Sperm-induced Ca2+ oscillations in mouse oocytes and eggs can be mimicked by photolysis of caged inositol 1,4,5-trisphosphate: Evidence to support a continuous low level production of inositol 1,4,5-trisphosphate during mammalian fertilization

During mouse fertilization the spermatozoon induces a series of low-frequen cy long-lasting Ca2+ oscillations. It is generally accepted that these osci llations are due to Ca2+ release through the inositol 1,4,5-trisphosphate ( InsP(3)) receptor. However, InsP(3) microinjection does not mimic sperm-ind uced Ca2+ oscillations, leading to the suggestion that the spermatozoon cau ses Ca2+ release by sensitizing the InsP(3) receptor to basal levels of Ins P(3). This contradicts recent evidence that the spermatozoon triggers Ca2oscillations by introducing a phospholipase C or else an activator of phosp holipase C. Here we show for the first time that sperm-induced Ca2+ oscilla tions may be mimicked by the photolysis of caged InsP(3) in both mouse meta phase II eggs and germinal vesicle stage oocytes. Eggs, and also oocytes th at had displayed spontaneous Ca2+ oscillations, gave long-lasting Ca2+ osci llations when fertilized or when caged InsP(3) was photolyzed. In contrast, oocytes that had shown no spontaneous Ca2+ oscillations did not generate m any oscillations when fertilized or following photolysis of caged InsP(3). Fertilization in eggs was most closely mimicked when InsP(3) was uncaged at relatively low amounts for extended periods. Here we observed an initial C a2+ transient with superimposed spikes, followed by a series of single tran sients with a low frequency; all characteristics of the Ca2+ changes at fer tilization. We therefore show that InsP(3) can mimic the distinctive patter n of Ca2+ release in mammalian eggs at fertilization. It is proposed that a sperm Ca2+-releasing factor operates by generating a continuous small amou nt of InsP(3) over an extended period of time, consistent with the evidence for the involvement of a phospholipase C. (C) 2000 Academic Press.