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Determination of synthesis, recycling and body mass of glucose in rats and rabbits in vivo 3H-and 14C-labelled glucose.

ABSTRACT: 1. Glucose labelled with (3)H in position 2 and uniformly with (14)C was administered simultaneously to rabbits and rats either as a single injection or by continuous infusion. Plasma glucose specific radioactivity and the yield of (3)H in the plasma water were monitored. 2. The rates of synthesis, recycling of carbon and total body mass of glucose were calculated, without assuming a multicompartmental model and without fitting data by exponential expressions. 3. The rate of synthesis of glucose in starved-overnight rabbits was 4mg/min per kg (range 3-4.5mg/min per kg) and 25-35% of the glucose carbon was recycled. The mass of total body glucose in starved rabbits was 290mg/kg (range 220-390mg/kg). About one-third of the total body glucose equilibrates nearly instantaneously with plasma glucose. 4. In rats starved overnight, glucose synthesis was about 10mg/min per kg and recycling of carbon ranged from 30-40%. Total body mass (per kg body weight) is similar to that in rabbits. 5. The activity in plasma water after injection of [2-(3)H]glucose was determined. The initial rate of (3)H(2)O formation is rapid, indicating that the major site of glucose catabolism is in the rapidly mixing pool. The curve of total body glucose radioactivity was obtained from the (3)H(2)O yield, and total mass of glucose was calculated. This agrees with that obtained from the (3)H specific-radioactivity curve.

SUBMITTER: Katz J

PROVIDER: S-EPMC1168223 | biostudies-other | 1974 Jul

REPOSITORIES: biostudies-other

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Project description:1. [2-(3)H,U-(14)C]- or [3-(3)H,U-(14)C]-Lactate was administered by infusion or bolus injection to overnight-starved rats. Tracer lactate was injected or infused through indwelling cannulas into the aorta and blood was sampled from the vena cava (A-VC mode), or it was administered into the vena cava and sampled from the aorta (V-A mode). Sampling was continued after infusion was terminated to obtain the wash-out curves for the tracer. The activities of lactate, glucose, amino acids and water were followed. 2. The kinetics of labelled lactate in the two modes differed markedly, but the kinetics of labelled glucose were much the same irrespective of mode. 3. The kinetics of (3)H-labelled lactate differed markedly from those for [U-(14)C]lactate. Isotopic steady state was attained in less than 1h of infusion of [(3)H]lactate but required over 6h for [U-(14)C]lactate. 4. (3)H from [2-(3)H]lactate labels glucose more extensive than does that from [3-(3)H]lactate. [3-(3)H]Lactate also labels plasma amino acids. The distribution of (3)H in glucose was determined. 5. Maximal radioactivity in (3)HOH in plasma is attained in less than 1min after injection. Near-maximal radioactivity in [(14)C]glucose and [(3)H]glucose is attained within 2-3min after injection. 6. The apparent replacement rates for lactate were calculated from the areas under the specific-radioactivity curves or plateau specific radioactivities after primed infusion. Results calculated from bolus injection and infusion agreed closely. The apparent replacement rate for [(3)H]lactate from the A-VC mode averaged about 16mg/min per kg body wt. and that in the V-A mode about 8.5mg/min per kg body wt. The apparent rates for [(14)C]lactate (;rate of irreversible disposal') were 8mg/min per kg body wt. for the A-VC mode and 5.5mg/min per kg body wt. for the V-A mode. Apparent recycling of lactate carbon was 55-60% according to the A-VC mode and 35% according to the V-A mode. 7. The specific radioactivities of [U-(14)C]glucose at isotopic steady state were 55% and 45% that of [U-(14)C]lactate in the A-VC and V-A modes respectively. We calculated, correcting for the dilution of (14)C in gluconeogenesis via oxaloacetate, that over 70% of newly synthesized glucose was derived from circulating lactate. 8. Recycling of (3)H between lactate and glucose was evaluated. It has no significant effect on the calculation of the replacement rate, but affects considerably the areas under the wash-out curves for both [2-(3)H]- and [3-(3)H]-lactate, and calculation of mean transit time and total lactate mass in the body. Corrected for recycling, in the A-VC mode the mean transit time is about 3min, the lactate mass about 50mg/kg body wt. and the lactate space about 65% of body space. The V-A mode yields a mass and lactate space about half those with the A-VC mode. 9. The area under the wash-out curve for [(14)C]lactate is some 20-30 times that for [(3)H]lactate, and apparent carbon mass is 400-500mg/kg body wt. and presumably includes the carbon of glucose, pyruvate and amino acids, which are exchanging rapidly with that of lactate.

Project description:In confirmation of the findings of Gaitonde et al. (1974), a decrease in the brain concentration of threonine and serine, and an increase in glycine, were observed in rats maintained on a thiamin-deficient diet. Similar changes were found in the blood, and the concentration of several other amino acids in the blood decreased significantly. There was a correlation between the concentrations of threonine, serine, aspartate and asparagine in the brain and blood. In experiments in which [U-14C]threonine was injected into rats most of the radioactivity in the brain and blood of control rats was, as expected, in threonine in the acid soluble metabolites. In contrast, a considerable proportion of radioactivity was also found in other amino acids, namely glutamate, glutamine, aspartate, gamma-aminobutyrate and alanine, in the brain of thiamin-deficient rats. [U-14C]Threonine was also converted into 14C-labelled lactate and glucose, but the extent of this conversion was severalfold higher in thiamin-deficient than in control rats. This finding gave evidence of the stimulation in thiamin-deficient rats of the catabolism of [U-14C]threonine to [14C]lactate by the aminoacetone pathway catalysed by threonine dehydrogenase, and into succinate via propionate by the alpha-oxobutyrate pathway catalysed by threonine dehydratase (deaminase). The measurement of specific radioactivities of glutamate, aspartate and glutamine after injection of [U-14C]threonine, indicated a stimulation of the activities of threonine dehydrogenase and threonine dehydratase (deaminase) in the brain of thiamin-deficient rats. The specific radioactivities of glutamate, asparatate and glutamine int he brain were consistent with an alteration in the metabolism of threonine, mainly in the 'large' compartment of the brain of thiamin-deficient rats. The measurement of relative specific radioactivity of proteins after injection of [U-14C]threonine indicated a marked decrease in the synthesis of proteins, mainly in the liver of thiamin-deficient rats.

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Determination of synthesis, recycling and body mass of glucose in rats and rabbits in vivo 3H-and 14C-labelled glucose.

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