Shift of anaerobic to aerobic metabolism in the rats acclimatized to hypoxia

1. Metabolic acclimatization by repeated exposure to a simulated altitude of 4000, 5000 and 6000 m for 2 hr per day throughout 2 to 11 days was evaluated by the increased formation of ketone bodies as a marker of fatty acid oxidation and the decreased production of lactate and uric acid, the indicators of anaerobic metabolism in rats exposed to an altitude of 8000 m. 2. Pre-exposure of rats to an altitude of 5000 m and over caused an acclimatization to hypoxia. The rise of the altitude to which rats were pre-exposed reduced the period until the acquisition of metabolic acclimatization. 3. Acclimatized rats showed an increased activity of mitochondrial glutamate dehydrogenase without changes in glycolytic enzyme activity in skeletal muscle, heart and liver. 4. Acclimatization to high altitude hypoxia is concluded to involve a shift of the anaerobic glycolysis to aerobic metabolism by the increase in the oxidative enzymes.     

The role of hepatic glycogen stores in anaerobic metabolism in rats

Plasma levels of lactate and oxypurines markedly increased in both fed and fasted rats exposed to three acute anoxic states, cyanide poisoning, carbon monoxide poisoning and inhalation of oxygen-deficient gas, suggesting that the transition of aerobic to anaerobic metabolism occurred similarly in both groups. Plasma glucose level of fed rats increased 1.8-2.5 times after exposure to anoxia, whereas a remarkable hypoglycemia was induced by the exposure of fasted rats to anoxia. Hepatic glycogen stores in fed rats induced hyperglycemia, while exhaustion of the stores in fasted rats resulted in severe hypoglycemia during acute anoxia.     

Effect of sodium fluoride on cyclic AMP production in rat hepatocytes.

The effect of NaF on cAMP production was studied in hepatocytes isolated from fed and fasted rats. A four-six fold increase in hepatocyte cAMP production was observed in the presence of 10-20 mM NaF in cells isolated from either fed or fasted rats. The maximal stimulation of cAMP production was observed after a 10 min incubation in the presence of 1 mM theophylline. However, as little as 0.05-0.15 mM NaF induced a significant increase in cAMP production. It was also found that NaF would alter the production of glucose in isolated rat hepatocytes. When hepatocytes from fed rats were incubated with 0.05-5 mM NaF there was an increase in amount of glucose released from endogenous sources. Also NaF resulted in a decrease in lactate and pyruvate production. Similarly NaF stimulated glucose production in hepatocytes from fasted rats. The maximal stimulation was observed with about 0.15-0.25 mM NaF. At NaF concentrations greater than 1.5 mM a decrease in glucose production was observed. It is concluded that NaF increases the level of cAMP and alters glucose metabolism in intact hepatocytes.     

Long-chain acylcarnitine content determines the pattern of energy metabolism in cardiac mitochondria

In the heart, a nutritional state (fed or fasted) is characterized by a unique energy metabolism pattern determined by the availability of substrates. Increased availability of acylcarnitines has been associated with decreased glucose utilization; however, the effects of long-chain acylcarnitines on glucose metabolism have not been previously studied. We tested how changes in long-chain acylcarnitine content regulate the metabolism of glucose and long-chain fatty acids in cardiac mitochondria in fed and fasted states. We examined the concentrations of metabolic intermediates in plasma and cardiac tissues under fed and fasted states. The effects of substrate availability and their competition for energy production at the mitochondrial level were studied in isolated rat cardiac mitochondria. The availability of long-chain acylcarnitines in plasma reflected their content in cardiac tissue in the fed and fasted states, and acylcarnitine content in the heart was fivefold higher in fasted state compared to the fed state. In substrate competition experiments, pyruvate and fatty acid metabolites effectively competed for the energy production pathway; however, only the physiological content of acylcarnitine significantly reduced pyruvate and lactate oxidation in mitochondria. The increased availability of long-chain acylcarnitine significantly reduced glucose utilization in isolated rat heart model and in vivo. Our results demonstrate that changes in long-chain acylcarnitine contents could orchestrate the interplay between the metabolism of pyruvate–lactate and long-chain fatty acids, and thus determine the pattern of energy metabolism in cardiac mitochondria.     

Acclimatization to hypoxia modulates the tryptophan 2,3-dioxygenase activity in rats exposed to simulated high altitude.

