Hepatic glucokinase is induced by dietary carbohydrates in rainbow trout, gilthead seabream, and common carp

Dietary carbohydrates induce changes in glucosensing capacity and food intake of rainbow trout

We hypothesize that variations in dietary carbohydrate levels produce changes in glucosensor parameters in previously characterized glucosensing areas (hypothalamus and hindbrain) related with the regulation of food intake of a carnivorous fish species like rainbow trout. Therefore, we fed trout with standard, carbohydrate-free (CF) or high-carbohydrate (HC) diets for 10 days to assess changes in glucosensing system and food intake. Fish fed CF diet displayed hypoglycemia and increased food intake. Fish fed a HC diet displayed hyperglycemia and decreased food intake. Changes in food intake due to dietary carbohydrates were accompanied in hypothalamus and hindbrain of fish fed with HC diet by changes in parameters involved in glucosensing, such as increased glucose, glucose 6-phosphate, and glycogen levels and increased glucokinase (GK), glycogen synthase, and pyruvate kinase activities as well as increased GK and GLUT2 expression. All those results address for the first time in fish, despite the relative intolerance to glucose of carnivorous species, that dietary carbohydrates are important regulators of the glucosensing system in carnivorous fish, suggesting that the information generated by this system can be associated with the changes observed in food intake.

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.

Changes in food intake and glucosensing function of hypothalamus and hindbrain in rainbow trout subjected to hyperglycemic or hypoglycemic conditions

To evaluate the possible role of glucose in the control of food intake (FI) in fish and the involvement of glucosensing system in that role, we have subjected rainbow trout (via intraperitoneal injections) to control, hyperglycemic (500 mg kg(-1) glucose body mass) or hypoglycemic (4 mg kg(-1) bovine insulin) conditions for 10 days. The experimental design was appropriate since hypoglycemia and hyperglycemia were observed the first 5 days after treatment and changes observed in metabolic parameters in liver were similar to those of fish literature. Hyperglycemic conditions elicited small changes in FI accompanied by increased glucose and glycogen levels, glucokinase (GK) activity and glycolytic potential in hypothalamus and hindbrain. In contrast, hypoglycemic conditions elicited a marked increase in FI accompanied by decreased glucose and glycogen levels and GK activity in the same brain regions whereas both regions displayed different responses in glycolytic potential. These results allow us to hypothesize that, despite the relative intolerance to glucose of carnivorous fish, changes in plasma glucose levels in rainbow trout detected by glucosensing areas in brain regions (hypothalamus and hindbrain) are integrated in those or near areas eliciting a response in FI, which was more important under hypoglycemic than under hyperglycemic conditions.

Sexual dimorphism in hepatic gene expression and the response to dietary carbohydrate manipulation in the zebrafish (Danio rerio)

In this study, we tested for the presence of sexual dimorphism in the hepatic transcriptome of the adult zebrafish and examined the effect of long term manipulation of dietary carbohydrate on gene expression in both sexes. Zebrafish were fed diets comprised of 0%, 15%, 25%, or 35% carbohydrate from the larval stage through sexual maturity, then sampled for hepatic tissue, growth, proximate body composition, and retention efficiencies. Using Affymetrix microarrays and qRT-PCR, we observed substantial sexual dimorphism in the hepatic transcriptome. Males up-regulated genes associated with oxidative metabolism, carbohydrate metabolism, energy production, and amelioration of oxidative stress, while females had higher expression levels of genes associated with translation. Restriction of dietary carbohydrate (0% diet) significantly affected hepatic gene expression, growth performance, retention efficiencies of protein and energy, and percentages of moisture, lipid, and ash. The response of some genes to dietary manipulation varied by sex; with increased dietary carbohydrate, males up-regulated genes associated with oxidative metabolism (e.g. hadhbeta) while females up-regulated genes associated with glucose phosphorylation (e.g. glucokinase). Our data support the use of the zebrafish model for the study of fish nutritional genomics, but highlight the importance of accounting for sexual dimorphism in these studies.

