Regulation of calcium content in bovine spermatozoa

Plasma membrane vesicles isolated from bovine epididymal and ejaculated spermatozoa have widely different capabilities for transporting Ca²⁺. Spermatozoa were ruptured by nitrogen cavitation, and the plasma membrane fraction was harvested after low speed and sucrose gradient centrifugation; purity was assessed by marker enzyme analyses, electron microscopy, and sedimentation properties. Plasma membrane vesicles isolated from epididymal sperm accumulate Ca²⁺ passively at a faster rate and to a greater extent than vesicles prepared from ejaculated sperm. Ca²⁺ transport across bovine sperm plasma membranes is an ATP-independent, Na⁺-dependent process that obligatory exchanges intravesicular Na⁺ for external Ca²⁺. The rate of Na⁺/Ca²⁺ exchange is significantly lower in ejaculated sperm vesicles than in those of epididymal sperm. Bovine plasma membranes contain little or no Ca²⁺-dependent ATPase activity. It is suggested that, at the time of ejaculation, calcium flux into bovine sperm is prevented by the interaction of the plasma membrane with putative factors in seminal fluid that specifically interfere with Na⁺/Ca²⁺ exchange. We have isolated a protein from seminal plasma that prevents calcium accumulation by bovine epididymal sperm (Rufo, G.A. Jr., Singh, J.P., Babcock, D.F., and Lardy, H.A. (1982) J. Biol. Chem. 257, 4627-4632). A protein with properties resembling those of the seminal calcium transport inhibitor is found on the membrane vesicles from ejaculated sperm but not on membranes from epididymal sperm. We conclude that this protein binds strongly to the plasma membrane of bovine sperm and is responsible for preventing calcium uptake by ejaculated sperm.