Lithium transport across isolated frog skin epithelium

Transepithelial Li⁺ influx was studied in the isolated epithelium from abdominal skin of Rana catesbeiana. With Na⁺-Ringer's as inside medium and Li⁺-Ringer's as outside medium, the Li⁺ influx across the epithelium was 15.6 μA/cm². This influx was considerably reduced by removal of either Na⁺ or K⁺ from the inside bath or by the addition of ouabain or amiloride. Epithelial K⁺ or Na⁺ concentration was respectively lower in epithelia bathed in K⁺-free Ringer's or Na⁺-free Ringer's. In conditions of negligible Na⁺ transport, a 20 mm Li⁺ gradient (out→in) produced across the short-circuited epithelium a Li⁺ influx of 11.8 μA/cm² and a mean short-circuit current of 10.2 μA/cm². The same Li⁺ gradient in the opposite direction produced a Li⁺ outflux of only 1.9 μA/cm². With equal Li⁺ concentration (10.3 and 20.6 mm) on both sides of the epithelium, plus Na⁺ in the inside solution only, a stable Li⁺-dependent short-circuit current was observed. Net Li⁺ movement (out→in) was also indirectly determined in the presence of an opposing Li⁺ gradient. Although Li⁺ does not substitute for Na⁺ as an activator of the (Na⁺+K⁺)-ATPase from frog skin epithelium, Li⁺ influx appears to be related to Na⁺-K⁺ pump activity. It is proposed that the permeability of the "outer barrier" to Na⁺ and Li⁺ is regulated by the electrical gradient produced by electrogenic Na⁺-K⁺ pumps located in the membrane of the deeper epithelial cells.