Inhibition of Glyceraldehyde 3-Phosphate Dehydrogenase by Adenine Nucleotides in Pachytene Primary Spermatocytes from Rat Testes

The effect of adenine nucleotides on the catalytic activity of spermatocyte glyceraldehyde 3-phosphate dehydrogenase (GA3PDH) was studied for its significance to metabolic regulation. In the presence of glucose (10 mM), the ATP level declined whereas the ADP and AMP levels remained unchanged. During incubation with glucose, fructose 1,6-bisphosphate (fruc 1,6-P2) and dihydroxyacetone phosphate (DHAP) were accumulated markedly. GA3PDH was inhibited by ATP (Ki = 2.27 mM), ADP (Ki = 1.21 mM) and AMP (Ki = 0.73 mM) competitively with NAD (Km = 0.24 mM). The results suggest that glycolysis in spermatocytes is regulated by GA3PDH.     

Tolbutamide stimulates fructose-2, 6-bisphosphate formation in perfused rat liver

Effect of tolbutamide on liver fructose-2,6-bisphosphate (F-2,6-P2) was examined in isolated perfused rat liver in situ with a flow-through method. Tolbutamide (1 mM) gradually increased liver F-2,6-P2 level from 7.4 +/- 1.6 to 21.2 +/- 1.6 pmol/mg wet wt for 20 min perfusion. The increase of liver F-2,6-P2 induced by tolbutamide was dose dependent and was significantly observed at 10 min perfusion. The maximum plateau level of F-2,6-P2 induced by 16.7 mM glucose was further increased with 1 mM tolbutamide. Glucagon (10(-11) M) decreased the elevated level induced by 16.7 mM glucose, but this effect was completely inhibited with 2 mM tolbutamide. Cyclic AMP level of the liver throughout the perfusion with tolbutamide did not change. Carboxytolbutamide or gliclazide perfusion did not change significantly the liver F-2,6-P2 level; however, the results suggest that tolbutamide may increase the liver F-2,6-P2 level by affecting the phosphorylation state of fructose-6-phosphate, 2-kinase/fructose-2,6-bisphosphatase through cyclic AMP-dependent protein kinase, resulting in the stimulation of glycolysis and the inhibition of gluconeogenesis in the liver. Thus, the extrapancreatic action and the mechanism of action of different sulfonylureas may differ.     

Inhibition of fructose 1,6-bisphosphatase reduces excessive endogenous glucose production and attenuates hyperglycemia in Zucker diabetic fatty rats

Gluconeogenesis is increased in type 2 diabetes and contributes significantly to fasting and postprandial hyperglycemia. We recently reported the discovery of the first potent and selective inhibitors of fructose 1,6-bisphosphatase (FBPase), a rate-controlling enzyme of gluconeogenesis. Herein we describe acute and chronic effects of the lead inhibitor, MB06322 (CS-917), in rodent models of type 2 diabetes. In fasting male ZDF rats with overt diabetes, a single dose of MB06322 inhibited gluconeogenesis by 70% and overall endogenous glucose production by 46%, leading to a reduction in blood glucose of >200 mg/dl. Chronic treatment of freely feeding 6-week-old male Zucker diabetic fatty (ZDF) rats delayed the development of hyperglycemia and preserved pancreatic function. Elevation of lactate ( approximately 1.5-fold) occurred after 4 weeks of treatment, as did the apparent shunting of precursors into triglycerides. Profound glucose lowering ( approximately 44%) and similar metabolic ramifications were associated with 2-week intervention therapy of 10-week-old male ZDF rats. In high-fat diet-fed female ZDF rats, MB06322 treatment for 2 weeks fully attenuated hyperglycemia without evidence of metabolic perturbation other than a modest reduction in glycogen stores ( approximately 20%). The studies confirm that excessive gluconeogenesis plays an integral role in the pathophysiology of type 2 diabetes and suggest that FBPase inhibitors may provide a future treatment option.     

