Exogenous Glucose Utilization Research Articles

Neutrophil NADPH oxidase regulation by oxidative phosphorylation and accessory glucose metabolic pathways

Neutrophils produce reactive oxygen species (ROS) to extinguish pathogens and regulate intracellular signaling. The major producer of ROS in neutrophils is the NADPH oxidase, which is fueled by NADPH generated via the oxidative pentose phosphate pathway. It remains unclear how other accessory pathways of glucose metabolism and mitochondrial activity influence the oxidative burst in neutrophils. The respiratory burst in bone marrow-derived neutrophils was stimulated with phorbol 12-myristate 13-acetate (PMA) in the presence or absence of an inhibitor of glycogen breakdown (glycogen phosphorylase inhibitor; GPi), inhibitors of mitochondrial respiration (antimycin A and rotenone), or an inhibitor of serine biosynthesis (NCT503). Respiratory burst was also measured under conditions of limited glucose and glutamine. ROS production was measured via extracellular flux analysis or via dihydrorhodamine fluorescence. In both male and female mouse neutrophils, inhibition of glycogen breakdown by GPi delayed initiation of the oxidative burst in response to PMA (n = 6, p<0.05, Student’s T-test and Repeated Measure’s ANOVA). Inhibition of mitochondrial oxidative phosphorylation or inhibition of the serine biosynthetic pathway sustained ROS production in PMA-stimulated neutrophils (n = 6, p<0.05, Student’s T-test). Removal of glucose and glutamine showed a truncated oxidative burst (n = 6, p<0.05, Student’s T-test). Ultimately, these findings suggest that glycogen breakdown fuels the initial activation of the oxidative burst, while mitochondrial activity and the serine biosynthesis pathway control the magnitude and duration of neutrophil ROS production. Furthermore, utilization of exogenous glucose and glutamine are necessary for the entire oxidative burst. Future studies will continue to illustrate the mechanisms of neutrophil metabolic plasticity as well as connect their significance in the context of wound healing, exercise, and chronic inflammatory diseases. NIH: R01HL141191, NIH: P01 HL078825. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Effect Of Wearing Compression Tights On Muscle Oxygenation And Exogenous Glucose Utilization During Prolonged Cycling

PURPOSE: To determine effect of wearing compression tights on muscle oxygenation and exogenous glucose utilization during prolonged cycling in endurance athletes. METHODS: Ten triathletes (19.9 ± 1.2 years, 172.2 ± 4.4 cm, 62.0 ± 3.8 kg,BMI; 20.9 ± 0.7 kg·m-2; VO2max; 59.3 ± 4.4 ml·kg -1·min -1) completed 3 trials under respective conditions: 1) wearing a compression tight with approximately 15 mmHg [MED trial], 2) wearing a compression tight with approximately 30 mmHg [HIGH trial], and 3) wearing a tight with below 5 mmHg [CON trial]. The exercise consisted of 90 min of cycling at 65% of VO2max. Changes in exogenous glucose utilization (13C excretion in expired gas after consuming 13C labeled glucose), muscle oxygenation (oxy-hemoglobin, deoxy-hemoglobin, total hemoglobin and tissue saturation index), blood parameters (blood glucose and lactate, and serum glycerol, insulin and total ketone body concentrations), blood gas parameters (pH, pO2, pCO2, HCO3-, Na+ and K+), respiratory variables (VO2, VCO2, VE, RER), heart rate and rating of perceived exertion were evaluated throughout the exercise. Time to exhaustion test (at 85% of VO2max) was performed after completing 90 min of cycling to evaluate endurance performance. RESULTS: Exercise rapidly increased 13C excretion, but highest value of 13C excretion was significantly shown earlier in the MED trial (47.1 ± 9.9 min) compared with in the other trials (HIGH trial: 67.3 ± 23.7 min, CON trial: 54.0 ± 9.5 min, P = 0.033). Although MED trial showed significant lower oxy-hemoglobin throughout the exercise (P = 0.003), the changes in deoxy-hemoglobin, total hemoglobin and tissue oxygenation index did not differ among the trials. No significant difference was observed for changes in other variables among the trials. CONCLUSION: Wearing a compression tights exerting approximately 15 mmHg on thigh facilitated exogenous glucose utilization during 90 min of cycling.

