DESIGN OF THE OXYGEN AND SUBSTRATE PATHWAYS .4. PARTITIONING ENERGY PROVISION FROM FATTY-ACIDS

This paper quantifies the fluxes of fatty acids through the pathways s upplying muscle mitochondria with oxidative fuel in exercising dogs an d goats, We used continuous infusions of 1-[C-14]palmitate and indirec t calorimetry to measure fatty acid supply from two sources: the circu lation and the triglyceride stores within the muscle cells, Our goal w as to determine maximal flux through these two branches of the lipid p athway as key functional parameters for testing the principle of symmo rphosis, i.e. that structural capacity is quantitatively matched to fu nctional demand in the oxidative substrate pathways, It is under these rate-limiting conditions that we predict that all of the structural c apacity will be used. Maximal rates of fatty acid oxidation were reach ed at low exercise intensities of 40 % M(O2max). Fatty acids from the circulation supplied only a small fraction (15-25 %) of the total fat oxidized under these conditions, Although dogs were able to oxidize ci rculatory fatty acids faster than goats, maximal rates were not in pro portion to the 2.2-fold difference in aerobic capacity between the two species, Dogs compensated for their relatively lower use of circulato ry fatty acids by oxidizing more triglycerides from lipid droplets in their muscle cells, We conclude that fatty acids from intramuscular tr iglyceride stores are a major source of fuel during maximal rates of l ipid oxidation, Furthermore, it is this branch of the fatty acid pathw ay that is adapted to the large difference in aerobic capacity between dogs and goats.