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Sperm capacitation and acrosome reaction (AR) have been known to be Ca2+-dependent events. Sperm capacitation accompanies with cholesterol efflux fiom plasma membrane, that eventually stimulates AR. However, whether the AR mediated by cholesterol efflux is Ca2+ dependent has not been verified yet. Recently, methyl beta cyclodextrin (MBCD) was found to evoke AR by stimulating the cholesterol efflux fiom sperm membrane. In the present study, we examined the requirement of Ca2+ in the MBCD-induced AR. During incubation of sperm in the bicarbonate buffered media MBCD increased AR in a dose-dependent manner regardless of the Ca2+ presence. In the presence of low molar concentration of Ca2+ (100 μM), MBCD-induced AR was slightly increased compared to Ca2+-free condition. In the absence of Ca2+ supplement, spontaneous AR was slightly increased during the incubation but inhibited by 100 μM EGTA. MBCD potentiated AR even the presence of EGTA. However, EGTA attenuated MBCD-induced AR, suagesting the functional involvement of intracellular Ca2+ in the MBCD-induced AR. Taken together, it was suggested that cholesterol efflux from the sperm plasma membrane was sufficient for induction of AR even in the absence of extracellular Ca2+and that a condition permissive for mobilization of intracellular Ca2+ is important for MBCD-induced AR.
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