Quantitation of positional isomers of deuterium-labeled glucose by gas chromatography/mass spectrometry.

A method for determining the site and extent of deuterium (D) labeling of glucose by GC/MS and mass fragmentography was developed. Under chemical and electron impact ionization, ion clusters m/z 328, 242, 217, 212, and 187 of glucose aldonitrile pentaacetate and m/z 331 and 169 of pentaacetate derivative were produced. From the mass spectra of 13C- and D-labeled reference compounds, glucose carbon and hydrogen (C-H) positions included in these fragments were deduced to be m/z 328 = C1-C6, 2,3,4,5,6,6-H6; m/z 331 = C1-C6, 1,2,3,4,5,6,6-H7; m/z 169 = C1-C6, 1,3,4,5,6,6-H6; m/z 187 = C3-C6, 3,4,5,6,6-H5; m/z 212 = C1-C5, 2,3,4,5-H4; m/z 217 = C4-C6, 4,5,6,6-H4; and m/z 242 = C1-C4, 2,3,4-H3. After correction for isotope discrimination and deuterium-hydrogen exchange, the D enrichment of these fragments can be quantitated using selective ion monitoring, and the D enrichment of all C-H positions can be obtained by the difference in enrichment of the corresponding ion pairs. The validity of this approach was tested by examining D enrichment of known mixtures of 1-d1-, 2-d1-, 3-d1-, and 5,6,6-d3-glucose with unlabeled glucose and D enrichment of perdeuterated glucose using these fragments. This method was used to determine deuterium incorporation in C1 through C6 of blood glucose in fasted (24 h) rats infused with deuterated water. The distribution of deuterium was similar to that found by Postle and Bloxham (1980, Biochem. J. 192, 65-73). Approximately one deuterium atom was incorporated into C5 and only 75% deuterium atom was incorporated into C2. The enrichment of C2 and C6 of glucose relative to that of water indicated that 74 +/- 9% of plasma glucose was newly formed 4 h after the onset of deuterium infusion, and gluconeogenesis accounted for about 76 +/- 7% of the glucose 6-phosphate flux.     

Determination of glucose turnover in sea bass Dicentrarchus labrax. Comparative aspects of glucose utilization

1. Parameters of in vivo glucose utilization by sea bass (132 +/- 6 g, mean +/- SEM) acclimated at 15 degrees C in sea-water were measured after single injection of labelled glucose. 2. Glucose turnover rate (RG; mumol . min-1 . kg-1) was found to be 0.55-065 (2-3H glucose) and 0.34 +/- 0.42 (U14C glucose). 3. Glucose transit time was 443-449 min, glucose mass 233-261 mumol . kg-1, and glucose recycling 37%. 4. Oxygen consumption (MO2) amounted to 94 +/- 6.2 mumol . min-1 . kg-1. 5. The comparison with other fish species, mammals and birds, taking into account body size, temperature, diet, exercise, in poikilotherms and homeotherms leads to the calculation of a glucose turnover index (RGI = RG x 6 x 100 x MO2(-1)). 6. Value of this, generally lower in ectotherm teleosts (2-9), than in endotherms: mammals, birds and thunidae (22-60), confirms the minor quantitative importance of glucose in the metabolism of most fish.     

Effect of dihydroxyacetone and pyruvate on plasma glucose concentration and turnover in noninsulin-dependent diabetes mellitus

Consumption of dihydroxyacetone and pyruvate (DHP) increases muscle extraction of glucose in normal men. To test the hypothesis that these three-carbon compounds would improve glycemic control in diabetes, we evaluated the effect of DHP on plasma glucose concentration, turnover, recycling, and tolerance in 7 women with noninsulin-dependent diabetes. The subjects consumed a 1,500-calorie diet (55% carbohydrate, 30% fat, 15% protein), randomly containing 13% of the calories as DHP (1/1) or Polycose (placebo; PL), as a drink three times daily for 7 days. On the 8th day, primed continuous infusions of [6-3H]-glucose and [U-14C]-glucose were begun at 05.00 h, and at 09.00 h a 3-hour glucose tolerance test (75 g glucola) was performed. Two weeks later the subjects repeated the study with the other diet. The fasting plasma glucose level decreased by 14% with DHP (DHP = 8.0 +/- 0.9 mmol/l; PL = 9.3 +/- 1.0 mmol/l, p less than 0.05) which accounted for lower postoral glucose glycemia (DHP = 13.1 +/- 0.8 mmol/l, PL = 14.7 +/- 0.8 mmol/l, p less than 0.05). [6-3H]-glucose turnover (DHP = 1.50 +/- 0.19 mg.kg-1.min-1, PL = 1.77 +/- 0.21 mg.kg-1.min-1, p less than 0.05) and glucose recycling, the difference in [6-3H]-glucose and [U-14C]-glucose turnover rates, decreased with DHP (DHP = 0.25 +/- 0.07 mg.kg-1.min-1, PL = 0.54 +/- 0.10 mg.kg-1.min-1, p less than 0.05). Fasting and postoral glucose, plasma insulin, glucagon, and C peptide levels were unaffected by DHP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Acute anemia results in an increased glucose dependence during sustained exercise

