Kinetics of13CO2elimination after ingestion of13C bicarbonate: the effects of exercise and acid base balance

Abstract

In order to investigate the effects of muscular work and preceding exercise on the retention of exogenous labelled bicarbonate, we studied the effects of oral administration of [¹³C]bicarbonate (0·1 mg kg^‐1) in five subjects at rest before exercise and during and after 1 h of treadmill walking at 73% VO_2max on three separate occasions. Elimination of CO₂ from labelled bicarbonate was 62·6±8·1% at rest, 103·6±11·3% during exercise (PP= 0·01). During exercise mean residence time (MRT) was shorter than at rest (35±7 min vs. 54±9min, P < 0·02) and CO₂ pool size was larger (998±160 ml CO₂kg^‐1, vs. 194±28ml CO₂kg^‐1, P < 0·001). Compared to values obtained at rest, during recovery from exercise, MRT and CO₂ pool size were reduced (34±5min, P < 0·05; 116±19 ml CO₂kg^‐1, P < 0·02, respectively). In an additional five subjects acidosis and alkalosis were induced prior to administration of oral [¹³C]bicarbonate at rest. Elimination of bicarbonate was lower during acidosis (46·1±5·6%, P < 0·01) but was unaltered (50·9±5·6%, NS) during alkalosis, compared to the values obtained at resting pH. During acidosis MRT and CO₂ pool size decreased (37±3min, P2kg^‐1, P < 0·01, respectively) whereas in alkalosis MRT was unchanged (65±8 min NS) but CO₂ pool size was increased (230±23ml CO₂kg^‐1, P < 0·05). The kinetics of elimination of ¹³CO₂ from administered bicarbonate after exercise are different to those at rest and resemble acidosis. The appropriate correction factor for sequestered ¹³C should be used in metabolic studies of the post‐exercise state when using ¹³C tracers.