Glucose Recycling Research Articles

The metabolic and hormonal response to acute normoglycaemia in type 1 (insulin-dependent) diabetes: studies with a glucose controlled insulin infusion system (artificial endocrine pancreas).

Twelve insulin deficient Type 1 (insulin-dependent) diabetic subjects were studied over an 11 1/2 h period during both subcutaneous insulin therapy and closed loop insulin delivery, using a glucose controlled insulin system (Biostator) programmed to maintain normoglycaemia. Results were compared with those from 21 age and weight-matched normal subjects. Using the Biostator, normoglycaemia was achieved in all diabetic subjects within 3.5 h and normal profiles maintained thereafter. Blood metabolite and hormone values were evaluated during the subsequent 8 h normoglycaemic period. Subcutaneous therapy resulted in abnormal glucose levels throughout the study period (mean 8 h value 8.3 +/- 0.7 compared with 5.6 +/- 0.3 mmol/l on feedback control and 5.5 +/- 0.1 mmol/l in normal subjects). The mean value of lactate and pyruvate over the final 8 h period was 25% higher in diabetic patients than in normal subjects with no difference between the two insulin treatments (blood lactate: 0.94 +/- 0.04 on subcutaneous insulin, 0.91 +/- 0.04 on feedback control and 0.74 +/- 0.03 mmol/l in control subjects). The pre-prandial peaks of blood glycerol and plasma non-esterified fatty acids were significantly decreased or absent during both feedback control and subcutaneous therapy in comparison with the normal subjects, whereas after the midday and evening meals, total ketone body levels were significantly higher in the diabetic patients. Peripheral serum free insulin levels were two-to fourfold greater in the diabetic than in the normal subjects. There were no significant differences between levels in diabetic patients receiving subcutaneous insulin or on the Biostator. Glucose turnover (1600-1800 h) was normal on feedback control (1.41 +/- 0.20 versus 1.55 +/- 0.18 mg X kg-1 X min-1 in the normal subjects) but was significantly decreased during subcutaneous insulin (1.04 +/- 0.09 mg X kg-1 X min-1). There was, in addition, a decrease in glucose recycling during both subcutaneous insulin therapy and feedback control in the diabetic subjects. These data suggest that although fine control of glucose metabolism both in terms of circulating concentrations and rates of production can be achieved by feedback-control, insulin infusion by the peripheral route is associated with significant metabolic abnormalities, at least in the short term. Longer term studies and examination of portal insulin delivery seem warranted.

Glucose turnover and glucose-lactate interrelations in the newborn rat

A technique of continuous infusion of (6-3H) and (U-14C)glucose and (U-14C) lactate was developed in the 1-day-old suckling rat, allowing the calculation of true and apparent glucose turnover and glucose-lactate interrelations under steady-state conditions. True glucose turnover rate in suckling newborns (16.9 ± 0.4 mg/min/kg) was 50% higher than in fasted adult rats. A 20 ± 3% glucose recycling was found which corresponded approximately to the Cori cycle activity. Although lactate was contributing for 25% to glucose turnover rate, this did not represent a net glucose synthesis since more lactate was formed from glucose (8.6 mg/min/kg) than glucose from lactate (4.1 mg/min/kg). However, recycling from lactate may be physiologically important as it could be the expression of a decrease in glucose oxidation due to an inhibition of pyruvate dehydrogenase by the elevated concentrations of non-esterified fatty acids and ketone bodies found in the plasma of 1-day old suckling rats.This glucose sparing effect could in turn diminish the requirement of aminoacids for glucose synthesis, thus allowing a high rate of protein synthesis.

