INJECTION OF A PORCINE SPERM FACTOR TRIGGERS CALCIUM OSCILLATIONS IN MOUSE OOCYTES AND BOVINE EGGS

Fertilization in mammals is associated with the generation of intracel lular calcium ([Ca2+]i) oscillations. The site of, or mechanism(s) uti lized by, the sperm to initiate and maintain these Ca2+ responses is n ot known. in this study, we tested the hypothesis that a factor from t he sperm is capable, upon release into the oocyte's cytosol, of initia ting oscillations. A sperm factor, prepared from porcine semen, was in jected into mouse oocytes and bovine eggs that had been loaded with fu ra-2 dextran, a fluorescent Ca2+ indicator. The resulting Ca2+ respons es were monitored and compared to those characteristic of each species . Our results show that injection of sperm factor triggered long-lasti ng [Ca2+]i oscillations, and that the observed patterns were species-s pecific. in mouse oocytes, sperm factor-induced [Ca2+]i rises exhibite d high frequency, whereas in bovine eggs, Ca2+ responses were separate d by long intervals. Further characterization of the sperm factor reve aled that it was predominantly present in sperm preparations, that it contained a protein moiety, and that it was unlikely to be a protease. The intracellular Ca2+ channels/receptors through which the sperm fac tor mediated Ca2+ release was investigated by using heparin, a competi tive inhibitor of the inositol 1,4,5 trisphosphate receptor (InsP3R), and ryanodine, which binds the ryanodine receptor (RyR). The sperm fac tor appeared to stimulate InsP3R, at least in mouse oocytes, because s perm factor-induced oscillations were delayed or blocked in all oocyte s by injection of heparin. RyR may be involved in the modulation of th ese oscillations, since addition of ryanodine modified Ca2+ responses to the sperm factor. The present results support the hypothesis that a factor from the sperm is involved in the generation of fertilization associated [Ca2+]i oscillations. (C) 1997 Wiley-Liss, Inc.