Turnover of plasma-free arachidonic and oleic acids in resting and exercising human subjects

Turnover rates and metabolism in the leg region and the splanchnic region, of free arachidonic and oleic acid have been examined in five healthy subjects at rest and during bicycle exercise. A continuous intravenous infusion of tritiated arachidonic acid and 14-C-labeled oleic acid was given. The rate constant for arachidonic acid turnover at rest was 0.44 plus or minus 0.004/min as compared to 0.29 plus or minus 0.02 for oleic acid. Significant correlations between turnover rate and arterial concentrations were observed for both acids in the resting state. The turnover of arachidonic acid was not significantly altered during exercise which caused an eight- to ninefold rise in pulmonary oxygen uptake. In contrast, the turnover of oleic acid rose markedly with exercise; its rate constant increased by approximately 90% to 0.57 plus or minus 0.05/min. The fractional uptakes of the two acids in the leg region were similar in the resting state. The splanchnic fractional uptake for arachidonic acid significantly exceeded that for oleic acid at rest. There was a net uptake of arachidonic acid in the splanchnic region in all subjects studied. In vitro incubations of whole blood deomonstrated a significant exchange of arachidonic as well as oleic acid between plasma and blood cells. We conclude that the metabolism of plasma free arachidonic acid differs from that of oleic acid in that (1) its fractional turnover at rest is about 50% higher, (2) its splanchnic fractional uptake is about 60% higher, and (3) its turnover rate is unaffected by physical exercise. It is further suggested that the high turnover rate of arachidonic acid, and to a less extent that of oleic acid, could be due in part to an exchange of fatty acids between plasma and endothelial cells.