Intraretinal oxygen distribution in the rat with graded systemic hyperoxia and hypercapnia.

PURPOSE To describe the nature of oxygen level changes in specific layers in the rat retina under graded levels of systemic hyperoxia, with and without hypercapnia. METHODS Oxygen-sensitive microelectrodes were used to measure oxygen tension as a function of depth through the retina of anesthetized, mechanically ventilated rats. Breathing mixtures were manipulated to produce stepwise increments in systemic oxygen levels, with or without 5% CO2. Retinal arteriovenous oxygen differences were also measured as an indicator of oxygen delivery through the retinal circulation. Systemic blood gas levels were measured under each condition. RESULTS Hyperoxia increases PO2 throughout the retina to a varying extent in different retinal layers, with the increase more pronounced in the outer retina than in the inner retina. Simultaneous hypercapnia results in further increases in retinal oxygen levels. The lowest intraretinal oxygen level was consistently found in the inner plexiform layer (IPL), between the two capillary layers that support this region. There was a greater than fourfold increase in oxygen supply from the choroid with hyperoxia but, remarkably, the retinal circulation continued to provide a net delivery of oxygen to the retina. CONCLUSIONS Hyperoxia results in a significant but nonuniform increase in oxygen level in all layers of the rat retina, which is augmented by hypercapnia. The persistence of a minimum oxygen level in the IPL, despite the dramatic increase in oxygen flux from the choroid, suggests that oxygen consumption increases significantly in the IPL under hyperoxic conditions.