Intraretinal oxygen distribution and choroidal regulation in the avascular retina of guinea pigs

O-2-sensitive microelectrodes were used to measure PO2 as a function of depth through the retina and choroid of anesthetized and artificially ventilated guinea pigs. The guinea pig retina is of particular interest, because it has a typically mammalian structure but no retinal circulation; it relies totally on choroidal delivery of O-2 and other nutrients. Measurements of intraretinal O-2 distribution in an avascular mammalian retina have not previously been reported. Under normal ventilation conditions, PO2 decreased monotonically from the choroid (33.6 +/- 2.9 mmHg, n = 11) to near zero (0.4 +/- 0.1 mmHg) at the retina-vitreous boundary. The inner half of the retina had an average PO2 of only 0.6 +/- 0.1 mmHg. Stepwise increases in inspired O-2 (from 20 to 40 to 60 to 80 to 100%) had surprisingly little effect on choroidal PO2. Rapid changes (20-100%) produced overshoot-type responses in the choroid before recovery to levels only slightly above those found in normoxia. This indicates the presence of an active O-2-regulatory mechanism in the guinea pig choroid. Addition of CO2 (5%) to O-2 ventilation appeared to break down this control mechanism and led to dramatic and sustained increases in PO2 throughout the retina and choroid. The demonstration of an O-2-regulating mechanism in the guinea pig choroid that maintains choroidal PO2 well below that in the systemic arterial blood, coupled with the observation of very low O-2 levels throughout the inner retina, suggests that the O-2 requirement of the inner retina in the guinea pig is small and that O-2 levels in the choroid are deliberately constrained.