Caffeine‐induced inhibition of calcium channel current in cultured smooth muscle cells from pregnant rat myometrium

The inhibitory effect of caffeine on the calcium channel current was investigated in cultured myometrial cells isolated from pregnant rats. Caffeine inhibited the calcium channel current elicited from a holding potential of — 70 mV in a concentration‐dependent manner. The IC50 was estimated to be 35 mM. The caffeine inhibition was not enhanced when calcium channels were opened by a conditioning depolarizing pulse sequence or when the number of inactivated calcium channels was increased at depolarized holding potentials. Caffeine antagonized the specific binding of (+)‐[3H]‐isradipine to myometrial membranes. The IC50 value found in binding experiments was similar to the IC50 value for half‐maximal inhibition of calcium channel current. Caffeine decreased the maximal binding capacity of (+)‐[3H]‐isradipine to myometrial membranes without any significant change in the dissociation constant. The results indicate that caffeine interacts with a site closely associated with the voltage‐dependent calcium channels in myometrial cells and, in turn, inhibits calcium influx.