FREE FATTY-ACID OXIDATION BY FOREARM MUSCLE AT REST, AND EVIDENCE FOR AN INTRAMUSCULAR LIPID POOL IN HUMAN FOREARM

The extent to which oleic acid removed from plasma by forearm muscles was oxidized immediately, and evidence for an intramuscular lipid pool which may be composed of triglycerides synthesized from plasma free fatty acids which may supply substantial portions of lipid substrates for oxidation by muscle, were investigated. [1-14C]oleic acid was infused at a constant rate into the brachial artery of 7 healthy young men at rest in the postabsorptive state. The muscle respiratory quotient (0.76) implied that about 80% of the O2 consumed was for the oxidation of lipid. Muscle free fatty acid uptake, had it been oxidized directly, could account for more than 100% of the observed O2 uptake. There was a lag of at least 30 min before 14CO2 appeared in forearm venous blood. 14CO2 release from the forearm muscle tended to reach an apparent plateau after 3 h of infusion of [14C]oleic acid. During the time of plateau 14CO2 release, oleic acid extracted from plasma could account for only 20% of O2 consumption; most of the oleic acid taken up was not oxidized directly. 14CO2 release persisted at a high level during the 1-3 h follow-up period after the infusion ended. Neither the delay in initial appearance of 14CO2 nor its continued release after cessation of infusion was due to delay and distribution in a forearm CO2 pool, since intra-arterial infusion of NaH14CO3 in 2 additional subjects demonstrated much more rapid distribution of 14CO2 in the forearm. Most, if not all, free fatty acids taken up by resting muscle are not oxidized directly, but probably enter an intramuscular pool which has a slow turnover during resting metabolism and is the immediate source of oxidized lipid substrate.