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 norma lly 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 salmo n females throughout an annual reproductive cycle. While zr-proteins w ere detectable at low levels in female adult salmon plasma prior to se xual maturation, zr-protein levels increased dramatically as sexual ma turation proceeded. The strong correlation between gonado-somatic inde x (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 o f gonadotropin I (GtH I) and estradiol-17 beta, as determined by radio immuno-assays. However, during final sexual maturation, only the plasm a level of gonadotropin II (GtH II) was positively correlated to GSI. In contrast, zr-proteins and estradiol-17 beta were both negatively co rrelated to plasma level of GtH II during this period. In view of estr adiol-17 beta-induced hepatic synthesis and secretion of zr-proteins ( Oppen-Berntsen et al.: Journal of Endocrinology 135:293-302, 1992a) an d the established role of gonadotropins in regulating ovarian estradio l synthesis, we interpret the observed correlations among plasma level s of GtH I, estradiol-17 beta and zr-proteins in Atlantic salmon to si gnify 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.