Metabolism of Round Spermatids in the Rat: Effect of Gossypol on the Glucose Transport/Stoffwechsel der runden Spermatiden bei der Ratte: Der Einfluß von Gossypol auf den Glukose-Transport

Low dose of gossypol (5 microM) completely inhibited uptake of 2-deoxy-D-glucose by rat spermatids. With gossypol 2-deoxy-D-glucose transport into spermatids increased curvelinearly for 10 minutes after the start of incubation, but reached a plateau afterwards, whereas without gossypol it continuously increased. The levels of fructose-1,6-bis-phosphate (FBP) and ATP were inversely correlated when the cells were exposed to glucose. However, in the presence of gossypol the levels of ATP and FBP decreased simultaneously. In addition, gossypol at 10 microM stimulated oxygen consumption when lactate was present. These results suggest that gossypol probably inhibits glucose transport into spermatids by lowering the ATP level by uncoupling respiratory chain-linked phosphorylation.     

Regulation of gluconeogenesis in hepatocytes from fasted alloxan-diabetic rats

Hepatocytes from fasted, alloxan-diabetic rats were incubated in the absence of gluconeogenic substrates to deplete residual glycogen stores. Glucose production from lactate and pyruvate was enhanced in cells from diabetic rats relative to similarly treated hepatocytes from fasted, nondiabetic control rats. Gluconeogenesis from dihydroxyacetone, fructose, or glycerol was not increased but the formation of lactate plus pyruvate from dihydroxyacetone was decreased. The stimulation of gluconeogenesis by exogenous fatty acids was decreased by diabetes. The rates of gluconeogenesis in the presence of lactate plus pyruvate plus oleate were equal in hepatocytes from diabetic and control rats and indicate that the maximal rate of gluconeogenesis was not increased. With lactate plus pyruvate as substrates, stimulation of gluconeogenesis by norepinephrine or dibutyryl-cAMP was not altered by diabetes. The catecholamine stimulation of gluconeogenesis from glycerol also was unaffected. In contrast, diabetes decreased the maximal stimulation of gluconeogenesis from dihydroxyacetone by dibutyryl-cAMP, glucagon, or norepinephrine and this decrease was proportional to the decreased production of lactate plus pyruvate. The concentrations of glucagon or norepinephrine required for half-maximal stimulation were not altered by diabetes. Thus, the hormonal stimulation of gluconeogenesis from dihydroxyacetone is decreased by diabetes, probably because of decreased pyruvate kinase activity, but the interaction of glucagon and norepinephrine with hepatocytes and the subsequent stimulation of gluconeogenesis from physiologic substrates is not impaired.     

Effects of neuroprotective cocktails on hippocampal neuron death in an in vitro model of cerebral ischemia

Cocktails of neuroprotectants acting at different parts of the ischemic injury cascade may have advantages over single agents. This study investigated, singly and in combination, the neuroprotective efficacy of an energy substrate (3.5 mM fructose 1,6-bisphosphate, FBP), an antagonist of NMDA receptors (1 and 10 microM MK-801), a free-radical scavenger (100 microM ascorbate), an adenosine A1 receptor agonist (10 microM 2-chloroadenosine), and an inhibitor of neurotransmission (2% isoflurane). These agents were evaluated for their ability to prevent loss and morphologic damage of CA1 neurons in rat hippocampal slices when these agents were administered during 30 minutes in vitro ischemia (combined oxygen/glucose deprivation at 37 degrees C) followed by 5 hours of recovery. Ten microM MK-801, alone or in combination with the other compounds, prevented loss of CA1 neurons and preserved their histologic appearance. Isoflurane, which prevents glutamate receptor-dependent cell death in this model, was also protective. Protection against neuron loss was also found when a subtherapeutic concentration of MK-801 (1 microM) was combined with 2-chloroadenosine (which indirectly causes NMDA receptor suppression), but not FBP or ascorbate. The authors conclude that in this model, the strategy of antagonizing NMDA receptors appears more protective than fructose-1,6-bisphosphate, 2-chloroadenosine or ascorbate.     

