Dietary starch promotes hepatic lipogenesis in barramundi (Lates calcarifer).

Barramundi (Lates calcarifer) are a highly valued aquaculture species, and, as obligate carnivores, they have a demonstrated preference for dietary protein over lipid or starch to fuel energetic growth demands. In order to investigate how carnivorous fish regulate nutritional cues, we examined the metabolic effects of feeding two isoenergetic diets that contained different proportions of digestible protein or starch energy. Fish fed a high proportion of dietary starch energy had a higher proportion of liver SFA, but showed no change in plasma glucose levels, and few changes in the expression of genes regulating key hepatic metabolic pathways. Decreased activation of the mammalian target of rapamycin growth signalling cascade was consistent with decreased growth performance values. The fractional synthetic rate (lipogenesis), measured by TAG 2H-enrichment using 2H NMR, was significantly higher in barramundi fed with the starch diet compared with the protein diet (0·6 (se 0·1) v. 0·4 (se 0·1) % per d, respectively). Hepatic TAG-bound glycerol synthetic rates were much higher than other closely related fish such as sea bass, but were not significantly different (starch, 2·8 (se 0·3) v. protein, 3·4 (se 0·3) % per d), highlighting the role of glycerol as a metabolic intermediary and high TAG-FA cycling in barramundi. Overall, dietary starch significantly increased hepatic TAG through increased lipogenesis. Compared with other fish, barramundi possess a unique mechanism to metabolise dietary carbohydrates and this knowledge may define ways to improve performance of advanced formulated feeds.

New Insights on Intermediary Metabolism for a Better Understanding of Nutrition in Teleosts.

The rapid development of aquaculture production throughout the world over the past few decades has led to the emergence of new scientific challenges to improve fish nutrition. The diet formulations used for farmed fish have been largely modified in the past few years. However, bottlenecks still exist in being able to suppress totally marine resources (fish meal and fish oil) in diets without negatively affecting growth performance and flesh quality. A better understanding of fish metabolism and its regulation by nutrients is thus mandatory. In this review, we discuss four fields of research that are highly important for improving fish nutrition in the future: ( a) fish genome complexity and subsequent consequences for metabolism, ( b) microRNAs (miRNAs) as new actors in regulation of fish metabolism, ( c) the role of autophagy in regulation of fish metabolism, and ( d) the nutritional programming of metabolism linked to the early life of fish.

High or low dietary carbohydrate:protein ratios during first-feeding affect glucose metabolism and intestinal microbiota in juvenile rainbow trout.

Based on the concept of nutritional programming in mammals, we tested whether an acute hyperglucidic-hypoproteic stimulus during first feeding could induce long-term changes in nutrient metabolism in rainbow trout. Trout alevins received during the five first days of exogenous feeding either a hyperglucidic (40% gelatinized starch + 20% glucose) and hypoproteic (20%) diet (VLP diet) or a high-protein (60%) glucose-free diet (HP diet, control). Following a common 105-day period on a commercial diet, both groups were then challenged (65 days) with a carbohydrate-rich diet (28%). Short- and long-term effects of the early stimuli were evaluated in terms of metabolic marker gene expressions and intestinal microbiota as initial gut colonisation is essential for regulating the development of the digestive system. In whole alevins (short term), diet VLP relative to HP rapidly increased gene expressions of glycolytic enzymes, while those involved in gluconeogenesis and amino acid catabolism decreased. However, none of these genes showed persistent molecular adaptation in the liver of challenged juveniles (long term). By contrast, muscle of challenged juveniles subjected previously to the VLP stimulus displayed downregulated expression of markers of glycolysis and glucose transport (not seen in the short term). These fish also had higher plasma glucose (9 h postprandial), suggesting impaired glucose homeostasis induced by the early stimulus. The early stimulus did not modify the expression of the analysed metabolism-related microRNAs, but had short- and long-term effects on intestinal fungi (not bacteria) profiles. In summary, our data show that a short hyperglucidic-hypoproteic stimulus during early life may have a long-term influence on muscle glucose metabolism and intestinal microbiota in trout.