The role of hepatic, renal and intestinal gluconeogenic enzymes in glucose homeostasis of juvenile rainbow trout

Rainbow trout is unable to utilize high levels of dietary carbohydrates and experiences hyperglycemia after consumption of carbohydrate-rich meals. Carbohydrates stimulate hepatic glycolytic activity, but gene expression of the rate-limiting gluconeogenic enzymes glucose-6-phosphatase (G6Pase), fructose-1,6-bisphosphatase (FBPase) and phosphoenolpyruvate carboxykinase (PEPCK) remains high. Although there is significant mRNA expression and activity of gluconeogenic enzymes in trout intestine and kidney, the regulation of these enzymes by diet is not known. We tested the hypothesis that dietary carbohydrate modulates intestinal and renal G6Pase, FBPase and PEPCK. Fish were either fasted or fed isocaloric carbohydrate-free (CF) or high carbohydrate (HC) diets for 14 days. As expected, fish fed HC exhibited postprandial hyperglycemia and enhanced levels of hepatic glucokinase mRNA and activity. Dietary carbohydrates had no significant effect on the expression and activity of PEPCK, FBPase and G6Pase in all three organs. In contrast, fasting enhanced the activity, but not the mRNA expression of both hepatic and intestinal PEPCK, as well as intestinal FBPase. Therefore, the activity of rate-limiting gluconeogenic enzymes in trout can be modified by fasting, but not by the carbohydrate content of the diet, potentially causing hyperglycemia when fed high levels of dietary carbohydrates. In this species consuming low carbohydrate diets at infrequent intervals in the wild, fasting-induced increases in hepatic and intestinal gluconeogenic enzyme activities may be a key adaptation to prevent perturbations in blood glucose during food deprivation.

Expression of the glucokinase gene in mule duck liver and glucokinase activities in chicken and mule duck livers

The presence of glucokinase (GK), a critical enzyme controlling glucose homeostasis, particularly liver glucose utilization in mammals, has long been a matter of debate in avian species because a number of investigators have failed to detect GK activity in the livers of chickens and several other avian species. In this study, we cloned a partial GK cDNA from mule duck livers and measured GK-like activity in the livers of mule ducks and broiler chickens under 2 nutritional states. Liver samples from 5-wk-old meal-fed male broiler chickens (Ross) were obtained from overnight-fasted chickens (BC) and 5 h after an oral saccharose load (6 mL/kg of BW of a 50% saccharose solution) given just before the meal (BS). Liver samples from 15-wk-old mule ducks were collected after an overnight fast (DC) and 12 h after the last overfeeding meal (DO). A partial cDNA ( approximately 600 bp) was obtained from duck livers. It presented 99% identity with chicken partial GK cDNA (gi 44888789) and 82% identity with human GK (gi 15967158). Chicken liver weights represented 1.8 and 3.3% of BW, respectively, for BC and BS (n = 8, P < 0.05). Glucokinase and low-Michaelis constant hexokinase (HK) activity levels were similar in BC (respectively, 0.88 and 1.00 mU/mg of protein). In response to the meal load, GK activity increased significantly (+57%), whereas HK decreased (-46%) in BS. Duck liver weights represented 1.4 and 7.6% of BW, respectively, for DC and DO (n = 8, P < 0.05). In DC livers, GK activity was significantly higher than HK activity (respectively, 1.76 and 0.63 mU/mg of protein). Both activities were significantly increased in DO (2 times, n = 8, P < 0.05). In conclusion, GK is present in ducks as well as chickens, and it is nutritionally regulated in avian species as well as in mammals. Further work will determine whether the higher liver GK activity and GK:HK ratio in DC compared with BC is related to age or BW or linked to the high lipogenic capacity of the duck liver.

