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.

Glucose-6-/-14C was administered to rats and the specific activity and z/14C ratio of plasma glucose and tissue glycogens were determined. The decay rates of the tritium and carbon-14 specific activities of plasma glucose both followed first-order kinetics but the decay rate of tritium was faster than that of carbon-14. The half-times of tritiated and 14C glucose were 32 and 41 min, respectively. The fractional turnover rate as measured with tritium was 2.2% of the glucose pool/min in contrast to 1.7%/ min for 14C.The plasma glucose z/14C ratio decreased linearly to about 65% of its initial value in 100 min. The //'14C ratio of muscle glycogen was the same as the average ratio of circulating glucose, while that of liver glycogen was lower than the ratio of plasma glucose at the time of sacrifice. The rate of tritium loss from plasma glucose approximated the rate of 14C recycling as measured by incorporation of 14C from C-6 into C-l through C-3 of glucose. It is proposed that loss of tritium occurs in the liver during glucose resynthesis from lactate. The use of glucose-6-/ in turnover studies corrects for recycling and measures the production of new glucose from three-carbon units. The simultaneous use of glucose-6-/ and glucose-6-14C provides an estimation of the Cori cycle.c V^Jlucose labeled with carbon-14 has been widely used to study glucose turnover in intact animals.

The glucose replacement rate, percent carbon recycling, mean glucose transit time, and the glucose mass were determined in fasted unanesthetized rabbits after administration of [2-3H,U-14C]-, [3-3H,U-14C]-, [5-3H,U-14C]- or [6-3H,U-14C]glucose using the procedures of Katz et al. (10). The glucose replacement rates and carbon recycling determined with [2-3H,U-14C] and [5-3H,U-14C]glucose are equivalent and greater than those obtained with [3-3H,U-14C]- and [6-3H,U-14C]glucose. Although the means of the glucose replacement rates and percent carbon recycling obtained using [3-3H,U-14C]- and [6-3H,U-14C]glucose are similar, greater variation resulted using the former tracer. Comparisons of detritiation rates and percent carbon recycling using [2-3H,U-14C]- and [6-3H,U-14C]glucose suggest that about 10% of tritium is lost from carbon 2 via futile cycling at the glucose 6-phosphate level. Similarly, comparisons of [5-3H,U-14C]- and [6-3H,U-14C]glucose metabolism suggest that about 10% of tritium lost from carbon 5 occurs via futile cycling at the fructose diphosphate level and/or via the transaldolase reaction. Our results indicate that [6-3H,U-14C]glucose is the more suitable tracer for determining the glucose replacement rate and carbon recycling in vivo.

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...

L-[3-3H,U-14C]Lactate was administered to starved rats either as a bolus or by continuous infusion. Tracer administration was performed two ways: injection into the vena cava and sampling from the aorta (V-A mode), or injection into the aorta and sampling from the vena cava (A-VC mode). The specific-radioactivity curves after infusion or injection differed markedly with the two procedures. However, the specific radioactivities of 14C-labelled glucose derived from [U-14C]lactate were similar in the two modes. The apparent turnover rates of lactate calculated from the 3H specific-radioactivity curves in the V-A mode were about half those obtained from the 3H specific-radioactivity curves in the A-VC mode. The apparent contribution of lactate carbon to glucose carbon calculated from specific-radioactivity curves of the A-VC mode was greater than that obtained from the V-A mode. The apparent recycling of lactate carbon calculated from the specific radioactivities for [U-14C]- and [3-3H]-lactate was greater in the A-VC mode than the V-A mode. [U-14C] Glucose was administered in the two modes, but in contrast with lactate the specific radioactivities were only slightly different. An analysis to account for these observations is presented. It is shown that the two modes represent sampling from different pools of lactate. The significance of sites of tracer administration and sampling for the interpretation of tracer kinetics of compounds present in intracellular and extracellular spaces, and with a high turnover rate, is discussed. We propose that for such compounds, including lactate, alanine and glycerol, the widely used V-A mode leads to a marked underestimate of replacement, mass and carbon recycling, and that the A-VC mode is the preferred method for the assessment of these parameters.