Changes in plasma glucagon and insulin associated with fasting in sea bass (Dicentrarchus labrax)

Early starvation in European seabass (Dicentrarchus labrax) larvae has no drastic effect on hepatic intermediary metabolism in juveniles

The present study aims to investigate nutritional programming through early starvation in the European seabass (Dicentrarchus labrax). European seabass larvae were fasted at three different developmental periods for three durations from 60 to 65 dph (F1), 81 to 87 dph (F2), and 123 to 133 dph (F3). Immediate effects were investigated by studying gene expression of npy (neuropeptide Y) and avt (Arginine vasotocin) in the head, while potential long-term effects (i.e., programming) were evaluated on intermediary metabolism later in life (in juveniles). Our findings indicate a direct effect regarding gene expression in the head only for F1, with higher avt mRNA level in fasted larved compared to controls. The early starvation periods had no long-term effect on growth performance (body weight and body length). Regarding intermediary metabolism, we analyzed related key plasma metabolites which reflect the intermediary metabolism: no differences for glucose, triglycerides, and free fatty acids in the plasma were observed in juveniles irrespective of the three early starvation stimuli. As programming is mainly linked to molecular mechanisms, we then studied hepatic mRNA levels for 23 key actors of glucose, lipid, amino acid, and energy metabolism. For many of the metabolic genes, there was no impact of early starvation in juveniles, except for three genes involved in glucose metabolism (glut2-glucose transporter and pk-pyruvate kinase) and lipid metabolism (acly-ATP citrate lyase) which were higher in F2 compared to control. Together, these results highlight that starvation between 81 to 87 dph may have more long-term impact, suggesting the existence of a developmental window for programming by starvation. In conclusion, European seabass appeared to be resilient to early starvation during larvae stages without drastic impacts on intermediary metabolism later in life.

Interactions of long-term food ration variation and short-term fasting on insulin-like growth factor-1 (IGF-1) pathways in copper rockfish (Sebastes caurinus)

Variation in food intake affects somatic growth by altering the expression of hormones in the somatotropic endocrine axis including insulin-like growth factor-1 (IGF-1). Here, we examined IGF-1 pathway responses to long- and short-term variation in food availability in copper rockfish (Sebastes caurinus), a nearshore Pacific rockfish important for commercial and recreational fisheries. Juvenile copper rockfish were raised under differing ration amounts (3% or 9% mass feed·g^-1 fish wet mass·day^-1) for 140 d to simulate 'long-term' feeding variation, after which some fish from both rations were fasted for 12 d to generate 'short-term' conditions of food deprivation. Rockfish on the 9% ration treatment grew more quickly than those on the 3% ration and were larger in mass, length, and body condition (k) after 152 d. Fish on the 9% ration had higher blood glucose than those on the 3% ration, with fasting decreasing blood glucose in both ration treatments, indicating that both long-term and short-term feed treatments altered energy status. Plasma IGF-1 was higher in rockfish from the 9% ration than those in the 3% ration and was also higher in fed fish than fasted fish. Additionally, plasma IGF-1 related positively to individual variation in specific growth rate (SGR). The positive association between IGF-1 and SGR showed discordance in fish that had experienced different levels of food and growth over the long-term but not short-term, suggesting that long-term nutritional experience can influence the relationship between IGF-1 and growth in this species. Rockfish on the 3% ration showed a lower relative abundance of gene transcripts encoding igf1 in the liver, but higher hepatic mRNAs for IGF binding proteins igfbp1a and igfbp1b. Fasting similarly decreased the abundance of igf1 mRNAs in the liver of fish reared under both the 9% and 3% rations, while concurrently increasing mRNAs encoding the IGF binding proteins igfbp1a, -1b, and -3a. Hepatic mRNAs for igfbp2b, -5a, and -5b were lower with long-term ration variation (3% ration) and fasting. Fish that experienced long-term reduced rations also had higher mRNA levels for igfbp3a, -3b, and IGF receptors isoforms A (igf1rA) and B (igf1rB) in skeletal muscle, but lower mRNA levels for igf1. Fasting increased muscle mRNA abundance for igfbp3a, igf1rA, and igf1rB, and decreased levels for igfbp2a and igf1. These data show that a positive relationship between circulating IGF-1 and individual growth rate is maintained in copper rockfish even when that growth variation relates to differences in food consumption across varying time scales, but that long- and short-term variation in food quantity can shift basal concentrations of circulating IGF-1 in this species.