In vitro evidences for glucosensing capacity and mechanisms in hypothalamus, hindbrain, and Brockmann bodies of rainbow trout

We aimed to support in vitro the glucosensing capacity observed in vivo in rainbow trout hypothalamus, hindbrain, and Brockmann bodies (BB) and to obtain preliminary evidence of the mechanisms involved. The response of parameters involved in the glucosensing capacity [hexokinase, hexokinase IV (glucokinase), and pyruvate kinase activities and glucose and glycogen levels] was assessed in these tissues incubated for 1 h with 2, 4, or 8 mM d-glucose alone (control) or with specific agonists/inhibitors of the steps involved in glucosensing capacity in mammals. These agents were a competitor for glucose phosphorylation (15 mM mannose), sulfonylurea receptor-1 effectors (500 μM tolbutamide or diazoxide), glycolytic intermediates (15 mM glycerol, lactate, or pyruvate), and inhibitors of glucose transport (10 μM cytochalasin B), glycolysis [20 mM 2-deoxy-d-glucose (2-DG)], and L-type calcium channel (1 μM nifedipine). Control incubations of the three tissues displayed increased glucose and glycogen levels and glucokinase activities in response to increased medium glucose, thus supporting our previous in vivo studies. Furthermore, critical components of the glucosensing mammalian machinery are apparently functioning in the three tissues. The responses in brain regions to all substances tested (except 2-DG and nifedipine) were similar to those observed in mammals, suggesting a similar glucosensing machinery. In contrast, in BB, only the effects of 2-DG, lactate, pyruvate, diazoxide, and nifedipine were similar to those of mammalian β-cells, suggesting that some of the components of the piscine glucosensing model are different than those of mammals. Such differences may relate to the importance of amino acids rather than glucose signaling in the trout BB.

Evidence for the presence of a glucosensor in hypothalamus, hindbrain, and Brockmann bodies of rainbow trout

The aim of this study was to evaluate the existence of a glucosensor in different regions of the brain and in the Brockmann bodies (BB) of the rainbow trout, Oncorhynchus mykiss. Five groups ( n = 12) of trout were injected intraperitoneally with saline alone (control) or saline-containing bovine glucagon (100 μg/kg), bovine insulin (4 mg/kg), 2-deoxy-d-glucose (100 mg/kg), or d-glucose (500 mg/kg) to promote hyperglycemia (glucagon, d-glucose, 2-deoxy-d-glucose) or hypoglycemia (insulin). Six hours after injection, samples from four brain regions (hypothalamus, telencephalon, hindbrain, and midbrain) and the entire BB were taken. Our results demonstrate within the BB and both the hypothalamus and hindbrain a metabolic response different to that observed in other tissues (midbrain, telencephalon) but similar to that described in tissues known to be glucosensors in mammals. The metabolic responses of these areas to changes in plasma glycemia were characterized by parallel changes in GLUT-2 expression, hexokinase-IV, or glucokinase activity and expression, glycolytic potential, and levels of glycogen and glucose. These changes are similar to those reported in mammalian pancreatic β-cells and glucose-excited (GE) neurons, two cell types containing glucosensors. This study provides evidence for the presence of glucosensors responsive to hyper- and hypoglycemia in rainbow trout BB, hypothalamus, and hindbrain.

Early feeding of carnivorous rainbow trout (Oncorhynchus mykiss) with a hyperglucidic diet during a short period: effect on dietary glucose utilization in juveniles

