Synergistic activation of the type I adenylyl cyclase by Ca²⁺ and G_s-coupled receptors in vivo

The type I adenylyl cyclase is directly stimulated by Ca²⁺ and calmodulin in vivo (Choi, E. J., Wong, S. T., Hinds, T. R. and Storm, D. R. (1992) J. Biol. Chem. 267, 12440-12442; Wu, Z., Wong, S. T., and Storm, D. R. (1993) J. Biol. Chem. 268, 23766-23768). In this study, we examined the sensitivity of the type I adenylyl cyclase expressed in HEK-293 cells to β-adrenergic agonists or glucagon when intracellular Ca²⁺ was elevated by Ca²⁺ ionophore or carbachol. Although previous studies have shown that this enzyme can be directly stimulated by activated G_s in vitro, we demonstrate that it is not stimulated by G_s-coupled receptors in vivo. However, the enzyme was stimulated by G_s-coupled receptors in vivo when it was activated by intracellular Ca²⁺. For example, the Ca²⁺ ionophore A23187 stimulated the enzyme 3 ± 0.5-fold (n = 9) and isoproterenol alone did not stimulate the enzyme, but the combination of the two stimulated type I adenylyl cyclase 13 ± 2-fold (n = 9) in vivo. Similarly, 500 nM glucagon alone did not stimulate the enzyme but the combination of A23187 and glucagon activated the enzyme 90 ± 8-fold (n = 4). Synergistic stimulation of type I adenylyl cyclase activity was also obtained with combinations of carbachol and isoproterenol or glucagon. This phenomenon was not observed with a mutant enzyme that is insensitive to Ca²⁺ and calmodulin, suggesting that conformational changes caused by binding of calmodulin to the type I adenylyl cyclase enhance binding or coupling to activated G_s. These data illustrate that this adenylyl cyclase can couple Ca²⁺ and neurotransmitter signals to generate optimal cAMP levels, a property of the enzyme that may be important for its role in learning and memory in mammals.