Insulin-receptor binding in skeletal muscle of trout

Regulation of omega-3 fatty acids production by different genes in freshwater fish species: a review

The present study aims to compare the gene expression of three different fish species (common carp, tilapia, and trout) with varying levels of fatty acids (FA). Based on transcriptome analysis and RNA sequencing, various genes and their associated metabolic pathways are identified. Pathways are categorized based on the genes they encode. Genes that were differentially expressed and their promoter's methylation patterns were revealed by RNA-seq analysis in common carp. Furthermore, fatty acid-enriched pathways, such as ARA4 and adipocytokine signaling, were also identified. Many genes and pathways may influence tilapia's growth and omega-3 content. Using the mTOR pathway, trout with differential expression were discovered to be involved in producing omega-3 fatty acids. This study revealed major pathways in fish species to produce omega-3 fatty acids.

The mammalian insulin antagonist S961 does not exhibit insulin receptor antagonism in rainbow trout in vivo

Due to their reported 'glucose-intolerant' phenotype, rainbow trout have been the focus of comparative studies probing underlying endocrine mechanisms at the organismal, tissue and molecular level. A particular focus has been placed on the investigation of the comparative role of insulin, an important glucoregulatory hormone, and its interaction with macronutrients. A limiting factor in the comparative investigation of insulin is the current lack of reliable assays to quantify circulating mature and thus bioactive insulin. To circumvent this limitation, tissue-specific responsiveness to postprandial or exogenous insulin has been quantified at the level of post-translational modifications of cell signalling proteins. These studies revealed that the insulin responsiveness of these proteins and their post-translational modifications are evolutionarily highly conserved and thus provide useful and quantifiable proxy indices to investigate insulin function in rainbow trout. While the involvement of specific branches of the intracellular insulin signalling pathway (e.g., mTor) in rainbow trout glucoregulation have been successfully probed through pharmacological approaches, it would be useful to have a functionally validated insulin receptor antagonist to characterize the glucoregulatory role of the insulin receptor pathway in its entirety for this species. Here, we report two separate in vivo experiments to test the ability of the mammalian insulin receptor antagonist, S961, to efficiently block insulin signalling in liver and muscle in response to endogenously released insulin and to exogenously infused bovine insulin. We found that, irrespective of the experimental treatment or dose, activation of the insulin pathway in liver and muscle was not inhibited by S961, showing that its antagonistic effect does not extend to rainbow trout.

Different roles of insulin receptor a and b in maintaining blood glucose homeostasis in zebrafish

An inability of insulin to signal glycolysis and gluconeogenesis would largely result in type 2 diabetes. In this study, the physiological roles of zebrafish insulin receptor a and b in maintaining blood glucose homeostasis were characterized. We observed that, though blood glucose in insra-/- fish and insrb-/- fish were comparable with the control siblings at 0 h postprandium (hpp), the most evident hyperglycemia have been observed in insra-/- fish from 1 hpp to 3 hpp. A mild increase of blood glucose in insrb-/- fish has been seen only at 1.5 hpp. The down-regulated expressions of glycolytic enzymes were observed in insra-/- fish and insrb-/- fish liver and muscle, together with the significantly decreased activities or concentrations of glycolytic enzymes. These results suggest that both Insra and Insrb were critical in glycolysis. Intriguingly, the up-regulated expressions of gluconeogenic enzymes, pck1 and g6pca.1, along with the elevated enzyme activities, were observed in insra-/- fish liver at 1 hpp and 1.5 hpp. Compared with the control fish, the elevated plasma insulin and lowered phosphorylated AKT were observed in insra-/- fish and insrb-/- fish, suggesting that there is an insulin resistance in insra-/- fish and insrb-/- fish. The increased levels of both transcriptions of foxo1a and Foxo1a protein abundance in the insra-/- fish liver have been found. When insra-/- fish treated with the Foxo1 inhibitor, the postprandial blood glucose levels could be normalized, accompanied with the normalized expression levels and enzyme activities of both pck1 and g6pca.1. Therefore, Insra and Insrb demonstrate a similar role in promoting glycolysis, but Insra is involved in inhibiting gluconeogenesis via down-regulating the expression of foxo1a. Our results indicate that Insra and Insrb exhibit diversified functions in maintaining glucose homeostasis in zebrafish.