Based on the concept of nutritional programming in higher vertebrates, we tested whether an acute hyperglucidic stimulus during early life could induce a long-lasting effect on carbohydrate utilization in carnivorous rainbow trout. The trout were fed a hyperglucidic diet (60% dextrin) at two early stages of development: either at first feeding (3 days, stimulus 1) or after yolk absorption (5 days, stimulus 2). Before and after the hyperglucidic stimulus, they received a commercial diet until juvenile stage (>10 g). Fish that did not experience the hyperglucidic stimuli served as controls. The short- and long-term effects of the stimuli were evaluated by measuring the expression of five key genes involved in carbohydrate utilization: alpha-amylase, maltase (digestion), sodium-dependent glucose cotransporter (SGLT1; intestinal glucose transport), and glucokinase and glucose-6-phosphatase, involved in the utilization and production of glucose, respectively. The hyperglucidic diet rapidly increased expressions of maltase, alpha-amylase, and glucokinase in stimulus 1 fish and only of maltase in stimulus 2 fish, probably because of a lower plasticity at this later stage of development. In the final challenge test with juveniles fed a 25% dextrin diet, both digestive enzymes were upregulated in fish that had experienced the hyperglucidic stimulus at first feeding, confirming the possibility of modification of some long-term physiological functions in rainbow trout. In contrast, no persistent molecular adaptations were found for the genes involved in glucose transport or metabolism. In addition, growth and postprandial glycemia were unaffected by the stimuli. In summary, our data show that a short hyperglucidic stimulus during early trout life may permanently influence carbohydrate digestion.

Does overfeeding enhance genotype effects on liver ability for lipogenesis and lipid secretion in ducks?

We evaluated the effects of genotype (Muscovy, Pekin and their crossbreed hinny and mule ducks) and feeding levels (overfeeding between 12 and 14 weeks of age vs ad libitum feeding) on liver ability for lipogenesis and lipid secretion in ducks. Samples of liver and blood were collected at 14 weeks of age from 8 birds per group. Plasma levels of insulin was considerably increased in overfed ducks (1.9-fold), stimulating the hepatic activity of the main enzymes involved in lipogenesis from glucose (glucokinase, GK, glucose-6-phosphate dehydrogenase, G6PDH, malic enzyme, ME, acetyl CoA carboxylase, ACX), while cytochrome-c oxidase (COX) activity, indicating overall oxidation ability of energy-yielding substrates, remained unchanged. Plasma levels of triglycerides, phospholipids and total cholesterol were therefore increased (1.9, 3.7, 1.6 and 1.6-fold, respectively). Glycaemia also significantly increased (+8%). Pekin ducks exhibited higher levels of GK and G6PDH activity in the liver than Muscovy ducks, suggesting a greater ability to use glucose consistent with their lower glycaemia. Muscovy ducks had greater ACX activity, suggesting greater ability to synthesise lipids. However, plasma lipid levels were much higher in Pekin ducks than in Muscovy ducks, suggesting a greater ability to export lipids from the liver. Values for the different criteria measured in this study were intermediate or similar in hinny and mule ducks to those of parental species. The high values for GK, G6PDH, ME and ACX activity in hybrid ducks enabled them to produce heavy fatty livers with the same chemical and lipid composition as Muscovy ducks and characterised by high amounts of triglycerides (around 96% of total lipids), and saturated and mono-unsaturated fatty acids.

Oxygen availability regulates metabolism and gene expression in trout hepatocyte cultures

We studied the metabolic rate, cellular energetic state, hypoxia-inducible factor-1 (HIF-1) activation, and expression of enzymes involved in energy metabolism using rainbow trout (Oncorhynchus mykiss) hepatocytes over the oxygen range from 21 to 1 kPa. Oxygen dependence of these factors was assessed by gradually reducing oxygen supply to cells from 21 kPa to 10, 5, 2, and 1 kPa. Moreover, time course experiments for up to 20 h at oxygen tensions of 1 and 2 kPa were carried out. Reduction of oxygen from 21 kPa to 10, 5, 2, and 1 kPa decreased metabolic rate of the cells by 14, 24, 37, and 46%, respectively. This response was instantaneous and fully reversible upon reoxygenation. Cellular ATP content and the expression of all mRNAs studied decreased when oxygen was reduced from 21 to 5 and 2 kPa. The lowest ATP levels, approximately 43% of the initial value, were measured at 5 kPa of oxygen, whereas the reduction in mRNA amounts was most pronounced at 2 kPa. At 1 kPa oxygen tension, both ATP content and mRNA amounts returned to normoxic (21 kPa) levels with a concomitant activation of HIF-1, indicating reorganization of energy metabolism in adaptation of cells to low oxygen supply. These results show that oxygen has a direct regulatory effect on metabolism of trout hepatocyte cultures, supporting the view that oxygen has a profound role in metabolic regulation in cells.

