Oxygen consumption and distribution in the Long-Evans rat retina

The purpose of this study was to investigate the oxygen distribution and consumption in the pigmented Long-Evans rat retina in vivo during dark and light adaptation, and to compare these results to previous work on cat and albino rat. Double-barreled microelectrodes recorded both intraretinal PO₂ depth profiles and the electroretinogram (ERG), which was used to identify the boundaries of the retina. Light adaptation decreased photoreceptor oxygen consumption per unit volume (Q_av) from 3.0 ± 0.4 ml·100 g^-1 min^-1 (mean ± SEM) in darkness to 1.8 ± 0.2 ml·100 g^-1 min^-1 and increased minimum outer retinal PO₂ at the inner segments (P_min) from 17.4 ± 3.0 to 29.9 ± 5.3 mmHg. The effects of light on outer retinal PO₂ and Q_av were similar to those previously observed in cat, monkey, and albino rats; however, dark-adapted P_min was higher in rat than cat. The parameters derived from fitting the oxygen diffusion model to the rat data were compared to those from cat. Oxygen consumption of the inner segments (Q₂) and choroidal PO₂ (P_C) in rat and cat were similar. P_min was higher in rat than in cat for two reasons: first, rat photoreceptors have a shorter oxygen consuming region; and second, the retinal circulation supplied a greater fraction of consumed oxygen to rat photoreceptors. The average PO₂ across the inner retina (P_IR) was not different in dark adaptation (25.4 ± 4.8 mmHg) and light adaptation (28.8 ± 5.4 mmHg) when measured from PO₂ profiles. However, with the microelectrode stationary at 9-18% retinal depth, a small consistent decrease in PO₂ occurred during illumination. Flickering light at 6 Hz decreased inner retinal PO₂ significantly more than an equivalent steady illumination, suggesting that changes in blood flow did not completely compensate for increased metabolism. This study comprehensively characterized rat retinal oxygenation in both light and dark, and determined the similarities and differences between rat and cat retinas.