1. Exposure of rats to an 8000 m altitude increased the hepatic tryptophan 2,3-dioxygenase (EC 1.13.1.12) activity. 2. Acclimatization to hypoxia by a repeated exposure to an altitude of 5000 m induced a marked decrease in liver tryptophan dioxygenase activity after the rats were subjected to an 8000 m altitude, but a pre-exposure to 4000 m altitude showed no effect on the enzyme activity. 3. Plasma tryptophan was rapidly decreased by exposure to 8000 m altitude to the same extent in the acclimatized and non-acclimatized rats. 4. Plasma tryptophan may be utilized as the substrate for tryptophan dioxygenase in liver of the non-acclimatized rats under highly hypoxic conditions; however, acclimatized rats can reserve tryptophan as the substrate for the alternative metabolism other than the degradation pathway in liver.     

Acute hormonal control of pyruvate kinase and lactate formation in the isolated rat hepatocyte.

The activity of pyruvate kinase from the isolated rat hepatocyte was studied under conditions which allow investigation into the hormonal regulation of the enzyme. Incubating hepatocytes from fed or fasted rats with 1 μm glucagon gives approximately 60% inhibition of the enzyme activity determined at 1.6 mm P-enolpyruvate. A good correlation between the regulation of pyruvate kinase and lactate formation from 10 mm dihydroxyacetone is observed in hepatocytes from fasted rats. When hepatocytes are incubated in a Krebs-Ringer phosphate buffer, the inhibition of the pyruvate kinase activity by 1 μm glucagon is not accompanied by a marked inhibition of lactate production from fructose. Half-maximal regulation is observed at 0.26 ± 0.02 nm glucagon and 0.37 ± 0.05 nm glucagon for the enzyme and lactate formation from dihydroxyacetone respectively. Incubating hepatocytes with 10 mm l-alanine enhances inhibition of pyruvate kinase by physiological concentrations of glucagon, lowering the half-maximally effective concentration of glucagon from 0.3 nm to approximately 0.1 nm. A small but consistent inhibition of pyruvate kinase by 10 μm epinephrine is also observed and this inhibition is enhanced by 0.5 mm theophylline and by 10 mm l-alanine. The inhibition of pyruvate kinase by epinephrine both in the absence and presence of theophylline is blocked by the α-adrenergic antagonist phenoxybenzamine. The β-adrenergic blocker propranolol has no influence on the inhibition of the enzyme by epinephrine. Adenosine 3′:5′-monophosphate, N⁶O²-dibutyryl adenosine 3′:5′-monophosphate, and guanosine 3′:5′-monophosphate also inhibit glycolysis from dihydroxyacetone and modulate pyruvate kinase activity in hepatocytes from fasted rats. Oleate, ethanol, and 3-hydroxybutyrate inhibit dihydroxyacetone glycolysis, but they do not influence the activity of pyruvate kinase. The divalent metal ionophore A23187 slightly stimulates lactate synthesis from dihydroxyacetone, but it has no influence on pyruvate kinase activity.     

Repartition of ATP, ADP and PO4 in isolated hepatocytes from fed and fasted rats

1. The digitonin fractionation procedure [Zuurendonk, P. F. and Tager, J. M. (1974) Biochim. Biophys. Acta, 333, 393-399] was used to determine the repartition of adenine nucleotides and inorganic phosphate in isolated hepatocytes from fed and fasted rats. 2. This repartition is not significantly modified in the presence of pyruvate or alanine or lactate + pyruvate for isolated hepatocytes from fasted rats. 3. In isolated hepatocytes from fasted rats, the mitochondrial ATP/ADP X PO4 ratio is two-fold lower than in isolated hepatocytes from fed rats. 4. The cytosolic ATP/ADP X PO4 ratio depends on the nutritional state and (or) on the added substrate for neoglucogenesis.     

