Potassium-dependent volume regulation in retinal pigment epithelium is mediated by Na,K,Cl cotransport.

Abstract

Changes in retinal pigment epithelial (RPE) cell volume were measured by monitoring changes in intracellular tetramethylammonium (TMA) using double-barreled K-resin microelectrodes. Hyperosmotic addition of 25 or 50 mM mannitol to the Ringer of the apical bath resulted in a rapid (-30 s) osmometric cell shrinkage. The initial cell shrinkage was followed by a much slower (minutes) secondary shrinkage that is probably due to loss of cell solute. When apical [K +] was elevated from 2 to 5 mM during or before a hyperosmotic pulse, the RPE cell regulated its volume by reswelling towards control within 3-10 min. This change in apical [K +] is very similar to the increase in subretinal [K+]o that occurs after a transition from light to dark in the intact vertebrate eye. The K-dependent regulatory volume increase (RVI) was inhibited by apical Na removal, CI reduction, or the presence of bumetanide. These results strongly suggest that a Na(K),CI cotransport mechanism at the apical membrane mediates RVI in the bullfrog RPE. A unique aspect of this cotransporter is that it also functions at a lower rate under steady-state conditions. The transport requirements for Na, K, and C1, the inhibition of RVI by bumetanide, and thermodynamic calculations indicate that this mechanism transports Na, K, and CI in the ratio of 1:1:2.

The Retinal Pigment Epithelium in Visual Function

“…At the apical membrane this gradient facilitates uptake of HCO 3 Ϫ via the Na ϩ -HCO 3 Ϫ cotransporter and uptake of K ϩ and Cl Ϫ via the Na ϩ -K ϩ -2Cl Ϫ cotransporter (5,236,267,290,295,297,298,327,346). Accumulation of Cl Ϫ results in a high intracellular Cl Ϫ activity of ϳ20 -60 mM (290, 325 , 402, 477, 632).…”

Molecular Physiology of Bestrophins: Multifunctional Membrane Proteins Linked to Best Disease and Other Retinopathies

“…Transepithelial Cl Ϫ transport is the driving force for control of hydration and composition of the subretinal space between photoreceptors and the RPE (61). Although the exact mechanisms by which fluid transport is coupled to electrolyte transport remain poorly understood, it seems clear that the channels and transporters that mediate RVD also participate in fluid transport across the epithelium (1,2,18,61,81,93,107). Fluid in the subretinal space is absorbed by the RPE and transported towards the choroid at a rate of several microliters per square centimeter per hour (62).…”

Impact of SLC6A Transporters in Physiological Taurine Transport at the Blood–Retinal Barrier and in the Liver

“…Earlier studies showed that the volume of cells is controlled through the loss or gain of osmolyte, such as inorganic ions. 38,39) However, it is notable that even slight alterations in cellular inorganic ion levels can lead to changes in membrane potential, the rate of enzymatic reactions and membrane transport. 40) This suggests the importance of organic molecules in cell volume regulation because of their compatibility or non-perturbation, and taurine has been assumed to be an ideal organic osmolyte in cell volume regulation since taurine is at high concentrations in the retina.…”