Triglyceride synthesis in rat skeletal muscles

In order to gain first insight into triglyceride (TG) synthesis in skeletal muscle, conscious Sprague Dawley rats were intravenously infused with [U-¹³C]palmitate and [1-¹³C]oleate simultaneously at 12 and 18 nmol·kg· min respectively, or at quadrupled rates. Gastrocnemius and soleus muscles were biopsied after 2 and 4 h of the infusion. Muscle TG and non-esterified fatty acid (NEFA) were purified by HPLC and adsorption chromatography. ¹³C enrichments and concentrations of the constituent palmitate and oleate were determined by gas chromatography/combustion/isotope ratio mass spectrometry. Muscle TG fractional synthesis rates (FSR) were calculated using NEFA as the precursor pool: Gastrocnemius 0.267±0.075·h by [U-¹³C]palmitate and 0.278±0.049·h by [1-¹³C]oleate; Soleus 0.100±0.030·h by [U-¹³C]palmitate and 0.075±0.013·h by [1-¹³C]oleate. TG fatty acid pool sizes of gastrocnemius and soleus were: palmitate 4.4±0.8 and 18.0±3.1 μmol·g dry muscle respectively (P=0.002); Oleate 3.8±0.8 and 21.8±4.9 μmol·g dry muscle respectively (P=0.006). The data suggested that plasma free fatty acids are an important source for muscle TG synthesis and muscle TG is extremely active metabolically; TG FSRs in the two muscles are different (P=0.039 by [U-¹³C]palmitate and P=0.022 by [1-¹³C]oleate) and the differences are due to the much larger TG pool in soleus than that in gastrocnemius muscle.