Light and choroidal Po-2 modulation of intraretinal oxygen levels in an avascular retina

PURPOSE. To determine the influence that choroidal oxygen level and outer r etinal oxygen demand have on oxygen availability to the inner retina when t he choroid is the only source of retinal oxygenation. This condition prevai ls in avascular retinas and in vascularized retinas suffering vascular occl usion. METHODS. Oxygen-sensitive microelectrodes were used to measure the oxygen t ension as a function of depth in the naturally avascular retina of anesthet ized and mechanically ventilated guinea pigs (n = 6). Choroidal Po-2 was ma nipulated by varying the ventilation gas mixture, and outer retinal oxygen consumption was modulated by light-dark adaptation. Individual Po-2 profile s were fitted to a multilayer mathematical model of Po-2 distribution, and pairs of profiles at different choroidal Po-2 levels, or under light and da rk conditions, were fitted to an intraretinal Po-2 difference model. Both m odels reflect the purely choroidal supply of retinal oxygenation. RESULTS. An increase in choroidal Po-2 produced an equivalent increase in a ll retinal layers. tight induced a decreased oxygen consumption in the regi on of the inner segments of the photoreceptors which resulted in a signific ant increase in Po-2 in this layer, flowing on unattenuated to all inner re tinal layers. The intraretinal Po-2 distribution and the light- and ventila tor-induced changes in Po-2 were consistent with theoretical predictions of the mathematical models. CONCLUSIONS. The present experimental studies confirm that when the choroid is the only source of retinal oxygenation, the full effect of increased ch oroidal oxygen level or reduced uptake in the outer retina passes through t o the inner retinal layers if the oxygen utilization by the inner retina re mains constant.