Gastrointestinal tract, hepatic, hindlimb, and renal recovery of CO₂ in vivo

Whole body oxidative rates of labeled substrates are often measured by collecting expired air and determining the amount of labeled CO₂ that is produced. However, the CO₂ produced may not be completely recovered under all circumstances, and there is a wide variation in values reported under different experimental conditions (~50-100%). The potential contribution of specific organs to this variation has not been defined. In vivo studies using healthy, postabsorptive, multicatheterized conscious canines were conducted to determine gastrointestinal tract, hepatic, hindlimb, and renal recoveries of NaH¹⁴CO₃ during a 180-min constant infusion [0.022 ± 0.002 (SE) μCi·kg^-1·min^-1]. Before the constant infusion period, a bolus infusion of NaH¹⁴CO₃ (1.76 ± 0.16 μCi/kg) was given, and the rate of decay in blood was measured over a 15-min period to determine pool size. The pool size for the distribution of ¹⁴CO₂ was ~80% of the total body pool (16.0 ± 1.7 liters). Whole body recovery was 97.2 ± 6.7%. The recoveries across the liver, gut, leg, and kidney were 99.9 ± 1.3, 98.0 ± 1.4, 96.7 ± 2.6, and 99.9 ± 2.1%, respectively. In conclusion, hepatic, gastrointestinal tract, hindlimb, and renal recoveries of CO₂ in vivo were near 100%, suggesting that CO₂ loss is not greater in gluconeogenic organs and that corrections for incomplete recovery of CO₂, when measuring oxidation of substrates across these organs under normal postabsorptive conditions, would be very minor.