The gluconeogenetic ability of hepatocytes in various types of acute uraemia

Liver cells were prepared from untreated controls, rats with various models of acute uraemia (uranyl nitrate-treated, bilaterally nephrectomised and ureter-ligated rats, rats with acute ischaemic renal failure) and sham-operated animals. Hepatocyte glucose output, pyruvate utilisation and lactate production were determined in the presence of Krebs-Ringer bicarbonate buffer with different pH values (7.1, 7.4, 7.6) using pyruvate, dihydroxyacetone, serine and fructose as substrates. In the presence of pyruvate and dihydroxyacetone a significant increase of glucose production in hepatocytes from bilaterally nephrectomised and ureter-ligated rats was observed. However, pyruvate-generated glucose production in the hepatocytes of uranyl nitrate-treated animals was unchanged, while a diminished glucose output was seen in the presence of dihydroxyacetone. A marked increase in glucose and lactate production in the presence of serine was observed in the hepatocytes of uranyl nitrate-treated, ureter-ligated and binephrectomised rats. However, lactate production from dihydroxyacetone in the liver cells of uranyl nitrate-treated animals was inhibited. In contrast to other types of uraemia, in acute ischaemic renal failure there is significantly lower hepatocyte glucose production using pyruvate as a substrate, but unchanged glucose generation from dihydroxyacetone or serine.     

Myocardial glycolytic and gluconeogenic enzyme activity during cardiopulmonary bypass in humans

Through a study of the activity of the regulatory enzyme of glycolysis phosphofructokinase and the regulatory enzyme of gluconeogenesis fructose-1,6-diphosphatase, myocardial metabolism has been evaluated in the human heart during cardiopulmonary bypass. Three ventricular transmural biopsy specimens were obtained at open-heart surgery before cardiopulmonary bypass was instituted and three similar specimens were taken 30 minutes after cardiopulmonary bypass was started. The enzyme assays were performed on the supernatant obtained from myocardial cells disrupted by sonication. The activity of both enzymes increased during the anoxic conditions of cardiopulmonary arrest. The increase in phosphofructokinase activity is likely due to an overall increase in glycolytic activity, but the presence and increased activity of fructose-1,6-diphosphatase was unexpected.     

Diabetes-induced glomerular dysfunction: links to a more reduced cytosolic ratio of NADH/NAD+.

These studies were undertaken to examine effects of elevated glucose levels on glycolysis, sorbitol pathway activity, and the cytosolic redox state of NADH/NAD+ in isolated glomeruli. Blood-free glomeruli were isolated from kidneys of male, Sprague-Dawley rats using standard sieving techniques, then incubated for one hour at 37 degrees C, pH 7.4, pO2 approximately 500 torr, in Krebs bicarbonate/Hepes buffer containing 5 or 30 mM glucose. Elevated glucose levels increased glucose 6-phosphate, fructose 6-phosphate, total triose phosphates, lactate, the lactate/pyruvate ratio, sorbitol, and fructose, but did not affect sn-glycerol 3-phosphate, pyruvate, or myo-inositol levels. The more reduced glomerular cytosolic redox state (manifested by the tissue lactate/pyruvate ratio) induced by 30 mM glucose was completely abrogated by aldose reductase inhibitors added to the diet two to seven days prior to glomerular isolation. These observations, coupled with evidence linking glucose- and diabetes-induced glomerular dysfunction to increased sorbitol pathway metabolism, support the hypothesis that metabolic imbalances associated with a more reduced ratio of cytosolic NADH/NAD+ (resulting from increased glucose metabolism via the sorbitol pathway) play an important role in mediating glucose- and diabetes-induced glomerular dysfunction.     

