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 approximately 7.40), [H+]o steadily increased to a maximum concentration in the proximal portion of the outer nuclear layer (pH approximately 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. Stoichiometrically, one H+ is produced for each lactate produced, so we concluded that Q(OR-H+) is a measure of net rather than total H+ production. Because retinal acid production is so high, the retina must contain efficient H+ clearance and/or neutralization mechanisms that prevent severe acidosis. The effect of light on retinal extracellular [H+] and Q(OR-H+) was also examined. As expected, light adaptation caused a retinal alkalinization that resulted from a 52% reduction in Q(OR-H+). This is in agreement with previous studies that have shown that both oxidative (e.g. Haugh et al., 1990) and glycolytic metabolism (Wang et al., 1997a,c) in the photoreceptor are decreased by a factor of 2 during light adaptation. Although we could not obtain absolute values for outer retinal glycolysis, changes in Q(OR-H+) appear to directly reflect changes in glycolytic metabolism.

Retinal pH Reflects Retinal Energy Metabolism in the Day and Night

“…A similar pH gradient, in which the retina is more acidic than the superfusate, has routinely been reported in the literature, beginning with the very first attempts to characterize the retinal pH (Oakley and Wen 1989;Tsacopoulos and Levy 1976). In vivo measurements on intact cat (Padnick-Silver and Linsenmeier 2002;Yamamoto et al 1992) have also shown that the pH in the retina is lower than in the choroidal blood supply. The vertebrate retina is not unique in this respect: the brain slice preparation is also more acidic than the superfusate (Balestrino and Somjen 1988;Brockhaus et al 1993;Chen and Chesler 1992;Walz 1989).…”

“…A similar pH gradient in which the tissue has a lower pH than the superfusate has also been recorded when pH-selective microelectrodes were used to measure pH in isolated vertebrate retinas Mangel 2000, 2001;Oakley and Wen 1989). In vivo measurements also show that intact retina is more acidic than the choroidal blood supply (Padnick-Silver and Linsenmeier 2002;Yamamoto et al 1992). In fact, a consistent extrusion of acid can be detected even in the case of individual isolated retinal cells (Malchow et al 1998).…”

Effect of Hypoxemia and Hyperglycemia on pH in the Intact Cat Retina

“…7 To prevent acidosis, the tissue must possess extremely efficient H ϩ clearance/neutralization mechanisms, possibly involving retinal pigment epithelial and/or Müller cell active H ϩ and/or bicarbonate ion (HCO 3 − ) transport. Hypoxemia, a condition known to increase outer retinal glycolysis, 4,8,9 was examined to study retinal pH during hypoxemia and to aid in the interpretation of changes during hyperglycemia.…”

“…A complete experimental description was published previously. 7 All recordings were made while the animal was paralyzed and receiving urethane anesthesia. The animal was artificially ventilated at a rate and volume suitable for maintaining arterial blood gas values within a normal range (ie, PaO 2 Ͼ90 mm Hg; PaCO 2~3 0 mm Hg; 7.35ϽpH a Ͻ7.45).…”

Regulation of vascular endothelial growth factor and pigment epithelium-derived factor in rat retinal explants under retinal acidification

“…After dark adaptation, the lowest retinal pH may decrease to 7.15. 15 Thus, the acceptable pH for a healthy retina is considered to be about 7.2. Under the pathologic states, the lowest pH level retina can get is unknown.…”