Glucose, glycogen, and insulin responses in the hypothermic rat

The rat appears to be unable to utilize glucose during hypothermia. The objective of this study was to examine carbohydrate homeostasis during induction, hypothermia, and rewarming phases. Groups of normothermic animals were euthanized to serve as time controls for comparison. Hypothermia (15 degrees C) was produced by exposure to helox (80% helium:20% oxygen) at 0 +/- 1 degree C. Hyperglycemia was noted during the induction process (169 +/- 8 in control vs 326 +/- 49 mg/dl). Serum glucose increased further during 4 hr of hypothermia, but following rewarming (Tre of 33 +/- 1 degrees C) was reduced (153 +/- 16 mg/dl) significantly (P less than 0.05). Serum insulin was depressed during hypothermic induction (from 48 +/- 4 in controls to 19 +/- 3 microU/ml in hypothermic rats) and increased only slightly during the arousal process, remaining significantly lower than in normothermic subjects. Initial hepatic, skeletal muscle, and cardiac glycogen concentrations were reduced 34, 68, and 75%, respectively, during hypothermic induction. While liver glycogen decreased further during 4 hr of hypothermia, skeletal and cardiac stores increased markedly. During rewarming, hepatic glycogen was markedly decreased, while skeletal and cardiac stores were maintained. These data suggest that hyperglycemia in the hypothermic rat can be accounted for by glycogenolysis and hypoinsulinemia. In addition, this study indicates repletion of skeletal and cardiac muscle glycogen during maintained hypothermia and sparing of muscle glycogen during rewarming.     

Evidence against glycogen cycling of gluconeogenic substrates in various liver preparations

The effect of inhibition of glycogen phosphorylase by 1,4-dideoxy-1,4-imino-d-arabinitol on rates of gluconeogenesis, gluconeogenic deposition into glycogen, and glycogen recycling was investigated in primary cultured hepatocytes, in perfused rat liver, and in fed or fasted rats in vivo clamped at high physiological levels of plasma lactate. 1,4-Dideoxy-1,4-imino-d-arabinitol did not alter the synthesis of glycerol-derived glucose in hepatocytes or lactate-derived glucose in perfused liver or fed or fasted rats in vivo. Thus, 1,4-dideoxy-1,4-imino-d-arabinitol inhibited hepatic glucose output in the perfused rat liver (0.77 +/- 0.19 versus 0.33 +/- 0.09, p < 0.05), whereas the rate of lactate-derived gluconeogenesis was unaltered (0.22 +/- 0.09 versus 0.18 +/- 0.08, p = not significant) (1,4-dideoxy-1,4-imino-d-arabinitol versus vehicle, micromol/min * g). Overall, the data suggest that 1,4-dideoxy-1,4-imino-d-arabinitol inhibited glycogen breakdown with no direct or indirect effects on the rates of gluconeogenesis. Total end point glycogen content (micromol of glycosyl units/g of wet liver) were similar in fed (235 +/- 19 versus 217 +/- 22, p = not significant) or fasted rats (10 +/- 2 versus 7 +/- 2, p = not significant) with or without 1,4-dideoxy-1,4-imino-d-arabinitol, respectively. The data demonstrate no glycogen cycling under the investigated conditions and no effect of 1,4-dideoxy-1,4-imino-d-arabinitol on gluconeogenic deposition into glycogen. Taken together, these data also suggest that inhibition of glycogen phosphorylase may prove beneficial in the treatment of type 2 diabetes.     

Postexercise muscle glycogen resynthesis in obese insulin-resistant Zucker rats.