We evaluated whether acute anemia results in altered blood glucose utilization during sustained exercise at 26.8 m/min on 0% grade, which elicited approximately 60-70% maximal O2 consumption. Acute anemia was induced in female Sprague-Dawley rats by isovolumic plasma exchange transfusion. Hemoglobin and hematocrit were reduced 33% by exchange transfusion to 8.6 +/- 0.4 g/dl and 26.5 +/- 1%, respectively. Glucose kinetics were determined by primed continuous infusion of [6-3H]glucose. Rates of O2 consumption were similar during rest (pooled means 25.1 +/- 1.8 ml.kg-1.min-1) and exercise (pooled means 46.8 +/- 3.0 ml.kg-1.min-1). Resting blood glucose and lactate concentrations were not different in anemic animals (pooled means 5.1 +/- 0.2 and 0.9 +/- 0.02 mM, respectively). Exercise resulted in significantly decreased blood glucose (4.0 +/- 0.2 vs. 4.6 +/- 0.1 mM) and elevated lactate (6.1 +/- 0.4 vs. 2.3 +/- 0.5 mM) concentrations in anemic animals. Glucose turnover rates (Rt) were not different between anemic and control animals at rest and averaged 58.8 +/- 3.6 mumol.kg-1.min-1. Exercise resulted in a 30% greater increase in Rt in anemic (141.7 +/- 3.2 mumol.kg-1.min-1) than in control animals (111.2 +/- 5.2 mumol.kg-1.min-1). Metabolic clearance rates (MCR = Rt/[glucose]) were not different at rest (11.6 +/- 7.4) but were significantly greater in anemic (55.2 +/- 5.7 ml.kg-1.min-1) than in control animals (24.3 +/- 1.4 ml.kg-1.min-1) during exercise.(ABSTRACT TRUNCATED AT 250 WORDS)     

Increased dependence on blood glucose after acclimatization to 4,300 m

To evaluate the hypothesis that altitude exposure and acclimatization result in increased dependency on blood glucose as a fuel, seven healthy males (23 +/- 2 yr, 72.2 +/- 1.6 kg, mean +/- SE) on a controlled diet were studied in the postabsorptive condition at sea level (SL), on acute altitude exposure to 4,300 m (AA), and after 3 wk of chronic altitude exposure to 4,300 m (CA). Subjects received a primed continuous infusion of [6,6-2D]glucose and rested for a minimum of 90 min, followed immediately by 45 min of exercise at 101 +/- 3 W, which elicited 51.1 +/- 1% of the SL maximal O2 consumption (VO2 max; 65 +/- 2% of altitude VO2 max). At SL, resting arterial glucose concentration was 82.4 +/- 3.2 mg/dl and rose significantly to 91.2 +/- 3.2 mg/dl during exercise. Resting glucose appearance rate (Ra) was 1.79 +/- 0.02 mg.kg-1.min-1; this increased significantly during exercise at SL to 3.71 +/- 0.08 mg.kg-1.min-1. On AA, resting arterial glucose concentration (85.8 +/- 4.1 mg/dl) was not different from sea level, but Ra (2.11 +/- 0.14 mg.kg-1.min-1) rose significantly. During exercise on AA, glucose concentration rose to levels seen at SL (91.4 +/- 3.0 mg/dl), but Ra increased more than at SL (to 4.85 +/- 0.15 mg.kg-1.min-1; P less than 0.05). Resting arterial glucose was significantly depressed with CA (70.8 +/- 3.8 mg/dl), but resting Ra increased to 3.59 +/- 0.08 mg.kg-1.min-1, significantly exceeding SL and AA values.(ABSTRACT TRUNCATED AT 250 WORDS)     

Enteral infusion of glucose at rates approximating EGP enhances glucose disposal but does not cause hypoglycemia