Intrinsic gluconeogenesis is enhanced in renal proximal tubules of Zucker diabetic fatty rats

Recent studies indicate that renal gluconeogenesis is substantially stimulated in patients with type 2 diabetes, but the mechanism that is responsible for such stimulation remains unknown. Therefore, this study tested the hypothesis that renal gluconeogenesis is intrinsically elevated in the Zucker diabetic fatty rat, which is considered to be an excellent model of type 2 diabetes. For this, isolated renal proximal tubules from diabetic rats and from their lean nondiabetic littermates were incubated in the presence of physiologic gluconeogenic precursors. Although there was no increase in substrate removal and despite a reduced cellular ATP level, a marked stimulation of gluconeogenesis was observed in diabetic relative to nondiabetic rats, with near-physiologic concentrations of lactate (38%), glutamine (51%) and glycerol (66%). This stimulation was caused by a change in the fate of the substrate carbon skeletons resulting from an increase in the activities and mRNA levels of the key gluconeogenic enzymes that are common to lactate, glutamine, and glycerol metabolism, i.e., mainly of phosphoenolpyruvate carboxykinase and, to a lesser extent, of glucose-6-phosphatase and fructose-1,6-bisphosphatase. Experimental evidence suggests that glucocorticoids and cAMP were two factors that were responsible for the long-term stimulation of renal gluconeogenesis observed in the diabetic rats. These data provide the first demonstration in an animal model that renal gluconeogenesis is upregulated by a long-term mechanism during type 2 diabetes. Together with the increased renal mass (38%) observed, they lend support to the view so far based only on in vivo studies performed in humans that renal gluconeogenesis may be stimulated by and crucially contribute to the hyperglycemia of type 2 diabetes.     

Additional Evidence That Transaldolase Exchange, Isotope Discrimination During the Triose-Isomerase Reaction, or Both Occur in Humans: Effects of Type 2 Diabetes

OBJECTIVE

To determine whether deuterium enrichment on carbons 5 and 3 (C5/C3) in plasma glucose is influenced by processes other than gluconeogenesis and, if so, whether these processes are altered by type 2 diabetes.

RESEARCH DESIGN AND METHODS

In this study, 10 obese diabetic and 10 obese nondiabetic subjects were infused intravenously with [3,5-²H₂] galactose enriched at a C5-to-C3 ratio of 1.0 as well as the enrichment of deuterium on C5 and C3 of plasma glucose, measured with nuclear magnetic resonance using the acetaminophen glucuronide method.

RESULTS

The ratio of deuterium enrichment on C5 and C3 of glucose was <1 (P < 0.001) in all of the diabetic and nondiabetic subjects, resulting in a means ± SE C5-to-C3 ratio that did not differ between groups (0.81 ± 0.01 vs. 0.79 ± 0.01, respectively).

