Salt-induced changes in sodium transport across the skin of the euryhaline toad, Bufo viridis.

1. Euryhaline toads (Bufo viridis) were adapted to either distilled water, 115 or 200 mM-NaCl solutions. Na transport across the skin of these animals was studied in in vitro preparations. 2. Salinity adaptation caused a reduction in transepithelial potential and short-circuit current across the skin, and increased its electrical resistance. 3. Na influx across the skin was decreased. The reduction accounted fairly well for the measured reduction in short-circuit current. 4. Short-term Na uptake at the outer barrier of the skin was greatly reduced in preparations from salt adapted animals. Amiloride, which usually inhibits Na uptake, did not affect it in skins from salt adapted toads. 5. Sodium conductance decreased by a factor of three in skins from 115 mM-NaCl adapted toads, and to zero in skins from 200 mM-NaCl adapted animals. Shunt conductance was only little affected by the salinity of adaptation. 6. 'Amiloride-sensitive component' of transepithelial Na transport was 55, 21-5, and 4-5 muA/cm-2 in skins from toads which were adapted to distilled water, 115, and 200 mM-NaCl solutions, respectively. 7. It is concluded that the effect of salinity adaptation on the skin of the toad is to reduce the number of Na selective sites at the outer barrier. This change is reflected in a decrease in the 'amiloride-sensitive component' of Na transport across the skin, and could not be increased by aldosterone. Other changes may occur as secondary to that.