Effects of insulin and its related signaling pathways on lipid metabolism in the yellow catfish Pelteobagrus fulvidraco

The influence of insulin on hepatic metabolism in fish is not well understood. The present study was therefore conducted to investigate the effects of insulin on lipid metabolism, and the related signaling pathways, in the yellow catfish Pelteobagrus fulvidraco. Hepatic lipid and intracellular triglyceride (TG) content, the activity and expression levels of several enzymes and the mRNA expression of transcription factors (PPARα and PPARγ) involved in lipid metabolism were determined. Troglitazone, GW6471, fenofibrate and wortmannin were used to explore the signaling pathways by which insulin influences lipid metabolism. Insulin tended to increase hepatic lipid accumulation, the activity of lipogenic enzymes (6PGD, G6PD, ME, ICDH and FAS) and mRNA levels of FAS, G6PD, 6PGD, CPT IA and PPARγ, but down-regulated PPARα mRNA level. The insulin-induced effect could be stimulated by the specific PPARγ activator troglitazone or reversed by the PI3 kinase/Akt inhibitor wortmannin, demonstrating that signaling pathways of PPARγ and PI3 kinase/Akt were involved in the insulin-induced alteration of lipid metabolism. The specific PPARα pathway activator fenofibrate reduced insulin-induced TG accumulation, down-regulated the mRNA levels of FAS, G6PD and 6PGD, and up-regulated mRNA levels of CPT IA, PPARα and PPARγ. The specific PPARα pathway inhibitor GW6471 reduced insulin-induced changes in the expression of all the tested genes, indicating that PPARα mediated the insulin-induced changes of lipid metabolism. The present results contribute new knowledge on the regulatory role of insulin in hepatic metabolism in fish.

Effects of dietary arginine levels and carbohydrate-to-lipid ratios on mRNA expression of growth-related hormones in largemouth bass, Micropterus salmoides

Utilizing the tissue samples and growth data collected from our two preceding researches in largemouth bass (LMB), we have investigated effects of dietary arginine (Arg) levels and carbohydrate-to-lipid (CHO/LIP) ratios on the GH, IGF-I and insulin expression in related tissues to find possible relationships between the nutrient intake, growth performance and transcript level. Hepatic IGF-I and pituitary GH mRNA levels were significantly up-regulated by lower dietary Arg levels from 1.94% to 3.01% and by higher levels from 2.76% to 3.01%, respectively, while Brockmann body (BB)-containing tissue insulin mRNA expression was not affected. Dietary CHO/LIP ratios ranging from 0.32 to 5.17 (w/w) affected pituitary GH, liver IGF-I and BB-containing tissue insulin mRNA expression in a ratio-specific pattern. The lower ratios from 0.32 to 2.36 significantly up-regulated GH and insulin transcript levels, but significantly down-regulated IGF-I transcript levels; the higher ratios did no longer exert any further effects on them. Meanwhile, two strong positive correlations (r=0.892, r=0.885) between hepatic IGF-I transcript levels and specific growth rates of tested fish were observed with varying dietary Arg levels and CHO/LIP ratios, respectively. These findings indicate that in LMB dietary Arg levels and CHO/LIP ratios regulate differentially the endocrine system of GH, IGF-I and insulin at transcription level; this system, in turn, plays a fundamental role in the regulation of the nutrient metabolism and somatic growth; and that hepatic IGF-I mRNA abundance should be a more reliable index to assess growth and nutritional fitness than the others, at least in LMB.

