Whole-cell and single-channel calcium currents of isolated smooth muscle cells from saphenous vein.

Whole-cell and single-channel calcium currents in enzymatically isolated dog saphenous vein cells were recorded by the patch-clamp method. Test pulses to negative potentials from holding potentials of -90 to -40 mV elicited currents that inactivated quickly and in a voltage-dependent manner (called I low for low threshold). A second calcium current persisted even at relatively positive holding potentials of -30 to -10 mV, required stronger depolarizations for maximum current, and inactivated slowly (I high for high threshold). I high transported barium more than calcium, whereas I low transported the two ions equally. Single-channel current records (90 mM barium) showed a larger conductance that activated at relatively positive potentials and a smaller (about one-third) conductance that activated at weak depolarizations. Nitrendipine suppressed I high, and the effect was voltage dependent as observed in cardiac cells, although block of resting channels was much greater in vein cells (KR approximately 10(-8) M). Exposure to the stereoisomer (-)Bay K 8644 increased I high but not I low. The (-)Bay K 8644 also increased the channel activity and prolonged the open time of the larger conductance current. Thus, two types of calcium channels, differing in potential-dependence of activation and inactivation, calcium/barium selectivity, single-channel conductance, and sensitivities to dihydropyridines were identified in smooth muscle cells isolated from a large cutaneous vein.