Induction of the acrosome reaction in guinea pig spermatozoa by calmodulin antagonist W-7

The effect of the calmodulin antagonist W-7 on the capacitation and the acrosome reaction of guinea pig spermatozoa was examined. The characteristic features of the acrosome reaction induced by W-7 were the dependence on the composition and pH of the medium and on the presence of sodium bicarbonate. The most effective concentration of W-7 for inducing the acrosome reaction was approximately 5 μM, which is far less than the Kd for calmodulin. Moreover, W-7 enhanced the ability of spermatozoa to acquire capacitation in a Ca2+-free medium. The spermatozoa induced to undergo the acrosome reaction by W-7 were capable of penetrating the zona-free hamster eggs. W-5, which has a lower affinity for calmodulin than W-7, also induced the acrosome reaction in the same manner as W-7. These results suggest that the naphthalenesulfonamide derivatives W-7 and W-5 can induce the acrosome reaction in guinea pig spermatozoa via capacitation in a pH-dependent, Ca2+-calmodulin-independent manner.