CONCLUSIONS

That the C5-to-C3 glucose ratio is <1 indicates that transaldolase exchange, selective retention of deuterium at the level of the triose-isomerase reaction, or both occur in humans. This also indicates that the net effect of these processes on the C5-to-C3 ratio is the same in people with and without type 2 diabetes. The possible effects of transaldolase exchange or selective retention of deuterium (or tritium) at the level of the triose-isomerase reaction on tracee labeling and tracer metabolism should be considered when the deuterated water method is used to measure gluconeogenesis or [3-³H] glucose is used to measure glucose turnover in humans.The deuterated water method is extensively used to measure gluconeogenesis in humans (1–7). One premise of this method is that there is negligible exchange of the lower three carbons of fructose or sedoheptulose via the transaldolase exchange reaction (8). If such exchange does occur, then glucose can be labeled on the fifth carbon (C5) by simple exchange with a labeled carbon three (C3) precursor without net hexose synthesis (9–11). Whereas it is established that transaldolase exchange can occur in vitro (9,12), until recently it was not known whether exchange also occurs in vivo. To address this question, we infused [3,5-²H₂] glucose enriched at a C5-to-C3 ratio of 1.07 intravenously in nondiabetic subjects (13). We observed that the C5-to-C3 deuterium enrichment in uridine-diphosphoglucose glucose measured using the acetaminophen glucuronide method was <1 in all six subjects studied, averaging 0.75 before and 0.67 during a 4-h hyperinsulinemic-euglycemic clamp. Jones et al. (14) have observed similar effects of transaldolase activity on overestimation of the indirect pathway of glycogen synthesis in five healthy humans.These observations are both surprising and disconcerting because they indicate that deuterium on C5 of fructose-1,6-phosphate was lost during exchange with unlabeled C3 precursors, presumably via transaldolase exchange; that deuterium was selectively retained on C3, presumably due to a kinetic isotope effect at the level of the triose-isomerase reaction; or that both occurred (Fig. 1). If substantial transaldolase exchange does occur in humans, then the extent of labeling of C5 glucose with deuterium following administration of deuterated water will be determined by both the rate of transaldolase exchange and the rate of gluconeogenesis (8). Therefore, the plasma C5 glucose–to–C2 glucose ratio (which is labeled by both glycogenolysis and gluconeogenesis) would overestimate the percent of glucose derived from gluconeogenesis. This also would preclude accurate measurement of gluconeogenesis with any other tracer method because all assume negligible transaldolase exchange (15,16).Open in a separate windowOpen in a separate windowFIG. 1.The C5-to-C3 ratio of plasma glucose can decrease if unlabeled glyceraldhyde-3-phosphate exchanges with the bottom three carbons of C5– and C3–labeled fructose-1,6-phosphate via the transaldolase reaction (A), if the deuterium on dihydroxyacetone originating from the third carbon of frucose-1,6-posphate is retained relative to that originating from the fifth carbon of fructose-1,6-phosphate due to kinetic isotope effect (i.e., a slower removal) during the triose isomerase reaction (B), or if a combination of both occurs. Hydrogens and oxygens have been omitted for the sake of clarity. C, carbon; D, deuterium; DHAP, dihydroxyacetone phosphate; fructose-1,6-P, fructose-1,6-phosphate; GAP, glyceraldehyde-3-phosphate; glucose-1-P, glucose-1-phosphate; glucose-6-P, glucose-6-phosphate; UDP, uridine-diphosphoglucose.Alternatively, selective retention of deuterium on C3 also would reduce the C5-to-C3 ratio. This would be consistent with in vitro studies that have shown slower removal of deuterium during the triose-isomerase reaction due to a kinetic isotope effect (9–11,17). If this were to also occur in humans, then it would call into question the ability of [3-³H] glucose to accurately measure glucose turnover because retention of tritium due to a kinetic isotope effect could result in an underestimation of glucose turnover if it caused the hepatic glucose-6-phosphate pool to be enriched with tracer. If the rate of transaldolase exchange or the degree of retention of tritium during the triose-isomerase reaction differs in diabetic and nondiabetic humans, this would be particularly problematic because it would confound comparison of gluconeogenesis measured with the deuterated water method and glucose turnover measured with [3-³H] glucose between groups.The present study was undertaken to confirm or refute, in a larger number of subjects, our previous observation that the C5-to-C3 ratio measured in the plasma glucose pool is lower than that of the intravenously infused tracer (13). We also sought to determine whether the degree of reduction of the C5-to-C3 plasma glucose ratio differs in diabetic and nondiabetic humans. We addressed these questions by infusing [3,5-²H₂] galactose to directly label the plasma-glucose pool in obese nondiabetic and obese diabetic subjects after an 18-h fast in order to reduce hepatic glycogen, thereby maximizing plasma glucose enrichment by minimizing the rate of entry of unlabeled plasma glucose into the pool. We report that the C5-to-C3 plasma glucose ratio was <1 in all subjects. We also report that the degree of reduction in the C5-to-C3 ratio did not differ for diabetic and nondiabetic subjects, indicating that the net effect of transaldolase exchange or retention of deuterium on C3 was comparable in the two groups.     

