Calcium and phospholipase A2 are both required for the acrosome reaction mediated by G-proteins stimulation in human spermatozoa

G-proteins, calcium, and phospholipase A2 (PLA2) have all been implicated in the cascade of signaling events leading to the acrosome reaction in human spermatozoa. In order to study the role of Ca+2 and PLA2 during the acrosome reaction triggered by G-proteins, we treated human spermatozoa incubated for 3 hr under capacitating conditions with several reagents (GTPgammaS, A23187, ONO-RS-082, arachidonic acid, BAPTA-AM, and TPEN), alone or in different combinations. Our results suggest that GTP-binding proteins require Ca+2 and PLA2 to accomplish their stimulatory effect, and that Ca+2 is also required when the acrosome reaction--bypassing the action of PLA2--is stimulated by AA. Accordingly, when treated with GTPgammaS or AA, the cells loaded with Fura 2-AM showed a steady increase of Ca+2]i. On the other hand, a massive influx of Ca+2 was completely unable to induce the acrosome reaction if PLA2 was inhibited, suggesting that both an increase of Ca+2]i and PLA2 activation are required for the acrosome reaction to occur.