Methionine restriction affects the phenotypic and transcriptional response of rainbow trout (Oncorhynchus mykiss) to carbohydrate-enriched diets

Mammalian studies report that methionine restriction (MR) as a dietary regimen extends life span, delays the onset of age-related diseases and enhances fat oxidation in obese subjects with metabolic syndromes. However, the underlying cellular signalling pathways are poorly understood. Rainbow trout (Oncorhynchus mykiss) is a glucose-intolerant species, providing an excellent model for the study of carbohydrate metabolism. MR diets in combination with 12 % (+/-) and 22 % (+/-) carbohydrate-rich meals were fed to rainbow trout for a period of 8 weeks and phenotypic and transcript expression changes in the liver and white muscle were assessed. Fish fed MR diets, irrespective of carbohydrate load, were shown to abolish the glucose-intolerant phenotype 6 h post-feeding. There was a distinct switch in glucose and glycogen content in the liver of fish fed MR diets, with a significantly higher concentration of glycogen, suggesting reduced glycolytic capacity. Transcriptional responses to MR demonstrated decreased expression of hepatic fatty acid synthase, sterol regulatory binding protein 1, PPARγ coactivator 1-α and PPARα, indicative of a reduction in the de novo synthesis of fatty acids and cholesterol, and a potential decrease in hepatic fat oxidative capacity. Muscle adenylate charge was depressed under MR, and increased expression of AMP-activated protein kinase α1 was detected, indicative of reduced energy availability. Total DNA methylation showed that carbohydrate load, rather than MR, dictated hypomethylation of genomic DNA. This is the first study which demonstrates that MR can abolish a glucose-intolerant phenotype in trout, and identifies trout as a suitable model for studying metabolic syndromes.

Glucose homeostasis in rainbow trout fed a high-carbohydrate diet: metformin and insulin interact in a tissue-dependent manner

Carnivorous fish species such as the rainbow trout ( Oncorhynchus mykiss ) are considered to be “glucose intolerant” because of the prolonged hyperglycemia experienced after intake of a carbohydrate-enriched meal. In the present study, we use this species to study glucose homeostasis in fish chronically infused with the hypoglycemic agents, insulin, and metformin, and fed with a high proportion of carbohydrates (30%). We analyzed liver, skeletal muscle, and white adipose tissue (WAT), which are insulin- and metformin-specific targets at both the biochemical and molecular levels. Trout infused with the combination of insulin and metformin can effectively utilize dietary glucose at the liver, resulting in lowered glycemia, increased insulin sensitivity, and glucose storage capacity, combined with reduced glucose output. However, in both WAT and skeletal muscle, we observed decreased insulin sensitivity with the combined insulin + metformin treatment, resulting in the absence of changes at the metabolic level in the skeletal muscle and an increased potential for glucose uptake and storage in the WAT. Thus, the poor utilization by rainbow trout of a diet with a high proportion of carbohydrate can at least be partially improved by a combined treatment with insulin and metformin, and the glucose intolerance observed in this species could be, in part, due to some of the downstream components of the insulin and metformin signaling pathways. However, the predominant effects of metformin treatment on the action of insulin in these three tissues thought to be involved in glucose homeostasis remain exclusive in this species.

Role of insulin and IGF-I on the regulation of glucose metabolism in European sea bass (Dicentrarchus labrax) fed with different dietary carbohydrate levels

The roles of insulin and insulin-like growth factor-I (IGF-I) in the regulation of glucose metabolism were assessed in European sea bass juveniles fed with distinct dietary carbohydrate levels. Three isonitrogenous diets were formulated to contain 10% (10%PGS) or 30% (30%PGS) pregelatinized starch or no starch (control). The highest plasma glucose and insulin levels were observed 6h after feeding in fish receiving the 30%PGS diet. Although plasma IGF-I was higher at 6h than at 24h after feeding, no effect of dietary carbohydrate level was noticed within each sampling time. Increasing dietary carbohydrate level resulted in an increase of liver but not of muscle glycogen content. Hepatic glucokinase (GK) and glucose-6-phosphate dehydrogenase (G6PD) activities increased with the dietary carbohydrate content, whereas pyruvate kinase (PK) activity was higher in fish fed the carbohydrate containing diets than the carbohydrate-free diet. GK activity was higher 6h than 24h after feeding, whereas the opposite was observed for G6PD activity. Data suggest that under the nutritional conditions assayed plasma glucose is an insulin secretagogue. Furthermore, insulin appears to have a more important role than IGF-I in stimulating hepatic glucose uptake, thus enhancing GK activity and leading to an increase in liver glycogen content to maintain glucose homeostasis.

