Stimulation of the type III olfactory adenylyl cyclase by calcium and calmodulin.

Characterization of adenylyl cyclases has been facilitated by the isolation of cDNA clones for distinct adenylyl cyclases including the type I and type III enzymes. Expression of type I adenylyl cyclase activity in animal cells has established that this enzyme is stimulated by calmodulin and Ca2+. Type III adenylyl cyclase is enriched in olfactory neurons and is regulated by stimulatory G proteins. The sensitivity of the type III adenylyl cyclase to Ca2+ and calmodulin has not been reported. In this study, type III adenylyl cyclase was expressed in human kidney 293 cells to determine if the enzyme is stimulated by Ca2+ and calmodulin. The type III enzyme was not stimulated by Ca2+ and calmodulin in the absence of other effectors. It was, however, stimulated by Ca2+ through calmodulin when the enzyme was concomitantly activated by either GppNHp or forskolin. The concentrations of free Ca2+ for half-maximal stimulation of type I and type III adenylyl cyclases were 0.05 and 5.0 microM Ca2+, respectively. These data suggest that the type III adenylyl cyclase is stimulated by Ca2+ when the enzyme is activated by G-protein-coupled receptors and that increases in free Ca2+ accompanying receptor activation may amplify the primary cyclic AMP signal.