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.

Effects of insulin infusion on glucose homeostasis and glucose metabolism in rainbow trout fed a high-carbohydrate diet.

The origin for the poor glucose utilization in carnivorous fish species fed high carbohydrate diets remains under debate. In the present study, we have fed rainbow trout a diet containing 30% carbohydrate for 1 or 5 days. In both cases, fish were implanted with mini-osmotic pumps releasing 0.7 i.u. kg(-1) day(-1) bovine insulin, and mRNA transcripts and the protein phosphorylation status of proteins controlling glycemia and glucose-related metabolism were studied in fish killed 6 h after the last meal. We demonstrate that when the exposure occurs over a short term (30 h), insulin exerts beneficial actions on trout glucose homeostasis, including a lowered glycemia and increased hepatic lipogenic and glycogenic potentials. However, when trout were fed for 5 days, these beneficial actions of insulin infusion were no longer observed. Thus, the increased lipogenic potential observed after one single meal was not present, and this together with the increased glycogenesis and the decreased glucose exported to the blood from the liver explains the lack of hypoglycemic action of insulin. The fact that insulin improved glucose homeostasis when administrated over a short time period implies that endogenous insulin secretion is inadequate in trout to deal with this amount of dietary carbohydrates. Moreover, the fact that a longer exposure to insulin resulted in a reduced response indicates that the rainbow trout is sensitive to insulin, re-enforcing the hypothesis that the hyperglycemia observed following a high carbohydrate meal is an insulin secretion issue rather an insulin action issue.

Molecular regulation of lipid metabolism in liver and muscle of rainbow trout subjected to acute and chronic insulin treatments.

Although the metabolic actions of insulin in fish have been investigated widely in the past several years, lipid metabolism has received little attention, especially in tissues like the liver or white muscle. In the present study, rainbow trout received insulin treatments both acutely (intraperitoneal injection) and chronically (through mino-osmotic pumps) to elucidate hormone metabolic actions at molecular levels on the 2 main insulin target tissues in trout, namely, liver and muscle. Plasma and free fatty acid concentrations in plasma, as well as mRNA measurements of some key enzymes involved in lipid metabolism, were assessed in these tissues after 6h and 4 d of acute and chronic insulin treatments, respectively. Our results showed that although fish received the same final total amount of hormone in both treatments, the actions of insulin on lipid metabolism were both time and tissue dependent. After the acute insulin treatment, the main anabolic role of insulin was reflected in decreased plasma free fatty acid concentrations linked to enhanced hepatic lipogenesis. We also found that insulin increased the mRNA levels of enzymes involved in lipid oxidation, perhaps to counteract insulin-induced hypoglycemia. In contrast, our data show that after chronic insulin treatment, liver and muscle exhibit different metabolic strategies: whereas in the liver chronic insulin-induced hypoglycemia may stimulate lipolytic processes to spare glucose stores, the muscle responds directly to the anabolic hormone action by increasing its lipogenic capacity and by inhibiting pathways of lipid oxidation.

Glucose homeostasis is impaired by a paradoxical interaction between metformin and insulin in carnivorous rainbow trout.

Utilizing rainbow trout (Oncorhynchus mykiss) as a known model of a "glucose-intolerant" and poor dietary glucose user, we assessed glucose utilization in fish chronically receiving two molecules able to improve glucose homeostasis: insulin and metformin. Our objectives were to assess the ability of rainbow trout to deal with a glucose load and to improve glucose utilization in fish receiving a chronic administration of insulin plus metformin treatments. Fish received (implanted miniosmotic pumps) saline, insulin, metformin, and insulin plus metformin solution for 4 days and then were subjected to a glucose challenge (intraperitoneal injection) to study glucose homeostasis, analyzing plasma glycemia, mRNA levels of glucose metabolism-related proteins, insulin signaling, and glycogen levels in liver and muscle. Control fish received a saline pump implantation and saline intraperitoneal injection. We found no evidence that the "glucose intolerance" in this species could be linked to any of the molecular markers of metabolism in the tissues analyzed. By contrast, very interestingly, we show for the first time, that metformin is not only unable to improve glucose homeostasis in trout, but, in fact, its counteracts the effects of insulin, creating an "insulin resistance," especially in the muscle. These results make trout an attractive original model to study both insulin and metformin effect on biological systems.

