Free fatty acid oxidation in bovine muscle in vivo: effects of cold exposure and feeding.

A mixture of (1-14C)-labeled free fatty acids (FFA), complexed in bovine plasma, was infused into the abdominal aorta of conscious young steers exposed to thermoneutral or moderately cold conditions for several hours and fed 6 or 22 h before the experiment. The uptake, release, and oxidation of FFA in one hindlimb was calculated from simultaneous measurements of leg blood flow and arteriovenous difference in the specific activities of plasma 14C-FFA and blood 14CO2. Despite an invariable net release of FFA from the resting leg, uptake of 14C-FFA was considerable; of this only 14 and 3% was immediately converted to 14CO2 in fasted and fed steers, respectively. During cold exposure, increases in whole-body oxygen consumption (VO2), arterial concentration and turnover rate of plasma FFA, and a decrease in respiratory quotient were accompanied by much greater increases in VO2, uptake, and oxidation of FFA by the shivering leg. Even so, most FFA taken up were apparently not immediately oxidized to CO2, and possible alternatives for FFA metabolism in shivering muscle are discussed.