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Isthmin‐1 (Ism1) plays roles in glucose uptake in mammals as an adipokine. To investigate its role in the glucose metabolism of common carp (Cyprinus carpio. L), the Ism1 sequence was cloned, and its expression and distribution in tissues were detected. In addition, we prepared and purified the recombinant Ism1 protein using the E. coli expression system and assessed changes in the expression of key genes related to glucose metabolism through both in vivo injection experiments and primary hepatocyte experiments in vitro. The results revealed that the open reading frame of Ism1 was 1377 bp long, encoding 458 amino acids. Similarity analysis indicated that Ism1 exhibited a close evolutionary relationship with goldfish (Carassius auratus), sharing 98.35% amino acid similarity. Ism1 was expressed in all tissues of common carp, with the highest level observed in the heart, followed by the gill, head kidney, and hepatopancreas. Distinct patterns of Ism1 expression were identified during the oral glucose tolerance test and long‐term high‐carbohydrate and high‐fat diet feeding experiments. In vivo studies demonstrated that the serum glucose concentration was reduced on treatment with Ism1, accompanied by a significant upregulation of mRNA levels for gk, hk, and pfk genes in hepatopancreas; conversely pepck and g6pase mRNA levels were significantly downregulated in the hepatopancreas under these conditions as well. Furthermore, our primary hepatocyte experiment confirmed that Ism1 could inhibit pepck and g6pase mRNA expression, while promoting gk, hk, and pfk mRNA expression levels. In conclusion, Ism1, in common carp, could participate in the glucose metabolism, which provides essential information for future studies on the function of Ism1. [ABSTRACT FROM AUTHOR]

It is widely acknowledged that glucose serves as the primary energy source for organisms. However, fish exhibit persistent postprandial hyperglycemia and are thought to have low glucose tolerance. Glycolysis serves as the ubiquitous pathway for glucose catabolism, with phosphofructokinase (PFK) acting as a crucial rate-limiting enzyme in this process and playing an indispensable role. PFK is classified into three isoforms based on their major expression sites, i.e., PFKM (skeletal muscle type), PFKL (liver type) and PFKP (platelet type). In this study, grass carp (Ctenopharyngodon idella) was used as animal model and the open reading frame (ORF) sequences of six PFK genetic isoforms of grass carp were cloned. Real-time PCR was used to detect its tissue distribution, and expression changes in oral glucose tolerance test (OGTT), insulin and glucagon injection experiments. The results showed that the ORF of pfkla , pfklb , pfkma , pfkmb , pfkpa and pfkpb genes was 2343, 2340, 2355, 2331, 2364 and 2349 bp in length, respectively. The results of tissue distribution showed that pfkla and pfklb , homologous to mammalian pfkl , exhibited low expression levels in the liver of grass carp, but were expressed at the highest level in the brain. Muscle-type pfkma and pfkmb mRNA were found to be highly expressed in both red and white muscle, with pfkmb also exhibiting high expression levels in the heart, while platelet type pfkpa and pfkpb showed high mRNA abundances in the brain and heart. Oral glucose administration stimulated pfkma and pfkmb mRNA expression in the red muscle, and up-regulated pfklb mRNA levels in the liver at 3 h post treatment, but it suppressed liver-type and platelet-type PFK genes expression in the brain. The expression of pfkmb in white muscle and pfkmb and pfkpb in heart were promoted by insulin, whereas the expression of pfkla and pfkpb in the brain, pfkma and pfkmb in the red muscle, pfkma in the white muscle, and pfklb in the liver was suppressed by insulin. As for glucagon, it inhibited pfkma and pfkmb mRNA expression in the red muscle, as well as pfklb in the liver, but it up-regulated PFK genes expression in most tissues detected, such as brain (pfklb , pfkpa and pfkpb), white muscle (pfkma and pfkmb), liver (pfkla) and heart (pfkmb and pfkpb). Our results suggest that PFK family genes have different or even opposite expression patterns in response to glucose, insulin and glucagon stimulation in various tissues of grass carp, which may contribute to glucose intolerance in fish. [Display omitted] • Brain, heart, red and white muscle are the main tissues for PFK expression. • Glucose promoted PFK expression in muscle (M-type), liver (pfklb), heart (pfkpb). • Insulin up-regulated expression of pfkmb in white muscle, pfkmb and pfkpb in heart. • Glucagon up-regulated PFK expression in most tissues, except for red muscle. • PFK family genes have different or even opposite expression patterns. [ABSTRACT FROM AUTHOR]