cs(+)](o) to 4, 8 and 16 mM gradually hyperpolarized less, depolarized more and abolished the undershoot. We conclude that in Purkinje fibers Cs(+) hyperpolarizes the membrane by stimulating the activity of the electrogenic Na(+)-K(+) pump (and not by suppressing I(f)), and blocks the pacemaker potential by blocking the undershoot, consistent with a Cs(+) block of a potassium pacemaker current. Copyright 1995 S. Karger AG, Basel