Effects of benfluorex on fatty acid and glucose metabolism in isolated rat hepatocytes: from metabolic fluxes to gene expression

The effects of benfluorex and two of its metabolites (S 422-1 and S 1475-1) on fatty acid and glucose metabolic fluxes and specific gene expression were studied in hepatocytes isolated from 24-h fasted rats. Both benfluorex and S 422-1 (0.1 or 1 mmol/l) reduced beta-oxidation rates and ketogenesis, whereas S 1475-1 had no effect. At the same concentration, benfluorex and S 422-1 were more efficient in reducing gluconeogenesis from lactate/pyruvate than S 1475-1. Benfluorex inhibited gluconeogenesis at the level of pyruvate carboxylase (45% fall in acetyl-CoA concentration) and of glyceraldehyde-3-phosphate dehydrogenase (decrease in ATP/ADP and NAD(+)/NADH ratios). Accordingly, neither benfluorex nor S 422-1 inhibited gluconeogenesis from dihydroxyacetone, but both stimulated gluconeogenesis from glycerol. In hepatocytes cultured in the presence of benfluorex or S 422-1 (10 or 100 micromol/l), the expression of genes encoding enzymes of fatty acid oxidation (carnitine palmitoyltransferase [CPT] I), ketogenesis (hydroxymethylglutaryl-CoA synthase), and gluconeogenesis (glucose-6-phosphatase, PEPCK) was decreased, whereas mRNAs encoding glucokinase and pyruvate kinase were increased. By contrast, Glut-2, acyl-CoA synthetase, and CPT II gene expression was not affected by benfluorex or S 422-1. In conclusion, this work suggests that benfluorex mainly via S 422-1 reduces gluconeogenesis by affecting gene expression and metabolic status of hepatocytes.     

Glucose-induced accumulation of fructose-2,6-bisphosphate in pancreatic islets

Rat islets contain the acid-labile activator of phosphofructokinase, fructose-2,6-bisphosphate. The islet content in activator is higher in islets exposed to glucose (16.7 mM) than in islets deprived of glucose. The islets display fructose-6-phosphate, 2-kinase activity with a Km for fructose-6-phosphate close to 0.08 mM. Glucose fails to affect the activity of this enzyme. It is proposed that the effect of glucose to increase the islet content of fructose-2,6-bisphosphate is attributable, in part at least, to the glucose-induced increase in the concentration of fructose-6-phosphate in the islet cells.     

Effect of hormones on hepatocyte gluconeogenesis in different models of acute uraemia

Hepatocytes isolated from the livers of starved, sham-operated, bilaterally nephrectomised and ureter-ligated rats as well as rats with ischaemic acute renal failure were used for a comparative study of the effects of different hormones on gluconeogenesis. In all tested groups dibutyryl-3':5'-adenosine monophosphate inhibits glucose synthesis from pyruvate whereas this process is not affected by glucagon and only slightly activated by adrenalin. In contrast, gluconeogenesis from dihydroxyacetone was stimulated by all three hormones at the expense of the conversion of dihydroxyacetone to lactate. In the presence of l-serine adrenalin, glucagon and dibutyryl cAMP also stimulate glucose synthesis, which is more marked in bilaterally nephrectomised and ureter-ligated animals. In half of the experiments with bilaterally nephrectomised rats (group BN 2), lack of sensitivity of hepatocytes to all tested hormones on gluconeogenesis from serine or dihydroxyacetone was observed. The beta-adrenergic antagonist propranolol reduced the stimulatory effect of adrenalin on glucose synthesis from serine and abolished the influence of catecholamines in the presence of dihydroxyacetone and pyruvate. This suggests that both alpha- and beta-receptors are involved in the activation of hepatic gluconeogenesis. Insulin and parathyroid hormone did not change the rate of glucose synthesis in any of the experimental groups.     