We determined the effect of an acute bout of swimming (8 x 30 min) followed by either carbohydrate administration (0.5 mg/g glucose ip and ad libitum access to chow; CHO) or fasting (Fast) on postexercise glycogen resynthesis in soleus muscle and liver from female lean (ZL) and obese insulin-resistant (ZO) Zucker rats. Resting soleus muscle glycogen concentration ([glycogen]) was similar between genotypes and was reduced by 73 (ZL) and 63% (ZO) after exercise (P < 0.05). Liver [glycogen] at rest was greater in ZO than ZL (334 +/- 31 vs. 247 +/- 16 micromol/g wet wt; P < 0.01) and fell by 44 and 94% after exercise (P < 0.05). The fractional activity of glycogen synthase (active/total) increased immediately after exercise (from 0.22 +/- 0.05 and 0.32 +/- 0.04 to 0.63 +/- 0.08 vs. 0.57 +/- 0.05; P < 0.01 for ZL and ZO rats, respectively) and remained elevated above resting values after 30 min of recovery. During this time, muscle [glycogen] in ZO increased 68% with CHO (P < 0.05) but did not change in Fast. Muscle [glycogen] was unchanged in ZL from postexercise values after both treatments. After 6 h recovery, GLUT-4 protein concentration was increased above resting levels by a similar extent for both genotypes in both fasted (approximately 45%) and CHO-supplemented (approximately 115%) rats. Accordingly, during this time CHO refeeding resulted in supercompensation in both genotypes (68% vs. 44% for ZL and ZO). With CHO, liver [glycogen] was restored to resting levels in ZL but remained at postexercise values for ZO after both treatments. We conclude that the increased glucose availability with carbohydrate refeeding after glycogen-depleting exercise resulted in glycogen supercompensation, even in the face of muscle insulin-resistance.     

Effects of insulin and beta-adrenergic blockade on certain indicators of carbohydrate metabolism in the blood and tissues of rats during short-lasting hypothermia

Administration of insulin 1 i.u./100 g of body weight to hypothermic rats causes a fall of glucose and lactate levels in the serum and a rise in myocardial glycogen level in relation to the group of control rats kept at room temperature and to the group of rats subjected only to hypothermia. Beta-adrenergic blockade (propranolol 0.6-1 mg/kg) caused no changes in the levels of carbohydrate metabolites in the serum of hypothermic rats but raised the myocardial glycogen level by 42% in relation to the animals subjected only to hypothermia. Simultaneous administration of both these agents during hypothermia produces a fall of the serum levels of glucose and pyruvate with a rise in the level of lactate, and raises the glycogen level in the myocardium (by about 161%) and in the skeletal muscle (by 54%) in relation to the rats subjected to hypothermia alone. Insulin and/or propranolol fail to prevent glycogen reserve exhaustion in the liver of hypothermic rats which could be due to activation of non-blocked alpha-adrenergic receptors or to the action of yet another glycogenolytic agent, e.g. glucagon, during hypothermia.     

Reversal of diet-induced glucose intolerance by hepatic expression of a variant glycogen-targeting subunit of protein phosphatase-1.

Glycogen-targeting subunits of protein phosphatase-1 facilitate interaction of the phosphatase with enzymes of glycogen metabolism. Expression of one family member, PTG, in the liver of normal rats improves glucose tolerance without affecting other plasma variables but leaves animals unable to reduce hepatic glycogen stores in response to fasting. In the current study, we have tested whether expression of other targeting subunit isoforms, such as the liver isoform G(L), the muscle isoform G(M)/R(Gl), or a truncated version of G(M)/R(Gl) termed G(M)DeltaC in liver ameliorates glucose intolerance in rats fed on a high fat diet (HF). HF animals overexpressing G(M)DeltaC, but not G(L) or G(M)/R(Gl), exhibited a decline in blood glucose of 35-44 mg/dl relative to control HF animals during an oral glucose tolerance test (OGTT) such that levels were indistinguishable from those of normal rats fed on standard chow at all but one time point. Hepatic glycogen levels were 2.1-2.4-fold greater in G(L)- and G(M)DeltaC-overexpressing HF rats compared with control HF animals following OGTT. In a second set of studies on fed and 20-h fasted HF animals, G(M)DeltaC-overexpressing rats lowered their liver glycogen levels by 57% (from 402 +/- 54 to 173 +/- 27 microg of glycogen/mg of protein) in the fasted versus fed states compared with only 44% in G(L)-overexpressing animals (from 740 +/- 35 to 413 +/- 141 microg of glycogen/mg of protein). Since the OGTT studies were performed on 20-h fasted rats, this meant that G(M)DeltaC-overexpressing rats synthesized much more glycogen than G(L)-overexpressing HF rats during the OGTT (419 versus 117 microg of glycogen/mg of protein, respectively), helping to explain why G(M)DeltaC preferentially enhanced glucose clearance. We conclude that G(M)DeltaC has a unique combination of glycogenic potency and responsiveness to glycogenolytic signals that allows it to be used to lower blood glucose levels in diabetes.     

