Amphotericin B-induced active transport of K⁺ and the Na⁺-K⁺ flux ratio in frog corneal epithelium

Transepithelial unidirectional K⁺ fluxes across the isolated frog cornea were primarily paracellular and proportional to the K⁺ concentration in the bathing solution (from 2.5 to 25 mM). The net K⁺ flux was not different from zero. Amphotericin B (10^-5 M) elicited a large and sustained net K⁺ transport from the stroma- to the tear-side bathing solution of about 10 μA/cm². Concomitantly, a net Na⁺ transport occurs in the opposite direction with a short-circuit current from tear to stroma of about 25 μA/cm². The net K⁺ transport exhibited saturation, increasing only 35% when the K⁺ concentration in the bathing solution was augmented 5 times. Cellular K⁺ content measured analytically after scraping off the epithelium was reduced by amphotericin B from 0.56 to 0.10 μeq. The amphotericin B-induced K⁺ transport was inhibited by ouabain and low Na⁺ (5 mM) in the tear-side solution. Paracellular permeability determined with mannitol or estimated from the tear-to-stroma K⁺ flux increased 4 times with amphotericin B. From the net K⁺ transport and the short-circuit current, the Na⁺-K⁺ flux ratio was calculated and found to vary between 2.2 and possibly as high as 5.5 among corneas in the same experimental conditions. The Na⁺-K⁺ flux ratio determined in the same cornea decreased as the K⁺ concentration in the bathing solution increased. Such variability suggests that in corneal epithelial cells the Na⁺-K⁺ coupling ratio is sensitive to changes in the electrochemical gradient across the basolateral membrane of the cell.