Journal
JOURNAL OF PHYSIOLOGY-LONDON
Volume 549, Issue 2, Pages 453-469Publisher
WILEY
DOI: 10.1113/jphysiol.2003.040428
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Funding
- NIGMS NIH HHS [GM-60448, R01 GM060448] Funding Source: Medline
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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, I-Cl,I-ir, activates slowly with hyperpolarization (tau(act) = similar to1 s at -80 mV, V-1/2 = -94 +/- 3 mV), is blocked by Zn2+ (IC50 = 185 muM), is stimulated by acid (I-Cl,I-ir is 5 times larger at pH 6 than pH 8), and is blocked by DIDS in a voltage-dependent manner. I-Cl,I-ir closely resembles cloned ClC-2currents. (2) An outwardly rectifying Cl- current, I-Cl,I-Ca is stimulated by elevated cytosolic free [Ca2+]. With 1 mum free [Ca2+](i) in the patch pipette, I-Cl,I-Ca activates slowly with depolarization (tau(act) =325 ms at 100 mV) and deactivates upon hyperpolarization. I-Cl,I-Ca is not blocked by 1 mm Zn2+ or 10 mum Gd3+ but is blocked by DIDS. High extracellular [Ca2+] (10 mm) also activates I-Cl,I-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 (I-bkg) which has a linear I-V relationship and is voltage insensitive. This current is blocked by Zn2+ (IC50 of 5.3 mum) or 10 mum Gd3+. This description provides new insights into the physiology of Cl- channels involved in salt and fluid transport by the retinal pigment epithelium.
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