Characterization of Cl-/HCO3- exchange in cultured bovine pigmented ciliary epithelium

Many recent data indicate that transport of Cl- across the ciliary epithelium plays an important role in aqueous humor formation. We used 36Cl to investigate the pathways for Cl- transport in confluent monolayers of cultured bovine pigmented ciliary epithelial cells. Cl- uptake mainly occurred via a mechanism with typical characteristics of an anion exchanger, and could be stimulated by an outwardly directed HCO3- gradient. One mM SITS and 1 mM DIDS inhibited Cl- uptake by some 80-90%, the latter with an IC50 of about 20 microM. HCO3- stimulated Cl- uptake could be partly inhibited for furosemide and to a lesser extent by bumetanide, indicating an action of loop-diuretics on the anion exchanger. 36Cl- uptake was cis-inhibited by the halides Cl-, I- and Br-, by NO3-, formate and acetate. Inhibition of Cl- uptake by extracellular HCO3- was less effective in the absence of extracellular Na+, suggesting that not only HCO3- but also NaCO3- binds to the carrier. SO2/4-, cyclamate and gluconate did not significantly reduce Cl- uptake via the anion exchanger. DIDS-senstive Cl- uptake showed saturation kinetics with respect to the Cl- concentration with an apparent Km of 8 mM. Cl- efflux could be stimulated by external Cl- and HCO3- and was inhibited by DIDS. Thus, cultured bovine pigmented ciliary epithelial cells express a Cl-/HCO3- exchanger. A possible role of this carrier system for aqueous humor formation is discussed [corrected].