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

Abstract

Experiments were designed to determine the interrelationship between cycle[c]AMP and Ca2+ 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 h to capacitate in the presence of 1.7 mM Ca2+ and a minimum of 0.5-1.0 h to capacitate in the absence of added Ca2+. Sperm cAMP concentrations were increased by as much as 30-fold within 0.5 min after addition of cells to various media containing Ca2+, and the concentrations then remained increased for up to 4 h. When the cells were added to several Ca2+-deficient media, cAMP concentrations increased only .apprx. 3-fold within 0.5 min and then returned to basal concentrations within 2 min. D-600 (.alpha.-isopropyl-.alpha.-[N-methyl-N-homoveratryl-.gamma. aminopropyl]-3,4,5-trimethoxyphenylacetonitrile), a Ca transport antagonist, completely blocked the Ca2+-induced increase in sperm cAMP concentrations. In contrast to capacitation, the acrosome reaction failed to occur in the absence of extracellular Ca2+. After spermatozoa capacitation in a Ca2+-free medium, addition of Ca2+ caused an increase in sperm cAMP 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 Ca2+ had a synergistic effect on the increase in cAMP. Neither 1-methyl-3-isobutylxanthine nor 8-Br cAMP induced an acrosome reaction in capacitated spermatozoa in the absence of Ca2+ but both significantly decreased the time required for maximal expression of the acrosome reaction in the presence of Ca2+. The sperm acrosome reaction is evidently associated with both a primary transport of Ca2+ and a Ca2+-dependent increase in sperm cAMP concentrations. Because a cAMP analog did not induce an acrosome reaction in the absence of added Ca2+, the increase in sperm cAMP concentrations induced by Ca2+ probably reflects one of a number of Ca2+-dependent events associated with the acrosome reaction.