BRAIN EXTRACELLULAR POTASSIUM AND ENERGY METABOLISM DURING ISCHEMIA IN JUVENILE RATS AFTER EXPOSURE TO HYPOXIA FOR 24 h

Abstract— The ischemia-induced change of brain extracellular potassium concentration [K⁺]_e and brain energy metabolism was studied following decapitation of normal 7-dav-old rats and 7-day-old rats exposed to a simulated altitude of 6000 m ( p O₂= 70 mm Hg) for 24 h. Initially there was in both groups a slow rate of rise of [K ⁺]_e followed by a faster rise until a steep increase occurred. In the group exposed to hypoxia the initial rate of rise was lower and the duration until the steep increase twice as long as in the normal group. Energy consumption expressed as Δ# P and the rates of lactate accumulation were similar in the two groups in the first minute following decapitation. After 2 min the Δ# P and the rate of lactate accumulation was higher in the group exposed to hypoxia. These findings are explained by a doubling of the glucose concentration in the brain of the rats exposed to hypoxia, permitting anaerobic glycolysis to maintain ATP regeneration for a longer time during ischemia.     

Role of fructose 2,6-bisphosphate in the regulation of glycolysis and gluconeogenesis in chicken liver

Glucagon and dibutyryl cyclic AMP inhibited glucose utilization and lowered fructose 2,6-bisphosphate levels of hepatocytes prepared from fed chickens. Partially purified preparations of chicken liver 6-phosphofructo-1-kinase and fructose 1,6-bisphosphatase were activated and inhibited by fructose 2,6-bisphosphate, respectively. The sensitivities of these enzymes and the changes observed in fructose 2,6-bisphosphate levels are consistent with an important role for this allosteric effector in hormonal regulation of carbohydrate metabolism in chicken liver. In contrast, oleate inhibition of glucose utilization by chicken hepatocytes occurred without change in fructose, 2,6-bisphosphate levels. Likewise, pyruvate inhibition of lactate gluconeogenesis in chicken hepatocytes cannot be explained by changes in fructose 2,6-bisphosphate levels. Exogenous glucose caused a marked increase in fructose 2,6-bisphosphate content of hepatocytes from fasted but not fed birds. Both glucagon and lactate prevented this glucose effect. Fasted chicken hepatocytes responded to lower glucose concentrations than fasted rat hepatocytes, perhaps reflecting the species difference in hexokinase isozymes.     

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...     

Effect of maternal exercise on fetal liver glycogen late in gestation in the rat

To determine the effect of maternal exercise on fetal liver glycogen content, fed and fasted rats that were pregnant for 20.5 or 21.5 days were run on a rodent treadmill for 60 min at 12 m/min with a 0% grade or 16 m/min up a 10% grade. The rats were anesthetized by intravenous injection of pentobarbital sodium, and fetal and maternal liver and plasma samples were collected and frozen. Fetal liver glycogenolysis did not occur as a result of maternal exercise. Fetal blood levels of lactate increased 22-60%, but glucose, plasma glucagon, and insulin were unchanged during maternal exercise. Maternal liver glycogen decreased as a result of exercise in all groups of rats except the fasted 20.5-day-pregnant group. Plasma free fatty acids increased in all groups and blood lactate increased in fed (20.5 days) and fasted (21.5 days) pregnant rats. Maternal glucose, glucagon, and insulin values remained constant during exercise. The fetus appears to be well-protected from metabolic stress during moderate-intensity maternal exercise.     

The role of the cytoplasmic redox potential in the control of fatty acid synthesis from glucose, pyruvate and lactate in white adipose tissue

The metabolism of lactate, pyruvate and glucose was studied in epididymal adipose tissue of starved, normally fed and starved-re-fed rats. Lactate conversion into fatty acid occurred at an appreciable rate only in the adipocyte of starved-re-fed animals. NNN'N'-Tetramethyl-p-phenylenediamine, an agent that transports reducing power from the cytoplasm to the mitochondria, caused large increments of fatty acid synthesis from lactate and a smaller one from glucose but a decrease in that from pyruvate. Glucose (1.0mm) increased fatty acid synthesis from lactate 4.3-fold but only 1.67-fold from pyruvate in adipocytes from normally fed animals. 2-Deoxyglucose decreased fatty acid synthesis from lactate to a greater degree (threefold) compared to that from pyruvate in adipocytes from starved-re-fed animals. l-Glycerol 3-phosphate contents were approximately equal in epididymal fat-pads, incubated in the presence of lactate or pyruvate, from normally fed animals, whereas the addition of 1mm-glucose resulted in a tenfold increase in l-glycerol 3-phosphate content only in the presence of lactate. The l-glycerol 3-phosphate content was tenfold higher in adipose tissue from starved-re-fed animals incubated in the presence of lactate than in the presence of pyruvate. 2-Deoxyglucose caused these values to be slightly lowered in the presence of lactate. We suggest that lactate metabolism is limited by the rate of NADH removal from the cytoplasm. In the starved-re-fed state, this occurs by reduction of dihydroxyacetone phosphate formed from glycogen to produce l-glycerol 3-phosphate, thus permitting lactate conversion into fatty acid. When glucose is the substrate, and rates of transport are not limiting, the rate of removal of cytoplasmic NADH limits glucose conversion into fatty acid.     