Rainbow trout genetically selected for greater muscle fat content display increased activation of liver TOR signaling and lipogenic gene expression

Genetic selection is commonly used in farm animals to manage body fat content. In rainbow trout, divergent selection for low or high muscle fat content leads to differences in utilization of dietary energy sources between the fat muscle line (FL) and the lean muscle line (LL). To establish whether genetic selection on muscle fat content affects the hepatic insulin/nutrient signaling pathway, we analyzed this pathway and the expression of several metabolism-related target genes in the livers of the two divergent lines under fasting and then refeeding conditions. Whereas glycemia returned to basal level 24 h after refeeding in FL trout, it remained elevated in the LL trout. Target of rapamycin (TOR) protein was more abundant in the livers of FL trout than in LL trout, and refeeding activation of the hepatic TOR signaling pathway (TOR, S6K1, and S6) was therefore enhanced. Genes related to glycolysis (glucokinase and pyruvate kinase) and gluconeogenesis (glucose-6-phosphatase and phosphoenolpyruvate carboxykinase) were only slightly affected by refeeding and genetic selection. Refeeding stimulated expression of lipogenic genes and the sterol-responsive element binding protein (SREBP1), and expression of fatty acid synthase, glucose-6-phosphate dehydrogenase, and serine dehydratase was predominant in the livers of FL fish compared with LL fish. In agreement with recent findings linking TOR to lipogenesis control, we concluded that genetic selection for muscle fat content resulted in overactivation of the TOR signaling pathway-associated lipogenesis and probably also improved utilization of glucose.

Metformin improves postprandial glucose homeostasis in rainbow trout fed dietary carbohydrates: a link with the induction of hepatic lipogenic capacities?

Carnivorous fish are poor users of dietary carbohydrates and are considered to be glucose intolerant. In this context, we have tested, for the first time in rainbow trout, metformin, a common anti-diabetic drug, known to modify muscle and liver metabolism and to control hyperglycemia in mammals. In the present study, juvenile trout were fed with very high levels of carbohydrates (30% of the diet) for this species during 10 days followed by feeding with pellets supplemented with metformin (0.25% of the diet) for three additional days. Dietary metformin led to a significant reduction in postprandial glycemia in trout, demonstrating unambiguously the hypoglycemic effect of this drug. No effect of metformin was detected on mRNA levels for glucose transporter type 4 (GLUT4), or enzymes involved in glycolysis, mitochondrial energy metabolism, or on glycogen level in the white muscle. Expected inhibition of hepatic gluconeogenic (glucose-6-phosphatase, fructose-1,6-bisphosphatase, and phosphoenolpyruvate carboxykinase) mRNA levels was not found, showing instead paradoxically higher mRNA levels for these genes after drug treatment. Finally, metformin treatment was associated with higher mRNA levels and activities for lipogenic enzymes (fatty acid synthase and glucose-6-phosphate dehydrogenase). Overall, this study strongly supports that the induction of hepatic lipogenesis by dietary glucose may permit a more efficient control of postprandial glycemia in carnivorous fish fed with high carbohydrate diets.