Glucokinase and hexokinase expression and activities in rainbow trout tissues: changes with food deprivation and refeeding

The expression and activities of glucokinase (GK) and hexokinase (HK) were assessed in different tissues of rainbow trout (Oncorhynchus mykiss) under different feeding conditions (fed, fasted for 14 days, and refed for 7 days). Two different HK-I cDNAs were identified with different tissue distributions. One transcript named heart or H-HK-I was observed in the four brain regions assessed, white muscle, kidney, and gills but not in liver or erythrocytes. A second transcript named liver or L-HK-I was found in all tissues surveyed. GK mRNA was identified only in liver and the four brain regions. GK expression was altered by feeding conditions, especially in liver and hypothalamus where food deprivation decreased and re-feeding increased expression; changes in expression reflected activity changes and changes in tissue glycogen levels. In contrast, feeding conditions did not alter expression of either HK-I transcript but did alter tissue HK activities. The reduced phosphorylating capacity noted with food deprivation correlates primarily with changes in tissue HK, whereas increased capacity, as with refeeding, was associated with changes in GK; these changes fit with the different K(m) values of the GK and HK enzymes. These results provide evidence for the hypothalamus acting as a glucosensor in trout, as hyperglycemia produced increased GK expression and activity, as well as increased glycogen levels. Thus, even though trout use glucose poorly, none of the parameters tested here relate to this inability to use glucose and suggest that, at least, rainbow trout, if given an appropriate carbohydrate diet, could metabolically adjust to such a diet.

Food deprivation alters osmoregulatory and metabolic responses to salinity acclimation in gilthead sea bream Sparus auratus

The influence of acclimation to different environmental salinities (low salinity water, LSW; seawater, SW; and hyper saline water, HSW) and feeding conditions (fed and food deprived) for 14 days was assessed on osmoregulation and energy metabolism of several tissues of gilthead sea bream Sparus auratus. Fish were randomly assigned to one of six treatments: fed fish in LSW, SW, and HSW, and food-deprived fish in LSW, SW, and HSW. After 14 days, plasma, liver, gills, kidney and brain were taken for the assessment of plasma osmolality, plasma cortisol, metabolites and the activity of several enzymes involved in energy metabolism. Food deprivation abolished or attenuated the increase in gill Na+,K+-ATPase activity observed in LSW- and HSW-acclimated fish, respectively. In addition, a linear relationship between renal Na+,K+-ATPase activity and environmental salinity was observed after food deprivation, but values decreased with respect to fed fish. Food-deprived fish acclimated to extreme salinities increased production of glucose through hepatic gluconeogenesis, and the glucose produced was apparently exported to other tissues and served to sustain plasma glucose levels. Salinity acclimation to extreme salinities enhanced activity of osmoregulatory organs, which is probably sustained by higher glucose use in fed fish but by increased use of other fuels, such as lactate and amino acids in food-deprived fish.

Glucokinase in chicken (Gallus gallus). Partial cDNA cloning, immunodetection and activity determination

Chickens are more hyperglycaemic and insulin-resistant than mammals, and in efforts to understand their glucose metabolism we investigated whether glucokinase (GK) is present in chicken liver or pancreas. This enzyme plays a major role in glucose-sensing in mammals and we have examined whether it also contributes to glucose homeostasis in chickens. Using RT-PCR, we cloned and sequenced a partial cDNA fragment (750 bp) from liver and pancreas that showed a high degree of identity with mammalian GK. Using antibodies directed towards human GK, we immunodetected a 50 kDa band in chicken liver and pancreas. The molecular mass of the band and its specific interaction with the antibody suggest that this protein corresponds to a chicken homologue of human GK. We also determined by spectrophotometry a glucokinase-like activity in crude liver homogenates with an apparent half saturating concentration for glucose of 8.6 mM. GK gene and protein expression did not differ between fed and 24 h fasted states but GK-like activity was significantly increased in fed chickens. In conclusion, our study provides evidence for the presence of GK gene and protein in chicken liver and pancreas and shows that the liver enzyme is active.

