Evaluation of polyunsaturated fatty acid uptake, distribution, and incorporation into specific muscle types
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Polyunsaturated fatty acids (PUFA) affect key cellular and physiological processes in the body ranging from cell signalling to inflammation. Compositional, dietary refinement and bioassay studies have shown strong associations between the PUFA composition of skeletal muscle with various contractile properties as well as the development of obesity and insulin resistance. The incorporation of PUFAs into rat soleus (slow-twitch oxidative), red gastrocnemius (fast-twitch oxidative), and white gastrocnemius (fast-twitch glycolytic) muscle were examined using stable isotope-labelled fatty acids. Two separate tracer studies were conducted. In the first study, four groups of rats were orally dosed with one of three isotopes of 18:2n-6; 13C18-18:2n-6 ethyl ester, 13C18- 18:2n-6 nonesterified fatty acid or 2H5-18:2n-6 ethyl ester and a control group received the vehicle only (olive oil). Animals were sacrificed 8 hours post dosing and soleus, red and white gastrocnemius muscles were collected for lipid analysis. In the second study, rats were orally administered a single dose of a mixture of 4 isotopes (13C18-18:2n-6, 2H5-18:3n-3, 13C16-16:0, and 2H2-18:1n-9) or vehicle only (olive oil) as a control. Groups of animals were sacrificed at 8, 24, and 48 h after dosing and four muscle types (heart, soleus, red and white gastrocnemius) were collected and analyzed for isotopic signal of these fatty acids and their corresponding desaturation and/or elongation products. Soleus accumulated significantly higher concentrations of labelled 18:2n-6, 18:3n-3 and most of n-6 fatty acids derived from 18:2n-6 followed by red gastrocnemius and white gastrocnemius. Heart muscle accumulated 20:5n-3 and 22:6n-3 increasingly over time while skeletal muscle accumulation was variable across muscle types. Labelled 20:5n-3 was detected in red and white gastrocnemius at 8 and 24 h with levels declining by 48 h while no 20:5n-3 was detected in soleus at anytime. Labelled 22:6n-3 was not detected in white gastrocnemius, but 22:6n-3 appeared to be increasing in red gastrocnemius over time. Soleus demonstrated a large accumulation of 22:6n-3 at 8 h with no detectable levels at 48 h. In conclusion we were able to demonstrate that the distribution and metabolism of various PUFAs differ in muscle types with distinct fibre type composition.