Protein level affects the relative lysine requirement of growing rainbow trout (Oncorhynchus mykiss) fry

The effect of two digestible protein levels (310 and 469 g/kg DM) on the relative lysine (Lys; g Lys/kg DM or g Lys/100 g protein) and the absolute Lys (g Lys intake/kg0·75 per d) requirements was studied in rainbow trout fry using a dose–response trial. At each protein level, sixteen isoenergetic (22–23 MJ digestible energy/kg DM) diets were tested, involving a full range (2–70 g/kg DM) of sixteen Lys levels. Each diet was given to one group of sixty rainbow trout fry (mean initial body weight 0·78 g) reared at 15°C for 31 feeding d. The Lys requirements were estimated based on the relationships between weight, protein, and Lys gains (g/kg0·75 per d) and Lys concentration (g/kg DM or g/100 g protein) or Lys intake (g/kg0·75 per d), using the broken-line model (BLM) and the non-linear four-parameter saturation kinetics model (SKM-4). Both the model and the response criterion chosen markedly impacted the relative Lys requirement. The relative Lys requirement for Lys gain of rainbow trout estimated with the BLM (and SKM-4 at 90 % of the maximum response) increased from 16·8 (19·6) g/kg DM at a low protein level to 23·4 (24·5) g/kg DM at a high protein level. However, the dietary protein content affected neither the absolute Lys requirement nor the relative Lys requirement expressed as g Lys/100 g protein nor the Lys requirement for maintenance (21 mg Lys/kg0·75 per d).

Insulin regulates the expression of several metabolism-related genes in the liver and primary hepatocytes of rainbow trout (Oncorhynchus mykiss)

Rainbow trout have a limited ability to use dietary carbohydrates efficiently and are considered to be glucose intolerant. Administration of carbohydrates results in persistent hyperglycemia and impairs post-prandial down regulation of gluconeogenesis despite normal insulin secretion. Since gluconeogenic genes are mainly under insulin control, we put forward the hypothesis that the transcriptional function of insulin as a whole may be impaired in the trout liver. In order to test this hypothesis, we performed intraperitoneal administration of bovine insulin to fasted rainbow trout and also subjected rainbow trout primary hepatocytes to insulin and/or glucose stimulation. We demonstrate that insulin was able to activate Akt, a key element in the insulin signaling pathway, and to regulate hepatic metabolism-related target genes both in vivo and in vitro. In the same way as in mammals, insulin decreased mRNA expression of gluconeogenic genes, including glucose 6-phosphatase (G6Pase),fructose 1,6-bisphosphatase (FBPase) and phosphoenolpyruvate carboxykinase (PEPCK). Insulin also limited the expression of carnitine palmitoyltransferase 1 (CPT1), a limiting enzyme of fatty acid β-oxidation. In vitro studies revealed that, as in mammals,glucose is an important regulator of some insulin target genes such as the glycolytic enzyme pyruvate kinase (PK) and the lipogenic enzyme fatty acid synthase (FAS). Interestingly, glucose also stimulates expression of glucokinase (GK), which has no equivalent in mammals. This study demonstrates that insulin possesses the intrinsic ability to regulate hepatic gene expression in rainbow trout, suggesting that other hormonal or metabolic factors may counteract some of the post-prandial actions of insulin.

Receptors for insulin-like growth factor-I (IGF-I) predominate over insulin receptors in skeletal muscle throughout the life cycle of brown trout, Salmo trutta

