The uninhibited glucagon signaling pathway in Japanese flounder Paralichthys olivaceus: A link with the glucose intolerance?

The study aimed to explore the role of glucagon in regulating glucose metabolism and gain insight into the reasons for glucose intolerance in carnivorous fish. Firstly, Japanese flounder was injected intraperitoneally with glucagon and small interfering RNA (siRNA) targeted glucagon receptor a (GCGRa), cAMP-response element binding protein 1 (CREB1), and peroxisome proliferator-activated receptor-gamma-coactivator 1 alpha (PGC-1 alpha). Secondly, a two-week feeding trial was conducted with Japanese flounder fed diets with 11.95% and 31.96% dietary carbohydrates, respectively. At the start and 7th day of the feeding trial, flounders fed each diet were intraperitoneally injected with saline or siGCGRa. Furthermore, the fish were separated into two groups and fasted for 72 h. Saline and siGCGRa were injected respectively at 6 h after feeding. The results showed that glucagon activated gluconeogenesis in the liver, leading to an elevation of plasma glucose levels, while interference of GCGRa, CREB1, and PGC-1 alpha caused the opposite results. Elevated dietary carbohydrate intake increased plasma glucose and insulin levels. Meanwhile, the glucagon signaling pathway was activated. After interference with the GCGRa, plasma glucose decreased, and the glucagon pathway was inhibited. The plasma insulin increased, and the mRNA expression of protein kinase B 8 (akt2) was up-regulated accordingly. In addition, upon injecting siGCGRa, it was observed that gluconeogenesis in the liver and plasma glucose levels during fasting were reduced in comparison to the control group. In contrast, plasma insulin levels and the transcription levels of akt2 and glucokinase (gck) in the liver exhibited an increase. In summary, the uninhibited glucagon signaling pathway promoted gluconeogenesis and suppressed the action of insulin in Japanese flounder, which might be the reason for the glucose intolerance.