Protection to glycolysis by a combination of 5-hydroxy-L-tryptophan and 2-aminoethylisothiuronium bromide hydrobromide in lethally irradiated rats.

Rate of glycolysis in vivo at different time intervals following 8 Gy [LD100(30)] whole body gamma radiation (WBGR) was evaluated by estimating liver glycogen, blood sugar, serum lactic dehydrogenase (LDH) and blood lactic acid concentration in adult male Sprague Dawley rats. Within 1 hr of radiation exposure, a significant fall in liver glycogen was observed in rats fed food and water ad libitum. The glycogen content increased after 24 hr and had returned to control level on 7th day after radiation exposure. Blood sugar, serum LDH and blood lactate levels increased significantly as compared to non irradiated controls. Pretreatment with 5-hydroxy-L-tryptophan (5-HTP; 100 mg/kg) + 2-aminoethylisothiuronium bromide hydrobromide (AET; 20 mg/kg) ip 30 min before 8 Gy WBGR, modified these values and restored them to normal level on 7th day post-irradiation.     

Endocrine regulation of carbohydrate metabolism in hypometabolic animals

Experimental hypothermia and natural hibernation are two forms of hypometabolism with recognized physiological changes, including depression of endocrine and metabolic functions. To better understand functional changes, helox (i.e., helium and oxygen (80:20) mixtures) and low ambient temperatures have been used to induce hypothermia in hamsters and rats. Both clinical and biological survival, i.e., survival without recovery and survival with recovery from hypothermia, respectively, are related to depth and length of hypothermia. In the rat, body temperatures of 15 degrees C for periods greater than 6-10 h greatly restrict biological survival. The role of glucocorticoids in enhancing thermogenic capacity of rats was assessed using triamcinolone [correction of triamcinalone] acetonide. In the hamster, treatment with cortisone acetate prolonged both clinical and biological survival. Hypothermic hamsters continue utilizing circulating glucose until they become hypoglycemic and die. Hypothermic rats do not utilize glucose and respond with a significant hypoinsulinema. The role of endocrines in the regulation of carbohydrate homeostasis and metabolism differs in hibernation and hypothermia. Glucocorticoids influence the hypothermic response in both species, specifically by prolonging induction of hypothermia in rats and by prolonging survival in hypothermic hamsters.     

Hepatic Overexpression of a Constitutively Active Form of Liver Glycogen Synthase Improves Glucose Homeostasis

In this study, we tested the efficacy of increasing liver glycogen synthase to improve blood glucose homeostasis. The overexpression of wild-type liver glycogen synthase in rats had no effect on blood glucose homeostasis in either the fed or the fasted state. In contrast, the expression of a constitutively active mutant form of the enzyme caused a significant lowering of blood glucose in the former but not the latter state. Moreover, it markedly enhanced the clearance of blood glucose when fasted rats were challenged with a glucose load. Hepatic glycogen stores in rats overexpressing the activated mutant form of liver glycogen synthase were enhanced in the fed state and in response to an oral glucose load but showed a net decline during fasting. In order to test whether these effects were maintained during long term activation of liver glycogen synthase, we generated liver-specific transgenic mice expressing the constitutively active LGS form. These mice also showed an enhanced capacity to store glycogen in the fed state and an improved glucose tolerance when challenged with a glucose load. Thus, we conclude that the activation of liver glycogen synthase improves glucose tolerance in the fed state without compromising glycogenolysis in the postabsorptive state. On the basis of these findings, we propose that the activation of liver glycogen synthase may provide a potential strategy for improvement of glucose tolerance in the postprandial state.     

Substrate utilization during acute exercise in obese Zucker rats

The purpose of the present study was to compare the carbohydrate use of insulin-resistant obese Zucker rats with that of their lean littermates during steady-state exercise. Obese and lean rats were randomly assigned to a sedentary group or to a run group in which rats ran at 72-73% of their maximal O2 consumption, with the duration of exercise set to require an energy expenditure of 2.1-2.2 kcal. During the run the respiratory exchange ratio was significantly higher in the obese than in the lean rats [0.94 +/- 0.01 (SE) and 0.86 +/- 0.01, respectively], which indicate that the obese rats required 54% more carbohydrate than the lean rats. Total muscle glycogen utilization in the soleus, plantaris, and red and white gastrocnemius was not different between groups. Obese rats had total liver glycogen values five times greater than those of lean rats (833.38 +/- 101.4 and 152.8 +/- 37.5 mg, respectively) and utilized twice as much liver glycogen as their lean littermates (193.5 and 90.4 mg, respectively). The obese rats exhibited higher blood glucose and insulin concentrations than the lean rats during the run. These findings indicate that, despite their characteristic insulin resistance, the obese Zucker rats had a greater dependency on carbohydrate as a substrate during exercise than their lean littermates and that the major source of this carbohydrate was liver glycogen.     

