Responses of glycolysis, glycogen accumulation and glucose-induced lipogenesis in grass carp and Chinese longsnout catfish fed high-carbohydrate diet

Abstract This study investigated the different adaptive metabolic strategies of grass carp (Ctenopharyngodon idellus) and Chinese longsnout catfish (Leiocassis longirostris Gunther) to a high-carbohydrate diet (HCD). Two dietary carbohydrate levels were formulated for grass carp (normal-carbohydrate diet [NCD]: 30%; HCD: 50%) and Chinese longsnout catfish (NCD: 8%; HCD: 20%). Based on transcriptome data, the top twenty KEGG pathway enrichment analyses indicated that five pathways related to carbohydrate metabolism were enriched in grass carp that were fed an HCD, but were not in Chinese longsnout catfish. After 56 d, compared to the fish in the NCD group, grass carp fed an HCD exhibited significant increase in their whole-body lipid content and glucose-induced lipogenesis, including fatty acid synthase content and transcriptional levels of acetyl-CoA carboxylase, sterol regulatory element-binding protein-1, and carbohydrate response element-binding protein. Raised hepatic glycogen content was observed in both grass carp and Chinese longsnout catfish that were fed an HCD. The enhanced glycogen synthesis, including induced glycogen synthase activity and upregulation of mRNA levels of glycogen synthase and serine/threonine-protein phosphatase PP1-beta catalytic subunit, led to accumulation of glycogen in grass carp. However, reduced glycogenolysis with downregulation of the transcriptional levels of phosphorylase kinase regulatory subunit alpha and glycogen phosphorylase caused accumulation of glycogen in Chinese longsnout catfish. In addition, significantly increased intestinal amylase activity and induced glycolysis of glucokinase and pyruvate kinase were observed in grass carp that were fed an HCD. The results indicated that grass carp responded effectively to an HCD by induction of glycolysis, glycogen accumulation and glucose-induced lipogenesis, while Chinese longsnout catfish did not.