Insulin and IGF-I binding has been studied in brown trout (Salmo trutta) wheat germ agglutinin semipurified receptors from embryos (organogenesis), larvae (yolk sac), juveniles (2.98 +/- 0.21 g bw) and adults (111.6 +/- 6.92 and 522 +/- 53 g bw). Embryos and larvae were sampled at 5 and 12 weeks after fertilization (December 1999 and February 2000) and juvenile and adults were taken simultaneously (July 1999) and under the same feeding conditions to minimize potential nutritional and seasonal effects. Insulin receptor number was maximal at 12 weeks (144 fmol/mg glycoprotein) and progressively decreased in subsequent samplings. No alterations in affinity were detected (K(d) range, 0.21-0.32 nM) and changes in number of receptor paralleled changes in total specific binding. IGF-I receptor number was highest at 5 weeks (1044 fmol/mg) and was significantly higher than values for insulin in all samplings. The affinity of IGF-I receptor did not change (K(d) range, 0.11-0.18 nM) but was consistently higher than that for the insulin receptor. A more rapid decrease of IGF-I binding and receptor number was found with age. However, the ratio of insulin/IGF-I binding established in 12-week-old larvae (0.18 +/- 0.01) was thereafter maintained at very similar values in juveniles and adults (0.15-0.17). Tyrosine kinase activity (TKA) for insulin receptors ranged between 136 and 183% and there were no significant changes with age. For the IGF-I receptor, TKA ranged from 174 to 281% and was significantly higher in 5-week-old larvae coincident with the highest levels of receptor number and declined gradually in parallel with binding levels. In conclusion, the greater abundance of IGF-I receptors during embryonic and larval development is maintained throughout juvenile and adult stages. This would suggest a key role for IGF-I in the growth and metabolism of trout muscle.

Glucose tolerance and peripheral glucose utilization in rainbow trout (Oncorhynchus mykiss), American eel (Anguilla rostrata), and black bullhead catfish (Ameiurus melas)

This study tests the hypothesis that glucose tolerance in fish is related to nutrient preference and is correlated with white muscle glucose transporter and phosphorylation (hexokinase) activities. Glucose clearance was investigated in the carnivorous rainbow trout (Oncorhynchus mykiss) and American eel (Anguilla rostrata) (feeding and fasting) and the omnivorous black bullhead catfish (Ameiurus melas). Glucose tolerance was assessed by an intravenous glucose tolerance test, injecting 250 mg glucose/kg body weight and tracking blood glucose concentrations over 24 h. Both feeding eel and feeding catfish returned plasma glucose levels to baseline within 60 min of glucose injection. Glucose values remained elevated for more than 360 min in both the food-deprived eel and the feeding rainbow trout. Glucose transport studies in white muscle membrane vesicles provided evidence for the presence of a stereospecific, saturable glucose transporter in all three species. Affinity constants (K(m)) ranged from 8 to 14 mM while V(max) values ranged from 75 to 150 pmol/s/mg protein. Neither kinetic parameter differed significantly between species. Cytochalasin B and phloretin did not significantly inhibit glucose transport, implying that these transporters are unlike the mammalian muscle glucose transporters (GLUT). In fact, Northern and Western blot analyses of mRNA and protein from white and red muscles and heart did not detect a mammalian-type GLUT-1 or -4 in any of the species examined. Glucose phosphorylation indicated the presence of a hexokinase activity (low K(m) enzyme) but again there were no differences in kinetic parameters between species. These studies demonstrate that glucose tolerance in fish is species-dependent but none of the parameters examined clearly differentiate between the species examined. Certainly a stereospecific glucose transporter exists in white skeletal muscle of the fish studied but no molecular or kinetic similarities to the mammalian GLUTs were found. Whether these transporters are insulin-sensitive or contribute to glucose tolerance requires further molecular characterization.

Gene transfer for targeted modification of salmonid fish metabolism

The reviewed studies addressed the possibility of using gene transfer for correction of L-ascorbic acid biosynthesis and carbohydrate utilization in rainbow trout. Analyses of enzymatic activities in the L-AAB pathway indicated that reasons for the lack of L-AA production can be common in fish and scurvy-prone animals. Rat gulonolactone oxidase cDNA was transferred into trout. Regardless of the fact that rGLO transcription occurred in embryos, neither GLO protein, nor enzyme activity were detected. There was no production of L-AA in transgenic fish raised on vitamin C-free diets or injected with L-gulonolactone. These results indicated that the conditions required for translation or stability of rGLO were not present in trout tissues. To augment carbohydrates utilization, human glucose transporter 1 and rat hexokinase II cDNAs were tested. In the transfected embryos. HK activity, rates of hexose uptake and glucose oxidation were increased. The effect of hGLUT1 on glucose metabolism was greater than that of rHKII. Trout carrying hGLUT1 and rHKII with viral or piscine promoters were created. Though interpretation of the metabolic effects of the transgenes was complicated with mosaicism, a tendency to improved carbohydrate utilization was revealed in some of the transgenic individuals.

