STUDIES ON THE MECHANISM FOR CA-I-TRANSIENTS IN SEA-URCHIN ZYGOTES CAUSED BY REFERTILIZATION AND EXTERNAL APPLICATION OF SPERM EXTRACT

Sea urchin zygotes can be refertilized when they are deprived of the f ertilization membrane and the hyaline layer. We have earlier reported that a transient increase of the intracellular Ca2+ concentration (Ca- i-transient) is induced in zygotes refertilized by sperm or treated wi th a sperm extract (spex) (M. Osawa et al., 1994, Dev. Biol. 166, 268- 276), We investigated quantitative characteristics of the Ca-i-transie nt induced by sperm and spex, using a Ca2+ indicator, Indo-1. When spe rm or spex was applied to zygotes, the peak value of the Ca-i-transien t was 1.16 or 0.69 mu M, respectively, Although these values mere lowe r than the peak value of 1.95 mu M measured during normal fertilizatio n, the entire time courses of the three types of Ca-i-transients were similar. The Ca-i-transients during fertilization is known to be cause d both by the IP3-induced Ca2+ release (IICR) and by a mechanism indep endent of IICR. The Ca-i-transients during refertilization and fertili zation were not inhibited by an IP3 receptor inhibitor, heparin or by a G-protein inhibitor, GDP beta S. However, heparin delayed the time c ourses of both Ca-i-transients. These results suggest that there may b e two signal transduction pathways operating during refertilization, o ne dependent and the other independent of IICR, By contrast, both hepa rin and GDP beta S inhibited the spex-induced Ca-i-transient. The IP3 content in spex-treated zygotes increased, and the spex-induced Ca-i-t ransient occurred even in the absence of external Ca2+, Ca-i-transient was not observed when spex was injected into zygotes. These data sugg est that spex induces IICR in zygotes by activating certain cell surfa ce receptors coupled to G-proteins. (C) 1997 Academic Press.