Slow inward current in single cells isolated from adult human ventricles

Characteristics of the slow inward current (Isi) in human ventricular myocytes isolated from septal specimens obtained in patients undergoing corrective cardiac surgery were studied using the whole-cell clamp method. A first series of experiments was performed under normal standard superfusion. Clamping from −60 mV evoked an inward current with a threshold at about −35 mV, a maximum around +10 mV and an apparent reversal potential at about +55 mV. No overlapping transient or background outward currents were detected in the −60 to +30 mV potential range, but time-dependent and steady-state outward currents were elicited at potentials above +30 mV. An overlap of steady-state activation and inactivation curves was present between −30 and +10 mV and a slight relief from inactivation was observed for voltages positive to +10mV. The time course of inactivation consisted of fast and slow phases with time constants differing by a factor of eight. Slow time constants of inactivation were shorter at potentials that elicited larger Isi, and longer at potentials inducing smaller Isi. Recovery from inactivation evolved slowly with 100% reactivation occurring in about 4000 ms. Switching the holding potential from −60 to −40 mV led to a reversible decline of Isi without any change of the decay time constants. Isi was significantly increased by 0.1 μM isoproterenol. Total or partial inhibition by inorganic (2 mM Mn2+, 3 mM Co2+, 1 mM Cd2+) and organic (1 μM methoxyverapamil, 5 μM diltiazem) calcium antagonists did not unmask any transient outward current. However, a consistent increase of Isi was reversibly observed with 3 mM 4-aminopyridine while using standard solutions. A second series of experiments carried out with K+- and Na+-free solutions did not demonstrate any significant change from data observed with standard solutions except a reduction of outward currents at steps above +30 mV and alteration of inactivation kinetics. In this experimental setting, 4-aminopyridine also increased Isi but to a lesser degree. We conclude that Isi, as compared to the outward currents, is dominant in the diseased human ventricular cells we have studied.