Regulation of inwardly rectifying K+ channels in retinal pigment epithelial cells by intracellular pH

Inwardly rectifying K+ (Kir) channels in the apical membrane of the retinal pigment epithelium (RPE) play a key role in the transport of K+ into and out of the subretinal space (SRS), a small extracellular compartment surrounding photoreceptor outer segments. Recent molecular and functional evidence indicates that these channels comprise Kir7.1 channel subunits. The purpose of this study was to determine whether Kir channels in the RPE are modulated by extracellular (pH(o)) or intracellular pH (pH(i)), both of which change upon illumination of the dark-adapted retina. The Kir current (I-Kir) in acutely dissociated bovine RPE cells was recorded in the whole-cell configuration while altering pH(o) or pH(i). In cells dialysed with pipette solution buffered to pH 7.2, step changes in pH(o) from 7.4 to 8.0, 7.0 or 6.5 had little effect on I-Kir. Acidification to pH(o) 6.0, however, caused a transient activation of I-Kir, followed by a slower inhibition. To determine the dependence of I-Kir on pH(i), we altered pH(i) within individual RPE cells at constant pH(o) by imposing transmembrane acetate concentration gradients. These experiments revealed a biphasic relationship between I-Kir, and pH(i):I-Kir was maximal at about pH(i) 7.1, but decreased sharply at more acidic or alkaline levels. To evaluate the role of Kir7.1 channels in the pH(i)-dependent changes in I-Kir, we tested the effect of transmembrane acetate concentration gradients on Rb+ currents, which are 10-fold larger than K+ currents for this channel subtype. Inwardly rectifying Rb+ currents were maximal at about pHi 7.0 and were inhibited by intracellular alkalinization or acidification. We conclude that the Kir conductance in the RPE is modulated by intracellular pH in the physiological range and that this reflects the behaviour of Kir7.1 channels. This sensitivity to pH(i) may provide an important mechanism linking photoreceptor activity and RPE function.