Portal infusion of glucose at rates approximating endogenous glucose production (EGP) causes paradoxical hypoglycemia in wild-type but not GLUT2 null mice, implying activation of a specific portal glucose sensor. To determine whether this occurs in humans, glucose containing [3-3H]glucose was infused intraduodenally at rates of 3.1 mg. kg-1. min-1 (n = 5), 1.55 mg. kg-1. min-1 (n = 9), or 0/0.1 mg. kg-1. min-1 (n = 9) for 7 h in healthy nondiabetic subjects. [6,6-2H2]glucose was infused intravenously to enable simultaneous measurement of EGP, glucose disappearance, and the rate of appearance of the intraduodenally infused glucose. Plasma glucose concentrations fell (P < 0.01) from 90 +/- 1 to 84 +/- 2 mg/dl during the 0/0.1 mg. kg-1. min-1 id infusions but increased (P < 0.001) to 104 +/- 5 and 107 +/- 3 mg/dl, respectively, during the 1.55 and 3.1 mg. kg-1. min-1 id infusions. In contrast, insulin increased (P < 0.05) during the 1.55 and 3.0 mg. kg-1. min-1 infusions, reaching a peak of 10 +/- 2 and 18 +/- 5 micro U/ml, respectively, by 2 h. Insulin concentrations then fell back to concentrations that no longer differed by study end (7 +/- 1 vs. 8 +/- 1 micro U/ml). This resulted in comparable suppression of EGP by study end (0.84 +/- 0.2 and 0.63 +/- 0.1 mg. kg-1. min-1). Glucose disappearance was higher (P < 0.01) during the final hour of the 3.1 than 1.55 mg. kg-1. min-1 id infusion (4.47 +/- 0.2 vs. 2.6 +/- 0.1 mg. kg-1. min-1), likely because of the slightly, but not significantly, higher glucose and insulin concentrations. We conclude that, in contrast to mice, selective portal glucose delivery at rates approximating EGP does not cause hypoglycemia in humans.     

Dynamics of hepatic lactate and glucose balances during prolonged exercise and recovery in the dog

The present experiments were undertaken to assess dynamics of hepatic lactate and glucose balance in the over-night-fasted dog during 150 min of moderate-intensity treadmill exercise and 90 min of exercise recovery. Catheters were implanted chronically in an artery and portal and hepatic veins 16 days before experimentation. 3-3H-glucose was infused to determine hepatic glucose uptake, as well as tracer-determined glucose production by isotope dilution (Ra). At rest, net hepatic lactate output was 0.33 +/- 0.15 mg.kg-1.min-1 and increased to 2.26 +/- 0.82 mg.kg-1.min-1 after 10 min of exercise, after which it fell such that the liver was a net lactate consumer by the end of exercise and through recovery. In contrast to the rapid release of lactate, net hepatic glucose output rose gradually from 2.58 +/- 0.20 mg.kg-1.min-1 at rest to 8.87 +/- 0.85 mg.kg-1.min-1 after 60 min of exercise, beyond which it did not change significantly until the cessation of exercise. Hepatic glucose uptake at rest was 1.38 +/- 0.42 mg.kg-1.min-1 and did not change appreciably during exercise or recovery. Absolute hepatic glucose output (net glucose output plus uptake) rose from 3.96 +/- 0.45 mg.kg-1.min-1 at rest to 10.20 +/- 1.09 mg.kg-1.min-1 after 60 min of exercise and was 9.65 +/- 1.15 mg.kg-1.min-1 at 150 min of exercise. Ra rose from 3.34 +/- 0.21 mg.kg-1.min-1 to 7.58 +/- 0.73 and 8.59 +/- 0.77 mg.kg-1.min-1 at 60 and 150 min, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Glucagon chronically impairs hepatic and muscle glucose disposal

