Cellular and paracellular pathway resistances in the “tight” Cl−-secreting epithelium of rabbit cornea

Abstract

The high transverse resistance of the isolated rabbit cornea (6–12 kΩ·cm²) is associated with the corneal epithelium, a Cl⁻-secreting tissue which is modulated by β-adrenergic and serotonergic receptors. Three methods were employed to determine the resistances for the apical membrane, basolateral membrane, and paracellular conductive pathways in the epithelium. In the first method, the specific resistance of the apical membrane was selectively and reversibly changed. Epinephrine was used to increase apical Cl⁻ conductance and Ag⁺ was used to increase apical cation permeability. The second method utilized a direct measure of the spontaneous cellular ionic current. The third method obtained estimates of shunt resistance using transepithelial electrophysiological responses to changes in apical membrane resistance. The results of the first method were largely independent of the agent used. In addition, the three methods were in general agreement, and the ranges of mean values for apical membrane, basolateral membrane, and shunt resistances were 23–33, 3–4, and 12–16 kΩ·cm², respectively, for the normal cornea. The apical membrane was the major, physiologically-modulated barrier to ion permeation. The shunt resistance of the corneal epithelium was comparable to that found previously for other “tight” epithelia. Experiments using Ag⁺ in tissues that were bathed in Cl⁻ and HCO₃-free solutions indicated that under resting conditions the apical membrane is anion-selective.