1. The effects of diet composition and insulin treatment on blood glucose level, blood amino acid concentrations, the activities of two hepatic gluconeogenic enzymes (fructose diphosphatase, EC 22.214.171.124; phosphoenolpyruvate carboxykinase, EC 126.96.36.199) and of two hepatic glycolytic enzymes (hexokinase, EC 188.8.131.52; pyruvate kinase, EC 184.108.40.206) were examined in rainbow trout (Salmo gairdneri).
2. Blood glucose levels were much higher in trout given a high-carbohydrate (HC) diet than in those given a high-protein (HP) diet. Insulin reduced blood glucose concentration in the HC-fed fish but had no effect in HP-fed fish.
3. Plasma amino acid concentrations were higher in HP-fed trout than in HC-fed trout. Insulin reduced plasma amino acid levels in both groups.
4. Gluconeogenic enzyme activities were higher in HP-fed trout than in HC-fed trout. Insulin reduced phosphoenolpyruvate carboxykinase activity in HP-fed trout but had no effect on either enzyme activity in the HC-fed trout.
5. Pyruvate kinase activity was greater in HC-fed trout than in HP-fed trout. Insulin did not affect pyruvate kinase activity in the HC-fed trout but reduced it in HP-fed trout.
6. Hexokinase activity was not affected by the diet treatments used, but was enhanced in the trout treated with insulin. Glucokinase (EC 220.127.116.11) was not detected in any of the trout livers.
7. The results suggest that the inability of trout to control blood glucose concentration is partly due to a lack of glucose-phosphorylating capacity. Gluconeogenesis is controlled in response to diet and insulin by changes in enzyme level and by variation in concentration of gluconeogenic substrates.