Calcium-dependent increase in adenosine 3',5'-monophosphate and induction of the acrosome reaction in guinea pig spermatozoa.

Experiments were designed to determine the interrelationship between cyclic AMP and Ca(2+) during the processes of sperm capacitation and the acrosome reaction. In minimal culture media containing pyruvate and lactate as substrates, guinea pig spermatozoa required a minimum of 1.0-1.5 hr to capacitate in the presence of 1.7 mM Ca(2+) and a minimum of 0.5-1.0 hr to capacitate in the absence of added Ca(2+). Sperm cyclic AMP concentrations were increased by as much as 30-fold within 0.5 min after addition of cells to various media containing Ca(2+), and the concentrations then remained increased for up to 4 hr. When the cells were added to several Ca(2+)-deficient media, however, cyclic AMP concentrations increased only about 3-fold within 0.5 min and then returned to basal concentrations within 2 min. D-600, a calcium transport antagonist, completely blocked the Ca(2+)-induced increase in sperm cyclic AMP concentrations. In contrast to capacitation, the acrosome reaction failed to occur in the absence of extracellular Ca(2+). After capacitation of spermatozoa in a Ca(2+)-free medium, addition of Ca(2+) caused an increase in sperm cyclic AMP concentrations within 1 min and a maximal number of spermatozoa showing an acrosome reaction within 10 min. The addition of 1-methyl-3-isobutylxanthine along with Ca(2+) had a synergistic effect on the increase in cyclic AMP. Neither 1-methyl-3-isobutylxanthine nor 8-Br cyclic AMP induced an acrosome reaction in capacitated spermatozoa in the absence of Ca(2+), but both significantly decreased the time required for maximal expression of the acrosome reaction in the presence of Ca(2+). These results suggest that the sperm acrosome reaction is associated with both a primary transport of Ca(2+) and a Ca(2+)-dependent increase in sperm cyclic AMP concentrations. Because a cyclic AMP analogue did not induce an acrosome reaction in the absence of added Ca(2+), the increase in sperm cyclic AMP concentrations induced by Ca(2+) probably reflects one of a number of Ca(2+)-dependent events associated with the acrosome reaction.