Furosemide-sensitive Cl transport in embryonic chicken retinal pigment epithelium

Retinal pigment epithelium- (RPE) choroid-sclera preparations from embryonic chickens were mounted in an Ussing chamber. A spontaneous transepithelial voltage (Ve) of 5.1 mV (retinal side positive) and a resistance of 114 omega . cm2 can be attributed to the RPE. Furosemide and ouabain inhibited the Ve without affecting the resistance when applied to the retinal surface of the preparation but had no effect when applied to the scleral surface. Acetazolamide and amiloride were without effect when applied to either surface. The Ve fell by 100% in Na-free medium, by 70% in Cl-free medium, and was unchanged in HCO3-free medium. When Na and Cl were returned to the medium, the Ve recovered. Dilution potentials and unidirectional flux studies indicated that the RPE is more permeable to Na than to Cl. Isotope flux studies performed under open-circuit conditions showed a net retina-to-choroid flux for both Na and Cl with the net Cl flux abolished by furosemide. Analysis of the Na+ and Cl- electrochemical gradients and the Na permeability across the RPE suggests that the net retina-to-choroid Na+ flux is largely passive, whereas the net retina-to-choroid Cl- flux results from active transport. These results indicate that embryonic chicken RPE possesses furosemide-sensitive Cl transport function. A model of embryonic chicken RPE transepithelial transport is presented.