CO ₂-induced ion and fluid transport in human retinal pigment epithelium

Abstract

In the intact eye, the transition from light to dark alters pH, [Ca ²⁺], and [K] in the subretinal space (SRS) separating the photoreceptor outer segments and the apical membrane of the retinal pigment epithelium (RPE). In addition to these changes, oxygen consumption in the retina increases with a concomitant release of CO ₂ and H ₂O into the SRS. The RPE maintains SRS pH and volume homeostasis by transporting these metabolic byproducts to the choroidal blood supply. In vitro, we mimicked the transition from light to dark by increasing apical bath CO ₂ from 5 to 13%; this maneuver decreased cell pH from 7.37 ± 0.05 to 7.14 ± 0.06 ( n = 13). Our analysis of native and cultured fetal human RPE shows that the apical membrane is significantly more permeable (≈10-fold; n = 7) to CO ₂ than the basolateral membrane, perhaps due to its larger exposed surface area. The limited CO ₂ diffusion at the basolateral membrane promotes carbonic anhydrase–mediated HCO ₃ transport by a basolateral membrane Na/nHCO ₃ cotransporter. The activity of this transporter was increased by elevating apical bath CO ₂ and was reduced by dorzolamide. Increasing apical bath CO ₂ also increased intracellular Na from 15.7 ± 3.3 to 24.0 ± 5.3 mM ( n = 6; P < 0.05) by increasing apical membrane Na uptake. The CO ₂-induced acidification also inhibited the basolateral membrane Cl/HCO ₃ exchanger and increased net steady-state fluid absorption from 2.8 ± 1.6 to 6.7 ± 2.3 µl × cm ⁻² × hr ⁻¹ ( n = 5; P < 0.05). The present experiments show how the RPE can accommodate the increased retinal production of CO ₂ and H ₂O in the dark, thus preventing acidosis in the SRS. This homeostatic process would preserve the close anatomical relationship between photoreceptor outer segments and RPE in the dark and light, thus protecting the health of the photoreceptors.