A Thiamin Derivative Inhibits Oxidation of Exogenous Glucose at Rest, but Not during Exercise

Thiamin (vitamin B(1)) is known to activate carbohydrate metabolism in part through activation of pyruvate dehydrogenase. The purpose of this study was to investigate the effect of thiamin tetrahydrofurfuryl disulfide (TTFD), a thiamin derivative, on utilization of exogenous glucose by measuring oxidation of (13)C-glucose at rest and during prolonged exercise in mice under normal dietary conditions. Mice orally ingested TTFD (0.1 mg/g BW [body weight]) and (13)C-glucose (0.8 mg/g BW) or (13)C-lactate (0.1 mg/g BW) plus glucose (0.8 mg/g BW) at rest or before endurance running. The average percent of (13)C atoms in total (12)C+(13)C ((13)C atom%) in expired air after ingestion of (13)C-glucose at rest was significantly lower in the TTFD group than in the control group. No significant difference was found in (13)C atom% in expired air after ingestion of (13)C-glucose and prolonged exercise. In addition, no significant effect of TTFD was found in expired (13)C atom% after ingestion of (13)C-lactate plus glucose at rest. TTFD also had no effect on concentrations of muscle or liver glycogen at rest. These results suggest that TTFD, which is a thiamin derivative, decreases oxidation of exogenous glucose at rest, but not during exercise.

Substrate metabolism, nutrient balance and obesity development in children and adolescents: a target for intervention?

Obesity results from the organism's inability to maintain energy balance over a long term. Childhood obesity and its related factors and pathological consequences tend to persist into adulthood. A cluster of factors, including high energy density in the diet (high fat intake), low energy expenditure, and disturbed substrate oxidation, favour the increase in fat mass. Oxidation of three major macronutrients and their roles in the regulation of energy balance, particularly in children and adolescents, are discussed. Total glucose oxidation is not different between obese and lean children; exogenous glucose utilization is higher whereas endogenous glucose utilization is lower in obese compared with lean children. Carbohydrate composition of the diet determines carbohydrate oxidation regardless of fat content of the diet. Both exogenous and endogenous fat oxidation are higher in obese than in lean subjects. The influence of high fat intake on accumulation of fat mass is operative rather over a long term. Several future directions are addressed, such that a combination of increased physical activity and modification in diet composition, in terms of energy density and glycemic index, is recommended for children and adolescents.

Exercise training restores abnormal myocardial glucose utilization and cardiac function in diabetes.

Clinical and experimental studies have shown that a reduction in myocardial glucose utilization is a factor contributing to diabetic cardiomyopathy. This study determined whether exercise training could prevent the depression in glucose utilization observed in the diabetic rat heart. Diabetes was induced in Sprague-Dawley rats by an intravenous injection of streptozotocin (60 mg/kg). After 10 weeks of treadmill running, exogenous myocardial glucose utilization and cardiac function were determined in isolated working hearts perfused under aerobic conditions and then subjected to a 60-min period of low-flow ischemia followed by reperfusion. Compared to aerobically perfused sedentary control hearts, rates of myocardial glucose oxidation and glycolysis were lower in diabetic hearts. Diabetes was also characterized by a pronounced decrease in cardiac function. Following exercise training, rates of myocardial glucose oxidation and glycolysis were restored and cardiac performance was improved compared to sedentary diabetic hearts. During low-flow ischemia, the decrease in glycolysis observed in hearts of sedentary diabetic rats was attenuated following exercise training. Following ischemia, glucose oxidation and glycolysis returned to preischemic levels in all groups. However, hearts from trained diabetic animals had higher rates of glucose oxidation compared to their respective sedentary group. This was accompanied by an enhanced recovery of heart function following ischemia. Our results indicate that exercise training is effective in preventing the depression in myocardial glucose metabolism observed in the diabetic rat. This may explain the benefits of exercise in preventing cardiac dysfunction in diabetes.

