Determination of glucose utilization in coho salmon [Oncorhynchus kisutch (Walbaum)]with (6-3H)- and (U-14C)-glucose

Orange-spotted grouper (Epinephelus coioides) adiponectin receptors: molecular characterization, mRNA expression, and subcellular location

Adiponectin is an abundantly secreted adipokine from adipose tissue in mammals, which plays important roles in the regulation of glucose and lipid metabolism. The biological function of adiponectin is mediated by at least two receptors (AdipoR1 and AdipoR2). Although both of them were identified in mammals, there are few researches about adiponectin and its receptors in teleosts. In this study, two types of adiponectin receptors have been isolated and characterized in the orange-spotted grouper (Epinephelus coioides). The cDNAs of grouper AdipoR1 and AdipoR2 are 1444 and 2034 bp in length, encoding proteins of 376 amino acids and 375 amino acids, respectively. Multiple alignment results showed that there was a variable region at the N-terminal of AdipoR1/R2, which has never been reported. Both AdipoR1 and AdipoR2 were found to be widely expressed in various tissues of grouper. Compared to AdipoR2, AdipoR1 expressed at higher levels in the nervous system and pituitary gland, but at lower levels in some peripheral tissues, including heart, liver, adipose tissue, stomach, intestine and especially gonad. Fasting and refeeding experiments showed that the mRNA expressions of AdipoR1/R2 were up-regulated by fasting in the muscle and adipose tissue of grouper, and restored rapidly to normal levels after refeeding. However, the mRNA expressions of AdipoR1/R2 in the hypothalamus and liver of grouper were insensitive to fasting. By indirect immunofluorescence, we demonstrated that grouper AdipoR1/R2 were integral membrane proteins; the C-terminals were extracellular, while the N-terminals were intracellular.

Glucose metabolism in fish: a review

Teleost fishes represent a highly diverse group consisting of more than 20,000 species living across all aquatic environments. This group has significant economical, societal and environmental impacts, yet research efforts have concentrated primarily on salmonid and cyprinid species. This review examines carbohydrate/glucose metabolism and its regulation in these model species including the role of hormones and diet. Over the past decade, molecular tools have been used to address some of the downstream components of these processes and these are incorporated to better understand the roles played by carbohydrates and their regulatory paths. Glucose metabolism remains a contentious area as many fish species are traditionally considered glucose intolerant and, therefore, one might expect that the use and storage of glucose would be considered of minor importance. However, the actual picture is not so clear since the apparent intolerance of fish to carbohydrates is not evident in herbivorous and omnivorous species and even in carnivorous species, glucose is important for specific tissues and/or for specific activities. Thus, our aim is to up-date carbohydrate metabolism in fish, placing it to the context of these new experimental tools and its relationship to dietary intake. Finally, we suggest that new research directions ultimately will lead to a better understanding of these processes.

Glucosensing and glucose homeostasis: from fish to mammals

This review is focused on two topics related to glucose in vertebrates. In a first section devoted to glucose homeostasis we describe how glucose levels fluctuate and are regulated in different classes of vertebrates. The detection of these fluctuations is essential for homeostasis and for other physiological processes such as regulation of food intake. The capacity of that detection is known as glucosensing, and the different mechanisms through which it occurs are known as glucosensors. Different glucosensor mechanisms have been demonstrated in different tissues and organs of rodents and humans whereas the information obtained for other vertebrates is scarce. In the second section of the review we describe the present knowledge regarding glucosensor mechanisms in different groups of vertebrates, with special emphasis in fish.

Utilization of dietary starch and glucose tolerance in juvenile chinook salmon (Oncorhynchus tshawytscha) of different strains in seawater

Juvenile chinook salmon of three strains responded to inclusion of 28.7% of gelatinized starch in the diet with different degrees of reduction in growth rate and feed efficiency relative to control fish of the respective strains fed a low-starch, high-lipid diet of similar protein (46%) and estimated metabolizable energy content (16 mJ/kg). The productive protein value of the diet was not reduced to the same extent by the high intake of starch. Carcasses of fish fed the high-starch diet contained higher concentrations of protein and lower concentrations of lipid than control fish. The accumulation of liver glycogen in response to the high-starch diet differed among strains. Glucose tolerance curves also varied among strains but were poorly correlated with plasma concentrations of insulin. Tolerance to glucose loading was improved in fish previously fed the high-starch diet.

