| A synchronous acrosome reaction system has been developed for human spermatozoa in which capacitated sperm are induced to undergo the acrosome reaction. Rapid influx of calcium is believed to initiate the acrosome reaction. After a brief incubation with a calcium transporter, ionophore A23187, a significant stimulation of the acrosome reaction occurred in comparison to untreated sperm. The effect of oocytes on spermatozoa was then investigated. The acrosome reaction of spermatozoa was enhanced in the presence of oocytes in comparison to spermatozoa not exposed to oocytes, and was comparable to results with ionophore. The acrosome reaction appears to be similar to exocytotic events mediated by second messenger pathways. Therefore, studies were directed towards the adenylate cyclase system. Analogues of the second messenger cAMP stimulated an acrosome reaction comparable to ionophore. However, when sperm were incubated in calcium-free medium only dbcAMP-induced the acrosome reaction, implying that calcium is required prior to cAMP involvement. Treatment of spermatozoa with forskolin, an adenylate cyclase activator, resulted in a significant stimulation of the acrosome reaction. Inhibitors of adenylate cyclase, adenosine and adenosine analogues, prevented the forskolin-induced acrosome reaction. The dbcAMP-induced acrosome reaction was prevented by an inhibitor of protein kinase A, which is a target of cAMP. These data provide evidence for the adenylate cyclase/cAMP second messenger system in the human sperm acrosome reaction. Acrosin, a proteinase located in the sperm head, is believed to be essential for penetration of oocyte vestments. Serine proteinase inhibitors prevented the ionophore- and dbcAMP-induced acrosome reaction, implying that acrosin activation occurs after the involvement of cAMP. Another second messenger pathway, the diacylglycerol/protein kinase C system was also studied. It was found that activators of protein kinase C, phorbol esters and synthetic diacylglycerols, stimulated a greater percent acrosome reaction than non-treatment controls. Additionally, an inhibitor of protein kinase C prevented the diacylglycerol-induced acrosome reaction. These data imply a role for the diacylglycerol/protein kinase C pathway in the acrosome reaction. In conclusion, the current data support a model for the role of two second messenger pathways in the human sperm acrosome reaction. |
