Development of salmon GTH I and GTH II radioimmunoassays

Abstract

Radioimmunoassays (RIAs) for the measurement of two gonadotropins, GTH I and GTH II, in the plasma and pituitary of salmonid fish were developed using a rabbit antiserum to β-subunits of chum salmon GTH I and GTH II. Intact GTH I and GTH II were used as standards and radioactive competitors. The displacement curves for plasma of salmonids including chum salmon, amago salmon, and rainbow trout were parallel to chum salmon GTH I and GTH II standards. Parallel displacement curves were obtained for pituitary extracts of chum salmon and amago salmon. The cross-reactivities of growth hormone, prolactin, and proopiomelanocortin (POMC)-related hormones were less than 1% in both RIAs. However, cross-reactivities of GTH I in the GTH II RIA and GTH II in the GTH I RIA were 10 and 12%, respectively. Plasma concentrations of both GTHs from salmonids at various stages of reproductive development were compared. In immature rainbow trout of both sexes (males: average (AV) gonadosomatic index (GSI) = 0.05; females: AV GSI = 0.24), plasma levels of GTH I and GTH II were low (less than 2 ng/ml). During prematurational stages of spermatogenesis and vitellogenesis in rainbow trout (males: AV GSI = 0.43; females: AV GSI = 2.8), the predominant GTH in the pituitary and plasma was GTH I. In contrast, plasma concentrations of GTH II were significantly higher than those of GTH I in postovulatory amago and chum salmon. Similarly, pituitary concentrations of GTH II were significantly higher than those of GTH I in postovulatory and spermiating amago salmon and postovulatory chum salmon. These data suggest that the synthesis and secretion of GTH I and GTH II vary during salmonid reproductive development.