KINSEY, S.T.*; DA SILVA, M.; PATHI, P.; LOCKE, B.R.; Univ. of North Carolina Wilmington; Florida State University; Florida State University; Florida State University: Diffusion limitation of aerobic metabolism in muscle fibers

The net rate of metabolic processes in cells depends on the competition between the catalytic capacity of the system and the diffusive flow of substrates to the reaction center. While the diffusion component is typically ignored, it exerts greater control over metabolic flux as either rates of ATP turnover or intracellular diffusion distances increase. ATP turnover rates during steady-state metabolic work may differ by many fold among muscle fibers of different organisms. In addition, muscle fibers in different animal groups can range in size from a few microns to >5mm in diameter, and the intracellular diffusion distances between mitochondria can vary in a similar fashion. In the present study, we used a reaction-diffusion model to examine the interaction between diffusion distance, ATP supply and ATP demand in muscle fibers of different organisms in an effort to determine whether aerobic capacities are likely to be limited by intracellular metabolite diffusion. Our results indicate that metabolite diffusion probably exerts significant control over metabolism in some types of muscles, but many muscle types are not close to being limited by intracellular diffusion. In these latter cases, the maximal rate of sarcolemmal oxygen flux may be a more important determinant of fiber metabolic design.