LARSEN, D.A.*; COOPER, K.A.; SHIMIZU, M.; National Marine Fisheries Service, Seattle, WA.: In vivo effects of testosterone and 11-ketotestosterone on the GH-IGF-I axis (plasma GH, IGF-I, and putative IGFBP-3) in coho salmon, Oncorhynchus kisutch.

Studies in fish have demonstrated significant interactions among the endocrine factors regulating growth and reproduction including stimulatory and inhibitory effects of insulin-like growth factor-I and II (IGF-I and II) on gonadal steroidogenesis and stimulatory effects of gonadal steroids on growth hormone (GH) secretion. Plasma IGF-I levels have also been shown to increase markedly during final reproductive maturation in salmon. To further clarify these relationships we examined the in vivo effects of the androgens testosterone (T) and 11-ketotestosterone (11-KT) on plasma GH, IGF-I and salmon 41-kDa IGF binding protein (putative IGFBP-3) in coho salmon. Immature male and female, 2 year-old fish (avg. wt. 31.7�0.63 g) were injected with coco butter containing 0, 0.1, 0.25, or 1 mg/ml T or 11-KT and blood samples were taken after 1 and 2 weeks post injection and analyzed by radioimmunoassay for T, 11-KT, GH, IGF-I, and putative IGFBP-3. Steroid treatments elevated the plasma T and 11-KT levels to ranges that were physiologically relevant for maturing fish (0.9-35 ng/ml T, 0.3-73 ng/ml 11-KT). Plasma GH, IGF-I and putative IGFBP-3 all increased in response to both T and 11-KT in a dose dependent manner after 1 and 2 weeks post injection. The increases in IGF-I and putative IGFBP-3 were highly significant, but those for plasma GH were not significant due to high variation in plasma levels among individual fish. These data suggest that during reproductive maturation, in addition to the previously demonstrated effects of the IGF�s on steroidogenesis, the gonadal steroids may in turn play a major role in regulating IGF�s and their binding proteins.