Effects of thyroid states on the Cori cycle, glucose-alanine cycle, and futile cycling of glucose metabolism in rats

The effect of thyroid status on glucose recycling was measured in intact rats by comparing the fates of differently labeled [ 3H]- and [ 14C]glucose. Glucose recycling at the level of three-carbon compounds (i.e., Cori and glucose-alanine cycles) was measured by comparing the rates of turnover of [6- 3H]- and [6- 14C]glucose in the same animal. The rate of recycling increased (33–110%) in hyperthyroid rats and decreased (22–30%) in hypothyroid (thyroidectomized) rats. The relative importance of the Cori and glucose-alanine cycles was measured by analyzing the labeled glycolytic intermediates after the injection of labeled glucose; and by measuring the rate of glucose production from the infused labeled lactate and alanine. The results showed that the rate of the Cori cycle is much greater than the glucose-alanine cycle in rats. Substrate cycling at the level of glucokinase-glucose-6-phosphatase was measured by comparing the rates of turnover of [2- 3H]- and [6- 3H]glucose; and phosphofructokinase-fructose bisphosphatase was measured by comparing the rates of turnover of [3- 3H]- and [6- 3H]glucose. These cycles were also affected by thyroid states of the animals. The rate of the phosphofructokinase-fructose bisphosphatase cycle increased threefold in hyperthyroid rats and decreased by about half in hypothyroid rats. The glucokinase-glucose-6-phosphatase substrate cycle occurred at the rate of nearly 2 μmol/min/100 g body wt in the hyperthyroid, fasted rats; it was not detectable in hypo- or euthyroid rats. The contribution of the energy released by these cycles to thyroid thermogenesis was discussed. Effects of thyroid states on glucose metabolism in perfused muscles were also studied. There is an apparent shift in the source of energy for oxidation in the hyperthyroid rat. The ratio of lactate production to glucose uptake was significantly elevated in the hyperthyroid rats. This change predisposes for increased glucose recycling in hyperthyroid rats to avoid lactate accumulation and acidosis.

Carbohydrate metabolism in pregnancy XVI: Longitudinal estimates of the effects of pregnancy on D-(6 3H) glucose and D-(6- 14C) glucose turnovers during fasting in the rat

We measured blood glucose concentrations and glucose turnover rates in 24 hr fasted, conscious, unrestrained pregnant rats and nongravid controls on days 18, 19, and 20 of gestation. Turnover measurements were secured with simultaneous equilibrium infusions of D-(6- 3H) and D-(6- 14C) glucose so that gluconeogenic recycling could also be determined. “Steady state” values for blood glucose in the mother after 24 hr of fasting did not significantly differ on each of the days, and these concentrations were significantly lower than the values in 24 hr fasted nongravid rats. At 18 days gestation, glucose turnover did not differ from nongravid values. By contrast, values for glucose turnover after 24 hr fasting increased significantly and progressively in the 19 and 20 day pregnant rats. The increase in turnover correlated with the increasing growth of the conceptus. The ratio between D-(6- 14C) glucose and D-(6- 3H) turnover remained constant (and the same as in the nongravid rats) during all 3 days of gestation suggesting that rates of glucose recycling remained unaltered. These longitudinal studies indicate that the factors contributing to the pattern of “accelerated starvation” during dietary deprivation in pregnancy may vary as pregnancy progresses. The exaggerated lowering of blood glucose which accompanies fasting occurs before total glucose turnover increases. This could provide a potential mechanism for conserving maternal glucose. Since transplacental transfer of glucose is concentration-dependent, the early establishment of a lower “steady state” for circulating glucose could diminish the magnitude of loss of this key nutrient to the fetus.

The effects of progesterone on glucose and lactate metabolism in ovariectomized sheep.

In nine samples from four ovariectomized sheep the mean plasma concentration of progesterone was 0.19 +/- 0.04 ng/ml. After the subcutaneous implantation of 3 x 375 mg of progesterone in silicone rubber the concentration rose to 3.8 +/- 0.51 ng/ml at 48 h and fell steadily to 1.5 ng/ml at 21 days. Removal of the implants caused a reduction to pre-implantation values by 48 h. The increased concentration of circulating progesterone after implantation had no significant effect on the concentration of glucose in plasma, its total entry rate, irreversible loss or on plasma lactate concentration. Glucose recycling increased from 0.13 +/- 0.05 to 0.38 +/- 0.12 mg min-1 kg-1 (P less than 0.05). Lactate entry rate fell from 1.68 +/- 0.16 to 1.12 +/- 0.10 mg min-1 kg-1 (P less than 0.02). The total net formation of lactate from glucose fell from 0.56 +/- 0.05 to 0.41 +/- 0.08 mg min-1 kg-1 (P less than 0.05). The production of lactate from non-glucose substrates fell from 1.15 +/- 0.11 to 0.70 +/- 0.04 mg min-1 kg-1 (P less than 0.02). The utilization of lactate for purposes other than glucose synthesis fell from 1.50 +/- 0.14 to 0.97 +/- 0.09 mg min-1 kg-1 (P less than 0.05). The results support the view that progesterone produces an increase in glucose recycling by pathways involving compounds other than lactate.

