Hormonal action of relaxin-like gonad-stimulating substance (GSS) on starfish ovaries in growing and fully grown states

Gonad-stimulating substance (GSS) of starfish is the only known invertebrate peptide hormone responsible for final gamete maturation, rendering it functionally analogous to gonadotropins in vertebrates. Recently, we purified GSS from the radial nerves of the starfish Asterina pectinifera and identified the chemical structure as a relaxin-like peptide. This study examined the hormonal action of GSS on ovaries in the growing (stage IV) and fully grown states (stage V) of the starfish. The sensitivity of oocytes to 1-methyladenine (1-MeAde) as starfish maturation-inducing hormone was enhanced as oocytes enlarged in stage V. GSS-stimulated 1-MeAde production by ovarian follicle cells was also correlated with the size of oocytes. Although 1-MeAde production was observed in whole ovaries in stage V, GSS failed to induce 1-MeAde production in young ovaries (stage IV). This suggests that follicle cells in ovaries in a growing state (stage IV) are still unresponsive to the hormonal action of GSS. According to competitive experiments using radioiodinated and radioinert GSS, however, dissociation constant (K_d) values and the number of binding sites for GSS were mostly constant in the ovaries from stages IV to V. These results strongly suggest that GSS receptors are expressed in follicle cells of ovaries in the growing state. The failure of GSS to induce 1-MeAde production in young ovaries may be due to the uncoupling of signal transduction from the receptor to 1-MeAde biosynthesis in follicle cells.