Time-course changes in macronutrient metabolism induced by a nutritionally balanced low-calorie diet in obese women

The use of low-calorie diets is a common strategy for body-weight reduction purposes, but the time-course of the metabolic changes induced by moderately energy-restricted, otherwise balanced, diets is still poorly known. The aim of this nutritional intervention design was to study in obese women the effect of a balanced low-calorie diet on the metabolic rate, and metabolic fuel utilization changes during the weight loss process through the application of breath tests with stable isotope-labeled tracers.Seven obese (body mass index >30 kg/m2) women were assigned to a 10-week dietary hypoenergetic intervention regime supplying 55% of energy as carbohydrate, 30% as fat and 15% as protein. Metabolic rate and substrate utilization were evaluated for 6 h in separate occasions during the weight loss program by indirect calorimetry and after 13C-labeled glucose, triolein and leucine administration. Body weight loss after 10 weeks was 4.2±1.1 kg, while the percent body fat decrease was about 5%. Slimming was accompanied by a marked decrease in fasting leptin (about 25%). Postprandial carbohydrate utilization after the administration of a test meal with the same macronutrient distribution as the experimental low-energy diet was decreased (24.1%, P<0.05) as a consequence of the dietary restriction, which was associated with lower insulin plasma levels (P<0.05). Although protein and lipid oxidation were not significantly different after weight reduction (day 1 versus day 70), the metabolic utilization of these substrates tended to increase. Moreover, marginally significant indications obtained on days 15 and 45 suggest that the weight and body composition changes are attributable to a shift in endogenous and exogenous glucose utilization in favor of lipid burning. The breath tests determinations, which were performed on different occasions along the experimental trial, confirmed that the cumulative 13C output decreased for labeled tracers with time, being only statistically significant for the glucose utilization between days 15 and 45. In summary, the weight and fat mass losses were associated with a lower carbohydrate oxidation, which were probably compensated by an increase in lipid oxidation without major changes in protein mobilization.

MATERNAL NICOTINE EXPOSURE DURING PREGNANCY AND LACTATION: I. EFFECT ON GLYCOLYSIS IN THE LUNGS OF THE OFFSPRING

We investigated the effect of maternal nicotine exposure during pregnancy and lactation on carbohydrate metabolism in the neonatal lung. Female rats received nicotine (1 mg/kg body weight/day) subcutaneously from day 7 after mating. Control animals received saline. The suckling rats were killed 24 hours after the last dose of nicotine was administered to the mother on postnatal day 14. The lung tissue of 9 rat pups from 3 litters was surgically removed and the in vitro utilization of exogenous glucose (μmol/g wet lung tissue/h) was determined. Lactate production (μmol/g wet lung tissue/h) was also determined to assess glycolytic activity. Maternal nicotine exposure during pregnancy and lactation stimulated glucose turnover by 21.6% (P<.01), but suppressed glycolysis by 24.6% (P<.001) and glycogenolysis by 37.9% (P<.001). Maternal nicotine exposure during gestation and lactation had no effect on the activity of hexokinase (U/g wet lung tissue), but resulted in a lower phosphofructokinase activity (U/g wet lung tissue) in the lungs of the offspring. From the data, it appeared that the inhibition of the flux of glucose through the glycolytic pathway can be attributed to an inhibition of phosphofructokinase.

Epinephrine inhibits exogenous glucose utilization in exercising horses.

This study examined the effects of preexercise glucose administration, with and without epinephrine infusion, on carbohydrate metabolism in horses during exercise. Six horses completed 60 min of treadmill exercise at 55 +/- 1% maximum O(2) uptake 1) 1 h after oral administration of glucose (2 g/kg; G trial); 2) 1 h after oral glucose and with an intravenous infusion of epinephrine (0.2 micromol. kg(-1). min(-1); GE trial) during exercise, and 3) 1 h after water only (F trial). Glucose administration (G and GE) caused hyperinsulinemia and hyperglycemia ( approximately 8 mM). In GE, plasma epinephrine concentrations were three- to fourfold higher than in the other trials. Compared with F, the glucose rate of appearance was approximately 50% and approximately 33% higher in G and GE, respectively, during exercise. The glucose rate of disappearance was approximately 100% higher in G than in F, but epinephrine infusion completely inhibited the increase in glucose uptake associated with glucose administration. Muscle glycogen utilization was higher in GE [349 +/- 44 mmol/kg dry muscle (dm)] than in F (218 +/- 28 mmol/kg dm) and G (201 +/- 35 mmol/kg dm). We conclude that 1) preexercise glucose augments utilization of plasma glucose in horses during moderate-intensity exercise but does not alter muscle glycogen usage and 2) increased circulating epinephrine inhibits the increase in glucose rate of disappearance associated with preexercise glucose administration and increases reliance on muscle glycogen for energy transduction.

