Retinal pH and Acid Regulation During Metabolic Acidosis

Purpose: Changes in retinal pH may contribute to a variety of eye diseases. To study the effect of acidosis alone, we induced systemic metabolic acidosis and hypothesized that the retina would respond with altered expression of genes involved in acid/base regulation. Methods: Systemic metabolic acidosis was induced in Long-Evans rats for up to 2 weeks by adding NH4Cl to the drinking water. After 2 weeks, venous pH was 7.250.08 (SD) and [HCO3-] was 21.4 +/- 4.6mM in acidotic animals; pH was 7.41 +/- 0.03 and [HCO3-] was 30.5 +/- 1.0mM in controls. Retinal mRNAs were quantified by quantitative reverse transcription polymerase chain reaction. Protein was quantified with Western blots and localized by confocal microscopy. Retinal [H+](o) was measured in vivo with pH microelectrodes in animals subjected to metabolic acidosis and in controls. Results: NH4Cl in drinking water or given intravenous was effective in acidifying the retina. Cariporide, a blocker of Na+/H+ exchange, further acidified the retina. Metabolic acidosis for 2 weeks led to increases of 40-100% in mRNA for carbonic anhydrase isoforms II (CA-II) and XIV (CA-XIV) and acid-sensing ion channels 1 and 4 (ASIC1 and ASIC4) (all p<0.005). Expression of anion exchange protein 3 (AEP-3) and Na+/H+ exchanger (NHE)-1 also increased by 50% (both p<0.0001). Changes were similar after 1 week of acidosis. Protein for AEP-3 doubled. NHE-1 co-localized with vascular markers, particularly in the outer plexiform layer. CA-II was located in the neural parenchyma of the ganglion cell layer and diffusely in the rest of the inner retina. Conclusions: The retina responds to systemic acidosis with increased expression of proton and bicarbonate exchangers, carbonic anhydrase, and ASICs. While responses to acidosis are usually associated with renal regulation, these studies suggest that the retina responds to changes in local pH presumably to control its acid/base environment in response to systemic acidosis.