OBJECTIVE To evaluate the effect of cocaine on intracellular free calcium ([Ca2+]i) regulation in human myometrial cells by determining the sources of Ca2+ it might mobilize, as well as assess the role cocaine might play in the catecholamine's effect on the cell's [Ca2+]i. METHODS Primary culture of myometrial cells from pregnant women was used as an experimental model. [Ca2+]i relative changes in response to cocaine and norepinephrine were measured with fura-2 fluorometry and analyzed by means of one-way analysis of variance. RESULTS Cocaine alone (10(-8) to 10(-3) mol/L) increased [Ca2+]i by up to 43 +/- 18% over basal level in a dose-dependent manner. Norepinephrine also elevated [Ca2+]i in a concentration-dependent manner (202 +/- 24% over basal level at 10(-4) mol/L). The norepinephrine-evoked increase was inhibited in Ca(2+)-free media by 48%, whereas the cocaine response was not affected. The Ca(2+)-channel antagonist nifedipine caused decrease in the [Ca2+]i response to 10(-5) mol/L of norepinephrine by 84%, whereas the [Ca2+]i rise to 10(-5) mol/L cocaine was not significantly changed. Inhibitor of the sarcoplasmic reticulum Ca2+ pump, thapsigargin, completely blocked cocaine-evoked increases in [Ca2+]i, whereas norepinephrine responses were greatly reduced. At the same time, cocaine (10(-8) to 10(-3) mol/L) did not potentiate norepinephrine-evoked Ca2+]i increases in the cells. CONCLUSIONS These results indicate that cocaine increases [Ca2+]i in pregnant human myometrial cells, primarily by stimulating release of Ca2+ from intracellular stores rather than by direct stimulation of Ca2+ influx.