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 microA/cm2. Concomitantly, a net Na+ transport occurs in the opposite direction with a short-circuit current from tear to stroma of about 25 microA/cm2. The net K+ transport exhibited saturation, increasing only 35% when the K+ concentration in the bathing solution was augmented five times. Cellular K+ content measured analytically after scraping off the epithelium was reduced by amphotericin B from 0.56 to 0.10 mueq. 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 four 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.