Journal of Lipid Research, Vol. 8, 366-373, July 1967
Copyright © 1967 by Lipid Research, Inc.

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

Richard J. Havel , Lars-Göran Ekelund , and Alf Holmgren

Department of Clinical Physiology and the King Gustav V Research Institute, Karolinska Institute, Stockholm, Sweden, and the Cardiovascular Research Institute and the Department of Medicine, University of California San Francisco Medical Center, San Francisco, California 94122

Two healthy men with high working capacities were injected intravenously with palmitate-1-¹⁴C and NaH¹⁴CO₃ on two occasions while they were performing strenuous exercise on a bicycle ergometer. From analysis of ¹⁴CO₂ in expired air after injection of NaH¹⁴CO₃, rate constants and compartment sizes describing a three-compartment system for CO₂ were determined algebraically. These data were combined with those of a separate study in which ¹⁴C in free fatty acids of arterial blood plasma and in expired CO₂ were measured after injection of palmitate-1-¹⁴C 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 ¹⁴C 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-1-¹⁴C were similar during exercise in these two subjects to those obtained after pulse injection.

Supplementary key words free fatty acids • energy metabolism • fat metabolism • triglyceride metabolism • compartmental analysis

Submitted on February 6, 1967
Accepted on March 27, 1967

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