Evolution of the glucagon-like system across fish

In fishes, including the jawless lampreys, the most ancient lineage of extant vertebrates, plasma glucose levels are highly variable and regulation is more relaxed than in mammals. The regulation of glucose and lipid in fishes in common with mammals involves members of the glucagon (GCG)-like family of gastrointestinal peptides. In mammals, four peptides GCG, glucagon-like peptide 1 and 2 (GLP1 and GLP2) and glucose-dependent insulinotropic peptide (GIP) that activate four specific receptors exist. However, in lamprey and other fishes the glucagon-like family evolved differently and they retained additional gene family members (glucagon-related peptide, gcrp and its receptor, gcrpr) that are absent from mammals. In the present study, we analysed the evolution of the glucagon-like system in fish and characterized gene expression of the family members in the European sea bass (Dicentrarchus labrax) a teleost fish. Phylogenetic analysis revealed that multiple receptors and peptides of the glucagon-like family emerged early during the vertebrate radiation and evolved via lineage specific events. Synteny analysis suggested that family member gene loss is likely to be the result of a single gene deletion event. Lamprey was the only fish where a putative glp1r persisted and the presence of the receptor gene in the genomes of the elephant shark and coelacanth remains unresolved. In the coelacanth and elephant shark, unique proglucagon genes were acquired which in the former only encoded Gcg and Glp2 and in the latter, shared a similar structure to the teleost proglucagon gene but possessed an extra exon coding for Glp-like peptide that was most similar to Glp2. The variable tissue distribution of the gene transcripts encoding the ligands and receptors of the glucagon-like system in an advanced teleost, the European sea bass, suggested that, as occurs in mammals, they have acquired distinct functions. Statistically significant (p < .05) down-regulation of teleost proglucagon a in sea bass with modified plasma glucose levels confirmed the link between these peptides and metabolism. The tissue distribution of members of the glucagon-like system in sea bass and human suggests that evolution of the brain-gut-peptide regulatory loop diverged between teleosts and mammals despite the overall conservation and similarity of glucagon-like family members.

Effects of long-term feed deprivation on body weight loss, muscle composition, plasma metabolites, and intermediate metabolism of meagre (Argyrosomus regius) under different water temperatures

The effect of feed deprivation at four water temperatures (17, 20, 23, 26 °C) was investigated in meagre (Argyrosomus regius) of initial mean weight ± SD, 116.16 ± 4.74 g, in triplicate groups. Fish were deprived of feed for a period of 60 days and sampled on days 0, 14, 41, and 60, during which body weight, specific growth rate, somatic indices, muscle proximate composition, plasma metabolite levels (total lipids, proteins, cholesterol, triglycerides, glucose), and liver and muscle enzymatic activities [L-lactate dehydrogenase (L-LDH), citrate synthase (CS), malate dehydrogenase (MDH)] were evaluated. Long-term feed deprivation resulted in a significant decrease in body weight, condition factor (CF), hepatosomatic index (HSI), muscle lipids, and plasma metabolites (all except proteins) and increase in muscle moisture. Plasma glucose concentration decreased with time and became significantly lower at 41 and 60 days. Glucose concentration and weight loss expressed a different response in the short term (14 days) than in the long term (14 and 60 days) of feed deprivation, suggesting a change in glucose metabolic profile. After 60 days of feed deprivation, there was an increase in the L-LDH activity in the liver at all temperature levels, which reflects a rising glycolytic potential by activating the carbohydrate metabolism and an ATP-dependent demand. MDH activity increased in the liver and muscle, except at 17 °C in the muscle, which indicates aerobic glycolysis and lipolysis. CS activity in the liver increased after the 60 days, whereas that in the muscle decreased, indicating the muscle is less dependent on aerobic oxidation for energy reserves.