Glucose biokinetics in normal and urea-treated sheep ( Ovis aries)

1. 1. Effects of urea on glucose kinetics and plasma parameters were studied by means of radioisotopes in sheep administered intraruminally with different amounts of urea. 2. 2. Urea increased plasma concentrations of glucose, urea, ammonia, pyruvate, lactate and potassium, and the activities of GOT and GPT; the effect was more pronounced with higher doses. 3. 3. The increase in glucose concentration was accompanied with a decrease in glucose total entry rate and irreversible loss. Urea had no effect on glucose recycling. 4. 4. It is concluded that the elevation in glucose levels reflects under-utilization and not over-production of this compound. Mechanisms involved in glucose metabolism in ruminants are discussed.

Glucose kinetics and respiratory metabolism in fed and fasted chickens

1. 1. Glucose replacement rate, recycling and oxidation rates, as well as respiratory metabolism, were studied in unanaesthetized chickens which were fed or fasted, 0, 24 and 72 hr. (U[ 14C], 6[ 3H]) glucose was used for glucose kinetics study while oxygen consumption ( VO 2) and CO 2 production ( VCO 2) were determined simultaneously in an open circuit system. 2. 2. Glucose apparent replacement rate decreased during fasting from 17 to 9 mg/min/kg while glucose recycling did not change significantly. 3. 3. The net conversion of glucose carbon to expired CO 2 decreased from 2.1 to 0.89 mg/min/kg during the first 24 hr of fasting. Glucose oxidation contributed 31 and 12% of the total heat production ( M) in fed and fasted chickens respectively. 4. 4. The relations between glucose homeostasis and heat production in the chicken are discussed.

Determination of glucose turnover and recycling with 14C and 3H labelled tracers in pigs exposed to thermally neutral and cold environments

1. 1. Glucose turnover and recycling have been investigated in pigs fasted for 24hr, using either [2- 3H] or [3- 3H] or [5- 3H] or [6- 3H] glucose, together with the [U- 14C] labelled compound. 2. 2. Glucose mass, replacement rates, and transit times were similar to those which have been observed in other mammals. 3. 3. Glucose recycling was higher with [2- 3H], [3- 3H] and [5- 3H] glucose than with the [6- 3H] tracer, indicating the presence of substrate cycles between glucose/glucose-6-phosphate and fructose-6-phosphate/fructose-1,6-biphosphate. 4. 4. Glucose kinetics and recycling were not significantly altered by exposing pigs to the cold (10°C). It is concluded that “glucose substrate cycles” are not involved in thermoregulatory heat production in the pig.

Rates of glucose production and utilization by the foetus in chronically catheterized sheep.

1. Six well-nourished Scottish Blackface ewes were each prepared at 110-112 days of gestation with indwelling catheters in an umbilical artery and vein and at 2 days before experiment with catheters in each maternal jugular vein. Two-day experiments were conducted between days 124 and 134 of gestation when [2-3H,U-14C]glucose was infused over 9 h into either the maternal or the foetal circulations, separate days being used for each infusion. 2. Plasma glucose specific radioactivities at plateau were used to determine, for a two-pool model representing the glucose system of the mother and foetus, the rates of glucose production, utilization and recycling by the mother and by the foetus and the rates of transfer of glucose between the mother and foetus. 3. The net rate of glucose utilization by the foetus, which we have called the 'foetal glucose requirement', was 7.6 +/- 0.6 mg/min per kg of foetus (n = 8). Compared with the corresponding rate in the mother of 1.1 +/- 0.1 mg/min per kg of mother (n = 8), this indicated a high rate of foetal metabolism. 4. The net rate of foetal uptake of glucose from the mother accounted for only 69 +/- 5% of the foetal glucose requirement, and it has been argued that the remainder was supplied by foetal gluconeogenesis. 5. The rate of glucose recycling within the foetus was high, equal to 108 +/- 16% of the net foetal glucose-utilization rate. The significance of this in terms of either Cori or futile cycling is discussed.

