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Received January 29, 2003
Accepted after revision March 14, 2003
1 Department of Cell Biology, Whitehead Biomedical Research Building 535, 615 Michael St., Emory University School of Medicine, Atlanta, GA 30322-3030 USA
2 Department of Cell Biology, Whitehead Biomedical Research Building 535, 615 Michael Street, Emory University School of Medicine, Atlanta, GA 30322-3030, USA
* To whom correspondence should be addressed. E-mail: criss{at}cellbio.emory.edu.
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 (
act = ~1 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 (act =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.
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