Defects in insulin secretion and/or action contribute to the hyperglycemia of stressed and diabetic patients, and we hypothesize that failure to suppress glucagon also plays a role. We examined the chronic impact of glucagon on glucose uptake in chronically catheterized conscious depancreatized dogs placed on 5 days of nutritional support (NS). For 3 days of NS, a variable intraportal infusion of insulin was given to maintain isoglycemia (approximately 120 mg/dl). On day 3 of NS, animals received a constant low infusion of insulin (0.4 mU.kg-1.min-1) and either no glucagon (CONT), basal glucagon (0.7 ng.kg-1.min-1; BasG), or elevated glucagon (2.4 ng.kg-1.min-1; HiG) for the remaining 2 days. Glucose in NS was varied to maintain isoglycemia. An additional group (HiG+I) received elevated insulin (1 mU.kg-1.min-1) to maintain glucose requirements in the presence of elevated glucagon. On day 5 of NS, hepatic substrate balance was assessed. Insulin and glucagon levels were 10+/-2, 9+/-1, 7+/-1, and 24+/-4 microU/ml, and 24+/-5, 39+/-3, 80+/-11, and 79+/-5 pg/ml, CONT, BasG, HiG, and HiG+I, respectively. Glucagon infusion decreased the glucose requirements (9.3+/-0.1, 4.6+/-1.2, 0.9+/-0.4, and 11.3+/-1.0 mg.kg-1.min-1). Glucose uptake by both hepatic (5.1+/-0.4, 1.7+/-0.9, -1.0+/-0.4, and 1.2+/-0.4 mg.kg-1.min-1) and nonhepatic (4.2+/-0.3, 2.9+/-0.7, 1.9+/-0.3, and 10.2+/-1.0 mg.kg-1.min-1) tissues decreased. Additional insulin augmented nonhepatic glucose uptake and only partially improved hepatic glucose uptake. Thus, glucagon impaired glucose uptake by hepatic and nonhepatic tissues. Compensatory hyperinsulinemia restored nonhepatic glucose uptake and partially corrected hepatic metabolism. Thus, persistent inappropriate secretion of glucagon likely contributes to the insulin resistance and glucose intolerance observed in obese and diabetic individuals.     

Effects of training on glucose metabolism of gluconeogenesis-inhibited short-term-fasted rats

To evaluate the effects of endurance training on gluconeogenesis and blood glucose homeostasis, trained as well as untrained short-term-fasted rats were injected with mercaptopicolinic acid (MPA), a gluconeogenic inhibitor, or the injection vehicle. Glucose kinetics were assessed by primed-continuous venous infusion of [U-14C]- and [6-3H]glucose at rest and during submaximal exercise at 13.4 m/min on level grade. Arterial blood was sampled for the determination of blood glucose and lactate concentrations and specific activities. In resting untrained sham-injected rats, blood glucose and lactate were 7.6 +/- 0.2 and 1.3 +/- 0.1 mM, respectively; glucose rate of appearance (Ra) was 71.1 +/- 12.1 mumol.kg-1.min-1. MPA treatment lowered blood glucose, raised lactate, and decreased glucose Ra. Trained animals had significantly higher glucose Ra at rest and during exercise. At rest, trained MPA-treated rats had lower blood glucose, higher blood lactate, and similar glucose Ra and disappearance rates (Rd) than trained sham-injected animals. Exercising sham-injected untrained animals had increased blood glucose and glucose Ra compared with rest. Exercising trained sham-injected rats had increased blood glucose and glucose Ra and Rd but no change in blood lactate compared with untrained sham-injected animals. In the trained animals during exercise, MPA treatment increased blood lactate and decreased blood glucose and glucose Ra and Rd. There was no measurable glucose recycling in trained or untrained MPA-treated animals either at rest or during submaximal exercise. There was no difference in running time to exhaustion between trained and untrained MPA-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prior exercise enhances passive absorption of intraduodenal glucose.

The purpose of this study was to assess whether a prior bout of exercise enhances passive gut glucose absorption. Mongrel dogs had sampling catheters, infusion catheters, and a portal vein flow probe implanted 17 days before an experiment. Protocols consisted of either 150 min of exercise (n = 8) or rest (n = 7) followed by basal (-30 to 0 min) and a primed (150 mg/kg) intraduodenal glucose infusion [8.0 mg x kg-1x min-1, time (t) = 0-90 min] periods. 3-O-[3H]methylglucose (absorbed actively, facilitatively, and passively) and l-[14C]glucose (absorbed passively) were injected into the duodenum at t = 20 and 80 min. Phloridzin, an inhibitor of the active sodium glucose cotransporter-1 (SGLT-1), was infused (0.1 mg x kg-1 x min-1) into the duodenum from t = 60-90 min with a peripheral venous isoglycemic clamp. Duodenal, arterial, and portal vein samples were taken every 10 min during the glucose infusion, as well as every minute after each tracer bolus injection. Net gut glucose output in exercised dogs increased compared with that in the sedentary group (5.34 +/- 0.47 and 4.02 +/- 0.53 mg x kg-1x min-1). Passive gut glucose absorption increased approximately 100% after exercise (0.93 +/- 0.06 and 0.45 +/- 0.07 mg x kg-1 x min-1). Transport-mediated glucose absorption increased by approximately 20%, but the change was not significant. The infusion of phloridzin eliminated the appearance of both glucose tracers in sedentary and exercised dogs, suggesting that passive transport required SGLT-1-mediated glucose uptake. This study shows 1). that prior exercise enhances passive absorption of intraduodenal glucose into the portal vein and 2). that basal and the added passive gut glucose absorption after exercise is dependent on initial transport of glucose via SGLT-1.     