Ischemic preconditioning and glucose metabolism during low-flow ischemia: Role of the adenosine A1 receptor

Glycolysis-from-glycose may be more beneficial than glycogenolysis in protecting hearts against ischemia. We tested the hypothesis that ischemic preconditioning is mediated by increased exogenous glucose use during low-flow ischemia, an effect triggered by adenosine A1 receptor activation. Langendorff rat hearts were subjected to 25 min low-flow ischemia (0.6 ml/min) and 30 min reperfusion. Prior to underperfusion, hearts (n = 6 per group) were subjected to two cycles of either preconditioning ischemia (PC), infusion of the adenosine A1 agonist 2-chloro-N6-cyclopentyladenosine (CCPA; 0.25 mumol/l), or PC in the presence of the adenosine antagonist 8-(p-sulfophenyl)theophylline (SPT; 50 mumol/l). Glycolysis-from-glucose during underperfusion was measured using D-[2-3H]glucose. At the end of reperfusion, recovery of rate-force product was enhanced in the PC and CCPA groups (62 and 67% of preischemic value) compared to the ischemic control hearts (IC, 32%; P < 0.05), whereas protection was abolished in the SPT hearts (20%; P < 0.05 vs. PC). PC improved total glycolysis-from-glucose during underperfusion by 31% (P < 0.05 vs. IC); SPT abolished this increase. CCPA reduced total lactate release and glucose uptake during ischemia by 47% and 61%, respectively (P < 0.05 vs. IC). Abolishment of the preconditioning-associated increase in glucose uptake during underperfusion, by switching to a low glucose buffer, resulted in a loss of functional protection. This study strongly suggests that increased exogenous glucose utilization during low-flow ischemia mediates ischemic preconditioning without increasing total anaerobic glycolytic flux. Although adenosine A1 receptor activation reduces ischemic injury, it does not facilitate the increased glucose uptake observed with ischemic preconditioning, suggesting a different mechanism of protection.

A period of energy supplementation but not the type of supplement influences the insulin response to exogenous glucose in food-restricted post-partum suckler beef cows

AbstractTwo experiments were designed to study the influence of an increase in energy supply on the utilization of exogenous glucose in food-restricted post-partum Charolais suckler beef cows. In experiment 1, 18 cows were given a basal diet (BD) providing 0·70 of energy requirements from calving with or without an energy supplement (SD) of 2 kg of rolled triticale grain starting at day 32 post partum. On day 42 post partum an injection of glucose (1·0 mmol/kg live weight) lasting 20 min resulted in a similar plasma glucose and cortisol profile in each diet group but a smaller plasma insulin increase in SD than in BD cows (146 (s.e. 37)v.273 (s.e. 37) pmol/l,P&lt; 0·05). Plasma non-esterified fatty acids concentrations were reduced by the glucose injection (P&lt; 0·01). Experiment 2 compared the efficiency of an energy supplement in the form of concentrate (CS, maize grain, no. = 11) or forage (PS, maize silage, no. = 10) in oestrous-synchronized beef cows previously given a restricted diet. Glucose injection resulted in a small increase in insulin concentrations during infusion with a peak at 20 min (PS: 250 (s.e. 34)v.CS: 257 (s.e. 32) pmol/l,P&gt; 0·05). No differences were observed between the two groups of cows for glucose, insulin and cortisol profiles (P&gt; 0·05). Similar pregnancy rates after oestrous synchronization treatment were observed in CS and PS cows (6/11v.5/10). In conclusion, a period of supplementation sufficient to re-establish energy balance, but not the type of supplement, influences the insulin response to exogenous glucose in food-restricted post-partum Charolais beef cows.

MRNA Expression of Glycolytic Enzymes and Glucose Transporter Proteins in Ischemic Myocardium With and Without Reperfusion

