ABSTRACT
Within the oviduct, sperm are exposed to an elevated bicarbonate concentration, a crucial early event in capacitation. Bicarbonate is also required in in vitro capacitation systems[7] and provided either as a component of the medium or is the result of solutions reaching an equilibrium in a CO2 incubator. Bicarbonate exposure leads to the activation of adenylate cyclase, elevated cAMP, and the activation of protein kinase A. As a result, the asymmetrical distribution of phosphatidylserine and phosphatidylethanolamine in the inner leaflet of the lipid bilayer collapses, and these two phospholipids appear in the outer leaflet at the anterior head of the sperm.[8] The redistribution of these two phospholipids is presumably a result of scramblase activity. Collapse of asymmetry leads to a net loss of cholesterol from the plasma membrane. Albumin in the milieu serves as an acceptor for this cholesterol. At least in the boar, cholesterol is lost from the fluid bilayer, but capacitation appears to be accompanied by increased raft formation, with cholesterol maintained in the membrane rafts. Rafts may act to bring together protein assemblies for subsequent signaling, which eventually leads to the acrosome reaction.
