The pacemaker current in cardiac Purkinje myocytes

Abstract

It is generally assumed that in cardiac Purkinje fibers the hyperpolarization activated inward current i(f) underlies the pacemaker potential. Because some findings are at odds with this interpretation, we used the whole cell patch clamp method to study the currents in the voltage range of diastolic depolarization in single canine Purkinje myocytes, a preparation where many confounding limitations call be avoided. In Tyrode solution ([K+](o) = 5.4 mM), hyperpolarizing steps from V(h) = -50 mV resulted in a time-dependent inwardly increasing current in the voltage range of diastolic depolarization. This time-dependent current (i(Kdd)) appeared around -60 mV and reversed near E(K). Small superimposed hyperpolarizing steps (5 mV) applied during the voltage clamp step showed that the slope conductance decreases during the development of this time-dependent current. Decreasing [K+](o) from 5.4 to 2.7 mM shifted the reversal potential to a more negative value, near the corresponding E(K). Increasing [K+](o) to 10.8 mM almost abolished i(Kdd). Cs+ (2 mM) markedly reduced or blocked the time- dependent current at potentials positive and negative to E(K). Ba2+ (4 mM) abolished the time-dependent current in its usual range of potentials and unmasked another time-dependent current (presumably i(f)) with a threshold of ~-90 mV (>20 mV negative to that of the time-dependent current in Tyrode solution). During more negative steps, i(f) increased in size and did not reverse. During i(f), the slope conductance measured with small (8-10 mV) superimposed clamp steps increased. High [K+](o) (10.8 mM) markedly increased and Cs+ (2 mM) blocked i(f). We conclude that: (a) in the absence of Ba2+, a time-dependent current does reverse near E(K) and its reversal is unrelated to K+ depletion; (b) the slope conductance of that time- dependent current decreases in the absence of K+ depletion at potentials positive to E(K) where inactivation of i(f) is unlikely to occur. (c) Ba2+ blocks this time-dependent current and unmasks another time-dependent current (i(f)) with a more negative (>20 mV) threshold and no reversal at more negative values; (d) Cs+ blocks both time-dependent currents recorded in the absence and presence of Ba2+. The data suggest that in the diastolic range of potentials in Purkinje myocytes there is a voltage- and time-dependent K+ current (i(Kdd)) that can be separated from the hyperpolarization-activated inward current i(f).