Intramuscular fatty acid metabolism evaluated with stable isotopic tracers

We evaluated the applicability of stableisotopic tracers to the study of intramuscular fatty acid metabolism byinfusing bothU-¹³C]palmitateand 1-¹³C]oleateintravenously for 4 h into fasted conscious rats. Skeletal muscles weresequentially biopsied, and the concentration and ¹³C enrichment of fatty acids weremeasured by gas chromatography/combustion/isotope ratio massspectrometry. Throughout the study, the¹³C enrichment of plasma palmitateand oleate remained substantially greater than intramuscularnonesterified palmitate and oleate enrichment, which in turn wasgreater than intramuscular triglyceride palmitate and oleateenrichment. Fractional synthesis rates of intramuscular triglyceridesin gastrocnemius and soleus were 0.267 ± 0.075 and 0.100 ± 0.030/h (P = 0.04), respectively, asdetermined by usingU-¹³C]palmitate, andwere 0.278 ± 0.049 and 0.075 ± 0.013/h(P = 0.02), respectively, by using1-¹³C]oleate. Weconclude that plasma free fatty acids are a source for intramusculartriglycerides and nonesterified fatty acids; the latter are likely thesynthetic precursors of the former. Uniformly and singly labeled¹³C]fatty acidtracers will provide an important tool to study intramuscular fattyacid and triglyceride metabolism.