It is known that ischemia commonly increases exogenous glucose utilization by accelerating glucose uptake and flux rates through the Embden–Meyerhof pathway. Constitutive enzymes regulate the rate of glycolysis and in turn are regulated by product inhibition and allosteric controls. The purpose of this report was to test whether mRNA abundance for select glycolytic enzymes, and glucose transport proteins, is also modified. Six intact, working pig hearts with coronary flow controlled by extracorporeal perfusion were compared at the following conditions: (1) aerobic control perfusion; (2) ischemia affected by a 60% decrease in left anterior descending (LAD) coronary perfusion; (3) ischemia again affected by a 60% decrease in LAD flow followed by a 40-min interval of aerobic reflow; (4) an intermittent ischemia and reflow protocol including four cycles of similar LAD flow reductions (5 min per cycle) interspersed with 15–20 min of aerobic reperfusion; (5) a 4-day model designed to produce myocardial chronic hibernation; and (6) mild ischemia induced by a 40% decrease in LAD flow for 85 min to produce certain adaptations compatible with short-term hibernation. In each heart, mRNA abundance was measured from LAD and circumflex (LCF) perfused myocardium for hexokinase, phosphofructokinase, glyceraldehyde-3-phosphate dehydrogenase and the two glucose transporter isomers, GLUT 4 and GLUT 1. mRNA data from LAD myocardium in intervention hearts were normalized to those from LAD tissue in the control heart (LADc) and with LCF values in the same intervention hearts. Signal variance around unity in the LAD tissue, with respect to that of the LCF myocardium, in the control heart compared closely (44 and 41% in two separate runs, respectively). GLUT 1/GLUT 4 ratios in the LAD and LCF beds of this heart also agreed closely. LAD/LADc ratios were increased for hexokinase (1.69), phosphofructokinase (3.69), and glyceraldehyde-3-phosphate dehydrogenase (2.29) in the ischemia heart and for phosphofructokinase (3.90), glyceraldehyde-3-phosphate dehydrogenase (2.20), GLUT 4 (1.55) and GLUT 1 (2.20) in the ischemia/reflow heart. There was no evidence of excess signal in the intermittent ischemia/reflow, chronic hibernation, or mild ischemia hearts. Altered signal from LCF myocardium was also suggested. These data indicate that mRNA abundance for select glycolytic enzymes and transporter proteins is increased in ischemic myocardium with or without reperfusion and offers a possible mechanism for increased protein activity in settings of diminished regional coronary flow.

Detection of Viability of Dysfunctional Myocardium in Coronary Heart Disease. I. Radionuclide Imaging

The question of myocardial viability in the presence of severe regional or global left ventricular dysfunction in patients with coronary artery disease poses a formidable challenge to the clinician. If the dysfunctional myocardium is viable, revascularization procedures are associated with improved contractile function, whereas no benefits are expected if the myocardium is nonviable. Thus, assessment of myocardial viability is necessary before optimal decisions can be made regarding the use of bypass surgery, angioplasty, or cardiac transplantation in a timely and cost-effective manner. The most reliable methods to detect viable myocardium include myocardial metabolic imaging of exogenous glucose utilization using ⊃18 F-FDG in combination with myocardial perfusion/flow studies, assessment of the integrity of the oxidative metabolism using ⊃11 C-acetate, and measurement of regional blood flow using ⊃13 N-ammonia or ⊃15 O-water. However, the equipment and expertise necessary to perform positron emission tomography (PET) are expensive and not generally available. Fortunately, alternative imaging modalities using routine radionuclide and echocardiographic techniques are widely available and have nearly the same accuracy as PET metabolic imaging. Excellent alternatives include perfusion imaging with ⊃201 thallium using the stress-redistribution-reinjection or the rest-redistribution protocols, or ⊃99m technetium sestamibi using a rest-delayed imaging protocol. As old techniques are being refined and new methods developed, such as metabolic imaging using SPECT techniques that utilize ⊃18 F-FDG or fatty acids labeled with ⊃131 I, the challenge is to choose the most appropriate technique after a critical review of the institutional resources and the merits and weaknesses of the technique.

Parity influences the utilization of exogenous glucose in suckler anoestrous Charolais beef cows

AbstractThis work was designed to study the influence of parity (multiparous (MP, no. = 9) or primiparous (PP, no. = 10)) and feeding level (control diet (CD; 1·00) or restricted diet (RD; proportionately 0·70 of requirements) in a 2×2 factorial design on the utilization of exogenous glucose in the suckler anoestrous Charolais beef cow. Lack of ovarian activity was confirmed by low circulating progesterone concentrations measured in weekly blood samples taken after parturition. An injection of glucose (1·5 mmol/kg live weight) lasting 20 min was given 30 days and 50 days post partum. Blood samples were collected before and for 120 min after the start of the injection and plasma insulin, glucose, non-esterified fatty acids (NEFA), β-hydroxybutyrate and urea were measured. Although plasma glucose concentrations were not significantly different between treatment groups in the period immediately after the injection of glucose, CD PP animals had higher plasma insulin levels than CD MP animals (at 25 min: 596·4, 283·8, 435·1 and 309·6 pmol/l, P &lt; 0·05 for groups PP CD, MP CD, PP RD and MP RD respectively). Plasma NEFA concentrations were initially higher in the RD groups compared with the CD groups but were reduced by the glucose injection (P &lt; 0·001). However, the PP RD cows took longer to respond to the exogenous glucose compared with the MP RD cows (at 25 min: PP RD: 133·2 and MP RD: 57·5 eq/l, P &lt; 0·05). In conclusion, suckler anoestrous PP cows appear to require higher plasma insulin levels than MP cows to metabolize exogenous glucose and underfed PP cows were slower to reduce plasma NEFA after glucose injection than underfed MP cows.

