Oxygen distribution and consumption in the cat retina during normoxia and hypoxemia.

Abstract

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 Po~ measurements to a model of steady-state oxygen diffusion and consumption. Choroidal Po 2 fell linearly during hypoxemia, about 0.64 mmHg/mmHg decrease in arterial Po~ (Pao~). 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 Pao~'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 OJ 100 g.min during normoxia to 2.6 ml Off 100 g'min at a P,o 2 of 50 mmHg. When the retina was adapted to a rod saturating background, normoxic oxygen consumption was ~ 33% of the darkadapted 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 Po~ 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 Po~ 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 Po~ 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 P~o~'s in light adaptation.

Oxygen consumption in the isolated toad retina

“…In the dark-adapted cat retina, QoR (called Qav there) was 4.4 ml 02 (100 g min) -1, and decreased in light adaptation to 60% of the value in dark (Haugh et al, 1990). In other studies in the cat, OoR was 5.1 and 3-9mlo 2(lO0gmin) x in darkness, decreasing to 33 % and 36 % of those values respectively in light adaptation (Linsenmeier and Braun, 1992;. Clearly the effect of light on QoR is similar in toad and in cat, and given the difference in temperature of the retinas, toad at 22°C and cat at 38°C, it was expected that the toad retina would have a lower absolute OO 2 than the cat retina.…”

“…Tissue PO 2 would also be lower in vivo. Evidence from cat retina shows that a modest degree of hyperoxia does not affect retinal QO 2 (Linsenmeier and Yancey, 1989; whereas mild hypoxia could have affected QO 2 (Linsenmeier and Braun, 1992).…”

A role for photoreceptors in retinal oedema and angiogenesis: an additional explanation for laser treatment?

“…of simple models of diffusion difficult. 43 Oxygen levels in the inner retina are higher near arterioles, 25 and in the absence of a choroid, there would be an oxygen concentration gradient favouring oxygen diffusion towards the outer retina across the whole retina. Similarly, it has been shown that in eyes without an inner retinal circulation oxygen diffuses inwards from the choroid across the whole retina in amounts sufficient to meet the metabolic requirements of the normal inner retina.…”

Spare the rods and spoil the retina: revisited

“…Therefore, the partial pressure of oxygen [PO 2 ] at the level of photoreceptors reduces to near zero resulting in a relative hypoxia in this layer compared with other retinal layers. [9][10][11][12] Oxygen consumption by rod photoreceptors…”