Effects of manganese deficiency on pyruvate carboxylase and phosphoenolpyruvate carboxykinase activity and carbohydrate homeostasis in adult rats

The activities of two liver gluconeogenic enzymes, pyruvate carboxylase (PC) and phosphoenolpyruvate carboxykinase (PEPCK), as well as liver glycogen and plasma glucose, insulin, and glucagon were measured in first- and second-generation, manganese-sufficient (control) and manganese-deficient (Mn?) adult rats. Pyruvate carboxylase activity of first generation male Mn? rats was higher than that of controls in both the fed and fasted states. In contrast, PC activity in second generation male Mn? rats was lower than control levels. In female rats, PC activity was lower than controls in both fed, first- and second-generation Mn? rats; in the fasted state, PC activity was either the same or higher than controls. Phosphoenolpyruvate carboxykinase activity was lower in male first generation Mn? rats than in controls, but there was no difference in PEPCK activity in second-generation animals. Phosphoenolpyruvate carboxylase activity was lower in both fed and fasted Mn? female rats than in controls. Plasma insulin levels were lower in the deficient rats than in controls, whereas plasma glucagon levels were similar. Manganese-deficient rats had higher concentrations of liver glycogen than their controls. These findings provide further evidence that manganese affects carbohydrate homeostasis; however, the response of the animal to manganese deficiency depends on the parameter studied and the timing of the deficiency.     

Are Kupffer cells involved in the metabolic adaptation of the liver to dietary carbohydrates given after fasting?

In rats, a high carbohydrate fat-free (HCFF) diet, given after fasting, induces both hepatic lipogenic and glycogenic enzymes. In the present study, we evaluated the involvement of Kupffer cells in the metabolic events occurring in the liver during the fasting-refeeding transition. Male Wistar rats were fasted for 48 h and received an intravenous injection of either NaCl 0.9% (Gd-) or 10 mg/kg GdCl(3) (Gd+), an inhibitor of Kupffer cells, then fed for 12 h with a HCFF diet. The comparison of colloidal carbon uptake was similar in rats fasted and in rats fasted and then refed a HCFF diet, thus indicating that refeeding does not affect per se Kupffer cell phagocytic activity. The inhibition of Kupffer cells by GdCl(3) did not affect fatty acid synthase (FAS) induction, as shown by the analysis of both FAS mRNA and activity; refeeding a HCFF diet increased the hepatic triglyceride and glycogen content to the same extent in Gd+ and Gd- rats. Our results do not support the involvement of Kupffer cells in the metabolic events occurring in the liver tissue by feeding a HCFF diet after fasting. However, the discussion supports the involvement of Kupffer cells in the modulation of the hepatic lipid metabolism by other nutrients than carbohydrates.     

SITES OF GLYCOGEN SYNTHESIS IN RAT LIVER CELLS AS SHOWN BY ELECTRON MICROSCOPE RADIOAUTOGRAPHY AFTER ADMINISTRATION OF GLUCOSE-H3

Glycogen synthesis was investigated by giving tritium (H³)-labeled glucose with carrier to fasted rats in vivo or incubating liver slices from fasted rats in vitro using a glucose-H³-containing medium. After 15 min or 1 hr, pieces of liver were fixed and radioautographed for light and electron microscopy. In vivo and in vitro, radioautographic reactions appeared over "glycogen areas" and over zones transitional between these areas and ergastoplasm. Treatment of sections by alpha amylase removed all but about 5% of the radioactivity, so that about 95% of it consisted of glycogen (synthesized during the 15 min or 1 hr elapsing after administration of glucose-H³). Within glycogen areas and transitional zones, most silver grains were over or very close to glycogen granules and smooth (or partly smooth) vesicles. Presumably, much of the label was added onto growing glycogen granules, in accord with the biochemical view that glycogen may serve as substrate for further glycogen synthesis. The few silver grains located far from glycogen granules—15% at the 15 min interval in vivo—approximated smooth (or partly smooth) vesicles of endoplasmic reticulum. This observation raised the possibility that smooth membranes play a role in glucose uptake at an early stage in de novo formation of glycogen granules.     

