An investigation of the effects of nutritional status on the action of insulin in rainbow trout (Salmo gairdneri)

Graduation date: 1982Three studies were conducted to evaluate the relationship between\ud insulin and nutritional parameters in rainbow trout. In the first study\ud effects on growth parameters and tissue composition of rainbow trout were\ud investigated following injection of bovine insulin at two dose levels\ud every 48 hr for 56 days. In addition, [¹⁴C]-leucine incorporation into\ud plasma, liver, and skeletal muscle was studied for the two insulin treatments\ud and a group of the saline-injected controls given a single shock-dose\ud of insulin (5.0 IU/kg). Hypoglycemic responses were observed with\ud all insulin treatments. In comparison to controls, high insulin treatment\ud gave a significant body weight increase. At both levels, insulin increased the content of protein, lipid, and also the incorporation of [¹⁴C]-leucine activity in skeletal muscle. Simultaneous decreases in specific\ud activity of plasma and liver tissue indicated a net movement of [¹⁴C]-leucine toward the peripheral musculature. No effect of the hormone was\ud seen on the glycogen content of liver or muscle tissue over the 56-day\ud period.\ud In a second study, the effect of bovine insulin on tissue incorporation of [¹⁴C]-glucose and [³H]-leucine was investigated in fed and fasted\ud rainbow trout reared on a control and high-protein diet. Insulin produced\ud marked hypoglycemia and mobilization of liver glycogen in all treatments.\ud Although insulin gave no evidence of glycogenic stimulation it\ud did appear to promote oxidative clearance of [¹⁴C]-glucose. Compared to\ud [¹⁴C]-glucose much greater tissue incorporation of [³H]-leucine was\ud observed in fasted fish; insulin stimulated the incorporation of [³H]-leucine into skeletal muscle protein. In plasma, liver, and skeletal\ud muscle of all treatments, the summed specific activities of [³H]-leucine\ud was considerably greater than that of the summed values of [¹⁴C]-glucose\ud following insulin administration. Four weeks of fasting apparently\ud lowered basal metabolism but no changes were observed in plasma glucose\ud and glycogen stores. There was some evidence of gluconeogenic activity\ud in the high protein-fasted fish and the data indicated in all fasted\ud treatments a stimulation of [¹⁴C]-glucose and [³H]-leucine metabolism\ud following insulin administration.\ud As a third investigation, [¹²⁵I]-iodoinsulin binding studies in\ud the presence of a concentration range of bovine insulin were conducted\ud to establish specific insulin binding levels in erythrocytes, skeletal\ud muscle plasma membranes and isolated hepatocytes of rainbow trout\ud reared on control-, high-protein and high-carbohydrate diets. Negative\ud cooperativity was observed and receptor concentrations and apparent\ud dissociation constants established for each preparation. No differences\ud of specific binding attributed to diet were detected in either erythrocytes\ud or skeletal muscle plasma membrane preparations, However, the\ud receptor concentration of isolated hepatocytes from high-carbohydrate\ud reared trout was increased. This contrasted to comparable mammalian\ud studies. In view of an apparent depression of receptor concentration in\ud skeletal muscle plasma membranes and isolated hepatocytes of high-protein\ud reared trout, these data were interpreted according to the\ud reciprocal relationship observed between endogenous insulin and insulin\ud receptor concentrations established for mammals. Unlike mammals where\ud glucose was shown to be the primary insulin secretagogue, endogenous\ud insulin levels in rainbow trout have been closely correlated to circulatory\ud amino acids and are thought to be primarily controlled by these\ud protein metabolites