Optical mapping of inner retinal tissue PO₂

Abstract

A high resolution optical mapping procedure was developed to visualize oxygen concentration levels topographically within the tissue of the inner retina in viva The novel optical mapping procedure has the potential for describing oxygen metabolism in retinal and other body tissues and elucidating the coupling of metabolism to function. The method is based upon the fluorescence quenching by molecular oxygen of a lipid soluble probe substance which accumulates within the lipid bilayers of tissue cells. The optical mapping system can provide more than 300,000 values of tissue PO₂ in space with millisecond time resolution. Optical maps of inner retinal tissue PO₂ were imaged under conditions of normoxia, hyperoxia, and for a retina which received restricted panretinal photocoagulation. Moreover, the effects of transient increases in intraocular pressure were also investigated. An O₂ consumption rate of 5.48 ± 0.50 (SEM) × 10-⁻³ ml O₂/ml tissue/min for the light-adapted rat inner retina was estimated from the application of a Krogh cylinder diffusion model to tissue PO₂ gradients measured in the capillary-free zone around arterioles. Similarly, arterioles oxygenated a surrounding cylinder of tissue with a mean radius of 144.73 ± 5.52 (SEM) μm. Histograms of PO₂ values within inner retinal tissue (mean PO₂= 25.03 mm Hg, median=24.58 mm Hg) showed remarkable correspondence to those determined invasively in brain by others, using O₂ microcathodes, possibly suggesting a similarity in the underlying capillary architectures of the two neural tissues. Moreover, panretinal photocoagulation increased inner retinal tissue O₂ by > 10 mm Hg, supporting the hypothesis that the beneficial effects of this treatment in diabetic retinopathy result from relieving a hypoxic stimulus for neovascularization.