Hepatic glucose metabolic responses to digestible dietary carbohydrates in two isogenic lines of rainbow trout

Abstract

Rainbow trout (Oncorhynchus mykiss) was recognized as typical “glucose-intolerant” fish and poor dietary carbohydrate user. Our first objective was to test the effect of dietary carbohydrates itself (without modification of dietary protein intake) on hepatic glucose gene expression (taking into account of the paralogs. The second aim was to research if two isogenic trout lines had different responds to carbohydrate intake, showing one with a better use dietary carbohydrates. Thus, we used two isogenic lines of rainbow trout (named A32h and AB1h) fed with either high carbohydrate diet or low carbohydrate diet for 12 weeks. We analysed the zootechnical parameters, the plasma metabolites, the hepatic glucose metabolism at a molecular level and the hormonal-nutrient sensing pathway. Globally dietary carbohydrate intake was associated with hyperglycemia and down regulation of the energy sensor Ampk, but also with atypical regulation of glycolysis and gluconeogenesis in liver. Indeed, the first steps of glycolysis and gluconeogenesis catalysed by the glucokinase and the phospenolpyruvate carboxykinase are regulated at the molecular level by dietary carbohydrates as expected (i.e. induction of the glycolytic gck and repression of the gluconeogenic pck); by contrast, and surprisingly, for two other key glycolytic enzymes (phosphofructokinase enzyme – pfkl – and pyruvate kinase – pk -) some of the paralogs (pfklb and pklr) are inhibited by carbohydrates whereas some of the genes coding gluconeogenic enzymes (the glucose-6-phosphatase enzyme –g6pcb1b and g6pcb2a gene and the fructose1-6 biphosphatase paralog fbp1a) are induced. On the other way, some differences for the zootechnical parameters and metabolic genes were found also between the two isogenic lines, confirming the existence of genetic polymorphisms for nutritional regulation of intermediary metabolism in rainbow trout. In conclusion, our study determines some new unexpected molecular regulation of the glucose metabolism in rainbow trout which may partly led to the poor utilization of dietary carbohydrates and underlines the existence of differences in molecular regulation of glucose metabolism between two isogenic lines provide arguments for future selection of rainbow trout.