Leg gas exchange, release of glycerol, and uptake of fats after two minutes bicycling to exhaustion.

It is not known to what extent the muscles use fats and carbohydrates as substrate for oxidation after intense, anaerobic types of bicycling. Six healthy young men therefore bicycled at constant power for 2 min to exhaustion. Blood was drawn from indwelling catheters in the femoral artery and vein at intervals during the 1-h postexercise recovery. The blood samples were analysed for concentrations of O2 and CO2, and for free fatty acids (FFA), triacylglycerols (TG), and glycerol in plasma. The blood flow was also measured, and the rate of leg uptake of FFA, TG, and O2 and the release of CO2 and glycerol as well as its gas exchange ratio were calculated and integrated over the recovery period. The leg gas exchange ratio integrated over the exercise plus 1-h recovery period was 0.67 +/- 0.06 (mean +/- SEM ), suggesting pure fat oxidation. There was no statistically significant arterial-femoral-venous difference of FFA across the leg. The concentration of TG in plasma fell by 0.18 +/- 0.09 mmol L(-1) (32%) during the first 10 min of the recovery period, and the leg took up 18 +/- 8 micromol TG kg(-1) body mass (bm) during the whole 1-h recovery period. Free glycerol was released from the leg throughout the recovery period in excess of that released from hydrolysis of TG from plasma, suggesting that 30 +/- 10 micromol TG kg (-1) bm was hydrolysed, probably from intra-muscular stores. If fully oxidized, the triacylglycerols hydrolysed can account for 101% of the measured O2 uptake. Thus, muscle seems to use only triacylglycerols as substrate for its oxidative energy release after intense exercise.