Intracellular calcium regulation in sperm capacitation and acrosomal reaction.

Binding to the egg's zona pellucida stimulates the spermatozoon to undergo acrosome reaction, a process which enables the sperm to penetrate the egg. Prior to this binding, the spermatozoa undergo in the female reproductive tract a series of biochemical transformations, collectively called capacitation. The first event in capacitation is the elevation of intracellular calcium and bicarbonate to activate adenylyl cyclase (AC) to produce cyclic-AMP, which activates protein kinase A (PKA) to phosphorylate certain proteins. During capacitation, there is also an increase in actin polymerization and in the membrane-bound phospholipase C (PLC). Sperm binding to zona-pellucida causes further activation of cAMP/PKA and protein kinase C (PKC), respectively. PKC opens a calcium channel in the plasma membrane. PKA together with inositol-trisphosphate activate calcium channels in the outer acrosomal membrane, which leads to an increase in cytosolic calcium. The depletion of calcium in the acrosome will activate a store-operated calcium entry mechanism in the plasma membrane, leading to a higher increase in cytosolic calcium, resulting in membrane fusion and acrosome reaction.