Two types of calcium currents in single smooth muscle cells from rat portal vein.

1. Using the whole‐cell recording mode of the patch‐clamp technique, we investigated the calcium currents in isolated cells from rat portal vein in short‐term primary culture. 2. From a holding potential of ‐70 mV the cells presented two types of calcium currents with 5 mM‐extracellular calcium: one type was activated by small depolarizations and inactivated quickly (fast calcium current), whereas the other required stronger depolarizations for activation and inactivated more slowly (slow calcium current). 3. Isradipine (PN 200‐110) blocked the slow calcium current at concentrations 300 times lower than those used to block the fast inward current. The isradipine‐induced inhibition was voltage‐dependent for the slow calcium current and voltage‐independent for the fast calcium current. 4. The slow calcium current was lost during internal perfusion with a 0.5 microM‐Ca2+ containing solution, and during stimulation of the cell at high frequencies (0.1‐0.2 Hz) within 5‐10 min. The fast calcium current was unchanged under these experimental conditions. 5. Steady‐state inactivation curves for both fast and slow calcium currents showed differences in their voltage dependence. Half‐maximal and complete inactivations of the fast calcium current were obtained at ‐50 and ‐30 mV while those of the slow calcium current were obtained at ‐20 and +10 mV. 6. Studied with the two‐pulse protocol, inactivation of the slow calcium current was dependent on both membrane potential and calcium influx while that of the fast calcium current appeared only dependent on membrane potential. 7. Two types of calcium currents, differing in potential dependence of inactivation, and in sensitivities to dihydropyridines, stimulation frequency and intracellular calcium concentration were identified in cultured smooth muscle cells isolated from portal vein.