The role of liver fructose-1,6-bisphosphatase in regulating appetite and adiposity

Liver fructose-1,6-bisphosphatase (FBPase) is a regulatory enzyme in gluconeogenesis that is elevated by obesity and dietary fat intake. Whether FBPase functions only to regulate glucose or has other metabolic consequences is not clear; therefore, the aim of this study was to determine the importance of liver FBPase in body weight regulation. To this end we performed comprehensive physiologic and biochemical assessments of energy balance in liver-specific transgenic FBPase mice and negative control littermates of both sexes. In addition, hepatic branch vagotomies and pharmacologic inhibition studies were performed to confirm the role of FBPase. Compared with negative littermates, liver-specific FBPase transgenic mice had 50% less adiposity and ate 15% less food but did not have altered energy expenditure. The reduced food consumption was associated with increased circulating leptin and cholecystokinin, elevated fatty acid oxidation, and 3-β-hydroxybutyrate ketone levels, and reduced appetite-stimulating neuropeptides, neuropeptide Y and Agouti-related peptide. Hepatic branch vagotomy and direct pharmacologic inhibition of FBPase in transgenic mice both returned food intake and body weight to the negative littermates. This is the first study to identify liver FBPase as a previously unknown regulator of appetite and adiposity and describes a novel process by which the liver participates in body weight regulation.     

Hepatic glycolytic intermediates and glucoregulatory enzymes in septic shock due to peritonitis: experimental study in rats

The hypoglycemia in septic shock due to peritonitis indicates deranged carbohydrate metabolism. To determine if this metabolic failure could be attributed to changes of glucoregulatory enzymes and glycolytic intermediates, activities and changes of these substances in septic shock have been studied in rats. Liver tissue was sampled 5 hours after induction of peritonitis by cecal incision in fasted male rats. Hepatic glycolytic intermediates were assayed by UV-spectrophotometry. Peritonitis caused 33% decrease in glucose-6-phosphate (G6P), a 2.5 fold increase in fructose-1,6-diphosphate (FDP) and a 3.5 fold increase in lactate. Phosphoenolpyruvate (PEP) levels did not show a significant increase in peritonitis. We investigated activities of glucose-6-phosphatase (G6Pase), fructose-1,6-diphosphatase (FDPase), phosphofructokinase ( PFKase ) and pyruvate kinase ( PKase ) in mitochondria-free supernatants from rat liver homogenates. Tissue was sampled 5 hours after induction of peritonitis by cecal incision. Assays were conducted at optimal substrate levels at pH 7.4; NADH charges produced by coupled reactions were determined by UV-spectrophotometry. A significant increase of PFKase and PKase specific activity was observed. These changes were consistent with stimulated glycolysis. For gluconeogenesis to achieve maximum efficiency it would be necessary to inhibit PFKase and PKase completely.     

Fructose-1,6-bisphosphate for improved outcome after hypothermic circulatory arrest in pigs

Objective: Fructose-1,6-bisphosphate is a high-energy intermediate in the anaerobic metabolism. It enhances glycolysis, preserves cellular adenosine triphosphate, and prevents the increase of intracellular calcium during ischemia. The potential neuroprotective effect of fructose-1,6-bisphosphate during hypothermic circulatory arrest was evaluated in a surviving porcine model.Methods: Twenty-four pigs were randomly assigned to receive two intravenous infusions of either fructose-1,6-bisphosphate (500 mg/kg) or saline solution. The first infusion was given immediately before a 75-minute period of hypothermic circulatory arrest and the second was given immediately after hypothermic circulatory arrest.Results: The 7-day survivals were 83.3% in the fructose-1,6-bisphosphate group and 41.7% in the control group (P = .09). The treated animals had significantly better postoperative behavioral scores. The administration of fructose-1,6-bisphosphate was associated with higher venous phosphate and sodium levels, lower venous ionized calcium levels, higher blood osmolarity, and a better fluid balance. Intracranial pressure and venous creatine kinase isoenzyme MB were significantly lower in the fructose-1,6-bisphosphate group during rewarming (P = .01 and P = .001, respectively). Among the treated animals, brain glucose, pyruvate and lactate levels tended to be higher, brain glycerol levels tended to be lower, and the histopathologic score of the brain was significantly lower (P = .04).Conclusions: Intravenous administration of fructose-1,6-bisphosphate at 500 mg/kg before and after hypothermic circulatory arrest in a surviving porcine model was associated with better survival, behavioral outcome, and histopathologic score. The observed lower blood creatine kinase isoenzyme MB and brain glycerol levels and the higher brain glucose, pyruvate, and lactate levels in the fructose-1,6-bisphosphate group suggest that this drug has supportive effects on myocardial and brain metabolisms.     