Muscle insulin binding and plasma levels in relation to liver glucokinase activity, glucose metabolism and dietary carbohydrates in rainbow trout

Rainbow trout were fed for 10 weeks with either a carbohydrate-free diet (C-free) or with four experimental diets containing various levels (20 or 40%) and sources of starch (extruded wheat or peas) in order to examine metabolic utilisation of dietary vegetable carbohydrates and its endocrine control. The study was focused on the parameters described as limiting in glucose metabolism in fish. Feeding trials were conducted at 8 and 18 degrees C to establish whether carbohydrate-rich diets can be used in trout farming irrespective of water temperature. At both temperatures, pea diets (especially the highest level) resulted in a feed efficiency as high as the C-free diet. Fish had similar growth rates except when fed the low wheat content diet. Glycaemia values 6 h after feeding were significantly higher in trout fed carbohydrate diets than those given the C-free diet, whereas plasma insulin levels were similar independently of the levels of dietary starch. This study provides the first evidence that glucokinase (GK) activity and mRNA level in trout liver increase in proportion to the content of dietary starch. Nevertheless, these changes were not correlated with plasma insulin levels. Insulin-like growth factor-I (IGF-I) binding and number of receptors in skeletal muscle were consistently higher than those for insulin but no diet-induced differences were found for any of these parameters. Temperature clearly affected the postprandial profile of glucose and insulin, which both showed lower levels 6 h after feeding at 8 degrees C than at 18 degrees C, which was consistent with a lower feed intake. Glucose and insulin levels decreased markedly 24 h after feeding at 18 degrees C, while they were still high at 8 degrees C, an observation concordant with delayed transit rate. These findings indicate satisfactory adaptation of rainbow trout to diets with a relatively high vegetable starch content, especially when provided as extruded peas, and indicate that diets with increased levels of carbohydrates can be used in this species even when it is reared at low temperature.

Metabolic changes in Brycon cephalus (Teleostei, Characidae) during post-feeding and fasting

Metabolic changes during the transition from post-feeding to fasting were studied in Brycon cephalus, an omnivorous teleost from the Amazon Basin in Brazil. Body weight and somatic indices (liver and digestive tract), glycogen and glucose content in liver and muscle, as well as plasma glucose, free fatty acids (FFA), insulin and glucagon levels of B. cephalus, were measured at 0, 12, 24, 48, 72, 120, 168 and 336 h after the last feeding. At time 0 h (the moment of food administration, 09.00 h) plasma levels of insulin and glucagon were already high, and relatively high values were maintained until 24 h post-feeding. Glycemia was 6.42+/-0.82 mM immediately after food ingestion and 7.53+/-1.12 mM at 12 h. Simultaneously, a postprandial replenishment of liver and muscle glycogen reserves was observed. Subsequently, a sharp decrease of plasma insulin occurred, from 7.19+/-0.83 ng/ml at 24 h of fasting to 5.27+/-0.58 ng/ml at 48 h. This decrease coincided with the drop in liver glucose and liver glycogen, which reached the lowest value at 72 h of fasting (328.56+/-192.13 and 70.33+/-14.13 micromol/g, respectively). Liver glucose increased after 120 h and reached a peak 168 h post-feeding, which suggests that hepatic gluconeogenesis is occurring. Plasma FFA levels were low after 120 and 168 h and increased again at 336 h of fasting. During the transition from post-feeding to fast condition in B. cephalus, the balance between circulating insulin and glucagon quickly adjust its metabolism to the ingestion or deprivation of food.