In vivo glucose turnover in hypo- and hyperthyroid starved rat

The effect of hypo- and hyperthyroidism on glucose turnover in vivo was determined in unanesthetized rats starved for 48 h. Glucose pool and decay rate of specific radioactivity of blood glucose was measured after bolus injection of a mixture of 3H-(2)- and 14C-(U)-glucose under steady state conditions. Compared with euthyroid controls (= 100%), hypothyroidism resulted in a decrease of blood glucose concentration (81%), glucose pool (52%), glucose disappearance rate (39%), and total glucose recycling (12%). In contrast, hyperthyroidism led to an increase of blood glucose concentration (148%), glucose pool (121%), glucose disappearance rate (185%), and total glucose recycling (163%). T 1/2 for glucose was calculated to be 46 min in the hypo-, 34 min in the eu-, and 22 min in the hyperthyroid state. The concentration of circulating glucoregulatory hormones, corticosterone and glucagon were elevated in hyperthyroid rats, while glucagon was diminished in hypothyroid animals. No difference in the level of insulin was found. These data demonstrate that glucose turnover in vivo is a function of the thyroid state being reduced in hypo- and considerably increased in hyperthyroidism.

Metabolism of glucose in hyper- and hypo-thyroid rats in vivo. Glucose-turnover values and futile-cycle activities obtained with 14C- and 3H-labelled glucose.

1. A trace amount of glucose labelled with 14C uniformly and with 3H at position 2, 3 or 6 was injected intravenously into starved rats to measure the turnover rate of blood glucose. 2. Reliable estimates were made based on the semilogarithmic plot of specific radioactivity of the glucose contained in whole blood samples taken from the tail vein. 3. Glucose turned over more rapidly in hyperthyroid and more slowly in hypothyroid than in euthyroid rats. The percentage contribution of glucose recycling (determined from the difference in replacement rates between [U-14C]glucose and [6-3H]glucose) to the glucose utilization increased on induction of hyperthyroidism. 4. Futile cycles between glucose and glucose 6-phosphate (determined from the difference between replacement rates of [2-3H]glucose and [6-3H]glucose) were activated and inactivated by induction of hyperthyroid and hypothyroid states respectively. 5. The hepatic content of glycogen was much lower in hyper- and hypo-thyroid than in euthyroid rats. The enhanced glucose production in hyperthyroid rats resulted from not only activationof hepatic gluconeogenesis but also diversion of the final product of gluconeogenesis from liver glycogen to blood glucose. In hypothyroidism, the inhibition of gluconeogensis led to suppression of both glucose production and glycogenesis in the liver.

Metabolism of glucose in hyper- and hypo-thyroid rats in vivo. Relation of catecholamine actions to thyroid activity in controlling glucose turnover.

1. In euthyroid rats, treatment with reserpine of 6-hydroxydopamine, which deprived neuronal terminals of catecholamines, resulted in increases in rates and rate coefficients for blood glucose turnover in the starved states as determined by decay of [U-14C,6-3H]-glucose. Conversely, the injection of adrenaline or noradrenaline into starved euthyroid rats caused a marked decrease in rate coeeficients for glucose turnover. There was no change in the percentage glucose recycling under these conditions. 2. Adrenaline and noradrenaline caused more pronounced hyperglycaemia in hyperthyroid than in euthyroid rats owing to the greater activation of hepatic glucose production. 3. The increase in glucose turnover characteristics of hyperthyroidism was observed even after treatment with an alpha- or beta-adrenergic antagonist, showing the insignificant role of the balance between alpha- and beta-adrenergic receptors in the thyroid-dependent metabolic changes. 4. Rate coefficients for glucose turnover were not affected by reserpine treatment or catecholamine injections when rats had been rendered hyperthyroid. 5. Thus catecholamines are direct determinants of glucose-turnover rates in the starved state, and depend to some extent on the prevailing thyroid state.

