Oxygen tension (PO2) was measured with microelectrodes within the retina of anesthetized cats during normoxia and hypoxemia (i.e., systemic hypoxia), and photoreceptor oxygen consumption was determined by fitting PO2 measurements to a model of steady-state oxygen diffusion and consumption. Choroidal PO2 fell linearly during hypoxemia, about 0.64 mmHg/mmHg decrease in arterial PO2 (PaO2). The choroidal circulation provided ~91% of the photoreceptors’ oxygen supply under dark-adapted conditions during both normoxia and hypoxemia. In light adaptation the choroid supplied all of the oxygen during normoxia, but at PaO2‘s < 60 mmHg the retinal circulation supplied ~ 10% of the oxygen. In the dark-adapted retina the decrease in choroidal Po2 caused a large decrease in photoreceptor oxygen consumption, from ~ 5.1 ml O2/100 g · min during normoxia to 2.6 ml Off 100 g · min at a PO2 of 50 mmHg. When the retina was adapted to a rod saturating background, normoxic oxygen consumption was ~ 33% of the dark-adapted value, and hypoxemia caused almost no change in oxygen consumption. This difference in metabolic effects ofhypoxemia in light and dark explains why the standing potential of the eye and retinal extracellular potassium concentration were previously found to be more affected by hypoxemia in darkness. Frequency histograms of intraretinal PO2 were used to characterize the oxygenation of the vascularized inner half of the retina, where the oxygen distribution is heterogeneous and simple diffusion models cannot be used. Inner retinal PO2 during normoxia was relatively low: 18 ± 12 mmHg (mean and SD; n = 8,328 values from 36 profiles) in dark adaptation, and significantly lower, 13 ± 6 mmHg (n = 4,349 values from 19 profiles) in light adaptation. Even in the dark-adapted retina, 30% of the values were < 10 mmHg. The mean PO2 in the inner (i.e., proximal) half of the retina was well regulated during hypoxemia. In dark adaptation it was significantly reduced only at PaO2‘s < 45 mmHg, and it was reduced less at these PaO2‘s in light adaptation.
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