Besides acting as an oxygen store during times of reduced blood oxygen supply, myoglobin can also facilitate intracellular oxygen transport by diffusion of oxymyoglobin along a PO 2 gradient. We reassess the importance of myoglobin-facilitated oxygen diffusion by applying new findings on the intracellular diffusivity of myoglobin in a model calculation.

he role of myoglobin as an oxygen store is well established. In terrestrial mammals, it compensates for the disadvantage of a reduced blood flow in heart and skeletal muscle during contraction. In aquatic mammals, which are characterized by markedly higher myoglobin concentrations than terrestrial mammals, it serves as an oxygen reservoir during the time ventilation ceases. The physiological significance of myoglobin-facilitated oxygen diffusion, i.e., reversible combination of myoglobin with oxygen and translational diffusion of oxymyoglobin molecules, has been a matter of controversy since Wittenberg (15) first reported evidence for it. Several approaches have been used to explore the physiological importance of this mechanism of intracellular oxygen transport. It has been attempted to experimentally evaluate its role in isolated cells, in isolated muscle tissue, and in vivo, both directly by chemically or genetically abolishing myoglobin function in the cell and observing the consequences for oxygen consumption or for physical performance and indirectly by determining the oxygen conductance of muscle tissue from measurements of oxygen consumption and intracellular PO 2 gradients. Theoretically, the problem was assessed in numerous model calculations, all of which until recently suffered from the lack of a reliable value for the intracellular diffusivity of myoglobin. We summarize here the results of these studies, and, taking into account our data on the intracellular myoglobin diffusivity, reevaluate the physiological role of myoglobinfacilitated oxygen transport in heavily exercising muscles by applying a modified Krogh cylinder model (6). Evidence for facilitated oxygen diffusion from abolishing myoglobin function In an earlier study (12), we concluded that all experimental studies reporting evidence for a significant contribution of myoglobin-facilitated diffusion to total intracellular oxygen transport exhibit experimental insufficiencies. One group blocked myoglobin function with carbon monoxide but chose an experimental setup in which severe hypoxia or anoxia was present in the muscle tissue investigated. Therefore, these results essentially show that myoglobin-facilitated