NORBECK, L.; KITTILSON, J.; SHERIDAN, M.A.*; North Dakota State University, Fargo.; North Dakota State University, Fargo.; North Dakota State University, Fargo.: Nutritional Regulation of the Somatotropic Axis of Rainbow Trout (Oncorhynchus mykiss).

Previously, we isolated and characterized the mRNAs that encode for major elements of the somatotropic axis of rainbow trout, including two forms of growth hormone receptor (GHR1 and GHR2), insulin-like growth factor-1 (IGF-1), and two forms of insulin-like growth factor-1 receptor (IGFR1A and IGFR1B). In this study, the regulation of the expression these somatotropic axis components was studied in rainbow trout placed on varying nutritional regimes (fed continuously, fasted continuously, fasted then refed). Fish that fasted for 6 weeks displayed significantly reduced growth compared to their fed counterparts, while refeeding for 2 weeks following 4 weeks of fasting partially restored growth. The expression of hepatic GHR1 (48%), GHR2 (44%), and IGF-1 (85%) mRNAs decreased in fasted fish compared to fed fish. Fasting also reduced the mRNA levels of GHR 1 (47%), GHR2 (67%), IGF-1 (81%), IGFR1A (68%), and IGFR1B (65%) in gill. The expression of IGFR1A (75%) and IGFR1B (110%) increased in cardiac muscle of fasted fish, but levels of these mRNAs were unchanged in white muscle. These results indicate that nutritional state regulates the expression of somatotropic axis components in a tissue-specific manner. (Support by NSF grant IOB 0444860)