Gluconeogenesis during starvation and refeeding phase is affected by previous dietary carbohydrates levels and a glucose stimuli during early life in Siberian sturgeon (Acipenser baerii)

Gluconeogenesis responses was assessed during a short starvation period and subsequent refeeding in Siberian sturgeon (Acipenser baerii) previously fed different dietary carbohydrates levels and experienced to a glucose stimuli during early life. The sturgeon larvae were previously fed either a high glucose diet (G) or a low glucose diet (F) from the first feeding to yolk absorption (8 to 12 d post-hatching [dph]). Each group of fish was sub-divided into 2 treatments at 13 dph and was fed either a high-carbohydrate diet (H) or a low carbohydrate diet (L) until 20 wk. In the current study, the fish in 4 groups (GL, FL, GH and FH) were experienced to starvation for 21 d following by re-feeding of their corresponding diets for 21 d. Fish were sampled at postprandial 6 and 24 h before starvation (P6h and P24h), starvation 7, 14 and 21 d (S7, S14 and S21) and 1, 7, 14 and 21 d during refeeding (R1, R7, R14 and R21). Plasma samples during refeeding were taken at P6h at each time point. Glycaemia levels, liver and muscle glycogen contents, activities and mRNA levels of hepatic gluconeogenic enzymes were examined. We found that both dietary carbohydrate levels and early glucose stimuli significantly affected the metabolic responses to starvation and refeeding in Siberian sturgeon (P < 0.05). During prolonged starvation, Siberian sturgeon firstly mobilized the liver glycogen and then improved gluconeogenesis when the dietary carbohydrates were abundant, whereas preserved the liver glycogen stores at a stable level and more effectively promoted gluconeogenesis when the dietary carbohydrates are absent to maintain glucose homoeostasis. During refeeding, as most teleostean, Siberian sturgeon failed controlling the activities and mRNA levels of phosphoenolpyruvate carboxykinase cytosolic forms (PEPCK-C), fructose-1,6-bisphosphatase (FBPase), but particularly controlled phosphoenolpyruvate carboxykinase mitochondrial forms (PEPCK-M) activities and mRNA expression of glucose-6-phosphatase (G6Pase, except in GL group). Siberian sturgeon has a full compensatory ability on growth, but this ability would be obstructed by early glucose stimuli when refeeding the low carbohydrate diet after S21.

Integrative Physiology of Fasting

Extended bouts of fasting are ingrained in the ecology of many organisms, characterizing aspects of reproduction, development, hibernation, estivation, migration, and infrequent feeding habits. The challenge of long fasting episodes is the need to maintain physiological homeostasis while relying solely on endogenous resources. To meet that challenge, animals utilize an integrated repertoire of behavioral, physiological, and biochemical responses that reduce metabolic rates, maintain tissue structure and function, and thus enhance survival. We have synthesized in this review the integrative physiological, morphological, and biochemical responses, and their stages, that characterize natural fasting bouts. Underlying the capacity to survive extended fasts are behaviors and mechanisms that reduce metabolic expenditure and shift the dependency to lipid utilization. Hormonal regulation and immune capacity are altered by fasting; hormones that trigger digestion, elevate metabolism, and support immune performance become depressed, whereas hormones that enhance the utilization of endogenous substrates are elevated. The negative energy budget that accompanies fasting leads to the loss of body mass as fat stores are depleted and tissues undergo atrophy (i.e., loss of mass). Absolute rates of body mass loss scale allometrically among vertebrates. Tissues and organs vary in the degree of atrophy and downregulation of function, depending on the degree to which they are used during the fast. Fasting affects the population dynamics and activities of the gut microbiota, an interplay that impacts the host's fasting biology. Fasting-induced gene expression programs underlie the broad spectrum of integrated physiological mechanisms responsible for an animal's ability to survive long episodes of natural fasting.

Characterization, tissue distribution and regulation by fasting of the agouti family of peptides in the sea bass (Dicentrarchus labrax)

The melanocortin system is one of the most complex hormonal systems in vertebrates. Atypically, the signaling of melanocortin receptors is regulated by the binding of endogenous antagonists, named agouti-signaling protein (ASIP) and agouti-related protein (AGRP). Teleost specific genome duplication (TSGD) rendered new gene copies in teleost fish and up to four different genes of the agouti family of peptides have been characterized. In this paper, molecular cloning was used to characterize mRNA of the agouti family of peptides in sea bass. Four different genes were identified: AGRP1, ASIP1, AGRP2 and ASIP2. The AGRP1 gene is mainly expressed in the brain whereas ASIP1 is mainly expressed in the ventral skin. Both ASIP2 and AGRP2 are expressed in the brain and the pineal gland but also in some peripheral tissues. Immunocytochemical studies demonstrated that AGRP1 is exclusively expressed within the lateral tuberal nucleus, the homologue of the mammalian arcuate nucleus in fish. Long-term fasting (8-29 days) increased the hypothalamic expression of AGRP1 but depressed AGRP2 expression (15-29 days). In contrast, the hypothalamic expression of ASIP2 was upregulated during short-term fasting suggesting that this peptide could be involved in the short term regulation of food intake in the sea bass.

