KINETICS OF (CO2)-C-13 ELIMINATION AFTER INGESTION OF C-13 BICARBONATE - THE EFFECTS OF EXERCISE AND ACID-BASE-BALANCE

In order to investigate the effects of muscular work and preceding exe rcise on the retention of exogenous labelled bicarbonate, we studied t he effects of oral administration of [C-13]bicarbonate (0.1 mg kg(-1)) in five subjects at rest before exercise and during and after Ih of t readmill walking at 73% VO2max on three separate occasions. Eliminatio n of CO2 from labelled bicarbonate was 62.6 +/- 8.1% at rest, 103.6 +/ - 11.3% during exercise (P < 0.01) and 43.0 +/- 4.7% during recovery f rom exercise (P = 0.01). During exercise mean residence time (MRT) was shorter than at rest (35 +/- 7 min vs. 54 +/- 9 min, P < 0.02) and CO 2 pool size was larger (998 +/- 160 ml CO2 kg(-1), vs. 194 +/- 28 ml C O2 kg(-1), P < 0.001). Compared to values obtained at rest, during rec overy from exercise, MRT and CO2 pool size were reduced (34 +/- 5 min, P < 0.05; 116 +/- 19 ml CO2 kg(-1) P < 0.02, respectively). In an add itional five subjects acidosis and alkalosis were induced prior to adm inistration of oral [C-13]bicarbonate at rest. Elimination of bicarbon ate was lower during acidosis (46.1 +/- 5.6%, P < 0.01) but was unalte red (50.9 +/- 5.6%, NS) during alkalosis, compared to the Values obtai ned at resting pH. During acidosis MRT and CO2 pool size decreased (37 +/- 3 min, P < 0.01 and 123 +/- 10 ml CO2 kg(-1), P < 0.01, respectiv ely) whereas in alkalosis MRT was unchanged (65 +/- 8 min NS) but CO2 pool size was increased (230 +/- 23 ml CO2 kg(-1), P < 0.05). The kine tics of elimination of (CO2)-C-13 from administered bicarbonate after exercise are different to those at rest and resemble acidosis. The app ropriate correction factor for sequestered C-13 should be used in meta bolic studies of the post-exercise state when using C-13 tracers.