[Calcium-dependent regulation of cardiac adenyl cyclase by calmodulin]

Calmodulin (CM) was found to activate adenylate cyclase (AC) in plasma membrane preparations of the rabbit heart in the presence of micromolar Ca2+ concentrations. CM action on the enzyme is terminated by trifluoperazine, troponin I and high Ma2 +/Ca2+ ratio. Isoproterenol in the presence of GTP, guanosine-5'-beta-gamma-imidotriphosphate[Cpp(NH)p] and sodium fluoride increases CM-dependent heart AC activity without changing the enzyme affinity to CM and Ca ions. CM has no effect on the convertion of regulatory N-protein into Gpp(NH)p-activated state. CM action on AC is revealed only if catalytic subunit forms a complex with regulatory N-protein. The conclusion was drawn that there is no distinct CM-dependent AC form in the heart, but the same catalytic subunit of the enzyme is regulated both by N-protein and CM. In the presence of Ca2+ and guanye nucleotides AC of the heart exists in the form of a complex: CM-catalytic subunit-N-protein.