Calcium channel current and its sensitivity to (+) isradipine in cultured pregnant rat myometrial cells

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Abstract
Action of (+) isradipine (PN 200-110), a dihydropyridine derivative, was investigated on the Ca channel current in cultured cells obtained from the longitudinal layer of the pregnant rat myometrium (18–19 days of gestation). Under our experimental conditions, the inward current was attributed to L-type inward current since: (i) equimolar replacement of Ba for Ca induced an increase in the peak current and a decrease in inactivation rate; (ii) residual inward currents were recorded at the end of the pulse; (iii) meinbrane potential for mid inactivation was about −40 mV; (iv) the voltage dependencies of the peak current elicited from holding potentials of −40 mV and −80 mV were similar. The inward current could be reduced with nanomolar concentrations of (+) isradipine when cells were depolarized by pulses to positive potentials. This was characterized by a pronounced initial blockade, but by no increased in blockade when pulses were repeatedly applied at a frequency of 0.05 Hz. Using the double pulse procedure we confirmed that (+) isradipine did not bind to the openstate of the Ca channels. Voltage-dependence of (+) isradipine blockade was assessed by determining the steadystate availability of the Ca channels. From the shift of the inactivation curve in the presence of (+) isradipine we calculated a K1 value of 130 pM. Scatchard analysis of the specific binding of (+)[3H] isradipine resulted in a linear plot, thereby indicating specific binding to a single class of sites with a dissociation constantKd of about 100 pM. These findings support the idea that in rat myometrical cells isradipine not only binds to the resting state of the Ca channels but also that it has a higher affinity for the inactivated state.