Involvement of a Na+/HCO3- cotransporter in mouse sperm capacitation

Mammalian sperm are incapable of fertilizing eggs immediately after ejaculation; they acquire fertilization capacity after residing in the female tract for a finite period of time. The physiological changes sperm undergo in the female reproductive tract that render sperm able to fertilize constitute the phenomenon of sperm capacitation. We have demonstrated that capacitation is associated with an increase in the tyrosine phosphorylation of a subset of proteins and that these events are regulated by an HCO3-/cAMP-dependent pathway involving protein kinase A. Capacitation is also accompanied by hyperpolarization of the sperm plasma membrane. Here we present evidence that, in addition to its role in the regulation of adenylyl cyclase, HCO3- has a role in the regulation of plasma membrane potential in mouse sperm. Addition of HCO3- but not Cl- induces a hyperpolarizing current in mouse sperm plasma membranes. This HCO3--dependent hyperpolarization was not observed when Na+ was replaced by the non-permeant cation choline(+). Replacement of Na+ by choline(+) also inhibited the capacitation-associated increase in protein tyrosine phosphorylation as well as the zona pellucida-induced acrosome reaction. The lack of an increase in protein tyrosine phosphorylation was overcome by the presence of cAMP agonists in the incubation medium. The lack of a hyperpolarizing HCO3- current and the inhibition of the capacitation-dependent increase in protein tyrosine phosphorylation in the absence of Na+ suggest that a Na+/HCO3- cotransporter is present in mouse sperm and is coupled to events regulating capacitation.