Insulin regulates lipid and glucose metabolism similarly in two lines of rainbow trout divergently selected for muscle fat content.

Two experimental rainbow trout lines were developed through divergent selection for low (Lean 'L' line) or high (Fat 'F' line) muscle fat content. Previous nutritional studies suggested that these lines differed in their regulation of lipid and glucose metabolism. Since insulin acts as an anabolic hormone by regulating lipid and glucose metabolism, we put forward the hypothesis that F line might have a stronger sensitivity to insulin than L line. In order to test this hypothesis, bovine insulin was injected into rainbow trout of the two lines fasted for 48 h. As expected, insulin induced hypoglycemia and activated Akt-TOR signaling both in the liver and muscle of the two lines. We demonstrate that this was coupled with increased expression of insulin dependent glucose transporter (GLUT4) and transcription factors of fatty acid anabolism (LXR and SREBP1c) in the muscle and liver, respectively, and lower mRNA levels of fatty acid oxidation enzymes (CPT1a, CPT1b and HOAD) in the white muscle of both lines. Regarding the genotype effect, TOR signaling response to insulin was stronger in F line as reflected by the higher phosphorylation of S6 protein and elevated mRNA levels of lipogenic enzyme (FAS) in the liver of F line. This observation was concordant with the higher plasma concentrations of free fatty acids and triglycerides in F line. Moreover, mRNA levels of hepatic gluconeogenic enzymes (G6Pase2, FBPase and PEPCK) and muscle fatty acid oxidation enzymes (CPT1a, CPT1b, HOAD and ACO) were higher in the F line. However, very few insulin-genotype interactions were detected, indicating that insulin induced similar changes in lipid and glucose metabolism in both lines.