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Both in nature and in aquaculture, fish may experience periods of food scarcity or hunger. The metabolic regulation of fish when nutritional state changes is a complex process that involves many factors. To study glucose metabolism adaptability during fasting and re-feeding in the black carp (Mylopharyngodon piceus), we measured changes in some biochemical indicators related to glucose metabolism. Five fish were sampled on days 0, 1, 3, 5, and 10 of fasting (F, S1, S3, S5, and S10, respectively) and days 1, 3, and 5 of re-feeding (RF1, RF3, and RF5, respectively). The serum glucose concentration decreased significantly at S1, reached the lowest point at S10, and increased significantly at RF1 (P < 0.05). The concentration of liver glycogen decreased significantly at S1 and reached the lowest level at S3, whereas the muscle glycogen level decreased significantly at S5 and reached the lowest value at S10 (P < 0.05). Both liver and muscle glycogen levels returned to the pre-fasting level at RF5 (P < 0.05). Regarding glycolysis, the concentrations of pyruvate kinase (PK) and hexokinase (HK) decreased significantly at S5 and increased significantly at RF5 and RF1, respectively (P < 0.05). The concentrations of glucokinase (GCK) and insulin decreased significantly at S1 and increased significantly at RF1 and RF3, respectively (P < 0.05). The mRNA expression levels of liver GCK and glucose transporter 2 (GLUT2) decreased significantly at S1 and increased significantly at RF1 and RF5, respectively (P < 0.05). As for gluconeogenesis, the concentration of glucose-6-phosphatase (G6PC) increased significantly at S1 and decreased significantly at RF1 (P < 0.05). The concentrations of glucagon and glucocorticoid (GC) increased significantly at S3 and significantly decreased at RF1 and RF5, respectively (P < 0.05). The mRNA expression levels of liver G6PC and phosphoenolpyruvate carboxykinase (PEPCK) increased significantly at S3 and S1, and both decreased significantly at RF1 (P < 0.05). These results indicate that coordination between glycolysis and gluconeogenesis might be crucial for glucose homeostasis during fasting and re-feeding in the black carp. [ABSTRACT FROM AUTHOR]

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The mammalian target of rapamycin (mTOR) has been shown to play a central role in regulating cell growth and metabolism. However, little is known about the function of mTOR in nutrient metabolism in bivalve mollusks. In this study, the role of mTOR in the regulation of nutrient metabolism was investigated in Sinonovacula constricta. First, the activation of mTOR was assayed after starvation and refeeding. Afterwards, the role of mTOR in the regulation of nutrient metabolism was investigated using an activator (MHY1485) or inhibitor (rapamycin) of mTOR. The open reading frame of the S. constricta mTOR is 7416 bp in length and encodes a polypeptide consisting of 2471 amino acids. The mTOR amino acid sequence of S. constricta was highly conserved when compared with other species and had a close evolutionary relationship with the TOR proteins of Crassostrea gigas and Lingula anatine. mTOR was expressed in the intestine, exhalent siphon, labial palppus, muscle, inhalent siphon, gill, mantle, digestive land, and gonad tissue of S. constricta, with the highest expression in muscle. During starvation, the level of phosphorylated mTOR protein was relatively low, and the ratio of LC3II/LC3I protein and the AMPKα mRNA level significantly increased with the increase in starvation time. After feeding, the level of phosphorylated mTOR protein increased from 0.13 to 0.56, and the ratio of LC3II/I protein and AMPKα mRNA level decreased from 1.17 to 0.38. MHY1485 significantly increased the level of phosphorylated 4E-BP1 and significantly decreased the ratio of LC3II/I proteins. Furthermore, MHY1485 significantly increased the mRNA level of the glucose metabolism-related gene glucokinase (GK), significantly decreased the mRNA expression of the G6P gene, and significantly increased the mRNA expression of the lipid synthesis-related genes sterol-regulatory element-binding protein (SREBP) and stearoyl-CoA desaturase (SCD). Rapamycin significantly reduced the level of phosphorylated 4E-BP1 and the mRNA expression of mTOR, and the expression level of phosphorylated 4EBP1 decreased from 0.97 to 0.28. Meanwhile, it also significantly reduced the mRNA expression of glucose metabolism-related genes GK, pyruvate kinase (PK), glucose transporter 1 (GLUT1), and G6P, as well as lipid synthesis-related genes SCD and acetyl-CoA carboxylase (ACC). These results indicate a conserved role of mTOR in regulating nutritional metabolism, including glucose metabolism, lipid synthesis, and autophagy in S. constricta. [ABSTRACT FROM AUTHOR]

An 8-week feeding trial was conducted to evaluate the application of common carbohydrate sources, cornstarch (CS), wheat starch (WS), and wheat flour (WF), to different gibel carp genotypes, Dongting, CASIII, and CASV. The results of the growth and physical responses were analysed by data visualization and unsupervised machine learning. As revealed by a self-organizing map (SOM) and the cluster of growth and biochemical indicators, CASV had superior growth and feed utilization and better regulation of postprandial glucose, followed by CASIII, while Dongting showed a high level of plasma glucose with poor growth performance. CS, WS, and WF were differently utilized by the gibel carp, and WF was associated with greater zootechnical performance based on higher specific growth rate (SGR), feed efficiency (FE), protein retention efficiency (PRE), and lipid retention efficiency (LRE), induced hepatic lipogenesis, increased liver lipids, and enhanced muscle glycogen. Spearman's correlation analysis of the physiological responses indicated that plasma glucose had a significantly negative correlation with growth, feed utilization, glycogen storage, and plasma cholesterol level, and it was positively related to liver fat content in gibel carp. Transcriptional variabilities were observed: CASIII showed increased expression of pklr, which is involved in hepatic glycolysis, and pck and g6p, which are involved in gluconeogenesis. Interestingly, Dongting showed upregulation of genes involved in glycolysis and fatty acid oxidation in muscle. Furthermore, there were numerous interactions between carbohydrate sources and strains for growth, metabolites, and transcriptional control, confirming the existence of genetic polymorphisms in carbohydrate use in gibel carp. Globally, CASV showed relatively better growth and carbohydrate utilization, and wheat flour seemed to be more efficiently utilized by gibel carp. [ABSTRACT FROM AUTHOR]

