Differential Regulation by Calmodulin of Basal, GTP-, and Dopamine-Stimulated Adenylate Cyclase Activities in Bovine Striatum

Abstract

The concentration requirements of calmodulin in altering basal, GTP-, and dopamine-stimulated adenylate cyclase activities in an EGTA-washed paniculate fraction from bovine striatum were examined. In the bovine striatal paniculate fraction, calmodulin activated basal adenylate cyclase activity 3.5-fold, with an EC 50 of 110 n M . Calmodulin also potentiated the activation of adenylate cyclase by GTP by decreasing the EC 50 for GTP from 303 ± 56 n M to 60 ± 10 n M Calmodulin did not alter the maximal response to GTP. The EC 50 for calmodulin in potentiating the GTP response was only 11 n M as compared to 110 n M for activation of basal activity. Similarly, calmodulin increased the maximal stimulation of adenylate cyclase by dopamine by 50–60%. The EC 50 for calmodulin in eliciting this response was 35 n M . These data demonstrate that calmodulin can both activate basal adenylate cyclase and potentiate adenylate cyclase activities that involve the activating GTP-binding protein, N s . Mechanisms that involve potentiation of N s -mediated effects are much more sensitive to calmodulin than is the activation of basal adenylate cyclase activity. Potentiation of GTP-stimulated adenylate cyclase activity by calmodulin was apparent at 3 and 5 m M MgCl 2 , but not at 1 or 10 m M MgCl 2 . These data further support a role for calmodulin in hormonal signalling and suggest that calmodulin can regulate cyclic AMP formation by more than one mechanism.