Abnormal erythrocyte metabolism in hepatic disease: effect of NADP repletion.

Erythrocytes from ten patients with severe liver disease displayed low methylene blue-stimulated hexose monophosphate (HMP) shunt activity and glucose recycling despite elevated total glucose consumption when compared to controls. Heinz body formation was increased and reduced glutathione concentration significantly decreased. After hemolysis, no differences in methylene-blue estimulated HMP shunt activity or glucose recycling could be demonstrated between patients and controls. The addition of 2- and 4-mM NADP to the hemolysates produced significantly greater HMP shunt activity and glucose recycling in the patients' hemolysates. The addition of NADPH to the incubation mixture produced no significant stimulation of either HMP shunt activity or glucose recycling, unless methylene blue was also added. Omission of NAD or phosphate from the incubation mixture produced no change in shunt metabolism. The absence of supplemental ATP resulted in extremely low shunt metabolism and refractoriness to NADP stimulation in both patients and controls. In the absence of additional magnesium, a reduction of shunt metabolism was noted. These data suggest that the defect in stimulated shunt metabolism in the intact erythrocytes of patients with hepatic disease does not result from an absolute enzyme deficiency, but rather from an unavailability of NADP or other cofactor.

867 ESTIMATION OF GLUCOSE PRODUCTION IN THE HUMAN NEWBORN USING SIMULTANEOUS 13C-AND D-LABELLED GLUCOSE

In order to compare two methods of quantifying systemic glucose production (SGP), 5 newborn infants age 2 h-3 days were infused simultaneously with glucose-1-13C (G113C) and glucose-6-6-D2 (G6D2) tracers. The older infants were studied 6 h after a meal. G113C was infused at 6μg/Kg min according to the prime constant rate infusion technique and 13C enrichment of C-1 of glucose measured by enzymatic decarboxylation and mass spectrometry. G6D2 was infused at 60μg/Kg min and deuterium enrichment of glucose measured by gas chromatography-mass spectrometry. SGP was calculated from the tracer dilution assuming steady state kinetics, from 30 min for 13C and from 120 min for D. Although 13C was expected to randomize away from C-1, recycling occurred and was estimated from the difference in rate of SGP quantified by dilution of the two tracers (mean ± S.D.) SGP ranged from 3.5 to 5.1 mg/Kg min. Recycling of glucose C-1 was 11-15% of SGP and it did not change with the age of infant. Conclusion: 1) SGP is established in the human newborn by 2 hours of age. 2) The two stable tracers used give similar estimates of SGP.

The effects of hypoglycin on glucose metabolism in the rat. A kinetic study in vivo and [U-14C,2-3H]glucose.

1. The kinetics of glucose metabolism were evaluated in rats deprived of food 15-21 h after the administration of hypoglycaemic doses of hypoglycin (100 mg/kg body wt.) by following changes in the specific radioactivities of 14C and 3H in blood glucose after an intravenous dose of [U-14C,2-3H]glucose [Katz, Rostami & Dunn (1974) Biochem. J. 142, 161-170]. 2. During this time, recycling of glucose through the Cori cycle was virtually abolished, the rate of irreversible disposal of glucose and its total body mass were both decreased by about 70%, whereas there was little effect on the mean transit time for glucose. 3. It was concluded that hypoglycaemia is due to inhibition of gluconeogenesis.

Turnover and recycling of glucose in man during prolonged fasting

The effect of prolonged (3–5 wk) fasting on tracer-determined glucose turnover and of recycling radioactive glucose has been examined. We followed the specific activity of plasma glucose after the simultaneous administration of 1- 14C-glucose and 3- 3H-glucose. The rate of glucose turnover decreased during prolonged fasting. Recycling of radioactive glucose was estimated by two different techniques: (1) the appearance of 14C in positions 2 to 6 glucose was measured; (2) the difference in the slopes of specific activity decline for 1- 14C-glucose and for 3- 3H-glucose was calculated. The two methods of estimating the radioactive recycling gave results similar to each other. The amount of glucose recycled did not change during prolonged fasting. However, in view of the decline in glucose production during fasting, the proportion of glucose production which was represented by recycling increased. Based on weight and urinary nitrogen loss an estimate of the glucose production from amino acids and glycerol was obtained. The difference between the rate of glucose production from the contribution of amino acids and glycerol and that estimated by radioisotopic techniques was much larger than the measured rate of recycling. This finding suggests that either a large exchange of 12C with 14C occurred in some glycolytic intermediates or that a hitherto unknown source of carbon for glucose production appeared during prolonged fasting.

