Na⁺-K⁺ pump stoichiometry and basolateral membrane permeability of frog corneal epithelium

The Na⁺-K⁺ pump flux ratio and the Na⁺ and K⁺ permeability of the basolateral membrane of the isolated frog corneal epithelium were studied with the aid of microelectrodes by analyzing the effects of ouabain, Ba²⁺, and amphotericin B. The experiments were done in Cl^--free solutions, a situation that approximates that of static head. Ouabain produced a quick depolarization of the potential difference across the basolateral membrane (PD(b)) from -72 to -62 mV without a change in resistance. Ba²⁺ (3 mM) rapidly lowered PD(b) from -74 to -57 mV and decreased the apical-to-basolateral resistance ratio. The effects of ouabain and Ba²⁺ were additive. The Na⁺-K⁺ flux ratio at the pump was calculated to be 1.78, substantially less than when the tissue is in a level flow condition, suggesting a variable stoichiometry. The K⁺ and Na⁺ resistances of the basolateral membrane were 15.7 and 5.5 kΩ·cm², respectively, allowing K⁺ and Na⁺ currents that approximately matched those produced by the Na⁺-K⁺ pump. The resistance of the basolateral membrane (4.0 kΩ·cm²) was double that reported in Cl^--rich solutions, suggesting that Cl^- contributes to the conductance of this membrane.