Soluble sperm extract triggers inositol 1,4,5-trisphosphate-induced Ca2+ release in the oocytes of the sea urchin, Anthocidaris crassispina.

The substance in sperm which induces the activation of eggs at fertilization has yet to be elucidated. Osawa et al. reported that the soluble extract of sperm (spex) causes inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release (IICR) in fertilized eggs of sea urchins when externally applied [Osawa et al., (1992). Zool. Sci. 9: 1206a]. This paper reports that spex also caused IICR in immature oocytes of Anthocidaris crassispina at their germinal vesicle stage and induced a transient increase in their intracellular Ca2+ concentration (Cai-transient). The peak value of this Cai-transient was 160 +/- 16 nM, a small increase by about 60 nM from the basal intracellular Ca2+ concentration, [Ca2+]i, of 96 +/- 12 nM. This reaction was completely blocked by heparin, an IICR inhibitor. Furthermore, the active factor in spex was resistant to pronase and did not show species specificity. It was found that Cai-transient can also be induced in oocytes by sperm. The peak value of the Cai-transient in this case was dependent on sperm concentration and was a maximum of approximately 250 nM, which was significantly lower than that at normal fertilization, i.e., 941 +/- 136 nM. The Cai-transient induced by sperm in oocytes was not suppressed, though its time course was delayed, by heparin.