DESIGN OF THE OXYGEN AND SUBSTRATE PATHWAYS .7. DIFFERENT STRUCTURAL LIMITS FOR OXYGEN AND SUBSTRATE SUPPLY TO MUSCLE MITOCHONDRIA

This paper integrates the results of a series of studies on the supply of O-2 and substrates for oxidative muscle metabolism and draws concl usions on the role of structural design in partitioning and limiting s ubstrate supply, The studies compared dogs and goats exercising at dif ferent intensities and combined physiological, biochemical and morphom etric investigations, In both species, the rate of fatty acid oxidatio n reached an upper limit at low exercise intensities, and only glucose consumption was increased at higher exercise intensities, The supply of both glucose and fatty acids from the capillaries reached maximal r ates at low exercise intensities; this limitation is related to the de sign of the sarcolemma as calculations suggest that the endothelium in troduces only a small resistance to substrate flux, From these finding s, it appears that the capillaries are designed to satisfy O-2 supply up to maximal O-2 demand, The increase in substrate supply to the mito chondria at higher exercise intensities is achieved by drawing on intr acellular stores of glycogen and lipids. The size of these stores is l arger in dogs than in goats, providing the athletic species with twice the fuel reserves, These findings are interpreted on the basis of a n etwork model with fluxes partitioned between direct and indirect pathw ays and with some structures shared by more than one function, Whereas O-2 is supplied through a direct pathway, the supply of both substrat es is split temporally to allow, during exercise, immediate fuel suppl y to the mitochondria from intracellular stores; these are replaced fr om the vasculature, during periods of rest, to a size commensurate wit h high rates of combustion, Considering this complexity, we conclude t hat the results are compatible with the principle of symmorphosis appl ied to a network structure and that the adjustment of design to functi onal demand involves different structures for O-2 and for substrates.