Breath-by-breath measurements for the analysis of exogenous glucose oxidation during intense endurance exercise using [13C]-isotopes.

Up to now only the analysis of 13CO2 in separate breath-gas samples after administration of [13C]-labelled carbohydrates with intervals of several minutes in between has been available for analyses of the exogenous glucose oxidation during exercise. Our studies show the use of rapid respiratory mass spectrometry for breath-by-breath analysis to determine exogenous glucose utilization. Six male triathletes performed two exercise tests of 100-min duration each on a cycle ergometer. In both tests 5 min after the beginning of cycling glucose was administered. In test 1 30 g natural glucose was given, in test 2 a mixture of 1.5 g [13C1]-glucose and 28.5 g natural glucose. The work rate was regulated to keep constant 90% of the VO2 at the VO2 levelling off (VO2max) measured in a short ramp test. The resulting work rate was at 260.2 +/- 9.5 watts in test 1 and at 276.3 +/- 12.1 watts in test 2. Respiration gases and the end-tidal concentrations of 12CO2 (12C) and 13CO2 (13C) were calculated breath-by-breath online. The course of the ratio [13C/12C] reflects the course of exogenous glucose utilization. Onset of utilization was 10.5 +/- 5.5 min following oral administration. Maximum utilization was attained at 53.1 +/- 9.8 min with a maximal rate of oxidation of 0.36 +/- 0.05 g/min. The beginning of the energy supply of exogenous glucose could be determined quite soon after oral administration.

Glucose metabolism distal to a critical coronary stenosis in a canine model of low-flow myocardial ischemia.

Myocardial regions perfused through a coronary stenosis may cease contracting, but remain viable. Clinical observations suggest that increased glucose utilization may be an adaptive mechanism in such "hibernating" regions. In this study, we used a combination of 13C-NMR spectroscopy, GC-MS analysis, and tissue biochemical measurements to track glucose through intracellular metabolism in intact dogs infused with [1-13C]glucose during a 3-4-h period of acute ischemic hibernation. During low-flow ischemia [3-13C]alanine enrichment was higher, relative to plasma [1-13C]glucose enrichment, in ischemic than in nonischemic regions of the heart, suggesting a greater contribution of exogenous glucose to glycolytic flux in the ischemic region (approximately 72 vs. approximately 28%, P < 0.01). Both the fraction of glycogen synthase present in the physiologically active glucose-6-phosphate-independent form (46 +/- 10 vs. 9 +/- 6%, P < 0.01) and the rate of incorporation of circulating glucose into glycogen (94 +/- 25 vs. 20 +/- 15 nmol/gram/min, P < 0.01) were also greater in ischemic regions. Measurement of steady state [4-13C)glutamate/[3-13C]alanine enrichment ratios demonstrated that glucose-derived pyruvate supported 26-36% of total tricarboxylic acid cycle flux in all regions, however, indicating no preference for glucose over fat as an oxidative substrate in the ischemic myocardium. Thus during sustained regional low-flow ischemia in vivo, the ischemic myocardium increases its utilization of exogenous glucose as a substrate. Upregulation is restricted to cytosolic utilization pathways, however (glycolysis and glycogen synthesis), and fat continues to be the major source of mitochondrial oxidative substrate.

