Effects of medium fatty acid concentration, epinephrine, and glucose on palmitate-l-C14 oxidation and incorporation into neutral lipids by skeletal muscle in vitro

Abstract

The rate of oxidation of palmitate-l-C14 to C14O2 by rat skeletal muscle in vitro was shown to increase markedly as a function of the concentrations of free fatty acids (FFA) in the medium. Neither epinephrine nor glucose had any important effect. The rate of incorporation of palmitate-l-C14 into tissue neutral lipids was also increased by higher levels of FFA in the medium. At any given concentration of FFA this incorporation was also increased by glucose but was not altered by epinephrine. It is concluded that the deposition of triglyceride reported to occur in muscle after epinephrine or norepinephrine administration is due to the effects of these hormones on serum FFA levels and not to a direct effect on muscle metabolism. The profound effect of FFA concentration on the rate of FFA oxidation suggests that regulation of the serum concentrations of this labile lipid fraction may be of central importance in determining over-all rates of body metabolism. In particular, it is proposed that the elevated metabolic rates after epinephrine administration and in hyperthyroidism may, in part, be directly attributable to the elevations of serum FFA in these conditions.