Effect of verapamil on glycogenolysis and gluconeogenesis in the perfused rat liver

In perfused livers from fed rats, rates of glucose production (glycogenolysis) were 133 +/- 12 mumol/g/hr. Infusion of 2 microM verapamil into these livers decreased the rates of glucose production significantly to 97 +/- 15 mumol/g/hr within 10 min. Conversely, rates of production of lactate plus pyruvate (glycolysis) of 64 +/- 6 mumol/g/hr were not significantly altered by verapamil (60 +/- 3 mumol/g/hr). When 50 microM verapamil was infused, however, rates of both glycogenolysis and glycolysis were diminished to 56 +/- 11 and 43 +/- 5 mumol/g/hr, respectively. In perfused livers from fasted rats, infusion of 20 mM fructose increased the rates of production of glucose (gluconeogenesis) significantly from 11 +/- 7 to 121 +/- 17 mumol/g/hr. These rates reached 138 +/- 7 mumol/g/hr upon the simultaneous infusion of verapamil (2 microM). In these livers, fructose also increased rates of production of lactate from 6 +/- 2 to 132 +/- 11 mumol/g/hr, which were further increased to 143 +/- 8 mumol/g/hr when 2 microM verapamil was infused. The results show that calcium-dependent processes involved in hepatic carbohydrate metabolism respond differently to the calcium channel blocker verapamil. Low concentrations of verapamil inhibited glycogenolysis significantly while having no effect on either glycolysis or gluconeogenesis. These data suggest that these two processes have different sensitivities to changes in intracellular calcium concentrations and/or different sources of regulatory calcium.     

Glycogen metabolism as detected by in vivo and in vitro 13C-NMR spectroscopy using [1,2-13C2]glucose as substrate

The metabolism of glucose to glycogen in the liver of fasted and well-fed rats was investigated with 13C nuclear magnetic resonance spectroscopy using [1,2-(13)C2]glucose as the main substrate. The unique spectroscopic feature of this molecule is the 13C-13C homonuclear coupling leading to characteristic doublets for the C-1 and C-2 resonances of glucose and its breakdown products as long as the two 13C nuclei remain bonded together. The doublet resonances of [1,2-(13)C2]glucose thus provide an ideal marker to follow the fate of this exogenous substrate through the metabolic pathways. [1,2-(13)C2]Glucose was injected intraperitoneally into anesthetized rats and the in vivo 13C-NMR measurements of the intact animals revealed the transformation of the injected glucose into liver glycogen. Glycogen was extracted from the liver and high resolution 13C-NMR spectra were obtained before and after hydrolysis of glycogen. Intact [1,2-13C2]glucose molecules give rise to doublet resonances, natural abundance [13C]glucose molecules produce singlet resonances. From an analysis of the doublet-to-singlet intensities the following conclusions were derived. (i) In fasted rats virtually 100% of the glycosyl units in glycogen were 13C-NMR visible. In contrast, the 13C-NMR visibility of glycogen decreased to 30-40% in well-fed rats. (ii) In fed rats a minimum of 67 +/- 7% of the exogenous [1,2-(13)C2]glucose was incorporated into the liver glycogen via the direct pathway. No contribution of the indirect pathway could be detected. (iii) In fasted rats externally supplied glucose appeared to be consumed in different metabolic processes and less [1,2-(13)C2]glucose was found to be incorporated into glycogen (13 +/- 1%). However, the observation of [5,6-(13)C2]glucose in liver glycogen provided evidence for the operation of the so-called indirect pathway of glycogen synthesis. The activity of the indirect pathway was at least 9% but not more than 30% of the direct pathway. (vi) The pentose phosphate pathway was of little significance for glucose but became detectable upon injection of [1-(13)C]ribose.     

Studies on the alpha-adrenergic activation of hepatic glucose output. I. Studies on the alpha-adrenergic activation of phosphorylase and gluconeogenesis and inactivation of glycogen synthase in isolated rat liver parenchymal cells.