Enhanced cerebral substrate pools in the fetus and neonate of the diabetic canine mother

Energy stores and intermediates of carbohydrate metabolism were investigated in the freeze-clamped cerebral cortex of the fetus and fasted neonate born to a diabetic canine mother. Prior studies in these same pups demonstrated circulating hyperinsulinemia, depressed free fatty acid levels, and attenuated gluconeogenesis. Hepatic and muscle tissue also demonstrated augmented levels of glycogen, triglycerides, and amino acids. In the present investigation, cerebral tissue from these same pups of diabetic mothers also demonstrated enhanced fetal cerebral glucose and glycogen content. After 24 h of neonatal fasting, cerebral glycogen content declined to values lower than in control pups. Cerebral cortical levels of glucose-6-phosphate, fructose-6-phosphate, lactate, citrate, alpha-ketoglutarate, and malate were not altered, while oxaloacetate was higher at 3 and 9 h and fructose-1,6-diphosphate was higher at 9 and 24 h in the IDM pups. Adenine nucleotide levels and the energy charge were equivalent to those in control pups at each time interval. In contrast, cerebral cortical amino acids of the glutamate group were enhanced in the fetus or neonate of the diabetic mother. Cerebral cortical alanine was increased from 3 to 24 h while aspartate and glutamate were augmented in the fetus and fasted IDM newborn pup. Glutamine was increased at 6 and 24 h, while gamma-aminobutyrate was elevated in the fetus. Cerebral ammonia concentration was not altered. The augmented stores of cerebral carbohydrate and amino acid pools in the fetus and neonate after maternal canine diabetes may serve as oxidizable substrates for the brain during periods of attenuated systemic fuel availability.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chlorpropamide raises fructose-2,6-bisphosphate concentration and inhibits gluconeogenesis in isolated rat hepatocytes

The addition of chlorpropamide to hepatocytes isolated from fed rats raised the cellular concentration of fructose-2,6-bisphosphate (F-2,6-P2), a regulatory metabolite that plays a relevant role in the control of hepatic glucose metabolism. The effect of chlorpropamide was dose dependent; a statistically significant increase was already seen at 0.2 mM of the sulfonylurea. The accumulation of F-2,6-P2 caused by chlorpropamide (1 mM) was parallel to the stimulation of L-lactate production (36.6 +/- 4.8 versus 26.1 +/- 2.6 mumol of lactate/g of cells X 20 min; N = 5, P less than 0.05) and to the inhibition of gluconeogenesis (0.57 +/- 0.1 versus 0.94 +/- 0.09 mumol of [U-14C]pyruvate converted to glucose/g of cells X 20 min; N = 5, P less than 0.05). In addition, chlorpropamide enhanced the inhibitory action evoked by insulin on glucagon-stimulated gluconeogenesis. This combined effect of chlorpropamide and insulin seems to be correlated with the synergistic accumulation of F-2,6-P2 provoked by the simultaneous action of these two agents on glucagon-treated hepatocytes. Finally, neither 6-phosphofructo-2-kinase activity nor hepatocyte cyclic AMP levels were significantly changed by the presence of the sulfonylurea in the incubation medium. Our results support the concept that chlorpropamide, by a cyclic AMP-independent mechanism, increases the hepatic content of F-2,6-P2 and, in this way, enhances the glycolytic flux and inhibits glucose output by the liver.