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

Sources of blood glucose in European seabass (initial weight 218.0±43.0g; mean±S.D., n=18) were quantified by supplementing seawater with deuterated water (5%-(2)H2O) for 72h 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 3days 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.

Metabolic adjustments of Dentex dentex to prolonged starvation and refeeding

The particular metabolic strategies of the common dentex (Dentex dentex) to face a period of prolonged starvation and subsequent refeeding were assessed. Plasma metabolites, endogenous reserves, and the activity of key enzymes of intermediary metabolism in liver, white muscle, and heart were evaluated. Plasma glucose, total lipid, triglycerides, total-, HDL- and LDL-cholesterol, and protein levels, liver, and white muscle glycogen, and perivisceral, and muscle fat were significantly reduced by starvation, whereas liver lipid content was surprisingly increased. Those enzymes involved in phosphorylation and oxidation of glucose and lipid synthesis, as well as alanine aminotransferase activity, were significantly depressed in liver of starved fish. The increase in β-hydroxyacyl-CoA dehydrogenase (HOAD) indicated an enhanced fatty acid oxidation during starvation. Part of the acetyl-CoA generated by β-oxidation was oxidized in the hepatic Krebs cycle, as reflected the increased citrate synthase (CS) activity. The oxaloacetate required for the reaction catalized by CS activity would be supplied by aspartate aminotransferase (ASAT) activity whose activity was also enhanced. Glutamate dehydrogenase also increased to deaminate the glutamate produced by transaminases, especially by the increased ASAT activity. Liver gluconeogenesis of starved fish was maintained at the same rate that in controls, with glycerol playing an important role as glucogenic substrate. The increased hepatic β-hydroxybutyrate dehydrogenase (β-OHBDH) activity indicates that part of the acetyl-CoA arriving from β-oxidation was being diverted for ketone bodies production with dentex liver playing an important role in providing ketone bodies as fuels for other tissues under such circumstances. Most enzyme activities in white muscle of starved dentex were significantly depressed. In heart, starvation induced an important inhibition of those enzymes involved in glucose and protein metabolism, whereas CS, HOAD, and β-OHBDH activities were maintained at control levels. Although several biomarkers assayed returned to control values after refeeding, many others did not, which indicate that after 3 weeks of refeeding, pre-starved dentex is still experiencing a transient period of metabolic adjustments directed toward the restoration of body mass.

Glycemic and insulin responses in white sea bream Diplodus sargus, after intraperitoneal administration of glucose

A glucose tolerance test was performed in white sea bream Diplodus sargus, juveniles to evaluate the effect of a glucose load on plasma glucose, insulin, triacylglyceride levels, and on liver glycogen storage in order to study the capability of glucose utilization by this species. After being fasted for 48 h, fish were intraperitoneally injected with either 1 g of glucose per kg body weight or a saline solution. Plasma glucose rose from a basal level of 4 to a peak of 18-19 mmol l(-1), 2-4 h after glucose injection and fish exhibited hyperglycemia for 9 h. An insulin peak (from 0.5 to 0.8 ng ml(-1)) was observed 2-6 h after glucose injection, and basal value was attained within 9 h. Liver glycogen peaked 6-12 h after the glucose load and thereafter decreased to the basal value which was attained 24 h after injection. Plasma triacylglycerides in glucose-injected fish were only significantly higher than the basal value 12 h after injection. Glucose-injected fish generally showed lower plasma triacylglyceride levels than control fish. Our results indicate that under these experimental conditions, glucose acts as an insulin secretagogue in white sea bream juveniles. Moreover, insulin may have contributed to restoring basal plasma glucose levels by enhancing glucose uptake in the liver. Further studies are needed to corroborate the lipolytic action of glucose. Clearance of glucose from the blood stream was fast, comparatively to other species, indicating that white sea bream has a good capability of glucose utilization.

