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In mammals, Ca 2 + and 12ptminimal amsmath wasysym amsfonts amssymb amsbsy mathrsfs -69pt document equation*HCO₃^-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 12ptminimal amsmath wasysym amsfonts amssymb amsbsy mathrsfs -69pt document equation*HCO₃^-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 12ptminimal amsmath wasysym amsfonts amssymb amsbsy mathrsfs -69pt document equation*HCO₃^-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 12ptminimal amsmath wasysym amsfonts amssymb amsbsy mathrsfs -69pt document equation*HCO₃^-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 12ptminimal amsmath wasysym amsfonts amssymb amsbsy mathrsfs -69pt document equation*HCO₃^-equation*document in the stimulation of this enzyme, underscoring an important role for sAC in the control of sperm functions.
Jaiswal et al. (Thu,) studied this question.