Lack of oxygen sensing by mitochondria in platelets

The range over which cells are sensitive to changes in oxygen concentration remains uncertain. Wilson and colleagues [Wilson, D.F. (1994) Med. Sci. Sports Exerc. 26, 37-43] have suggested that cytochrome oxidase is sensitive to oxygen concentrations below about 40 microM, but proposed that this sensitivity is obscured in intact cells because an increase in reduction state of cytochrome c acts to maintain oxygen consumption. We have tested this hypothesis in platelets, which are small cells (2-4 micrometer diameter, < 0.5 micrometer thick) that do not decrease their rate of oxygen consumption until oxygen concentrations fall below 2.5 microM. Contrary to the expectations of the hypothesis, the reduction state of cytochrome c, the concentration of NADH and the rate of glycolytic output are not changed as oxygen concentration declines from 40 microM down to 5 microM. Therefore, we conclude that at least some cell types contain mitochondria that are not capable of sensing oxygen above 5 microM by the mechanism proposed by Wilson and colleagues.