The expression of insulin and insulin receptor mRNAs is regulated by nutritional state and glucose in rainbow trout (Oncorhynchus mykiss)

Many species of fish, including rainbow trout, possess multiple INS- and IR-encoding mRNAs. In this study, rainbow trout (Oncorhynchus mykiss) were used as a model to study the regulation of INS (INS1, INS2) and IR (IR1, IR2, IR3, and IR4) mRNA expression by nutritional state and glucose. In the nutritional state study, fish were either fed continuously, fasted (4 or 6 weeks), or fasted 4 weeks, then refed for 2 weeks. Nutritional state regulated INS and IR mRNA expression in a subtype- and tissue-specific manner. A 4-week fast reduced INS1 expression in endocrine pancreas (Brockmann body) and of INS1 and INS2 in brain, whereas a 6-week fast reduced the expression of both INS1 and INS2 in pancreas but only of INS1 in brain. Refeeding only restored INS2 levels in pancreas. In adipose tissue, by contrast, a 4-week fast increased INS1 expression, and a 6-week fast increased the expression of both INS1 and INS2. Nutritional state also modulated the pattern of IR mRNA expression. Fasting for 4 weeks resulted in no significant change in IR expression. Prolonged fasting (6 weeks) increased the expression of IR4 mRNA in the pancreas, adipose tissue, cardiac muscle, and gill; however, fasting decreased expression of IR3 mRNA in liver. Refeeding restored fasting-associated increases in IR4 expression in pancreas, adipose tissue, cardiac muscle, and gill, but had no effect on the fasting-associated decrease in IR3 expression in liver. Glucose differentially regulated the expression of INS and IR mRNAs in Brockmann bodies and liver pieces incubated in vitro, respectively. Low glucose (1 mM) reduced pancreatic expression of both INS1 and INS2 mRNAs compared to levels observed at 4 or 10 mM glucose. In the liver, IR1 and IR2 mRNA expression was insensitive to glucose concentration, whereas expression of IR3 and IR4 was attenuated at 1 and 10 mM compared to 4 mM glucose. These findings indicate that the pattern of INS and IR expression in selected tissues is regulated by nutritional state and glucose.

Analysis of glucose metabolism in farmed European sea bass (Dicentrarchus labrax L.) using deuterated water

Glucose metabolism in free-swimming fasted and fed seabass was studied using deuterated water ((2)H(2)O). After transfer to seawater enriched with 4.9% (2)H(2)O for 6-h or for 72-h, positional and mole percent enrichment (MPE) of plasma glucose and water were quantified by (2)H NMR and ESI-MS/MS. Plasma water (2)H-enrichment reached that of seawater within 6h. In both fasted and fed fish, plasma glucose MPE increased asymptotically attaining ~55% of plasma water enrichment by 72 h. The distribution of (2)H-enrichment between the different glucose positions was relatively uniform. The gluconeogenic contribution to glucose that was synthesized during (2)H(2)O administration was estimated from the ratio of position 5 and 2 glucose enrichments. For both fed and fasted fish, gluconeogenesis accounted for 98±1% of the glucose that was produced during the 72-h (2)H(2)O administration period. For fasted fish, gluconeogenic contributions measured after 6h were identical to 72-h values (94±3%). For fed fish, the apparent gluconeogenic contribution at 6-h was significantly lower compared to 72-h (79±5% versus 98±1%, p<0.05). This may reflect a brief augmentation of gluconeogenic flux by glycogenolysis after feeding and/or selective enrichment of plasma glucose position 2 via futile glucose-glucose-6-phosphate cycling.

Response to short term starvation of growth, haematological, biochemical and non-specific immune parameters in European sea bass (Dicentrarchus labrax) and blackspot sea bream (Pagellus bogaraveo)

Growth, haematological (haematocrit), biochemical (serum cortisol and glucose), and non-specific immune (lysozyme, serum haemolytic and haemagglutinating activities, extracellular respiratory burst activity) parameters, were monitored in European sea bass Dicentrarchus labrax and blackspot sea bream Pagellus bogaraveo subjected to a 31 days starvation compared to fed fish, to assess the responses to feed deprivation of these health status indicators. While haematocrit, serum cortisol, glucose and haemolytic activity of both species did not undergo significant variation following starvation, probably due to the short period applied, some non-specific immune parameters were affected significantly. In the starved sea bass, mucus lysozyme content doubled (1.8 U/mL) compared to the initial value. Haemagglutinating activity was significantly lower in starved sea bass than in fed fish after 31 days. In blackspot sea bream, a slight, not significant, reduction in haemagglutinating activity occurred 11 days after starvation. Respiratory burst activity decreased significantly in the starved fish. In spite of the limited number of examined parameters, the opportunity to use a panel of several indicators to obtain a more complete picture of health status in fish was underlined.