Epinephrine and the alpha-adrenergic agonist phenylephrine activated phosphorylase, glycogenolysis, and gluconeogenesis from lactate in a dose-dependent manner in isolated rat liver parenchymal cells. The half-maximally active dose of epinephrine was 10-7 M and of phenylephrine was 10(-6) M. These effects were blocked by alpha-adrenergic antagonists including phenoxybenzamine, but were largely unaffected by beta-adrenergic antagonists including propranolol. Epinephrine caused a transient 2-fold elevation of adenosine 3':5'-monophosphate (cAMP) which was abolished by propranolol and other beta blockers, but was unaffected by phenoxybenzamine and other alpha blockers. Phenoxybenzamine and propranolol were shown to be specific for their respective adrenergic receptors and to not affect the actions of glucagon or exogenous cAMP. Neither epinephrine (10-7 M), phenylephrine (10-5 M), nor glucagon (10-7 M) inactivated glycogen synthase in liver cells from fed rats. When the glycogen synthase activity ratio (-glucose 6-phosphate/+ glucose 6-phosphate) was increased from 0.09 to 0.66 by preincubation of such cells with 40 mM glucose, these agents substantially inactivated the enzyme. Incubation of hepatocytes from fed rats resulted in glycogen depletion which was correlated with an increase in the glycogen synthase activity ratio and a decrease in phosphorylase alpha activity. In hepatocytes from fasted animals, the glycogen synthase activity ratio was 0.32 +/- 0.03, and epinephrine, glucagon, and phenylephrine were able to lower this significantly. The effects of epinephrine and phenylephrine on the enzyme were blocked by phenoxybenzamine, but were largely unaffected by propranolol. Maximal phosphorylase activation in hepatocytes from fasted rats incubated with 10(-5) M phenylephrine preceded the maximal inactivation of glycogen synthase. Addition of glucose rapidly reduced, in a dose-dependent manner, both basal and phenylephrine-elevated phosphorylase alpha activity in hepatocytes prepared from fasted rats. Glucose also increased the glycogen synthase activity ratio, but this effect lagged behind the change in phosphorylase. Phenylephrine (10-5 M) and glucagon (5 x 10(-10) M) decreased by one-half the fall in phosphoryalse alpha activity seen with 10 mM glucose and markedly suppressed the elevation of glycogen synthase activity. The following conclusions are drawn from these findings. (a) The effects of epinephrine and phenylephrine on carbohydrate metabolism in rat liver parenchymal cells are mediated predominantly by alpha-adrenergic receptors. (b) Stimulation of these receptors by epinephrine or phenylephrine results in activation of phosphorylase and gluconeogenesis and inactivation of glycogen synthase by mechanisms not involving an increase in cellular cAMP. (c) Activation of beta-adrenergic receptors by epinephrine leads to the accumulation of cAMP, but this is associated with minimal activation of phosphorylase or inactivation of glycogen synthase...     

Post-exercise ketosis and the glycogen content of liver and muscle in rats on a high carbohydrate diet

Post-exercise ketosis is known to be suppressed by physical training and by a high carbohydrate diet. As a result it has often been presumed, but not proven, that the development of post-exercise ketosis is closely related to the glycogen content of the liver. We therefore studied the effect of 1 h of treadmill running on the blood 3-hydroxybutyrate and liver and muscle glycogen concentrations of carbohydrate-loaded trained (n = 72) and untrained rats (n = 72). Resting liver and muscle glycogen levels were 25%-30% higher in the trained than in the untrained animals. The resting 3-hydroxybutyrate concentrations of both groups of rats were very low: less than 0.08 mmol.l-1. Exercise did not significantly influence the blood 3-hydroxybutyrate concentrations of trained rats, but caused a marked post-exercise ketosis (1.40 +/- 0.40 mmol.l-1 h after exercise) in the untrained animals, the time-course of which was the approximate inverse of the changes in liver glycogen concentration. Interpreting the results in the light of similar data obtained after a normal and low carbohydrate diet it has been concluded that trained animals probably owe their relative resistance to post-exercise ketosis to their higher liver glycogen concentrations as well as to greater peripheral stores of mobilizable carbohydrate.     

