Chloride Currents in Acutely Isolated Xenopus Retinal Pigment Epithelial Cells

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Abstract
The retinal pigment epithelium (RPE) regulates the ionic composition of the fluid surrounding the photoreceptors by transport mechanisms that utilize Cl− channels. Cl− currents in RPE cells, however, remain incompletely characterized. The purpose of this study was to identify the Cl− currents in acutely isolated Xenopus RPE cells using whole‐cell patch clamp. We describe three different Cl− currents. (1) An inwardly rectifying Cl− current, ICl,ir, activates slowly with hyperpolarization (τact=μ1 s at −80 mV, V1/2=−94 ± 3 mV), is blocked by Zn2+ (IC50=185 μm), is stimulated by acid (ICl,ir is 5 times larger at pH 6 than pH 8), and is blocked by DIDS in a voltage‐dependent manner. ICl,ir closely resembles cloned ClC‐2currents. (2) An outwardly rectifying Cl− current, ICl,Ca, is stimulated by elevated cytosolic free [Ca2+]. With 1 μm free [Ca2+]i in the patch pipette, ICl,Ca activates slowly with depolarization (τact=325 ms at 100 mV) and deactivates upon hyperpolarization. ICl,Ca is not blocked by 1 mm Zn2+ or 10 μm Gd3+ but is blocked by DIDS. High extracellular [Ca2+] (10 mm) also activates ICl,Ca. (3) A non‐rectifying current is activated by elevation of cytoplasmic cAMP with forskolin and IBMX. In addition to these three Cl− currents, Xenopus RPE cells exhibit a non‐selective background current (Ibkg) which has a linear I‐V relationship and is voltage insensitive. This current is blocked by Zn2+ (IC50 of 5.3 μm) or 10 μm Gd3+. This description provides new insights into the physiology of Cl− channels involved in salt and fluid transport by the retinal pigment epithelium.