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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

CLCA protein and chloride transport in canine retinal pigment epithelium

Departments of ¹Veterinary Biomedical Sciences and ²Small Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5B4

Submitted 20 May 2003 ; accepted in final form 8 July 2003

Problems in ion and fluid transfer across the retinal pigment epithelium (RPE) are a probable cause of inappropriate accumulations of fluid between the photoreceptors of the retina and the RPE. The activities of Cl^- transporters involved in basal fluid transfer across the RPE have been compared to determine whether Ca²⁺- or cAMP-dependent channels may be responsible for basal housekeeping levels of secretory activity in this tissue. The role of a candidate Ca²⁺-dependent CLCA protein in the basal RPE transport of Cl^- has been investigated. Low concentrations of the Cl^- conductance inhibitors glibenclamide and 5-nitro-2-(3-phenylpropylamino)benzoate reduced the short-circuit current in dog RPE preparations mounted in Ussing chambers and decreased the Ca²⁺-dependent Cl^- efflux from fibroblasts expressing the pCLCA1 Cl^- conductance regulator. However, these same agents did not inhibit the rate of Cl^- release from cultured fibroblasts expressing the cystic fibrosis transmembrane regulator (CFTR) conductive Cl^- channel. Addition of ionomycin to primary cultures of canine RPE cells or to fibroblasts expressing the pCLCA1 channel regulator increased the rate of release of Cl^- from both types of cultured cells. However, the presence of pCLCA1 also increased cAMP-dependent Cl^- release from fibroblasts expressing CFTR. We conclude that Ca²⁺-dependent Cl^- transport may be more important than cAMP-dependent Cl^- transport for normal fluid secretion across the RPE. Furthermore, CLCA proteins expressed in the RPE appear to regulate the activity of other Cl^- transporters, rather than functioning as primary ion transport proteins.

chloride channel; cystic fibrosis transmembrane conductance regulator, calcium; fluid secretion

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