Failure of hyperketonemia to alter basal and insulin-mediated glucose metabolism in man

The effect of physiologic elevations of plasma hydroxybutyrate induced by the infusion of sodium D,L-beta-hydroxybutyrate (15 mumol X kg-1 X min-1) on carbohydrate metabolism was examined with the euglycemic insulin clamp technique in nine healthy volunteers. Plasma insulin concentration was acutely raised and maintained at 126 +/- 6 microU/ml and plasma glucose was held constant at the fasting level by a variable glucose infusion. Glucose uptake of 6.53 +/- 0.80 mg X kg-1 X min-1 was unchanged by hyperketonemia when compared with an intraindividual control study using saline instead of beta-OH-butyrate infusion (6.26 +/- 0.59 mg X kg-1 X min-1). In studies, in which the degree of metabolic alkalosis accompanying butyrate infusion was mimicked by the continuous administration of bicarbonate, glucose uptake was also unaffected (6.25 +/- 0.45 mg X kg-1 X min-1). Furthermore, hyperketonemia had no effect on basal glucose production or the suppression of hepatic glucose production following hyperinsulinemia. It is concluded that moderate elevations in plasma beta-hydroxy-butyrate do not alter hepatic or peripheral glucose metabolism.     

Glucose-induced suppression of endogenous glucose production: dynamic response to differing glucose profiles

To determine whether, in the presence of constant insulin concentrations, a change in glucose concentrations results in a reciprocal change in endogenous glucose production (EGP), glucagon ( approximately 130 ng/l) and insulin ( approximately 65 pmol/l) were maintained at constant "basal" concentrations while glucose was clamped at approximately 5.3 mM (euglycemia), approximately 7.0 mM (sustained hyperglycemia; n = 10), or varied to create a "postprandial" profile (profile; n = 11). EGP fell slowly over the 6 h of the euglycemia study. In contrast, an increase in glucose to 7.13 +/- 0.3 mmol/l resulted in prompt and sustained suppression of EGP to 9.65 +/- 1.21 micromol x kg-1 x min-1. On the profile study day, glucose increased to a peak of 11.2 +/- 0.5 mmol/l, and EGP decreased to a nadir of 6.79 +/- 2.54 micromol x kg-1 x min-1 by 60 min. Thereafter, the fall in glucose was accompanied by a reciprocal rise in EGP to rates that did not differ from those observed on the euglycemic study day (11.31 +/- 2.45 vs. 12.11 +/- 3.21 micromol x kg-1 x min-1). Although the pattern of change of glucose differed markedly on the sustained hyperglycemia and profile study days, by design the area above basal did not. This resulted in equivalent suppression of EGP below basal (-1,952 +/- 204 vs. -1,922 +/- 246 mmol. kg-1. 6 h-1). These data demonstrate that, in the presence of a constant basal insulin concentration, changes in glucose within the physiological range rapidly and reciprocally regulate EGP.     

Determination of steady state and nonsteady-state glycerol kinetics in humans using deuterium-labeled tracer