Role of a nonmitochondrial Ca2+ pool in the synergistic stimulation by cyclic AMP and vasopressin of Ca2+ uptake in isolated rat hepatocytes.

The subcellular distribution of 45Ca2+ accumulated by isolated rat hepatocytes exposed to dibutyryl cyclic AMP (dbcAMP) followed by vasopressin (Vp) was studied by means of a nondisruptive technique. When treated with dbcAMP followed by vasopressin, hepatocytes obtained from fed rats accumulated an amount of Ca2+ approximately fivefold higher than that attained under control conditions. Ca2+ released from the mitochondrial compartment by the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) accounted for only a minor portion of the accumulated Ca2+. The largest portion was released by the Ca2+ ionophore A23187 and was attributable to a nonmitochondrial compartment. DbcAMP + Vp-treatment also caused a maximal stimulation of glucose production and a twofold increase in cellular glucose 6-phosphate levels. In hepatocytes obtained from fasted rats, dbcAMP + Vp-stimulated Ca2+ accumulation was lower, although with the same subcellular distribution, and was associated with a minimal glucose production. In the presence of gluconeogenetic substrates (lactate plus pyruvate) hepatocytes from fasted rats were comparable to cells isolated from fed animals. However, Ca2+ accumulation and glucose 6-phosphate production could be dissociated in the absence of dbcAMP, in the presence of lactate/pyruvate alone. Under this condition in fact Vp induced only a minimal accumulation of Ca2+ in hepatocytes isolated from fasted rats, although glucose production was markedly increased. Moreover, treatment of fed rat hepatocytes with 1 mM ATP caused a maximal activation of glycogenolysis, but only a moderate stimulation of cellular Ca2+ accumulation. In this case, sequestration of Ca2+ occurred mainly in the mitochondrial compartment. By contrast, the addition of ATP to dbcAMP-pretreated hepatocytes induced a large accumulation of Ca2+ in a nonmitochondrial pool. Additional experiments using the fluorescent Ca2+ indicator Fura-2 showed that dbcAMP pretreatment can enlarge and prolong the elevation of cytosolic free Ca2+ caused by Vp. A nonmitochondrial Ca2+ pool thus appears mainly responsible for the Ca2+ accumulation stimulated by dbcAMP and Vp in isolated hepatocytes, and cyclic AMP seems able to activate Ca2+ uptake in such a nonmitochondrial pool.     

Lactate production in adipose tissue: a regulated function with extra-adipose implications.

Estimates of the quantitative contribution of adipose tissue to whole-body glucose metabolism, previously reported as 1-3%, have been revised to be on the order of 10-30%. These revised estimates come, in part, from a recognition that adipose tissue uses glucose to produce lactate and pyruvate, in addition to CO2 and triglycerides. Lactate production by adipose tissue is modulated in vitro by changes in glucose, insulin, and epinephrine concentrations. In vivo, lactate production is regulated acutely by the animal's nutritional state (fed or fasted) and chronically by the degree of obesity. A strong positive correlation exists between rat fat cell size and relative conversion of glucose to lactate (r = 0.89, P less than 0.001). Diabetes is also associated with markedly increased lactate production in adipocytes. Fat cells from obese or diabetic rats (or humans) can metabolize to lactate as much as 50-70% of the glucose taken up. From these recent studies, a picture is emerging in which the adipose organ may provide lactate for hepatic gluconeogenesis during fasting, and also lactate for hepatic glycogen synthesis after food ingestion. Modulation of adipocyte lactate production and contribution of adipose tissue lactate to the body's fuel economy in physiological and pathological states are the focus of this review.     

