Existence of a low-threshold and sustained inward current in rabbit atrio-ventricular node cells

Single myocytes were dissociated from the atrio-ventricular (AV) node of rabbit hearts. Spontaneous action potentials of the N or NH type were recorded from the fusiform AV node cells. Under whole-cell voltage clamp, these cells were characterized by the presence of hyperpolarization-activated cation current (I-f), delayed rectifier K+ current (I-K), and L-type Ca2+ current (I-Ca,I-L) After blocking I-f and I-K, depolarizing pulses from -80 mV revealed a sustained inward current, which was suppressed by 1 mu M nicardipine. The activation threshold of the sustained inward current was around -60 mV and the maximum amplitude was 25.4+/-20.1 pA at -40 mV (n=15). The current was also blocked by 1 mu M verapamil. The sustained inward current was not decreased (125+/-32% of control, N=5) by reducing [Ca2+](o) from 1.8 to 0.1 mM, while it was nearly suppressed by depleting [Na+](o) differently from I-Ca,I-L. We concluded that the nicardipine-sensitive sustained inward current is largely attributable to I-st described in the rabbit sino-atrial node pacemaker cells. I-f and I-st were not found in rod-shape AV node cells.