Influence of high-carbohydrate enriched diets on plasma insulin levels and insulin and IGF-I receptors in trout

Insulin and insulin growth factor-I (IGF-I) binding to skeletal muscle semipurified receptors were assessed in rainbow trout (Oncorhynchus mykiss) fed with different enriched carbohydrate diets. The animals were fed for 2 months, either in spring, summer or autumn with a control diet (C, commercial diet containing 21% raw carbohydrates) or with two diets supplied with highly digestible carbohydrates (E1, 22% expanded wheat; and E2, 37% expanded wheat). Insulin and IGF-I receptors were semipurified by affinity chromatography (WGA-agarose). Fish fed with a carbohydrate enriched diet did not show lower growth rates than those fed with the control diet. Independently of the season, rainbow trout fed E1 and E2 presented higher insulin and glucose plasma levels as well as higher tissue glycogen reserves than fish fed C. An increase in the number of insulin receptors during the diet adaptation was observed especially in fish fed with E2. No differences in the affinity of receptors were observed. IGF-I specific binding in skeletal muscle was higher than that of insulin in all groups and in all seasons. Furthermore, IGF-I receptors showed the same tendency as insulin receptors, with increases in their number in experimentally fed fish, especially those fed with E2. Insulin and IGF-I receptors TKA increased only slightly, as a consequence of E1 and E2 diet adaptation. In conclusion, rainbow trout can be fed high-carbohydrate levels and show good rates of growth. This adaptation determines increases in circulating glucose and insulin, and muscle insulin receptors, which indicate an adaptation of the fish to higher levels of glucose supply. The response of IGF-I receptors also suggests a possible role in the regulation of metabolism.

GLUT-4 Deficiency and severe peripheral resistance to insulin in the teleost fish tilapia

Teleost fish, in general, are glucose intolerant; this trait has been attributed to piscine islets secreting insulin primary in response to amino acid secretogogues rather than glucose. However, pancreatic islet from the teleost fish tilapia, when transplanted into diabetic nude mice, were glucose responsive even though tilapia were severely glucose intolerant. This suggested a strong peripheral resistance to the glucostatic effects of insulin. Using Western blotting with polyclonal antibodies as well as Northern analysis for mRNA, tilapia tissues were found to be devoid of GLUT-4, the insulin-sensitive glucose transporter responsible for the hypoglycemic effect of insulin in mammals. The absence of GLUT-4 in peripheral tissues may explain why tilapia, and possibly other teleost fish, are severely glucose intolerant. This suggests that tilapia islets have evolved along mammalian lines to be glucose sensitive while tilapia peripheral tissue have diverged widely. Using the same methods, tilapia were found to have a very limited tissue distribution of the insulin-independent glucose transporter, GLUT-1, which is responsible for basal glucose transport in mammalian cells. It is suggested that tilapia provide a naturally occurring GLUT-4 knockout model.

