Distribution of glucose and lactate in the interphotoreceptor matrix

The photoreceptor cells of the retina derive their nourishment from the choroidal blood supply behind the retina. If the extracellular interphotoreceptor matrix (IPM) mediates this retinal nutrition, then gradients in concentrations of nutrients such as glucose and waste products such as lactic acid would be expected across the IPM, from the neural retina to the retinal pigment epithelium (RPE); this hypothesis is examined here. Gentle methods were employed to prepare IPM from the retinal surface (IPM-R) and apical RPE surface (IPM-P) of bovine eyes, and contamination from adjacent tissues was very low. The actual intrinsic volumes of the IPM-R (180 microliters) and IPM-P (87 microliters) compartments were estimated from dilution of trypan blue dye added to the wash buffer used for sample preparation; these measurements were required for calculation of concentrations. In the IPM-R samples glucose was virtually undetectable and the lactate concentration was very high (13 mM). Near the RPE surface (IPM-P) both compounds were present at levels (glucose 0.9 mM, lactate 3.8 mM) more comparable to those in blood serum. Similar results were obtained with fresh rabbit eyes. This difference in distribution (nutrient more concentrated near RPE, waste product near retina) indicates gradients consistent with the utilization of the IPM as a pathway for outer-retinal nutrition.