Glucose transporter 4 (GLUT4) is comprehensively investigated in mammals, while the comparative research of GLUT4 in common carp is deficient. To investigate the function of GLUT4, carp glut4 was first isolated. The open reading frame of carp glut4 was 1518 bp in length, encoding 505 amino acids. A high‐sequence homology was identified in carp and teleost, and the phylogenetic tree displayed that the carp GLUT4 was clustered with the teleost. A high level of glut4 mRNA was analysed in fat, red muscle and white muscle. After fasting treatment, glut4 mRNA expression was increased significantly in muscle. In the oral glucose tolerance test experiment, glut4 mRNA was also significantly elevated in muscle, gut and fat. Furthermore, intraperitoneal injection of insulin resulted in the upregulation of glut4 gene expression significantly in white muscle, gut and fat. On the contrary, the glut4 mRNA level in the white muscle, gut and fat was markedly downregulated after glucagon injection. These results suggest that GLUT4 might play important roles in food intake and could be regulated by nutrient condition, insulin and glucagon in common carp. Our study is the first to report on GLUT4 in common carp. These data provide a basis for further study on fish GLUT4. [ABSTRACT FROM AUTHOR]

This study was performed to evaluate the effects of dietary distinct protein to starch ratios on the growth, feed utilization, blood biochemistry, hepatic glucose metabolism and hepatocyte apoptosis of Nile tilapia Oreochromis niloticus. Three iso-lipidic (ca 6.56%) diets were formulated with elevated corn starch levels at the cost of protein provision, being designated as diets P40S5 (40.4% protein and 4.62% starch), P30S25 (30.4% protein and 24.2% starch) and P20S50 (19.9% protein and 48.6% starch). Tilapia juveniles (initial mean body weight, 32.0 g/fish) were assigned to 12 rectangular tanks (250 L each) with 20 fish per tank and fed the experimental diets to apparent satiation for 8 weeks. The results showed that either excess (40%) or insufficient (20%) amount of dietary protein impaired the growth of tilapia. Excess dietary protein led to unsatisfactory protein catabolism (lower protein efficiency ratio and protein productive value, and higher mRNA levels of hepatic glutamic-pyruvic transaminase 1 and glutamic-oxaloacetic transaminase paralogs), while insufficient amount of protein resulted in lower whole-body protein percentage. An increase in dietary starch from 5% or 25% to 50% was associated with enhanced lipid and glycogen storages in the liver of tilapia. Consistently, dietary starch reception up to 50% upregulated the expression of hepatic representative genes involved with glycolysis (glucokinase and phosphofructokinase muscle type a) and lipogenesis (acetyl-CoA carboxylase α and fatty acid synthase) while downregulated the expression of the gluconeogenic glucose-6-phosphatase catalytic subunit paralogs. TUNEL staining suggested that stronger apoptosis signal was recorded at the bile duct epithelium of the P20S50 fish. Concomitantly, plasma bile acid level nearly doubled, and an incidence (50%) of 'green liver' was observed in this treatment as compared with the P40S5 and P30S25 fish, indicating that bile acid metabolism disorder might be induced. Though plasma glucose (4.66-5.11 mmol/L) was not responsive to different treatments, advanced glycation end-products accumulated in the blood of the P20S50 fish, suggesting that 50% starch reception was probably associated with the progression of diabetes-associated pathologies. The overall results in this study suggested that tilapia might possess capability to tolerate dietary corn starch level up to 50% in a short-term exposure, whereas a long-term exposure might induce detrimental effects on liver functions. [ABSTRACT FROM AUTHOR]

It was previously shown that the influence of high‐energy diets on growth and flesh quality of grass carp was closely related to energy resource. The mechanisms were explored in this study by investigating the hepatic lipid and glucose metabolism of grass carp. Juvenile grass carp (Ctenopharyngodon idella) with initial body weight (55.0 ± 2.5) g were fed with four purified diets for 11 weeks, with four replicates per diet and 20 fish per replicate. Three high‐energy diets with high level of carbohydrate, fat and protein were prepared, and their gross energy was 11% higher than that of control diet. Results showed that the highest contents of hepatic crude fat, total saturated fatty acids, n‐6 unsaturated fatty acids and glycogen, as well as the highest grade of hepatocyte steatosis were observed in grass carp fed the high‐carbohydrate diet. Data on hepatic mRNA expression indicated that the high‐carbohydrate diet increased the genes expression including pk, gck, g6pd, fas and acc‐1, while the high‐fat diet significantly increased the genes expression including pk, gck, fbp, pck, lpl and hsl. The high‐fat and high‐carbohydrate diets activated hepatic AMPK/ACC pathway, whereas inhibited TOR/SREBP‐1 pathway. In conclusion, the high‐carbohydrate diet had a stronger effect on the hepatic lipid accumulation than the high‐fat diet in grass carp. The acceleration of lipolysis, glycolysis and gluconeogenesis in liver improved the utilization of the high‐fat diet in grass carp. Besides, the high‐carbohydrate diet can promote hepatic glycolysis and pentose phosphate shunt and inhibit gluconeogenesis in grass carp. [ABSTRACT FROM AUTHOR]