Soluble Sperm Extract Triggers Inositol 1, 4, 5-Trisphosphate-induced Ca²⁺ Release in the Oocytes of the Sea Urchin, Anthocidaris crassispina

Abstract

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 (Ca_i-transient). The peak value of this Cat-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 Ca_i-transient can also be induced in oocytes by sperm. The peak value of the Ca_i-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 Ca_i-transient induced by sperm in oocytes was not suppressed, though its time course was delayed, by heparin.