PLASMA-LEVELS OF EGGSHELL ZR-PROTEINS, ESTRADIOL-17-BETA, AND GONADOTROPINS DURING AN ANNUAL REPRODUCTIVE-CYCLE OF ATLANTIC SALMON (SALMO-SALAR)

Eggshell zona radiata proteins (zr-proteins) were found to occur normally in plasma of sexually mature female (but not male) Atlantic salmon (Salmon. salar). In order to ascertain the physiological relevance of these findings, we developed a specific enzyme-linked immunoassay for zr-proteins and screened plasma from sexually maturing Atlantic salmon females throughout an annual reproductive cycle. While zr-proteins were detectable at low levels in female adult salmon plasma prior to sexual maturation, zr-protein levels increased dramatically as sexual maturation proceeded. The strong correlation between gonado-somatic index (GSI) and plasma zr-proteins indicates a reproductive role for blood borne zr-proteins. During the vitellogenic phase, levels of plasma zr-proteins were positively correlated with GSI and with plasma levels of gonadotropin I (GtH I) and estradiol-17 beta, as determined by radioimmuno-assays. However, during final sexual maturation, only the plasma level of gonadotropin II (GtH II) was positively correlated to GSI. In contrast, zr-proteins and estradiol-17 beta were both negatively correlated to plasma level of GtH II during this period. In view of estradiol-17 beta-induced hepatic synthesis and secretion of zr-proteins (Oppen-Berntsen et al.: Journal of Endocrinology 135:293-302, 1992a) and the established role of gonadotropins in regulating ovarian estradiol synthesis, we interpret the observed correlations among plasma levels of GtH I, estradiol-17 beta and zr-proteins in Atlantic salmon to signify that GtH I regulates ovarian estradiol-17 beta synthesis, which in turn regulates hepatic synthesis and secretion of both vitellogenin and zr-proteins during oogenesis. (C) 1994 Wiley-Liss, Inc.