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Summary 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 (J13Cl) and efflux (J31Cl), short-circuit current, transepithelial potential, and conductance (Gm).J13Cl as well asJ31Cl were found to have a saturating component and a component which is apparently linear with Cl concentration. The linear component ofJ31Cl appears only upon addition of Cl to external medium, and about 3/4 of this component does not contribute toGm. The saturating component ofJ31Cl is only 5% of totalJ31Cl with 115mm Cl in the serosal medium. Replacement of 115mm Cl- in external medium by SO4=, NO3-, HCO3- or I- results in 87–97% reduction ofJ31Cl, whereas replacement with Br- has no effect. As external Cl concentration is raised in steps from 2 to 115mm,J13Cl andJ31Cl increase by the same amount butJ13Cl is persistently 0.15 µeq/cm2 hr larger thanJ31Cl. These results indicate that at least 3/4 of linear components ofJ13Cl andJ31Cl proceed via an exchange diffusion mechanism which seems to be located at the outer cell border. The saturating component ofJ13Cl 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. |
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