Determination of glucose turnover in sea bass Dicentrarchus labrax. Comparative aspects of glucose utilization

1. Parameters of in vivo glucose utilization by sea bass (132 +/- 6 g, mean +/- SEM) acclimated at 15 degrees C in sea-water were measured after single injection of labelled glucose. 2. Glucose turnover rate (RG; mumol . min-1 . kg-1) was found to be 0.55-065 (2-3H glucose) and 0.34 +/- 0.42 (U14C glucose). 3. Glucose transit time was 443-449 min, glucose mass 233-261 mumol . kg-1, and glucose recycling 37%. 4. Oxygen consumption (MO2) amounted to 94 +/- 6.2 mumol . min-1 . kg-1. 5. The comparison with other fish species, mammals and birds, taking into account body size, temperature, diet, exercise, in poikilotherms and homeotherms leads to the calculation of a glucose turnover index (RGI = RG x 6 x 100 x MO2(-1)). 6. Value of this, generally lower in ectotherm teleosts (2-9), than in endotherms: mammals, birds and thunidae (22-60), confirms the minor quantitative importance of glucose in the metabolism of most fish.

Does the natural diet influence the intestine's ability to regulate glucose absorption?

Two fish species (rainbow trout and common carp) that differ in natural diet also exhibit differences in the adaptive flexibility of their intestinal nutrient transport mechanisms in response to changes in dietary nutrient composition. When carp ingested a feed that was 24% glucose by weight, there was an increase in both the intestinal length and rates of nutrient absorption, particularly for glucose, when compared to carp fed an isonitrogenous diet devoid of digestible carbohydrate. As a result, the intestine's uptake capacity (nmol of glucose and proline absorbed per min per g body weight) was higher in carp fed the 24% glucose feed. Due to the greater increase in glucose uptake, the ratio of glucose uptake relative to proline (G/P ratio) was higher in carp fed the 24% glucose. Thus, carp are able to adapt to the quantity, and apparently also to the type, of digestible carbohydrate in the diet. In contrast, glucose uptake by trout was unresponsive to the quantity of dietary carbohydrate. Instead, the elevated glucose paradoxically resulted in a greater uptake capacity for proline and a lower G/P ratio. Hence, the adaptive capabilities of the intestinal nutrient transport processes are matched to the potential variation in the carbohydrate content of the natural diet.

Effect of insulin on glycogen metabolism in isolated catfish hepatocytes

Insulin effect on carbohydrate metabolism in catfish hepatocytes consisted of a significant decrease of cell glycogen concentration both in the absence and in the presence of glucose in the medium. The hormone did not influence either the output of glucose from the cell or the intracellular glucose level. Experiments with radioactive glucose showed a very low uptake of the sugar by the hepatocytes; correspondingly the incorporation of radioactivity into glycogen was very low and not influenced by insulin. The glycogen content in catfish liver cells was influenced by the hormone in the opposite way to rat liver cells.

Response of rainbow trout (Salmo gairdneri) to increased levels of available carbohydrate in practical trout diets

1. The physiological response of rainbow trout (Salmo gairdneri) reared on different levels of available carbohydrate in practical trout diets having the same levels of energy and nitrogen for 16-24 weeks was determined. 2. Weight gain was significantly reduced in trout reared on the highest level of available carbohydrate, 210 g cerelose (alpha-glucose) kg, and there was a significant linear regression (R2 0.88 of dietary carbohydrate on weight gain. 3. Liver: body-weight values and liver glycogen levels increased in relation to increased dietary carbohydrate. 4. Liver glucose-6-phosphate dehydrogenase (EC 1.1.1.49) activity increased and liver phosphoenolpyruvate carboxykinase (EC 4.1.1.32) activity decreased per kg body-weight of fish with increasing dietary carbohydrate. However, no significant effect was noted on the activity of these liver enzymes above a dietary cerelose level of 140 g/kg. 5. Liver fructose diphosphatase (EC 3.1.3.11) activity increased with increasing dietary carbohydrate has been interpreted as meaning a recycling of triosephosphate to glucose-6-phosphate. 6. Dietary carbohydrate level had no significant effect on the liver pyruvate kinase (EC 2.7.1.40) activity, the rate of glucose utilization or the percentage conversion of [14C]alanine to glucose in the plasma of trout. 7. The results indicate that rainbow trout have a limited ability to adapt to increased dietary carbohydrate and a level in excess of 140 g/kg of the diet is not efficiently utilized.