Using deuterium-labeled glycerol as tracer and gas-liquid chromatography-mass spectrometry techniques for the determination of isotopic enrichment, we have developed a simple and ethically acceptable method of determining glycerol appearance rate in humans under steady-state and nonsteady-state conditions. In normal subjects, the appearance rate of glycerol in the post-absorptive state was 2.22 +/- 0.20 mumol X kg-1 X min-1, a value in agreement with those reported in studies with radioactively labeled tracers. The ratio nonesterified fatty acid (NEFA) appearance rate/glycerol appearance rate ranged from 1.95 to 3.40. In insulin-dependent diabetic patients with a mild degree of metabolic control, the appearance rate of glycerol was 2.48 +/- 0.29 mumol X kg-1 X min-1. The volume of distribution of glycerol, determined by the bolus injection technique, was (mean) 0.306 l X kg-1 in normal subjects and 0.308 l X kg-1 in insulin-independent diabetic patients. To evaluate the usefulness of the method for determination of glycerol kinetics in nonsteady-state conditions, we infused six normal subjects with natural glycerol and calculated the isotopically determined glycerol appearance rate using a single compartment model (volume of distribution 0.31 l X kg-1). During these tests, the expected glycerol appearance rates were successively 5.03 +/- 0.33, 7.48 +/- 0.39, 9.94 +/- 0.34, 7.48 +/- 0.39, and 5.03 +/- 0.33 mumol +/- kg-1 X min-1, whereas the corresponding isotopically determined appearance rates were 4.62 +/- 0.45, 6.95 +/- 0.56, 10.85 +/- 0.51, 7.35 +/- 0.34, and 5.28 +/- 0.12 mumol X kg-1 X min-1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chronic hepatic artery ligation does not prevent liver from differentiating portal vs. peripheral glucose delivery

Infusion of glucose into the hepatic artery blocks the stimulatory effect of the "portal signal" on net hepatic glucose uptake (NHGU) during portal glucose delivery. We hypothesized that hepatic artery ligation (HAL) would result in enhanced NHGU during peripheral glucose infusion because the arterial glucose concentration would be perceived as lower than that in the portal vein. Fourteen dogs underwent HAL approximately 16 days before study. Conscious 42-h-fasted dogs received somatostatin, intraportal insulin, and glucagon infusions at fourfold basal and at basal rates, respectively, and peripheral glucose infusion to create hyperglycemia. After 90 min (period 1), seven dogs (HALpo) received intraportal glucose (3.8 mg. kg-1. min-1) and seven (HALpe) continued to receive only peripheral glucose for 90 min (period 2). These two groups were compared with nine non-HAL control dogs (control) treated as were HALpe. During period 2, the arterial plasma insulin concentrations (24 +/- 3, 20 +/- 1, and 24 +/- 2 microU/ml) and hepatic glucose loads (39.1 +/- 2.5, 43.8 +/- 2.9, and 37.7 +/- 3.7 mg. kg-1. min-1) were not different in HALpe, HALpo, and control, respectively. HALpo exhibited greater (P < 0.05) NHGU than HALpe and control (3.1 +/- 0.3, 2.0 +/- 0.4, and 2.0 +/- 0.1 mg. kg-1. min-1, respectively). Net hepatic carbon retention was approximately twofold greater (P < 0.05) in HALpo than in HALpe and control. NHGU and net hepatic glycogen synthesis during peripheral glucose infusion were not enhanced by HAL. Even though there exists an intrahepatic arterial reference site for the portal vein glucose concentration, the failure of HAL to result in enhanced NHGU during peripheral glucose infusion suggests the existence of one or more comparison sites outside the liver.     

Venous versus arterialised venous blood for assessment of blood glucose levels during glucose clamping: comparison in healthy men.

The aim of this study was to investigate the influence of the arteriovenous (A-V) gradient in blood glucose concentrations at low and high insulin levels on the determination of glucose requirements during glucose clamping in 9 healthy, insulin sensitive, male volunteers. In a random order two clamps were performed, once using arterialised venous blood (A Clamp, mean pO2 = 11.5 +/- 0.36 kPa, 86 +/- 2.7 mmHg), and once using venous blood (V clamp, mean pO2 = 7.9 +/- 0.21 kPa, 59 +/- 1.6 mmHg). Insulin levels were maintained at 48 +/- 2.4 mU/l from 0-180 min and at 1054 +/- 114 mU/l from 180-360 min. Elevation of insulin levels caused a significant rise of the A-V gradient: from 0.3 +/- 0.1 to 0.5 +/- 0.1 mmol/l (p < 0.05) and from 0.2 +/- 0.1 to 0.3 +/- 0.1 mmol/l (p < 0.05) during the A and V clamps, respectively. Despite these A-V glucose gradients no significant differences were found for the glucose requirements during the last 30 min of each period of insulin infusion between the A and V clamps: 43.70 +/- 3.4 vs 44.8 +/- 2.8 mumol.kg-1.min-1 during the low insulin level and 77.3 +/- 5.0 vs 76.2 +/- 3.4 mumol.kg-1.min-1 during the high insulin level. We conclude that the A-V glucose gradient, even at high insulin levels, does not influence the assessment of glucose requirements to a measurable extent, allowing the use of the simpler technique of taking venous rather than arterialised venous blood for the measurements of glucose levels during glucose clamping.     

