Influence of extracellular Cl concentration on Cl transport across isolated skin of Rana pipiens.

The effect of changes in Cl concentration in the external and/or serosal bath on Cl transport across short-circuited frog skin was studied by measurements of transepithelial Cl influx (JCl13) and efflux (JCl31), short-circuit current, transepithelial potential, and conductance (Gm). JCl13 as well as JCl31 were found to have a saturating component and a component which is apparently linear with Cl concentration. The linear component of JCl31 appears only upon addition of Cl to external medium, and about 3/4 of this component does not contribute to Gm. The saturating component of JCl31 is only 5% of total JCl31 with 115 mM Cl in the serosal medium. Replacement of 115 mM Cl- in external medium by SO4=, NO3-, HCO-3 or I- results in 87-97% reduction of JCl31, whereas replacement with Br- has no effect. As external Cl concentration is raised in steps from 2 to 115 mM, JCl13 and JCl31 increase by the same amount but JCl13 is persistently 0.15 mu eq/cm2hr larger than JCl31. These results indicate that at least 3/4 of linear components of JCl13 and JCl31 proceed via an exchange diffusion mechanism which seems to be located at the outer cell border. The saturating component of JCl13 is involved in active Cl transport in an inward direction, and there is evidence suggesting that Cl uptake across outer cell border, which proceeds against an electrochemical gradient, is electroneutral but not directly linked to Na.