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

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Abstract
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.