Dietary fructose induces a wide range of genes with distinct shift in carbohydrate and lipid metabolism in fed and fasted rat liver

Dietary fructose has been suspected to contribute to development of metabolic syndrome. However, underlying mechanisms of fructose effects are not well characterized. We investigated metabolic outcomes and hepatic expression of key regulatory genes upon fructose feeding under well defined conditions. Rats were fed a 63% (w/w) glucose or fructose diet for 4 h/day for 2 weeks, and were killed after feeding or 24-hour fasting. Liver glycogen was higher in the fructose-fed rats, indicating robust conversion of fructose to glycogen through gluconeogenesis despite simultaneous induction of genes for de novo lipogenesis and increased liver triglycerides. Fructose feeding increased mRNA of previously unidentified genes involved in macronutrient metabolism including fructokinase, aldolase B, phosphofructokinase-1, fructose-1,6-bisphosphatase and carbohydrate response element binding protein (ChREBP). Activity of glucose-6-phosphate dehydrogenase, a key enzyme for ChREBP activation, remained elevated in both fed and fasted fructose groups. In the fasted liver, the fructose group showed lower non-esterified fatty acids, triglycerides and microsomal triglyceride transfer protein mRNA, suggesting low VLDL synthesis even though plasma VLDL triglycerides were higher. In conclusion, fructose feeding induced a broader range of genes than previously identified with simultaneous increase in glycogen and triglycerides in liver. The induction may be in part mediated by ChREBP.     

Influence of fasting on glycogen depletion in rats during exercise

We recently observed that a 24-h fasted group of rats could run longer than an ad libitum fed control group before becoming exhausted. Because of the demonstrated importance of glycogen levels and free fatty acid availability during endurance exercise, we have investigated several parameters of carbohydrate and lipid metabolism in exercised and nonexercised rats that were either fed ad libitum or fasted for 24 h. A 24-h fast depleted liver glycogen, lowered plasma glucose concentration, decreased muscle glycogen levels, and increased free fatty acid and beta-hydroxybutyrate concentrations in plasma. During exercise the fasted group had lower plasma glucose concentration, higher plasma concentration of free fatty acids and beta-hydroxybutyrate, and a lower muscle glycogen depletion rate than did the ad libitum fed group. Since fasted rats were able to continue running even when plasma glucose had dropped to levels lower than those of fed-exhausted rats, it seems unlikely that blood glucose level, per se, is a factor in causing exhaustion. These results suggest that fasting increases fatty acid utilization during exercise and the resulting "glycogen sparing" effect may result in increased endurance.     

Characteristics of the early recovery period and the level of carbohydrate-lipid metabolic substrates after 24-hour immobilization hypothermia

The peculiarities of 24-hour hypothermia were studied in rats during immobilization and a 5-hour recovery period. The levels of glucose in blood and liver, glycogen in liver, skeletal muscles and heart, total lipids in liver and blood, nonesterified fatty acids in blood have shown that the speed of rewarming is limited both by the level of carbohydrate and lipid energy substrates.     

Effect of host feeding and available glucose on glycogen synthase and phosphorylase activities in Hymenolepis diminuta and Vampirolepis microstoma

The influences of host feeding and the availability of glucose in vitro on the activities of glycogen synthase and glycogen phosphorylase in Hymenolepis diminuta and in Vampirolepis microstoma were studied. The worms were recovered from hosts that had been fed ad libitum, starved for 24 hr, or starved 24 hr and then refed for 1 hr immediately prior to worm recovery. The ratios of active to inactive glycogen synthase and phosphorylase were correlated with the host feeding regimen prior to recovery. Glycogen synthase in H. diminuta was predominately in the inactive D form in worms from both fed and fasted hosts. One hour after refeeding, up to 80% of the synthase was in the active I form. Phosphorylase in H. diminuta was predominantly in the active a form in worms from fed and fasted hosts, but activity of this enzyme was suppressed in worms from refed hosts. When H. diminuta from fasted hosts was incubated in a balanced salt solution containing 40 mM glucose, glycogen synthase I increased, and phosphorylase a decreased. Glycogen synthase in V. microstoma was predominantly in the inactive D form in worms from both the fed and fasted hosts, but the proportion in the active I form increased to over half the total synthase by 1 hr of host refeeding. The proportion of glycogen phosphorylase a was high in worms from fed hosts and decreased, but not dramatically, in worms from fasted hosts. The results suggested that the worms had access to another source of glucose, probably from the host bile, and we measured a low but significant concentration of carbohydrate in the gall bladder bile of mice.(ABSTRACT TRUNCATED AT 250 WORDS)