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 mm, 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 mm diameter, , 0.5 mm thick) that do not decrease their rate of oxygen consumption until oxygen concentrations fall below 2.5 mm. 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 mm down to 5 mm. Therefore, we conclude that at least some cell types contain mitochondria that are not capable of sensing oxygen above 5 mm by the mechanism proposed by Wilson and colleagues.Keywords: cytochrome c; hypoxia; mitochondria; oxygen sensing; platelets.Oxygen is a critical element in cellular energy metabolism, yet the range over which oxygen concentration can influence oxidative energy production in a whole cell system remains poorly defined. Part of the difficultly lies in the problem of accurately defining extracellular and intracellular oxygen concentrations and the rate of oxygen consumption in tissue [1]. In addition, there has been a continuing controversy over the sensitivity of cytochrome oxidase to oxygen [1±6].One hypothesis is that cytochrome oxidase is sensitive to a range of oxygen concentrations (below about 40 mm, see [5] [7], as oxygen concentration decreases, the oxygen dependence of mitochondrial oxidative phosphorylation becomes important. However, the rate of oxygen consumption should not decline substantially in this phase as compensatory (also termed adaptive) changes in phosphorylation potential and/or redox state act to increase cytochrome c reduction state, which acts in turn to maintain oxygen consumption. One consequence of these adaptive changes is an increase in glycolytic activity [7]. In the third phase, further changes in phosphorylation potential and/or redox state cannot maintain oxygen consumption, and a decline is observed.The key feature of this model is the adaptive region [5,7,8]. The oxygen concentrations over which the proposed adaptive phase occurs has not been well defined, but the upper bound at the mitochondria has been placed near 40 mm (4 kPa, [5]), while the lower bound appears to be , 0.7 mm [7,8]. This model would be supported if adaptive changes could be detected where oxygen concentration was changing but oxygen consumption was remaining steady.Here, we test for the presence of an adaptive phase by measuring the concentration of NADH, lactate output and the reduction state of cytochrome c as a function of extracellular oxygen concentration in a platelet incubation system. Platelets were chosen because they are small cells (2±4 mm diamet...
