The sustained inward current in sino-atrial node cells of guinea-pig heart

Single myocytes were dissociated from the sino-atrial (SA) node of guinea-pig hearts. Only a quite small fraction of the cell population showed spontaneous action potentials and these cells were characterized by the presence of the hyperpolarization-activated cation current I-f the delayed rectifier K+ current I-K and the L-type Ca2+ current I-Ca,I-L as well as by the absence of both the transient outward current I-Ca,I-L and the inward rectifier K+ current I-K,I-l. After blocking I-f and I-K depolarizing pulses from -80 mV revealed a large nicardipine-sensitive late current (NSLC). The NSLC was scarcely affected by decreasing extracellular [Ca2+] ([Ca2+](0)) from 1.8 to 0.1 mM, while it was decreased significantly by depleting [Na+](0), differently from I-Ca,I-L. NSLC was blocked by nicardipine and was increased by Bay K 8644. NSLC was increased by isoprenaline and the additional application of acetylcholine reversed the increase of this current. We conclude that NSLC is largely composed of I-st described in the rabbit SA node pacemaker cells, and that I-st is unique for the pacemaker cells in mammalian SA node cells. Most of the quiescent cells showed neither I-f nor I-st.