Calcium and muscarinic agonist stimulation of type I adenylylcyclase in whole cells

The type I adenylylcyclase which was originally purified and cloned from bovine brain is stimulated by Ca²⁺ and calmodulin in vitro. Although it has been proposed that this enzyme may couple elevations in intracellular Ca²⁺ to increases in cAMP in whole cells, this has not been demonstrated in vivo. In this study, the type I adenylylcyclase was expressed in human 293 cells, and the influence of extracellular Ca²⁺ and Ca²⁺ ionophore on intracellular cAMP levels was examined. The cAMP levels of control cells were unaffected by Ca²⁺ and A23187. In contrast, intracellular cAMP in 293 cells expressing type I adenylylcyclase was markedly elevated by addition of A23187 and extracellular Ca²⁺. In the presence of forskolin, the muscarinic agonist carbachol also increased cAMP in 293 cells expressing the type I adenylylcyclase. These data indicate that the type I adenylylcyclase can be stimulated by Ca²⁺ in vivo, and that muscarinic agonists may indirectly stimulate the enzyme by increasing intracellular free Ca²⁺.