Calcium regulation of the soluble adenylyl cyclase expressed in mammalian spermatozoa

In mammals, Ca 2 + and [12pt]{minimal} {amsmath} {wasysym} {amsfonts} {amssymb} {amsbsy} {mathrsfs} {}{-69pt} {document} {equation*}{HCO}₃⁻{equation*}{document} ions play a critical role in the regulation of sperm function, most likely by regulation of cAMP levels. Mammalian germ cells contain a soluble adenylyl cyclase (sAC) with properties distinct from the well characterized membrane-bound enzymes Here we investigated whether the cyclase expressed in mature spermatozoa has the properties of sAC and whether it is regulated by Ca 2 + . In addition to an [12pt]{minimal} {amsmath} {wasysym} {amsfonts} {amssymb} {amsbsy} {mathrsfs} {}{-69pt} {document} {equation*}{HCO}₃⁻{equation*}{document} -dependent activation, the cyclase endogenous to human spermatozoa is stimulated 2- to 3-fold by Ca 2 + in a concentration-dependent manner (EC 50 ≈ 400 nM). In a similar fashion, Ca 2 + activates the recombinant rat and human full-length sAC with similar EC 50 values. The Ca 2 + stimulation was also observed when sAC was activated with [12pt]{minimal} {amsmath} {wasysym} {amsfonts} {amssymb} {amsbsy} {mathrsfs} {}{-69pt} {document} {equation*}{HCO}₃⁻{equation*}{document} , was independent of calmodulin, and was associated with an increase in V max without changes in K m for ATP-Mg 2 + . An increase in intracellular Ca 2 + by ionophore or by a muscarinic cholinergic receptor agonist increases cAMP in cells transfected with FL-hsAC, but not in mock-transfected cells. Similarly, both Ca 2 + and [12pt]{minimal} {amsmath} {wasysym} {amsfonts} {amssymb} {amsbsy} {mathrsfs} {}{-69pt} {document} {equation*}{HCO}₃⁻{equation*}{document} stimulate cAMP accumulation in human spermatozoa. These findings provide evidence that human spermatozoa express a cyclase with the properties of sAC and that Ca 2 + can substitute for [12pt]{minimal} {amsmath} {wasysym} {amsfonts} {amssymb} {amsbsy} {mathrsfs} {}{-69pt} {document} {equation*}{HCO}₃⁻{equation*}{document} in the stimulation of this enzyme, underscoring an important role for sAC in the control of sperm functions.