EFFECTS OF GONADECTOMY AND STEROIDS ON PLASMA AND PITUITARY LEVELS OFSOMATOLACTIN IN ATLANTIC SALMON, SALMO-SALAR

Somatolactin (SL) is a hormone recently isolated and characterized fro m fish pituitaries. Although the functions of SL are still largely unk nown, it has been implicated in reproduction. In this study, the effec ts of gonadal steroids on SL secretion were investigated in Atlantic s almon male parr in two experiments. In both, mature males were gonadec tomized in the autumn and implanted with Silastic capsules containing testosterone (T), 11-ketoandrostenedione (11kA), or 17 alpha,20 beta-d ihydroxy-4-pregnene-3-one (20-P), gonadectomized alone, or sham-operat ed. In addition, immature males were implanted with T or 11kA in exper iment 1. After 4-5 weeks pituitaries and plasma were sampled and SL le vels measured by radioimmunoassay (RIA). Plasma levels of T, 11-ketote stosterone, and 20-P were also measured by RIA. In experiment 1, initi al immature males had lower (0.7 +/- 0.4 ng/ml) plasma SL levels than initial mature males (3.3 +/- 0.4 ng/ml), whereas pituitary content wa s not influenced. Gonadectomy significantly reduced both plasma SL lev els (experiment 1, sham controls 5.6 +/- 0.5 ng/ml, castrated 1.6 +/- 0.5 ng/ml; experiment 2, sham controls 6.5 +/- 1.1 ng/ml, castrated 3. 3 +/- 0.4 ng/ml) and the pituitary content of SL (experiment I, sham c ontrols 1206 +/- 187 ng/pituitary, castrated 663 +/- 104 ng/pituitary; experiment 2, sham controls 1043 +/- 199 ng/pituitary, castrated 629 +/- 70 ng/pituitary), suggesting that the testes stimulated the synthe sis and release of pituitary SL. Overall, the effects of steroid repla cement were inconsistent between the experiments, although in experime nt 2 castrated males receiving the highest dose of T had significantly higher plasma SL levels (8.2 +/- 1.2 ng/ml) than all other castrated groups (1.8-4.3 ng/ml). (C) 1998 Academic Press.