Tissue sequestration of C-labelled bicarbonate [HCO3-] in fed and fasted young sheep

Carbon dioxide entry rates (CER) based on isotopic activities in either exp ired air or blood following a 24-h intravenous infusion of [C-13]- and [C-1 4] sodium bicarbonate were compared with CO2 production quantified by respi ration hood in young sheep (28-30 kg) either fed (three animals) or fasted (three animals). CO2 production increased with intake (5.2 vs 10.3 mol/day; P < 0.001) as did CER values based on expired air (9.9 vs 18.6 mol/day; P < 0.001) or blood (7.5 vs 16.5 mol/day; P <0.001). The differences between air and blood CER values were significant (P < 0.001). There were no differ ences, however, when data were compared between [C-13] and [C-14] measureme nts. How much of these differences could be attributed to sequestration of label in body tissues was examined at the end of the infusion. The highest specific radioactivities (dpm/g dry matter) in acid-fast tissue material we re observed for the more metabolically active tissues, liver, jejunum and k idney, with the lowest values for fat and muscle. When tissue mass was take n into account, however, the largest proportions of the dose sequestered we re in bone muscle, skin and fat with significantly more retained for the fo rmer three (P < 0.01) during fasting. Separately, losses as urinary urea we re also quantified. Total measured sequestration of label only accounted fo r approximately 24-44% of the difference between CER and CO2 production. (C ) 1999 Elsevier Science Inc. All rights reserved.