Intramuscular fatty acid metabolism evaluated with stable isotopic tracers

We evaluated the applicability of stable isotopic tracers to the study of intramuscular fatty acid metabolism by infusing both [U-¹³C]palmitate and [1-¹³C]oleate intravenously for 4 h into fasted conscious rats. Skeletal muscles were sequentially biopsied, and the concentration and ¹³C enrichment of fatty acids were measured by gas chromatography/combustion/isotope ratio mass spectrometry. Throughout the study, the ¹³C enrichment of plasma palmitate and oleate remained substantially greater than intramuscular nonesterified palmitate and oleate enrichment, which in turn was greater than intramuscular triglyceride palmitate and oleate enrichment. Fractional synthesis rates of intramuscular triglycerides in gastrocnemius and soleus were 0.267 ± 0.075 and 0.100 ± 0.030/h (P = 0.04), respectively, as determined by using [U-¹³C]palmitate, and were 0.278 ± 0.049 and 0.075 ± 0.013/h (P = 0.02), respectively, by using [1-¹³C]oleate. We conclude that plasma free fatty acids are a source for intramuscular triglycerides and nonesterified fatty acids; the latter are likely the synthetic precursors of the former. Uniformly and singly labeled [¹³C]fatty acid tracers will provide an important tool to study intramuscular fatty acid and triglyceride metabolism.