The effects of recombinant bovine somatotropin on growth-related genes in rainbow trout (Oncorhynchus mykiss)

The mechanisms regulating fish muscle growth are poorly understood. Therefore, an experiment was conducted to examine the growth-promoting effects of rbST in rainbow trout. The main objectives of the study were to evaluate effects of rbST treatment on (1) circulating levels of GH, rbST, and IGF-I, (2) tissue GH, IGF-I, and IGF-IrA mRNA levels, and (3) muscle myosin and MSTN gene expression in rainbow trout. In study I, as expected, rbST treatment increased (P < 0.05) circulating rbST levels compared to controls. Overall rbST increased (P < 0.01) circulating IGF-I while not affecting (P = 0.28) circulating endogenous GH. These results suggest that the endogenous negative control feedback loop described in mammals is not activated by rbST in rainbow trout, as rtST was unaffected by increased circulating rbST and IGF-I. In study II, IGF-I mRNA levels increased in liver, gill, gonad, muscle, brain, and intestine in response to rbST treatment (P < 0.10), while heart IGF-IrA mRNA levels decreased (P = 0.02). Levels of GH mRNA in gonad tissue were increased following rbST treatment, but there was no evidence that rbST treatment affected IGF-IrA or GH expression in any of the other tissues evaluated. Since the primary source for endocrine IGF-I is liver, the increased IGF-I mRNA reported in extrahepatic tissues may indicate local paracrine/autocrine actions for IGF-I. Increased gonad GH and IGF-I mRNA levels after rbST treatment also supports a link between the GH-IGF axis and reproduction. In study III, myosin mRNA levels were unaffected (P = 0.57), myosin heavy chain protein concentrations increased (P = 0.06), MSTN-I mRNA levels were elevated (P = 0.09), and MSTN-II mRNA levels decreased (P = 0.05) after rbST treatment compared to control fish. These results suggest that rbST might have a direct action on muscle tissue because the most abundant muscle protein, myosin, increased after rbST treatment. These results also suggest that MSTN-I and -II might function differently from each other, because MSTN-decreased while MSTN-II increased following rbST treatment. Overall, the results presented here indicate that the GH-IGF axis in teleosts possesses both similarities and differences in regulation and function to mammals.