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-frequency long-lasting Ca2+ oscillations. It is generally accepted that these oscillations are due to Ca2+ release through the inositol 1,4,5-trisphosphate (InsP(3)) receptor. However, InsP(3) microinjection does not mimic sperm-induced Ca2+ oscillations, leading to the suggestion that the spermatozoon causes Ca2+ release by sensitizing the InsP(3) receptor to basal levels of InsP(3). This contradicts recent evidence that the spermatozoon triggers Ca2+ oscillations by introducing a phospholipase C or else an activator of phospholipase C. Here we show for the first time that sperm-induced Ca2+ oscillations may be mimicked by the photolysis of caged InsP(3) in both mouse metaphase II eggs and germinal vesicle stage oocytes. Eggs, and also oocytes that had displayed spontaneous Ca2+ oscillations, gave long-lasting Ca2+ oscillations when fertilized or when caged InsP(3) was photolyzed. In contrast, oocytes that had shown no spontaneous Ca2+ oscillations did not generate many 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 Ca2+ transient with superimposed spikes, followed by a series of single transients with a low frequency; all characteristics of the Ca2+ changes at fertilization. We therefore show that InsP(3) can mimic the distinctive pattern of Ca2+ release in mammalian eggs at fertilization. It is proposed that a sperm Ca2+-releasing factor operates by generating a continuous small amount of InsP(3) over an extended period of time, consistent with the evidence for the involvement of a phospholipase C. (C) 2000 Academic Press.