Dependence of the Ca2+-inhibitable adenylyl cyclase of C6-2B glioma cells on capacitative Ca2+ entry.

The ability of adenylyl cyclases to be regulated by physiological transitions in Ca2+ provides a key point for integration of cytosolic Ca2+ concentration ([Ca2+]i) and cAMP signaling. Ca2+-sensitive adenylyl cyclases, whether endogenously or heterologously expressed, require Ca2+ entry for their regulation, rather than Ca2+ release from intracellular stores (Chiono, M., Mahey, R., Tate, G., and Cooper, D. M. F. (1995) J. Biol. Chem. 270, 1149-1155; Fagan, K., Mahey, R., and Cooper, D. M. F. (1996) J. Biol. Chem. 271, 12438-12444). The present study compared the regulation by capacitative Ca2+ entry versus ionophore-mediated Ca2+ entry of an endogenously expressed Ca2+-inhibitable adenylyl cyclase in C6-2B cells. Even in the face of a dramatic [Ca2+]i rise generated by ionophore, Ca2+ entry via capacitative Ca2+ entry channels was solely responsible for the regulation of the adenylyl cyclase. Selective efficacy of BAPTA over equal concentrations of EGTA in blunting the regulation of the cyclase by capacitative Ca2+ entry defined the intimacy between the adenylyl cyclase and the capacitative Ca2+ entry sites. This association could not be impaired by disruption of the cytoskeleton by a variety of strategies. These results not only establish an intimate spatial relationship between an endogenously expressed Ca2+-inhibitable adenylyl cyclase with capacitative Ca2+ entry sites but also provide a physiological role for capacitative Ca2+ entry other than store refilling.