Effects of food-deprivation and refeeding on the regulation and sources of blood glucose appearance in European seabass (Dicentrarchus labrax L.)

Abstract

H 2 O Gluconeogenesis Glycogenolysis Glucose 6-phosphatase Glucokinase NMR Sources of blood glucose in European seabass (initial weight 218.0 ± 43.0 g; mean ± S.D., n = 18) were quantified by supplementing seawater with deuterated water (5%-2 H 2 O) for 72 h and analyzing blood glucose 2 H-enrichments by 2 H NMR. Three different nutritional states were studied: continuously fed, 21-day of fast and 21-day fast followed by 3 days of refeeding. Plasma glucose levels (mM) were 10.7 ± 6.3 (fed), 4.8 ± 1.2 (fasted), and 9.3 ± 1.4 (refed) (means ± S.D., n = 6), showing poor glycemic control. For all conditions, 2 H-enrichment of glucose position 5 was equivalent to that of position 2 indicating that blood glucose appearance from endogenous glucose 6-phosphate (G6P) was derived by gluconeogenesis. G6P-derived glucose accounted for 65 ± 7% and 44 ± 10% of blood glucose appearance in fed and refed fish, respectively, with the unlabeled fraction assumed to be derived from dietary carbohydrate (35 ± 7% and 56 ± 10%, respectively). For 21-day fasted fish, blood glucose appearance also had significant contributions from unlabeled glucose (52 ± 16%) despite the unavailability of dietary carbohydrates. To assess the role of hepatic enzymes in glycemic control, activity and mRNA levels of hepatic glucokinase (GK) and glucose 6-phosphatase (G6Pase) were assessed. Both G6Pase activity and expression declined with fasting indicating the absence of a classical counter-regulatory stimulation of hepatic glucose production in response to declining glucose levels. GK activities were basal during fed and fasted conditions, but were strongly stimulated by refeeding. Overall, hepatic G6Pase and GK showed limited capacity in regulating glucose levels between feeding and fasting states.

New insights into the nutritional regulation of gluconeogenesis in carnivorous rainbow trout (Oncorhynchus mykiss): a gene duplication trail

“…These expression data, combined with the maintenance of gluconeogenic enzyme activity, suggest that the gluconeogenesis pathway remained active under NoCHO nutrition according to the need for endogenous glucose production to sustain maintenance of normoglycemia due to the dietary habits and physiology of carnivorous species (9,12,61,62,65). Furthermore, this is consistent with previous in vivo and in vitro studies showing that the gluconeogenesis flux is never turned off in fed carnivorous teleosts (38,60). Moreover, when trout were submitted to a refasting period we showed that mRNA levels for most of the genes (pck1, fbp1b1, fbp1b2, g6pca, and g6pcb1b) were maintained at the same levels as in fed trout.…”

“…reviewed by Polakof et al 2012 (44)]. Moreover, another adaptive carnivorous trait is a higher rate of hepatic gluconeogenesis (48,54,56,60,63) than omnivorous species, allowing them to sustain endogenous glucose demands, i.e., maintenance of normoglycemia and production of energy using gluconeogenic amino acids and fat rather than dietary carbohydrates (9,12,61,62,65). Probably as a result of no dietary carbohydrate requirements, gluconeogenesis in adult fish is also poorly, if ever, regulated by carbohydrates (19,23,29,(33)(34)(35)37).…”

Effects of dietary carbohydrate on hepatic de novo lipogenesis in European seabass (Dicentrarchus labrax L.)

“…It has been applied so far to study glucose and glycogen synthesis in seabass (58,59), alanine metabolism in gilthead seabream (60), and muscle protein synthesis in catfish (Ictalurus punctatus) (18). In the present study, FA synthesis was affected by the type of starch (raw vs. gelatinized), as already demonstrated in rainbow trout force-fed with 13 C-labeled starch (53).…”

Effects of dietary carbohydrate levels on growth, glucose tolerance, glucose homeostasis and GLUT4 gene expression inTilapia nilotica

“…Insulin is considered as the most important factor for mammals to regulate and maintain the glucose equilibrium (Viegas et al, ). In experiments on the glucose tolerance in the snapper sea bream (Furuichi & Yasuo, ), the plasma insulin level increases with the increase in dietary glucose.…”