The metabolic effects of prolonged starvation and refeeding in sturgeon and rainbow trout

The present study examines the particular metabolic strategies of the sturgeon Acipenser naccarii in facing a period of prolonged starvation (72 days) and subsequent refeeding (60 days) compared to the trout Oncorhynchus mykiss response under similar conditions. Plasma metabolites, endogenous reserves, and the activity of intermediate enzymes in liver and white muscle were evaluated. This study shows the mobilization of tissue reserves during a starvation period in both species with an associated enzymatic response. The sturgeon displayed an early increase in hepatic glycolysis during starvation. The trout preferentially used lactate for gluconeogenesis in liver and white muscle. The sturgeon had higher lipid-degradation capacity and greater synthesis of hepatic ketone bodies than the trout, although this latter species also showed strong synthesis of ketone bodies during starvation. During refeeding, the metabolic activity present before starvation was recovered in both fish, with a reestablishment of tissue reserves, plasmatic parameters (glucemia and cholesterol), and enzymatic activities in the liver and muscle. A compensatory effect in enzymes regarding lipids, ketone bodies, and oxidative metabolism was displayed in the liver of both species. There are metabolic differences between sturgeon and trout that support the contention that the sturgeon has common characteristics with elasmobranchs and teleosts.

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.

Serine dehydratase and tyrosine aminotransferase activities increased by long-term starvation and recovery by refeeding in rainbow trout (Oncorhynchus mykiss)

Here, we study a cycle of long-term starvation followed by refeeding in relation to the kinetics of serine dehydratase (SerDH) and tyrosine aminotransferase (TyrAT) in rainbow trout (Oncorhynchus mykiss). We determine SerDH- and TyrAT- specific activity at different substrate concentrations in liver and white muscle of juvenile trout starved for 70 days and then refed for 6 hr, 32 hr, 4 days, and 9 days. SerDH showed a hyperbolic kinetic with a K(m) for L-serine of 77.07+/-8.78 mM in the liver of control trout. After 70 days of starvation, the SerDH activity at saturate substrate concentration rose 100% over control. No significant changes were found in the K(m) values of the enzyme. After refeeding, the SerDH activity declined to control values. TyrAT also showed a hyperbolic kinetic with a K(m) for L-tyrosine of 1.86+/-0.12 and 2.55+/-0.57 mM in liver and white muscle, respectively. In starved trout, TyrAT activity in liver and white muscle was about 64 and 267%, respectively, higher than control. After 9 days of refeeding, the control values recovered, although, at 6 hr of refeeding, hepatic TyrAT activity was higher than that for starvation. This work shows that SerDH and TyrAT are present in rainbow trout and that the two enzymes have regulatory functions in the catabolism of their respective amino acids in this species.

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.

Selective changes in the protein-turnover rates and nature of growth induced in trout liver by long-term starvation followed by re-feeding

We report upon the effects of a cycle of long-term starvation followed by re-feeding on the liver-protein turnover rates and nature of protein growth in the rainbow trout (Oncorhynchus mykiss). We determined the protein-turnover rate and its relationship with the nucleic-acid concentrations in the livers of juvenile trout starved for 70 days and then re-fed for 9 days. During starvation the total hepatic-protein and RNA contents decreased significantly and the absolute protein-synthesis rate (A(S)) also fell, whilst the fractional protein-synthesis rate (K(S)) remained unchanged and the fractional protein-degradation rate (K(D)) increased significantly. Total DNA content, an indicator of hyperplasia, and the protein:DNA ratio, an indicator of hypertrophy, both fell considerably. After re-feeding for 9 days the protein-accumulation rates (K(G), A(G)) rose sharply, as did K(S), A(S), K(D)), protein-synthesis efficiency (K(RNA)) and the protein-synthesis rate/DNA unit (K(DNA)). The total hepatic protein and RNA contents increased but still remained below the control values. The protein:DNA and RNA:DNA ratios increased significantly compared to starved fish. These changes demonstrate the high response capacity of the protein-turnover rates in trout liver upon re-feeding after long-term starvation. Upon re-feeding hypertrophic growth increased considerably whilst hyperplasia remained at starvation levels.

