Glucose and lipid metabolic adaptations during postprandial starvation of Japanese flounder Paralichthys olivaceus previously fed different levels of dietary carbohydrates

Abstract

Exogenous glucose loading is generally used to assess the capacity of glucose utilization in carnivorous fish. Examining the use of endogenous glucose during fasting provides a new perspective on assessing glucose metabolism in fish. Japanese flounder Paralichthys olivaceus, a marine carnivorous fish species, were starved for 3, 9, 24, 48 h, and 3, 7, 14, 21, 28 days, respectively, after a 10-week feeding trial with 12% (C12: 49.49% crude protein, 9.65% crude lipid and gross energy of 18.98 kJ g−1) and 20% (C20: 49.99% crude protein, 9.75% crude lipid and gross energy of 19.11 kJ g−1) of dietary carbohydrates (alpha-starch and corn starch). After feeding trial, fish fed different diets showed similar specific growth rate (3.62 vs 3.61) (P > .05). During starvation period, compared with 3 h, plasma glucose and insulin concentration increased at 9 h in both groups. A similar trend was found for the glycogen content and the mRNA levels of genes involved in glycolysis and pentose phosphate pathway in liver. And the gluconeogenic genes mRNA levels significantly increased since 9 h in C12 and 24 h in C20 (P < .05). Subsequently, liver glycogen in C12 fell significantly on the 3 d compared with 48 h and was depleted on the 7 d (P < .05). It significantly decreased on the 7 d compared with 3 d and was minimized on the 14 d in C20 (P < .05). Plasma glucose peaked at 48 h and 7 d in both groups. As for lipid metabolism, high levels of plasma triglyceride, cholesterol concentration and lipogenetic genes mRNA levels were observed within 9 h. Compared with 3 h, liver lipid significantly decreased on the 14 d in both groups (P < .05). The mRNA levels of genes related to beta-oxidation and ketogenesis was enhanced in later stages in both groups. In addition, in C20, greater changes of mRNA levels of genes involved in gluconeogenesis and lipolysis in liver were observed. It was concluded that primary substrate of energy sources switched from glycogen to lipid occurring on the 3 d-7 d in C12 and 7 d-14 d in C20. Besides, high dietary carbohydrate (C20) feeding activated greater gluconeogenesis and lipolysis during starvation. These observations demonstrated glucose and lipid metabolic responses of Japanese flounder during starvation were influenced by previous feeding with different levels of dietary carbohydrates.