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 muA/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 (outleads toin) produced across the short-circuited epithelium a Li+ influx of 11.8 muA/cm2 and a mean short-circuit current of 10.2 muA/cm2. The same Li+ gradient in the opposite direction produced a Li+ outflux of only 1.9 muA/cm2. With equal Li+ concentration (10.3 and 20.6 mM) on both sides of the epihelium, plus Na+ in the inside solution only, a stable Li+-dependent short-circuit current was observed. Net Li+ movement (outleads toin) was also indirectly determined in the presence of an opposing Li+ gradient. Although Li+ does not substitute for Na+ as an activator to 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.