EXOGENOUS GLUCOSE OXIDATION DURING EXERCISE IN ENDURANCE TRAINED AND UNTRAINED SUBJECTS 576

To investigate the effect of training on the oxidation of orally ingested glucose during exercise 7 endurance trained cyclists (T, VO2max 67±2.3 ml/kg.min) and 8 untrained subjects (UT, 46±2 ml/kg.min) were studied during 120 min of exercise at -60% VO2max. At the onset of exercise, 8 ml/kg of an 8% naturally labeled [13C]-glucose solution was ingested and 2 ml/kg every 20 min thereafter. Body weights of the T and UT were comparable (75.2±3.8 kg vs 72.3±1.2 kg). Breath samples were collected at rest and every 15 min during exercise and analyzed for13 CO2/12CO2 ratio by IRMS (Finnigan MAT 252). To allow background correction, all subjects performed a second test in which glucose of low natural 13C abundance was ingested. Tests were performed in random order. All results were calculated for the 60-120 min period. Since exercise was performed at the same relative intensity trained subjects had a higher energy expenditure (3404 kJ versus 2630 kJ; P<0.01). Fat contributed significantly more to energy expenditure (EE) in T (45%) compared to UT (33%) while CHO contributed 55% and 67% respectively. No differences were observed in exogenous glucose oxidation which contributed 24% in T and 26% in UT to EE respectively. As a consequence, the relative contribution of endogenous carbohydrates (CHO), as calculated by total minus exogenous CHO oxidation, was significantly less in T (31%) compared to UT (41%). In absolute terms, exogenous glucose oxidation was slightly, but not significantly higher in T compared to UT (50±2 g versus 45±3 g). In addition, peak exogenous glucose oxidation rates were not significantly different among the 2 groups (0.91 g/min in T and 0.86 g/min in UT). It was concluded that although the state of training had a major effect on endogenous substrate utilization, it had no influence on exogenous glucose utilization during submaximal exercise at the same relative intensity.

Preferential oxidation of glycogen in isolated working rat heart.

We tested the hypothesis that glycogen is preferentially oxidized in isolated working rat heart. This was accomplished by measuring the proportion of glycolytic flux (oxidation plus lactate production) specifically from glycogen which is metabolized to lactate, and comparing it to the same proportion determined concurrently from exogenous glucose during stimulation with epinephrine. After prelabeling of glycogen with either 14C or 3H, a dual isotope technique was used to simultaneously trace the disposition of glycogen and exogenous glucose between oxidative and non-oxidative pathways. Immediately after the addition of epinephrine (1 microM), 40-50% of flux from glucose was directed towards lactate. Glycogen, however, did not contribute to lactate, being almost entirely oxidized. Further, glycogen utilization responded promptly to the abrupt increase in contractile performance with epinephrine, during the lag in stimulation of utilization of exogenous glucose, suggesting that glycogen serves as substrate reservoir to buffer rapid increases in demand. Preferential oxidation of glycogen may serve to ensure efficient generation of ATP from a limited supply of endogenous substrate, or as a mechanism to limit lactate accumulation during rapid glycogenolysis.

Septic patients in multiple organ failure can oxidize infused glucose, but non-oxidative disposal (storage) is impaired.

1. Patients suffering trauma and sepsis are insulin resistant, but no studies have specifically been made of patients suffering multiple organ failure. 2. We have studied exogenous glucose utilization in multiple organ failure using a combination of the hyperglycaemic glucose clamp and indirect calorimetry to quantify glucose utilization in multiple organ failure, partitioning it into oxidative and nonoxidative disposal (storage). 3. Fourteen septic patients with multiple organ failure were studied. APACHE II (Acute Physiological and Chronic Health Evaluation Mark II) scores on the day of the study ranged from 11 to 31 (median 16). Twenty percent D-glucose was infused and blood glucose was clamped at 12 mmol/l for 3 h. The results were compared with those obtained on seven healthy control subjects. 4. Glucose utilization and energy expenditure were similar in the two groups for the first 90 min of the clamp, after which glucose utilization and energy expenditure increased steadily in the control subjects but did not change in the patients. Respiratory exchange ratio rose in both groups; considered over the whole of the clamp period, respiratory exchange ratio was slightly lower in the patients than in the control subjects (P < 0.05) but not at any specific time point. Glucose oxidation rose in both groups but non-oxidative glucose disposal (storage) rose only in the control subjects. Glucose oxidation was slightly lower in the patients (P < 0.05) but not at any specific time point and there was no difference between the groups in the amount by which glucose oxidation increased. Non-oxidative disposal in the patients fell significantly (P < 0.01) over the course of the clamp and was significantly lower than in the control subjects (P < 0.01). 5. Growth hormone increased in response to glucose infusion in the patients but not in the control subjects. 6. Like patients suffering uncomplicated sepsis or trauma, patients with multiple organ failure are also insulin resistant. The defect appears to lie in an impairment of the ability to store glucose rather than oxidize it, and this may be due in part to the increase in growth hormone in patients with multiple organ failure.

