Fast Na+ and slow Ca2+ channels in single uterine muscle cells from pregnant rats

Whole cell voltage-clamp method was applied to single smooth muscle cells freshly isolated from the longitudinal layer of 18-day pregnant rat uterus. Inward currents were isolated after outward currents were minimized by use of high Cs+ in the pipette solution and 4-aminopyridine (3 mM) in the bath solution. Depolarizing pulses, applied from a holding potential of -90 mV, evoked two types of inward current, fast and slow. The fast inward current decayed and disappeared within 30 ms and depended on extracellular Na+ concentration. This fast current was inhibited by tetrodotoxin (TTX) dose dependently (KD = 27 nM). These results suggest that the fast inward current was a TTX-sensitive Na+ channel current. In contrast, the slow inward current decayed slowly, dependent on extracellular Ca2+ (or Ba2+) concentration, and was inhibited by the Ca2+ channel blocker, nifedipine, dose dependently (10 nM-10 microM). These results suggest that the slow inward current was a Ca2+ channel current. A fast-inactivating Ca2+ channel current was not evident when Ba2+ was the charge carrier. We conclude that the major ion channels in the cell membrane of pregnant rat uterus, which generate inward currents, are TTX-sensitive fast Na+ channels and dihydropyridine-sensitive slow Ca2+ channels (L-type, high-threshold type).