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.

Regulation of fatty acid oxidation in chicken (Gallus gallus): interactions between genotype and diet composition.

To explore the mechanisms leading to excessive adiposity in chicken, we investigated the regulation of fatty acid oxidation depending on genotype-related body fatness and diet composition. mRNA expression and/or activity of proteins involved in mitochondrial energy metabolism were measured in liver and gastrocnemius muscle of genetically lean or fat chickens reared on a low-fat/high-protein diet or an isoenergetic high-fat/low-protein diet (HF/LP). Muscle expressions of the muscle isoform of carnitine-palmitoyltransferase 1 (M-CPT1) and PPARbeta/delta were higher in fat than in lean chickens. This was also observed in liver, although only with the HF/LP diet for M-CPT1. This could stimulate mitochondrial fatty acid oxidation in fat chickens. Up-regulations of liver and muscle CPT-1 hepatic isoform, and muscle cytochrome-c-oxidase mRNA expressions, and of beta-hydroxyacyl-CoA-dehydrogenase activities suggest higher fatty acid utilization with the HF/LP diet. PPARbeta/delta and PGC-1alpha could control fatty acid oxidation in muscle and liver, respectively. Regulation of avian uncoupling protein (avUCP) mRNA was tissue-dependent. Predominantly expressed in muscle, it was stimulated in fat and in HF/LP-fed chickens, where it could be associated to the special need in muscle anti-oxidant pathways of fatter animals. In liver it was lower in fat than in lean chickens, and its potential function remains to be clarified.

Effects of thermal manipulation during early and late embryogenesis on thermotolerance and breast muscle characteristics in broiler chickens.

Genetic selection has significantly improved the muscle development of fast-growing broiler chickens in the last 50 yr. However, improvement in muscle growth has coincided with relatively poor development of visceral systems, resulting in impaired ability to cope with high environmental temperatures. The aim of this study was to elucidate the effects of thermal manipulation (TM) during different periods of embryogenesis on chick hatchability, BW and thermoregulation upon hatching, on their ability to cope with thermal challenge at 42 d of age, and on carcass and breast meat traits. Control embryos were incubated at 37.8 degrees C. The TM embryos were incubated at 37.8 degrees C and treated for 3 h at 39.5 degrees C on the following days of embryogenesis: E8 to E10 [early (EA)], E16 to E18 [late (LA)], and both E8 to E10 and E16 to E18 (EA-LA). Body weight and body temperature (T(b)) were measured at hatching and throughout the growth period as well as during exposure of 42-d-old chickens to a thermal challenge at 35 degrees C for 6 h. The LA and EA chicks exhibited significantly lower T(b) than control chicks (37.9 vs. 38.2 degrees C) at hatching, but during the growth period, differences in T(b) between treated and control chicks decreased with age. Significant hyperthermia (over 44 degrees C) was monitored in all groups during the thermal challenge, but mortality was higher in treated than in control chickens. No effect of treatments on BW was found during the entire growth period. However, breast yield was higher in LA chickens than in controls at slaughter. The EA and EA-LA treatments slightly decreased the ultimate pH of breast meat, whereas the LA treatment had no effect. In conclusion, none of the TM conditions tested in the present study were able to improve long-term thermotolerance in chickens. Late treatment favored breast muscle growth without affecting ultimate pH and drip loss of breast meat.