Movements of radioactive carbon dioxide within the animal body during oxidation of 14C-labelled substances

Recovery of (13)CO2 during rest and exercise after [1-(13)C]acetate, [2-(13)C]acetate, and NaH(13)CO3 infusions

For estimating the oxidation rates (Rox) of glucose and other substrates by use of (13)C-labeled tracers, we obtained correction factors to account for label dilution in endogenous bicarbonate pools and TCA cycle exchange reactions. Fractional recoveries of (13)C label in respiratory gases were determined during 225 min of rest and 90 min of leg cycle ergometry at 45 and 65% peak oxygen uptake (VO(2 peak)) after continuous infusions of [1-(13)C]acetate, [2-(13)C]acetate, or NaH(13)CO(3). In parallel trials, [6,6-(2)H]glucose and [1-(13)C]glucose were given. Experiments were conducted after an overnight fast with exercise commencing 12 h after the last meal. During the transition from rest to exercise, CO(2) production increased (P < 0.05) in an intensity-dependent manner. Significant differences were observed in the fractional recoveries of (13)C label as (13)CO(2) at rest (NaH(13)CO(3), 77.5 +/- 2.8%; [1-(13)C]acetate, 49.8 +/- 2.4%; [2-(13)C]acetate, 26.1 +/- 1.4%). During exercise, fractional recoveries of (13)C label from [1-(13)C]acetate, [2-(13)C]acetate, and NaH(13)CO(3) were increased compared with rest. Magnitudes of label recoveries during both exercise intensities were tracer specific (NaH(13)CO(3), 93%; [1-(13)C]acetate, 80%; [2-(13)C]acetate, 65%). Use of an acetate-derived correction factor for estimating glucose oxidation resulted in Rox values in excess (P < 0.05) of glucose rate of disappearance during hard exercise. We conclude that, after an overnight fast: 1) recovery of (13)C label as (13)CO(2) from [(13)C]acetate is decreased compared with bicarbonate; 2) the position of (13)C acetate label affects carbon dilution estimations; 3) recovery of (13)C label increases in the transition from rest to exercise in an isotope-dependent manner; and 4) application of an acetate correction factor in glucose oxidation measurements results in oxidation rates in excess of glucose disappearance during exercise at 65% of VO(2 peak). Therefore, bicarbonate, not acetate, correction factors are advocated for estimating glucose oxidation from carbon tracers in exercising men.

Recovery of 13C in breath from NaH13CO3 infused by gut and vein: effect of feeding

Estimates of substrate oxidation obtained from appearance of 13C or 14C from tracers in breath must be corrected for retention of labeled carbon in the body. We aimed to determine the effect of a defined experimental diet and metabolic status on recovery of infused Na [13C]bicarbonate in breath. Six healthy male subjects consumed an experimental diet for 7 days before receiving a continuous infusion of formula without tracer on day 8 and received either an intragastric (ig) or intravenous (iv) infusion of Na [13C]bicarbonate on day 9 or 11 during a 4-h postabsorptive (PA), 4-h continuously fed period. A trend toward increasing PA breath enrichment during the first 7 diet days approached statistical significance (P = 0.051), whereas breath enrichments measured 3 h postbreakfast were consistently higher than PA values throughout and did not change over the 7-day period. Breath enrichments during a 4-h continuous ig infusion of formula without tracer on day 8 rose 2.0 +/- 0.5 atom percent excess (APE).10(-3) above base line (P less than 0.001, ANOVA). In the tracer studies, breath enrichments were similar for the ig and iv routes of tracer infusion. For the ig infusion the fraction of infused Na [13C]bicarbonate recovered in breath as 13CO2 was 0.74 +/- 0.02 for the PA period and 0.79 +/- 0.02 for the fed period. For the iv infusion the fraction recovered was 0.70 +/- 0.04 for the PA period and 0.82 +/- 0.03 for the fed period. Fractional recoveries were not significantly different for ig and iv routes of administration but were different for PA and fed periods (P less than 0.0001, 2-way ANOVA). The fractional recoveries for the fed period obtained here were similar to the value 0.81 reported in a number of other studies. Recovery of tracer in breath increased linearly with O2 uptake and CO2 production, suggesting that factors affecting respiratory gas exchange may alter recovery. We conclude that the primary factor determining label recovery is the immediate and recent nutritional status of the host.

The analysis of glucose kinetics using a single jugular catheter in awake rats

The rates of glucose production and utilization can be estimated by a primed-constant infusion technique using separate catheters for the infusion of radiolabelled glucose and periodic blood withdrawal. In rats, a carotid artery catheter is most often combined with a jugular or femoral venous catheter in such studies. We presently describe a method which utilizes a single jugular catheter for both infusion and sampling in the awake rat. This method is directly compared with simultaneous carotid artery sampling during both the dynamic steady state and a nonsteady state induced by a constant infusion of insulin. Our results demonstrate the validity of a single vein design for the analysis of glucose kinetics in either state. Rapid sampling and complete flushing prevent disruption of infusate equilibrium and sample contamination respectively. This single catheter method requires less technical skill for placement, reduces surgical intervention and enhances the comfort of the awake rat.