Effect of epinephrine upon irreversible disposal and recycling of glucose in dogs.

Irreversible utilization and recycling of glucose during epinephrine-induced hyperglycemia were studied in adrenal-demedullated dogs exposed to neutral or cold ambient temperature. Whatever the ambient temperature, most of the extra glucose mobilized by epinephrine is recycled rather than irreversibly utilized by the peripheral tissue.

61: Contribution to the pathogenesis of pseudotype I glycogenosis (P. typeI G): prolonged hyperinsulinism induced by glucagon administration

P.type I G and type I glyoogenosis (type I G) have identical clinical and biological features although the hepatic content of glucose 6 phosphatase is normal in P.type I G. Recently Hers et al showed that in this latter affection a shortening of the half life of glucose and an increase in the recycling of glucose were present (Biochem. Soc. Trans. 1975 P. 1051). Venous blood glucose, lactate, NEFA, pH, and plasma insulin levels were studied on several occasions before, 1, 2, 3, and 4 h after feeding in one case of type I G and one case of pseudo type I G, on basal conditions and after prolonged glucagon administration (1 to 2 mgIM every 8h. for several days to months). Both cases were severe forms with a fasting tolerance not exceeding 4 h. On basal condition the biological data were similar in both patients with low insulin levels (<7,5 to 40 μu/ml). Glucagon did not induce any significant changes in type I G and insulin levels were not modified ( <7,5 to 48 μu/ml). By contrast glucagon induced important increases in plasma insulin in P.type I G with post-feeding values > 200 μu/ml reaching on several occasions values > 1 000 μu/ml; parallel blood glucose levels varied from low to high normal (20 to 145 mg/100ml) This hyperinsulinism was still present (120 to 335 μu/ml) 2 months after discontinuation of 6 months glucagon injections. This finding suggest that in spite of normal basal insulin levels, an abnormal insulin/glucagon secretion ratio is present in P.typol G which may be involved in the abnormal glucose kinetics.

Effect of fasting on glucose recycling in rats

Effect of fasting on glucose recycling in rats

Abnormal erythrocyte metabolism in hepatic disease

Erythrocyte (RBC) metabolic studies were done on 114 patients with severe hepatic disease. Heinz body formation after incubation of RBCs with acetyl phenylhydrazine was found to be significantly higher in patients than in controls. RBC-reduced glutathione levels were lower than those of controls both before and after incubation with acetyl phenylhydrazine, and patients with the highest Heinz body counts had the lowest reduced glutathione levels. RBC methylene blue-stimulated hexose monophosphate (HMP) shunt metabolism and glucose recycling through the shunt were significantly lower in patients with active hepatic disease than in controls. There was no difference in resting HMP shunt activity or in resting recycling of glucose. Despite impairment of shunt metabolism, total glucose consumption was greater in patients than in controls. The patients with the lowest stimulated HMP shunt metabolism and glucose recycling had the highest Heinz body counts, lowest reduced glutathione, and highest total glucose consumption. A continuum of abnormal shunt metabolism was seen, from a mild reduction of stimulated HMP shunt activity to a severe combined decrease in both the HMP shunt and glucose recycling. When measured, glutathione reductase, glutathione peroxidase, glucose-6-phosphate dehydrogenase, and transketolase were normal or increased. Sequential studies were done on 11 patients who had abnormal metabolic studies. Coincident with improvement of HMP shunt metabolism, the Heinz body counts became lower, reduced glutathione higher, hematocrit higher, and liver function improved. Impaired HMP shunt metabolism appears to be a common, acquired RBC abnormality in patients with severe, active liver disease.