Fast and slowly inactivating components of Ca-channel current and their sensitivities to nicardipine in isolated smooth muscle cells from rat vas deferens.

(1) Fast and slowly inactivating components of Ca-channel current were compared to clarify whether more than one type of Ca-channel exists in smooth muscle cells from rat vas deferens using the whole cell variant of the patch clamp technique. The pipette was filled with 150 mM Cs solution to eliminate outward current and Ba was used as the charge carrier for Ca-channel current. (2) When activated by a 5 s test pulse to O mV from a holding potential of -60 mV, the inactivation process of Ba-current was well fitted by the sum of two exponentials. The time constant of the faster inactivating component was 100-300 s and that of the slower inactivating component was 1.5-3 s. Steady-state inactivation curves of the fast- and slow-components were very similar. (3) The inward current activated at O mV from -80 mV was inactivated faster than that from -30 mV. The voltage-dependencies of the peak current from holding potentials of -30 mV and -80 mV were similar. Both had voltage threshold at -30 mV and were maximal at +10 mV. (4) Low concentrations of nicardipine (10(-9) to 10(-7) M) preferentially inhibited the slow component while higher concentration (10(-6) to 10(-5) M) were required to block the fast component. The current activated from a holding potential of -30 mV was almost fully suppressed by 10(-7) M nicardipine whereas that from -80 mV was blocked only slightly.(ABSTRACT TRUNCATED AT 250 WORDS)