Regulation of a Ca2+-sensitive adenylyl cyclase in an excitable cell -: Role of voltage-gated versus capacitative Ca2+ entry

In nonexcitable cells, we had previously established that Ca2+-sensitive adenylyl cyclases, whether expressed endogenously or heterologously, were regulated exclusively by capacitative Ca2+ entry (Fagan, K, A, Mahey, R, and Cooper, D, M, F, (1996) J, Biol. Chem. 271, 12438-12444; Fagan, K, A., Mons, N., and Cooper, D. M. F, (1998) J. Biol. Chem, 273, 9297-9305), Relatively little is known about how these enzymes are regulated by Ca2+ in excitable cells, where they predominate. Furthermore, no effort has been made to determine whether the prominent voltage-gated Ca2+ entry, which typifies excitable cells, overwhelms the effect of any capacitative Ca2+ entry that may occur. In the present study, we placed the Ca2+-stimulable, adenylyl cyclase type VIII in an adenovirus vector to optimize its expression in the pituitary-derived GH(4)C(1) cell line. In these cells, a modest degree of capacitative Ca2+ entry could be discerned in the face of a dramatic voltage-gated Ca2+ entry. Nevertheless, both modes of Ca2+ entry were equally efficacious at stimulating adenylyl cyclase, A striking release of Ca2+ from intracellular stores, triggered either by ionophore or thyrotrophin-releasing hormone, was incapable of stimulating the adenylyl cyclase. It thus appears as though the intimate colocalization of adenylyl cyclase with capacitative Ca2+ entry channels is an intrinsic property of these molecules, regardless of whether they are expressed in excitable or nonexcitable cells.