Modulation of juvenile rat ovarian adenylate cyclase activity by calcium and calmodulin

The involvement of calcium and calmodulin in the regulation of juvenile rat ovarian adenylate cyclase activity was investigated. Both basal and LH-stimulated cAMP production were inhibited by adding Ca2+ to the incubation medium at concentrations higher than 10(-5) M. Conversely, up to 10(-3) M concentrations of EGTA increased cAMP production (basal, stimulated by LH, FSH, NaF and Gpp(NH)p); higher concentrations of the chelator led to an inhibition of cAMP formation. However, when the homogenates were previously deprived of Ca2+ by treatment with buffer containing EGTA, a biphasic response to LH and Gpp(NH)p stimulation was obtained in the presence of increasing concentrations of added Ca2+:cAMP production was first enhanced at low concentrations and then inhibited at higher concentrations. These observations suggest that the optimal concentration of Ca2+ needed to obtained maximal stimulation of the enzyme was much lower than the Ca2+ content in the homogenates and that a minimal concentration of Ca2+ was required to activate it. In the presence of micromolar concentrations of trifluoperazine and pimozide, two potent inactivators of calmodulin, LH-stimulated cAMP production was markedly decreased. Reactivation was obtained by adding exogenous calmodulin to the assay medium. The addition of Ca2+-free exogenous calmodulin (10(-6) M) caused a specific and significant enhancement of cAMP accumulation induced by an optimal dose of LH. These results suggest that calcium ions regulated the adenylate cyclase activity in the rat ovaries and had a dual effect that was first stimulatory at low concentration and mediated by calmodulin and then inhibitory at high (non-physiological) concentration.