Time-course studies on plasma glucose, insulin, and cortisol in sea bass (Dicentrarchus labrax) held under different photoperiodic regimes

Daily variations of insulin, cortisol, and glucose are studied in animals adapted to two different photoperiodic regimes, with the intervals between feeding times and photoperiodic events kept constant. Data support the existence of a daily rhythm of plasma glucose which seems to be photoperiodic. In contrast, the daily patterns of insulin in sea bass seem to be mainly influenced by feeding time; however, an effect of photoperiod can not be excluded. When the digestive tract is absent of food, insulin levels are generally minimal at feeding times and maximal during the inter-meals periods, suggesting the central control of insulin secretion during short-term food deprivation. Contrarily, the nadir values of plasma cortisol were reached at midday during the inter-meal period and peak plasma levels were evident at both light onset and offset. Disruption between metabolite and hormone patterns suggest that they are under different controls. Such results could be explained under the existence of a multioscillator system, including a food entrainable oscillator in addition to the master light entrainable oscillator.

Food deprivation and refeeding in Atlantic salmon,Salmo salar: effects on brain and liver carbohydrate and ketone bodies metabolism

The capacity of carbohydrate and ketone bodies metabolism in brain and liver was evaluated in fed and food-deprived Atlantic salmon (Salmo salar) in a time period covering from 1 to 7 days (Experiment I), and in Atlantic salmon food deprived for 6 weeks, and food deprived for 4 weeks and refed for 2 weeks (Experiment II). The results obtained demonstrate for the first time in a teleost the existence of changes in brain metabolism due to food deprivation. Thus, decreased glucose levels in plasma are reflected in the brain by an increased mobilization of glycogen reserves, and by a decreased glycolytic capacity. Also, ketone bodies appear to increase their importance as a metabolic fuel from day 7 of food deprivation onwards. A possible increase in the gluconeogenic potential in brain simultaneously is not discarded. All these metabolic changes are reversed under refeeding conditions.

Changes in plasma glucagon, insulin and tissue metabolites associated with prolonged fasting in brown trout (Salmo trutta fario) during two different seasons of the year

1. Pyrenean brown trout juveniles (Salmo trutta fario) were fasted for 50 days in late winter (experiment 1) and summer (experiment 2). Plasma insulin, glucagon and glucose and some metabolites in plasma and in tissues were analysed. 2. Glucagon increased significantly on the 3rd day of fasting in the winter experiment (controls 653.7 +/- 92.4 pg/ml, fasted 912.7 +/- 135.2 pg/ml), and the same tendency was observed on the 5th day in the summer experiment (controls 430.5 +/- 56.2 pg/ml, fasted 555.5 +/- 95.3 pg/ml). During this initial period of fasting, plasma glucose was maintained in both experiments (75.5 +/- 4.7-67.6 +/- 4.1 mg/100 ml), but from day 8, glucose and glucagon decreased simultaneously. 3. Insulin decreased from the beginning of fasting, reaching lowest values after 50 days of fasting (winter experiment: controls 6.4 +/- 0.3 ng/ml, fasted 1.6 +/- 0.1 ng/ml; summer experiment: controls 4.8 +/- 0.1 ng/ml, fasted 1.2 +/- 0.2 ng/ml). Glucagon/insulin molar ratio (G/I) increased after 3 days in the winter experiment (controls 0.21 +/- 0.02, fasted 0.39 +/- 0.05), while in the summer experiment, the ratio rose from day 5 and reached a peak at day 30 (controls 0.16 +/- 0.02, fasted 0.48 +/- 0.07). 4. Muscle proteins were significantly mobilized after 50 days of fasting. Visceral index decreased significantly after day 15 while liver glycogen was already significantly lower at day 8. However, in the summer experiment, a transitory increase of liver glycogen was observed at day 30, coinciding with the peak of G/I.