Myocardial Kinetics of a Putative Hypoxic Tissue Marker, 99m Tc-Labeled Nitroimidazole (BMS-181321), After Regional Ischemia and Reperfusion

A new nitroimidazole complex, 99mTc-propylene amine oxime-1,2-nitroimidazole (BMS-181321), has been developed to allow the positive imaging of hypoxic myocardium by standard gamma camera techniques. To determine the myocardial kinetics of BMS-181321 during myocardial ischemia and reperfusion, seven open-chest swine were prepared according to a model of extracorporeal coronary perfusion in which left ventricular wall thickening (percent end-diastolic thickness) and substrate use in the left anterior descending (LAD) region ([14C]palmitate and [3H]glucose infusions) were determined. Measurements were obtained at baseline, during 40 minutes of ischemia produced by reducing flow in the LAD distribution by 60%, and during 70 minutes of reperfusion. Three aerobic control hearts were also studied in which LAD blood flow was not reduced. Regional coronary circulation was further assessed in all hearts by use of radiolabeled microspheres injected during ischemia. BMS-181321 (20 to 30 mCi) was injected after 30 minutes of ischemia, and its myocardial uptake was assessed by dynamic planar gamma imaging. Ischemia was associated with declines in fatty acid metabolism (15 +/- 11 mumol.h-1.g dry wt-1, mean +/- SEM), systolic wall thickening (20 +/- 6%), and myocardial oxygen consumption (3 +/- 1 mL.min-1.100 g-1) and an increase in exogenous glucose utilization (75 +/- 13 mumol.h-1.g dry wt-1). Systolic wall thickening recovered by only 8 +/- 3% with reperfusion. Initial distribution of BMS-181321 in the aerobic hearts appeared homogeneous. Washout from the ischemic and reperfused LAD bed was slower than the aerobically perfused LAD bed in the control group (t1/2 = 136 +/- 1 versus 80 +/- 1 minutes, P < .05), allowing visualization of the LAD region during reperfusion. Tissue activity of BMS-181321 was inversely related to LAD blood flow during ischemia (r = -.68 +/- .05), and the ratio of BMS-181321 in the LAD region versus normal myocardium was 1.7 +/- 0.2. Control swine lacked regional deposition of the tracer in the normally perfused LAD distribution. Thus, acute regional ischemia in these studies was visualized as an increase in retention of BMS-181321, suggesting its applicability in the imaging of clinical conditions of myocardial hypoperfusion.

Mechanical and metabolic functions in pig hearts after 4 days of chronic coronary stenosis

This study sought to evaluate the functional and metabolic consequences of imposing a chronic external coronary stenosis around the left anterior descending coronary artery for 4 days in an intact pig model. A clinical condition termed hibernating myocardium has been described wherein as a result of chronic sustained or intermittent coronary hypoperfusion, heart muscle minimizes energy demands by decreasing mechanical function and thus avoids cell death. The use of chronic animal models to stimulate this disorder may assist in establishing causative associations among determinants to explain this phenomenon. A hydraulic cuff occluder was placed around the left anterior descending coronary artery in eight pigs. Coronary flow velocity was reduced by a mean (+/- SE) of 49 +/- 5% of prestenotic values, as estimated by a Doppler velocity probe. After 4 days the pigs were prepared with extracorporeal coronary circulation and evaluated at flow conditions dictated by the cuff occluder. Substrate utilizations were described using equilibrium labeling with [U-14C]palmitate and [5-3H]glucose. Results were compared with a combined group of 21 acute and chronic (4 day) sham animals. Four days of partial coronary stenosis significantly decreased regional systolic shortening by 54%. Myocardial oxygen consumption was maintained at aerobic levels, and rest coronary flows were normal. Fatty acid oxidation was decreased by 43% below composite sham values, and exogenous glucose utilization was increased severalfold. Alterations in myocardial metabolism were accompanied by a decline in tissue content of adenosine triphosphate. These data suggest that chronic coronary stenosis in the absence of macroscarring imparts an impairment in mechanical function, whereas coronary flow and myocardial oxygen consumption are preserved at rest. The increases in glycolytic flux of exogenous glucose are similar to observations on glucose uptake assessed by fluorine-18 2-deoxy-2-fluoro-D-glucose in patients with advanced coronary artery disease. We speculate that intermittent episodes of ischemia and reperfusion are the cause of this phenomenon.