Mouse sperm adenylyl cyclase: general properties and regulation by the zona pellucida

It has been shown that the cAMP concentrations in capacitated mouse sperm can be increased in response to the egg's extracellular matrix, the zona pellucida (ZP), during the acrosome reaction. However, it is not known whether this cAMP elevation is caused by a stimulation of adenylyl cyclase (AC) activity or a decrease in cyclic nucleotide phosphodiesterase activity. In the present study using capacitated mouse spermatozoa, 20% and 30% of the total AC activity in the presence of Mg2+ and Mn2+, respectively, was measured in the sperm membrane fraction that contained the highest specific AC activity. As reported in other studies of sperm AC, Mn(2+)-supported AC activity was 15-30 times higher than Mg(2+)-supported AC activity. Interestingly, when the membrane fraction was solubilized with Lubrol-PX, an increase in specific and total Mg(2+)-supported AC activity was obtained while a decrease in both the specific and total Mn(2+)-supported AC activity was observed. In the absence of detergent, the ZP stimulates the membrane AC in a concentration-dependent manner, but this stimulation is observed only when the enzyme activity is measured in the presence of Mg2+. Forskolin also stimulates the Mg(2+)-but not the Mn(2+)-supported AC activity, and this effects is not additive to or synergistic with the ZP stimulation of the membrane AC. Results reported in this study suggest that ZP-mediated stimulation of membrane-associated AC may represent a mechanism by which the ZP affects sperm functions.