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

Whole body oxidative rates of labeled substrates are often measured by coll ecting expired air and determining the amount of labeled CO2 that is produc ed. However, the CO2 produced may not be completely recovered under all cir cumstances, and there is a wide variation in values reported under differen t experimental conditions (similar to 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 con ducted to determine gastrointestinal tract, hepatic, hindlimb, and renal re coveries of (NaHCO3)-C-14 during a 180-min constant infusion [0.022 +/- 0.0 02 (SE) mu Ci.kg(-1).min(-1)]. Before the constant infusion period, a bolus infusion of NaH14CO3 (1.76 +/- 0.16 mu Ci/kg) was given, and the rate of d ecay in blood was measured over a 15-min period to determine pool size. The pool size for the distribution of (CO2)-C-14 was similar to 80% of the tot al 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, h epatic, gastrointestinal tract, hindlimb, and renal recoveries of CO2 in vi vo 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 co nditions, would be very minor.