Kinetic analysis of the oxidation of palmitate-1-14C in man during prolonged heavy muscular exercise.

Two healthy men with high working capacities were injected intravenously with palmitate-1-(14)C and NaH(14)CO(3) on two occasions while they were performing strenuous exercise on a bicycle ergometer. From analysis of (14)CO(2) in expired air after injection of NaH(14)CO(3), rate constants and compartment sizes describing a three-compartment system for CO(2) were determined algebraically. These data were combined with those of a separate study in which (14)C in free fatty acids of arterial blood plasma and in expired CO(2) were measured after injection of palmitate-1-(14)C to construct an eight-compartment model with an analogue computer that described precisely the observed data in each subject. The results indicate that under these conditions almost half of the free fatty acids leaving the blood are oxidized directly (i.e., are transferred to mitochondrial oxidative sites through small intermediate compartments). The remainder enters larger compartments apart from the direct pathway; most of this fraction reenters the direct oxidative pathway within 30 min. These observations suggest that certain esterified fatty acids contained in working muscle cells are rapidly renewed. Recycling of (14)C in plasma free fatty acids and triglyceride fatty acids was substantially reduced during exercise. Values for turnover rate and extent of oxidation of free fatty acids obtained by the method of continuous intravenous infusion of palmitate-1-(14)C were similar during exercise in these two subjects to those obtained after pulse injection.