Effects of bicarbonate ion on chick retinal pigment epithelium: membrane potentials and light-evoked responses

The purpose of this study was to determine how changes in [HCO3-] alter the electrical properties of the retinal pigment epithelium (RPE). Experiments were conducted on the isolated chick retina-RPE-choroid preparation. The chamber holding the preparation allowed independent perfusion of the retinal and the choroidal surfaces. The light-evoked trans-tissue potential (TTP), the trans-epithelial potential (TEP), the trans-retinal potentials, and the intracellularly-recorded apical and basal membrane potentials were studied. Increasing the [HCO3-]0 in the choroidal bath from 25 to 40 mEq/1 led to an increase in the TTP and TEP. The same change in the retinal bath decreased the TTP because of a biphasic change of the RPE membrane potentials. There was also an increase in the amplitudes of the TEP, the c-wave and the slow PIII. The light-evoked subretinal K+ decrease was greater which is consistent with an increase in the photoreceptor light response. These observations indicated that the decrease of TTP resulted from a basal membrane hyperpolarization followed by an apical membrane depolarization induced by an increase in retinal [HCO3-]0. The relationship of these potential changes to the human bicarbonate responses is discussed.