A reexamination of the role of the cytosolic alanine aminotransferase in hepatic gluconeogenesis

The relative importance of the mitochondrial and cytosolic alanine aminotransferase isozymes for providing pyruvate from alanine for further metabolism in the mitochondrial compartment was examined in the isolated perfused rat liver. The experimental rationale employed depends upon the supposition that gluconeogenesis from alanine and the decarboxylation of infused [1-14C]alanine should be diminished by pyruvate transport inhibitors (e.g., alpha-cyanocinnamate) in proportion to the contribution of the cytosolic alanine aminotransferase for generating pyruvate. alpha-Cyanocinnamate inhibited the endogenous rate of glucose production in perfused livers derived from 24-h-fasted rats. The rate of [1-14C]alanine decarboxylation at low (1 mM) and high (10 mM) perfusate alanine concentrations was inhibited by 9.5 and 42%, respectively, in the presence of alpha-cyanocinnamate. In livers from fasted animals perfused with either 1 or 10 mM alanine, alpha-cyanocinnamate caused a substantial increase in the rates of both lactate and pyruvate production. Elevating the hepatic ketogenic rate during infusion of acetate in livers, perfused with alanine, stimulated both the rates of alanine decarboxylation and glucose production; the extent of stimulation of these two metabolic parameters was determined to be a function of the alanine concentration in the perfusate. The stimulation of the rate of alanine decarboxylation during acetate-induced ketogenesis was reversed by co-infusion of alpha-cyanocinnamate with simultaneous increases in the rates of lactate and pyruvate production. The results indicate that during rapid ketogenesis, cytosolic transamination of alanine contributes at least 19% (at 1 mM alanine) and 55% (at 10 mM alanine) of the pyruvate for gluconeogenesis.     

Interstitial lactate,lactate/pyruvate and glucose in rat muscle before,during and in the recovery from global hypoxia

Background

Hypoxia results in an imbalance between oxygen supply and oxygen consumption. This study utilized microdialysis to monitor changes in the energy-related metabolites lactate, pyruvate and glucose in rat muscle before, during and after 30 minutes of transient global hypoxia. Hypoxia was induced in anaesthetised rats by reducing inspired oxygen to 6% O₂ in nitrogen.

Results

Basal values for lactate, the lactate/pyruvate ratio and glucose were 0.72 ± 0.04 mmol/l, 10.03 ± 1.16 and 3.55 ± 0.19 mmol/l (n = 10), respectively. Significant increases in lactate and the lactate/pyruvate ratio were found in the muscle after the induction of hypoxia. Maximum values of 2.26 ± 0.37 mmol/l for lactate were reached during early reperfusion, while the lactate/pyruvate ratio reached maximum values of 35.84 ± 7.81 at the end of hypoxia. Following recovery to ventilation with air, extracellular lactate levels and the lactate/pyruvate ratio returned to control levels within 30-40 minutes. Extracellular glucose levels showed no significant difference between hypoxia and control experiments.

Conclusions

In our study, the complete post-hypoxic recovery of metabolite levels suggests that metabolic enzymes of the skeletal muscle and their related cellular components may be able to tolerate severe hypoxic periods without prolonged damage. The consumption of glucose in the muscle in relation to its delivery seems to be unaffected.

     

Changes in biochemical properties of myocardial lactate dehydrogenase during exposure of rats to high altitude.

Exposure of normal rats to chronic natural hypoxia has shown the following effect upon catalytic properties of myocardial lactate dehydrogenase (LDH): significant increase of the two substrates pyruvate and lactate in tissue extracts; no changes in electrophoretic patterns of the enzyme; a slight enhancement of the activation energy of enzyme molecules; a significant increase in the Michaelis constant for pyruvate. Variations in biochemical properties of LDH appear after 12 ueeks of life in high altitude environment. These adjustments may be related to the stimulation of anaerobic metabolism induced by altitudinal hypoxia. Changes in LDH biochemical parameters seem adaptative.     