Up-regulation of insulin binding in fish skeletal muscle by high insulin levels

The effects of insulin titres on regulation of receptor binding were studied in several fish species. Insulin receptors were semi-purified by affinity chromatography (WGA-agarose) from skeletal muscle of carp, brown trout and rainbow trout that had been subjected to increases in insulinemia produced either by arginine injection, food administration, or adaptation to an experimental diet (extruded diet with high-digestibility carbohydrates). Arginine injection provoked acute hyperinsulinemia in both carp and trout. Specific binding of insulin to the skeletal muscle was significantly increased 3 h after injection (from 5.8 +/- 0.3 to 9.6 +/- 0.9%/10 micrograms protein in carp and from 0.8 +/- 0.2 to 1.5 +/- 0.4%/10 micrograms in trout). The same effect was observed in carp liver preparations (from 6.0 +/- 0.75 to 9.9 +/- 1.25%/10 micrograms). No alterations in tyrosine kinase activity of the receptors were detected in either carp or trout preparations: basal activities of the receptors were maintained (3100 +/- 200 fmol P/fmol receptors/30 min and 3700 +/- 400 fmol P/fmol receptors/30 min, in carp and trout, respectively), as were the percentage of stimulation over basal levels obtained by incubation with insulin (227 +/- 25% and 160 +/- 10% respectively). Food ingestion raised plasma insulin levels more steadily. Specific binding also increased in skeletal muscle preparations, especially in carp (from 5.7 +/- 0.3 to 11 +/- 1.7%/10 micrograms at 4 h and 10 +/- 0.7%/10 micrograms at 8 h). Tyrosine kinase activity was maintained without significant changes. Rainbow trout adapted for 2 months to an extruded diet presented higher insulin titres and higher glycogen reserves in liver and muscle. Insulin binding to skeletal muscle preparations was also significantly increased (from 0.36 +/- 0.02 to 0.77 +/- 0.1%/10 micrograms), as was tyrosine kinase activity (from 132 +/- 4% to 156 +/- 6%, without alterations in the basal activity). Results showed that fish can respond to both acute and maintained increases in insulinemia by increasing the number of insulin receptors. Tyrosine kinase activity, in contrast, is only modified after long-term adaptation.

Binding and bioactivity of insulin in primary cultures of carp (Cyprinus carpio) hepatocytes

Isolated carp hepatocytes cultured in serum-free, chemically defined medium were used to investigate within the same cell preparation characteristics of the binding of insulin as well as effects of insulin on cellular metabolism. The binding of human [(125)I]-insulin to carp hepatocytes was studied in kinetic, saturation and displacement experiments. A dependency of insulin binding on the collagenase used for cell isolation was demonstrated. Insulin binding decreased during the first 12h of culture but remained constant during the following 12h. The kinetic experiments revealed that [(125)I]-insulin binding reached a steady state within 20-30 min of incubation. The mathematical analysis of the saturation experiments demonstrated the existence of two populations of binding sites, one with high affinity (Kd1 = 5.5 pM) and low capacity (Bmax1 = 0.14 fmol/mg protein or 77 binding sites/cell) and one with low affinity (Kd2 = 2.4 nM) and high capacity (Bmax2 = 17.6 fmol/mg protein or 9623 binding sites/cell). In competition experiments, 312 pM [(125)I]-insulin was displaced by cold insulin, IGF-I and IGF-II with IC50 values of 2.2, 7.9 and 20.3 nM, respectively. Glucagon was without effect. Binding of insulin to carp hepatocytes resulted in a significant reduction of glucose release and a significant increase of protein synthesis as of de novo fatty acid synthesis.

Insulin and IGF-I receptors and tyrosine kinase activity in carp ovaries: changes with reproductive cycle

Insulin and insulin-like growth factor I (IGF-I) receptors from carp ovaries were semipurified with wheat germ agglutinin at different moments of the reproductive cycle and their binding characteristics and tyrosine kinase activity were studied. Specific receptors for insulin and IGF-I were found. IGF-I receptors presented higher binding (23.8 ± 1.5%), number of receptors (965 ± 20fm/mg) and affinity (KD 0.24 ± 0.03nM) than those shown for insulin receptors (4.1 ± 1%, 530 ± 85fm/mg and 0.85 ± 0.1nM, respectively). Insulin and IGF-I receptors have a tyrosine kinase activity which is not different from that found in muscle of the same species. Seasonal changes were found in binding, with maximum values for insulin and IGF-I reached at the end of pre-spawning period (June). However, while IGF-I binding was observed in all stages, insulin binding decreased in autumn and disappeared in winter, which suggests a different role for the two peptides in ovarian physiology.