Low protein intake is associated with reduced hepatic gluconeogenic enzyme expression in rainbow trout (Oncorhynchus mykiss)

Abstract

Our objective was to understand the reasons behind the persistent postprandial hyperglycemia in rainbow trout (Oncorhynchus mykiss). We hypothesized that in this species, high levels of dietary protein could increase the hepatic production of glucose, irrespective of the dietary carbohydrate supply. We fed juvenile rainbow trout four diets containing graded levels of protein for 14 d. Pair-feeding was employed to keep lipid and carbohydrate intakes constant. Six hours after feeding, as postulated, activities and mRNA levels of gluconeogenic enzymes (glucose-6-phosphatase, fructose-1,6-bisphosphatase) increased with increasing dietary protein (P < 0.05). However, in fish with a very low protein intake, there was a very strong increase in plasma glucose (18 mmol/L) that was also associated with a high capacity to store excess glucose as indicated by altered pyruvate kinase activity, glucokinase activity, and hepatic glycogen and fat concentrations (P < 0.05). In conclusion, at the same level of carbohydrate intake, a low dietary protein intake was associated with an unexplained increase in glycemia, which was probably responsible for the decrease in hepatic gluconeogenic enzyme expression. The effect of dietary protein on low carbohydrate utilization in this species remains unclear.