Regulation by calmodulin of adenylate cyclase activity in anterior pituitary

The control of adenylate cyclase activity is exerted through separate components: receptors, guanyl nucleotide-binding protein, catalytic subunit, calmodulin (CaM), and divalent cations. We examined the roles of CaM, Mg+2, and Ca+2 in the regulation of adenylate cyclase activity in plasma membranes from anterior pituitary. Adenylate cyclase activity was reduced with 2.5 mM EGTA, 125 micrograms/ml compound 48/80, and 200 microM trifluoperazine, which are known inhibitors of CaM in a variety of tissues. Mg+2, in excess of ATP, stimulated adenylate cyclase activity. Ca+2 produced a biphasic effect on adenylate cyclase activity over the concentration range of 0.1-10 mM, exhibiting inhibition up to 2.0 mM and stimulation above that. GTP, 5'-guanylylimidodiphosphate, and F- each enhanced adenylate cyclase activity, but activity stimulated after each of these agents was reduced or returned toward control values by administration of compound 48/80 or trifluoperazine. In the absence of free Mg+2 (i.e. Mg+2 in excess of ATP concentrations), 10 mM Ca+2 produced marked stimulation of adenylate activity which was not reduced by trifluoperazine. We concluded that the plasma membranes from anterior pituitary possess a CaM-dependent adenylate cyclase and that activation of adenylate cyclase by guanyl nucleotide-binding protein requires CaM. Ca+2 may have allosteric binding sites on the catalytic subunit, and Ca+2 and Mg+2 appear to have antagonistic effects at different binding sites.