Abstract
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 obligatorily 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.
Highlights
From the Institute for Enzvme Research and the Department of Biochemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706“
Plasma membrane vesicles isolated from bovinep- branes prepared from boar sper(m6) have been shownto bind ididymal and ejaculated spermatozoa havweidely dif- calcium with high affinity, especially in the presence of soferent capabilities for transportinCga2+
Plasma membrane vesicles from bovine epididymaland ejaculated sperm. isolated from epididymal sperm accumulate Ca2+pas- Previous work has shown that washed ejaculated bovine sively at a faster rate and to a greater extent than vesicles prepared from ejaculated sperm
Summary
Plasma membrane vesicles from bovine epididymaland ejaculated sperm. Isolated from epididymal sperm accumulate Ca2+pas- Previous work has shown that washed ejaculated bovine sively 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 obligatorily exchanges intravesiculaNr a+ for external Ca2+.The rate of Na+/Ca2+ exchangeis signifisperm areincapable of accumulatingcalcium in uitro, whereas epididymal sperm take up calcium rapidly [9]. We have isolated a protein from seminal plasma that cantly lower in ejaculated spervmesicles than inthose inhibits calcium uptake by ejaculated sperm (IO),and it is of of epididymal sperm. We haveisolatedaproteinfromseminal sperm plasma membrane Na+/Ca2+ antiporter that is ameplasma that prevents calcium accumulation by bovine nable toregulation by extrinsic factors presentin the seminal epididymal sperm A protein with properties resembling those of the seminal calcium transport inhibitor
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