Differential role of adrenoceptors in control of plasma glucose and fatty acids in carp, Cyprinus carpio (L.)

Carp were cannulated in the dorsal aorta, and after 2 days of recovery they were infused with 1) norepinephrine, 2) yohimbine (alpha(2)-antagonist) plus norepinephrine, 3) clonidine (alpha(2)-agonist), and 4) isoproterenol (nonselective beta-agonist). Norepinephrine lowered the plasma free fatty acid (FFA) level and raised the plasma glucose level for several hours. Norepinephrine in combination with the alpha(2)-antagonist yohimbine resulted in retardation of the FFA decrease, indicating the involvement of alpha(2)-adrenoceptors. Infusion with the partial alpha(2)-agonist clonidine had a smaller effect. Infusion with isoproterenol caused a marked increase of glucose levels, and unexpectedly a decline of plasma FFA levels, indicating a direct involvement of beta-adrenoceptors. Combination of isoproterenol with either atenolol (beta(1)-antagonist) or ICI-118,551 (beta(2)-antagonist) showed that both beta(1)- and beta(2)-adrenoceptors were involved in the glucose release by isoproterenol. Remarkably, the decline of FFA levels was augmented in the presence of ICI-118,551, whereas with atenolol present plasma FFA levels were increased by isoproterenol. Thus it is concluded that in carp both beta(1)- and beta(2)-adrenoceptors mediate glucose release, whereas lipolysis is controlled by inhibitory beta(1)-adrenoceptors and stimulatory beta(2)-adrenoceptors, as well as by inhibitory alpha(2)-adrenoceptors.

Nutritional regulation and tissue specificity of gene expression for proteins involved in hepatic glucose metabolism in rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss) are known to use dietary carbohydrates poorly. One of the hypotheses to explain the poor utilisation of dietary glucose by these fish is a dysfunction in nutritional regulation of hepatic glucose metabolism. In this study, we obtained partial clones of rainbow trout cDNAs coding for a glucose transporter (Glut2), and for the enzymes 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF-2K/F-2,6BPase), fructose-1,6-bisphosphatase (FBPase) and pyruvate kinase (PK). Their deduced amino acid sequences were highly similar to those of mammals (up to 80% similarity). In a study of nutritional regulation, the Glut2 gene was highly expressed in the liver irrespective of the nutritional status of the trout, in agreement with the role of this transporter in the input (during refeeding) and output (during fasting) of glucose from the liver. Moreover, whereas PK and FBPase gene expression was high irrespective of the nutritional status, levels of hepatic 6PF-2K/F-2,6BPase mRNA were higher in fish fed with carbohydrates than in fish deprived of food. The high levels of hepatic PK, Glut2 and 6PF-2K/F-2,6BPase gene expression observed in this study suggest a high potential for tissue carbohydrate utilisation in rainbow trout. The persistence of a high level of FBPase gene expression suggests an absence of regulation of the gluconeogenic pathway by dietary carbohydrates.

Glucose intolerance in teleost fish: fact or fiction?

Teleost fish are generally considered to be glucose intolerant. This mini-review examines some of the background and the possible mechanistic bases for this statement. Glucose intolerance is a clinical mammalian term meaning that a glucose load results in persistent hyperglycemia. Teleost fish show persistent hyperglycemia that is generally coincident with transient hyperinsulinemia. The fact that teleost generally have high plasma insulin compared with mammals implies insulin-deficiency is not a suitable explanation for this persistent hyperglycemia. Instead, peripheral utilization of glucose is probably the principle cause of hyperglycemia. Recent evidence for muscle insulin receptors, glucose transporters and hexokinase/glucokinase is reviewed and future experimental directions are suggested. If by altering peripheral glucose utilization fish could become more glucose tolerant, costs to the aquaculture industry may be substantially reduced.

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.