Metabolism of [1-(13)C)glucose and [2-(13)C]acetate in the hypoxic rat brain.

The effects of hypoxia on the metabolism of the central nervous system were investigated in rats submitted to a low oxygen atmosphere (8% O(2); 92% N(2)). [1-(13)C]glucose and [2-(13)C]acetate were used as substrates, this latter being preferentially metabolized by glial cells. After 1-h substrate infusion, the incorporation of 13C in brain metabolites was determined by NMR spectroscopy. Under hypoxia, an important hyperglycemia was noted. As a consequence, when using labeled glucose, the specific enrichment of brain glucose C1 was lower (48.2+/-5.1%) than under normoxia (66.9+/-2.5%). However, relative to this specific enrichment, the (13)C incorporation in amino acids was increased under hypoxia. This suggested primarily a decreased exchange between blood and brain lactate. The glutamate C2/C4 enrichment ratio was higher under hypoxia (0.62+/-0.01) than normoxia (0.51+/-0.06), indicating a lower glutamate turnover relative to the neuronal TCA cycle activity. The glutamine C2/C4 enrichment ratio was also higher under hypoxia (0.87+/-0.07 instead of 0.65+/-0.11), indicating a new balance in the contributions of different carbon sources at the acetyl-CoA level. When using [2-(13)C]acetate as substrate, no difference in glutamine enrichment appeared under hypoxia, whereas a significant decrease in glutamate, aspartate, alanine and lactate enrichments was noted. This indicated a lower trafficking between astrocytes and neurons and a reduced tricarboxylic acid cycle intermediate recycling of pyruvate.     

Glucose and lactate kinetics in the neonate.

To evaluate the ontogeny of neonatal glucose homeostasis, glucose production and lactate production have been measured in nine prematurely born appropriate for gestational age neonates [birth weight 1985 +/- 100 g, (SEM) gestational age 33.6 +/- 0.7 weeks] and five full term appropriate for gestational age neonates [birth weight 3254 +/- 111 g, gestational age 40.8 +/- 0.4 wks] and compared to six non pregnant, nondiabetic adults [weight of 57.7 +/- 2.2 kg, age 32 +/- 2 years]. Ra glucose (preterm) averaged 27.7 +/- 2.8 mumol.kg-1 min-1 (5.0 +/- 0.5 mg.kg-1 min-1) and Ra glucose (term) averaged 28.9 +/- 3.9 mumol.kg-1 min-1 (5.2 +/- 0.7 mg.kg-1 min-1); both were higher than the Ra glucose of the adult controls (16.1 +/- 2.8 mumol.kg-1 min-1 (2.9 +/- 0.5 mg.kg-1 min-1) (P less than 0.05 vs preterm and P less than 0.05 vs. term). Ra lactate (preterm) averaged 100 +/- 11.9 mumol.kg-1 min-1 (9.1 +/- 1.1 mg.kg-1 min-1) and Ra lactate (term) average 77.2 +/- 13.0 mumol.kg-1 min-1 (7.1 +/- 1.2 mg.kg-1 min-1); both were higher than the Ra lactate of the adult controls 35.9 +/- 6.5 mumol.kg-1 min-1 (3.3 +/- 0.6 mg.kg-1 min-1) (P less than 0.01 vs preterm and P less than 0.05 vs. term). The potential for gluconeogenesis from lactate was estimated by determining the ratio of [Ra Lactate/Ra Glucose]. The [Ra Lactate/Ra Glucose] (preterm) (187 +/- 12 (x10(-2)) was similar to that of the [Ra Lactate/Ra Glucose] (term) (136 +/- 16) (x10(-2)).(ABSTRACT TRUNCATED AT 250 WORDS)     

Seven days of bed rest decrease insulin action on glucose uptake in leg and whole body

