Gamete Biology

Na⁺/K⁺ATPase as a Signaling Molecule During Bovine Sperm Capacitation

Abstract
Na⁺/K⁺ATPase is a heteromeric, integral membrane protein composed of two polypeptides, alpha and beta, and found to be active in many cell types including testis and spermatozoa. Na⁺/K⁺ATPase is a well-known ion transporter, but ouabain (a specific inhibitor of Na⁺/K⁺ATPase)binding to the Na⁺/K⁺ATPase in somatic cells initiates responses that are similar to signaling events associated with bovine sperm capacitation. The objectives of this study were to demonstrate the presence of Na⁺/K⁺ATPase in bovine sperm and investigate its role in the regulation of bovine sperm capacitation. The presence of Na⁺/K⁺ATPase in sperm from mature Holstein bulls was demonstrated by immunoblotting and immunocytochemistry using a monoclonal antibody developed in mouse against the beta 1 polypeptide of Na⁺/K⁺ATPase. Ouabain binding to the Na⁺/K⁺ATPase inhibited motility (decreased progressive motility, average path velocity, and curvilinear velocity), induced tyrosine phosphorylation and capacitation, but did not increase intracellular calcium levels in spermatozoa. Furthermore, ouabain binding to the Na⁺/K⁺ATPase induced depolarization of sperm plasma membrane. Therefore, ouabain binding to the Na⁺/K⁺ATPase induced sperm capacitation through depolarization of sperm plasma membrane and signaling via the tyrosine phosphorylation pathway without an appreciable increase in intracellular calcium. To our knowledge this is the first report on the signaling role of Na⁺/K⁺ATPase in mammalian sperm capacitation.

Key words: Gamete Biology • Signal transduction • Sperm • Sperm capacitation • Sperm maturation

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