Quantification of in vivo anaerobic metabolism in the normal cat retina through intraretinal pH measurements

Abstract

We examined intraretinal [H⁺] in the intact retina of anesthetized cats using H⁺-sensitive microelectrodes to obtain spatial profiles of extracellular [H⁺]. One H⁺ is produced when an anaerobically generated ATP is utilized. We theorized that H⁺ production directly reflects anaerobic glucose consumption. From the choroid (pH ∼7.40), [H⁺]_o steadily increased to a maximum concentration in the proximal portion of the outer nuclear layer (pH∼7.20). The shape of the profile was always concave down, indicating that a net production of H⁺ occurred across the avascular outer retina. A three-layer diffusion model of the outer retina was developed and fitted to the data to quantify photoreceptor H⁺ extrusion into the extracellular space (Q_OR-H⁺). It was determined that the outer segment (OS) layer had negligible H⁺ extrusion. The data were then refitted to a special three-layer model in which the OS layer Q_H⁺ was set equal to zero, but in which the inner segments and outer nuclear layer produced H⁺. The resulting Q_OR-H⁺ was several orders of magnitude lower than previous measurements of Q_OR-lactate, which were based on choroidal mass balances of lactate.