Impaired glucose tolerance develops in normal humans after short-term bed rest. To elucidate the mechanism, insulin action on whole body glucose uptake rate (WBGUR) and leg glucose uptake rate (LGUR) was measured by sequential euglycemic clamp technique combined with femoral arterial and venous cannulation at insulin concentrations of 10 +/- 1, 18 +/- 1, 37 +/- 2, and 360 +/- 15 microU/ml. Studies were performed before (C) and after (BR) 7 days of strict bed rest. WBGUR was significantly lower after bed rest than before (5.5 +/- 0.4 and 7.2 +/- 0.8 mg.min-1.kg-1, respectively) when insulin was 37 microU/ml. LGUR was even more markedly depressed by bed rest, being 0.6 +/- 0.1, 0.9 +/- 0.2, and 2.8 +/- 0.4 mg.min-1.kg leg-1 (BR) compared with 0.9 +/- 0.1, 1.7 +/- 0.4, and 5.9 +/- 0.5 mg.min-1.kg leg-1 (C) (P less than 0.05) at the three lower insulin concentrations. At these insulin concentrations also, lactate release and glucose oxidation and glycogen storage estimated by indirect calorimetry were lower in the leg after bed rest. At the highest insulin dose WBGUR was similar on BR and C days, while LGUR was lower after bed rest. In conclusion, 7 days of bed rest decrease whole body insulin action, a fact that is explained by decreased insulin action in inactive muscle.     

Effects of maternal fasting on hepatic gluconeogenesis and glucose metabolism in fetal lambs.

In unstressed, normoglycaemic fetal lambs, the liver produces little glucose, and gluconeogenesis is insignificant. Indirect measurements have suggested that the fetus may produce glucose endogenously during hypoglycaemia induced by prolonged maternal starvation. In eight fetal lambs we directly measured total and radiolabelled substrate concentration differences across the liver to determine whether the fetal liver produces glucose after four days of fasting-induced hypoglycaemia. Simultaneously we measured umbilical glucose uptake and fetal glucose utilization. Glucose concentrations in ewes (1.78 +/- 0.44 mmol.-1) and fetuses (0.61 +/- 0.17 mmol.l-1) were decreased. Fetal glucose utilization rate (21.7 +/- 8.9 mumol.min-1.kg-1) was not significantly different from umbilical glucose uptake (17.2 +/- 8.9 mumol.min-1.kg-1). Hepatic glucose production (8.9 +/- 17.2 mumol.min-1.100 g-1) and gluconeogenesis (6.1 +/- 4.4 mumol.min-1.100 g-1) were present, but could account for only 13% and 8% of fetal glucose requirements, respectively. To determine whether glucose output by the fetal liver was limited by substrate availability, we infused lactate, acetate, and acetone into the umbilical veins of four fasted animals, increasing hepatic substrate delivery. Hepatic glucose output did not increase during infusion of gluconeogenic substrates, indicating that substrate availability did not limit gluconeogenesis. We conclude that the gluconeogenic pathway is intact in late-gestation fetal lambs and that the fetal liver is capable of gluconeogenesis. However, the primary change in fetal metabolism during maternal starvation is the reduction in fetal glucose utilization, obviating the need for substantial hepatic glucose production. The factors stimulating this modest increase in fetal hepatic glucose production remain to be elucidated.     

Decreased reliance on lactate during exercise after acclimatization to 4,300 m

We hypothesized that the increased exercise arterial lactate concentration on arrival at high altitude and the subsequent decrease with acclimatization were caused by changes in blood lactate flux. Seven healthy men [age 23 +/- 2 (SE) yr, wt 72.2 +/- 1.6 kg] on a controlled diet were studied in the postabsorptive condition at sea level, on acute exposure to 4,300 m, and after 3 wk of acclimatization to 4,300 m. Subjects received a primed-continuous infusion of [6,6-2D]glucose (Brooks et al. J. Appl. Physiol. 70:919-927, 1991) and [3-13C]lactate and rested for a minimum of 90 min followed immediately by 45 min of exercise at 101 +/- 3 W, which elicited 51.1 +/- 1% of the sea level peak O2 consumption (VO2peak; 65 +/- 2% of both acute altitude and acclimatization). During rest at sea level, lactate appearance rate (Ra) was 0.52 +/- 0.03 mg.kg-1.min-1; this increased sixfold during exercise to 3.24 +/- 0.19 mg.kg-1.min-1. On acute exposure, resting lactate Ra rose from sea level values to 2.2 +/- 0.2 mg.kg-1.min-1. During exercise on acute exposure, lactate Ra rose to 18.6 +/- 2.9 mg.kg-1.min-1. Resting lactate Ra after acclimatization (1.77 +/- 0.25 mg.kg-1.min-1) was intermediate between sea level and acute exposure values. During exercise after acclimatization, lactate Ra (9.2 +/- 0.7 mg.kg-1.min-1) rose from resting values but was intermediate between sea level and acute exposure values. The increased exercise arterial lactate concentration response on arrival at high altitude and subsequent decrease with acclimatization are due to changes in blood lactate appearance.(ABSTRACT TRUNCATED AT 250 WORDS)