Inhibition of gluconeogenesis and glycogenolysis by 2,5-anhydro-D-mannitol

2,5-Anhydro-D-mannitol (100 to 200 mg/kg) decreased blood glucose by 17 to 58% in fasting mice, rats, streptozotocin-diabetic mice, and genetically diabetic db/db mice. Serum lactate in rats was elevated 56% by 2,5-anhydro-D-mannitol, but this could be prevented by dichloroacetate (200 mg/kg) or thiamin (200 mg/kg). In hepatocytes from fasted rats, 1 mM 2,5-anhydro-D-mannitol inhibited gluconeogenesis from a mixture of alanine, lactate, and pyruvate. It also inhibited glucose production and stimulated lactate formation from glycerol or dihydroxyacetone. Glycogenolysis in hepatocytes from fed rats was markedly inhibited by 1 mM 2,5-anhydro-D-mannitol both in the presence or absence of 1 microM glucagon. 2,5-Anhydro-D-mannitol can be phosphorylated by fructokinase or hexokinase to the 1-phosphate and then by phosphofructokinase to the 1,6-bisphosphate. Rat liver glycogen phosphorylase was inhibited by 2,5-anhydro-D-mannitol 1-phosphate (apparent Ki = 0.66 +/- 0.09 mM) but was little affected by 2,5-anhydro-D-mannitol 1,6-bisphosphate. Rat liver phosphoglucomutase was inhibited by 2,5-anhydro-D-mannitol 1-phosphate (apparent Ki = 2.8 +/- 0.2 mM), whereas 2,5-anhydro-D-mannitol 1,6-bisphosphate served as an alternative activator (apparent K alpha = 7.0 +/- 0.5 microM). Rabbit liver pyruvate kinase was activated by 2,5-anhydro-D-mannitol 1,6-bisphosphate (apparent K alpha = 9.5 +/- 0.9 microM), whereas rabbit liver fructose 1,6-bisphosphatase was inhibited by 2,5-anhydro-D-mannitol 1,6-bisphosphate (apparent Ki = 3.6 +/- 0.3 microM). The phosphate esters of 2,5-anhydro-D-mannitol would, therefore, be expected to inhibit glycogenolysis and gluconeogenesis and stimulate glycolysis in liver.     

Glial Formation of Pyruvate and Lactate from TCA Cycle Intermediates: Implications for the Inactivation of Transmitter Amino Acids?

Abstract: Cerebral formation of lactate via the tricarboxylic acid (TCA) cycle was investigated through the labeling of lactate from [2-¹³C]acetate and [1-¹³C]glucose as shown by ¹³C NMR spectroscopy. In fasted mice that had received [2-¹³C]acetate intravenously, brain lactate C-2 and C-3 were labeled at 5, 15, and 30 min, reflecting formation of pyruvate and hence lactate from TCA cycle intermediates. In contrast, [1-¹³C]glucose strongly labeled lactate C-3, reflecting glycolysis, whereas lactate C-2 was weakly labeled only at 15 min. These data show that formation of pyruvate, and hence lactate, from TCA cycle intermediates took place predominantly in the acetate-metabolizing compartment, i.e., glia. The enrichment of total brain lactate from [2-¹³C]acetate reached ∼1% in both the C-2 and the C-3 position in fasted mice. It was calculated that this could account for 20% of the lactate formed in the glial compartment. In fasted mice, there was no significant difference between the labeling of lactate C-2 and C-3 from [2-¹³C]acetate, whereas in fed mice, lactate C-3 was more highly labeled than the C-2, reflecting adaptive metabolic changes in glia in response to the nutritional state of the animal. It is hypothesized that conversion of TCA cycle intermediates into pyruvate and lactate may be operative in the glial metabolism of extracellular glutamate and GABA in vivo. Given the vasodilating effect of lactate on cerebral vessels, which are ensheathed by astrocytic processes, conversion of glutamate and GABA into lactate could be one mechanism mediating increases in cerebral blood flow during nervous activity.     

急性低氧暴露小鼠外周血代谢组变化分析

目的：探讨急性低氧对小鼠外周血代谢组的影响。方法：将14只小鼠随机分为正常组和低氧组。用基础饲料喂养2周后,将低氧组减压至6000m模拟高度停留8h,实验结束后,采集静脉血制备血浆待测。在核磁共振波谱仪进行^1H NMR检测,采用模式识别分析方法处理数据。结果：与正常组相比,低氧组乳酸含量明显增加,肉碱水平明显降低;脂类、丙氨酸、丙酮酸、谷氨酰胺、胆碱、牛磺酸和葡萄糖含量升高,缬氨酸、肛羟丁酸、谷氨酸、甘油、甘氨酸和丝氨酸含量下降。结论：急性低氧暴露使小鼠血浆碳水化合物、脂肪代谢和氨基酸代谢谱发生变化,表明低氧后能量代谢以及相关物质含量发生改变。