Abstract
According to one definition, hypoxia exists when, because of inadequate oxygen supply, various cellular oxidation-reduction systems must shift toward a more reduced state. This definition was tested in experiments on rat and turtle brain by comparing hypoxic changes in two oxidation-reduction systems (lactate: pyruvate and NADH-NAD+) with changes in tissue PO2, ATP and creatine phosphate. The results indicate that: (1) measurements of tissue PO2 are most useful when the electrode tip is intracellular, (2) creatine phosphate and lactate: pyruvate ratio are sensitive indicators of cellular hypoxia, (3) ATP and NADH: NAD+ ratio change significantly only in severe hypoxia, (4) tests of learning and performance show impairment at levels of hypoxia which do not significantly lower brain ATP concentration. It is concluded that intracellular hypoxia cannot be defined solely in terms of increased reduction of cellular oxidation-reduction systems.
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