Effects of gonadectomy and steroids on plasma and pituitary levels of somatolactin in Atlantic salmon, Salmo salar.

Somatolactin (SL) is a hormone recently isolated and characterized from fish pituitaries. Although the functions of SL are still largely unknown, it has been implicated in reproduction. In this study, the effects of gonadal steroids on SL secretion were investigated in Atlantic salmon male parr in two experiments. In both, mature males were gonadectomized in the autumn and implanted with Silastic capsules containing testosterone (T), 11-ketoandrostenedione (11kA), or 17 alpha,20 beta-dihydroxy-4-pregnene-3-one (20-P), gonadectomized alone, or sham-operated. In addition, immature males were implanted with T or 11kA in experiment 1. After 4-5 weeks pituitaries and plasma were sampled and SL levels measured by radioimmunoassay (RIA). Plasma levels of T, 11-ketotestosterone, and 20-P were also measured by RIA. In experiment 1, initial 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 was not influenced. Gonadectomy significantly reduced both plasma SL levels (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 1, sham controls 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 synthesis and release of pituitary SL. Overall, the effects of steroid replacement were inconsistent between the experiments, although in experiment 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).