Slow inward current in single cells isolated from adult human ventricles

Characteristics of the slow inward current (I_si) 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 I_si, and longer at potentials inducing smaller I_si. 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 I_si without any change of the decay time constants. I_si was significantly increased by 0.1 M isoproterenol. Total or partial inhibition by inorganic (2 mM Mn²⁺, 3 mM Co²⁺, 1 mM Cd²⁺) and organic (1 M methoxyverapamil, 5 M diltiazem) calcium antagonists did not unmask any transient outward current. However, a consistent increase of I_si 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 I_si but to a lesser degree. We conclude that I_si, as compared to the outward currents, is dominant in the diseased human ventricular cells we have studied.