The effect of aldosterone on sodium transport and membrane conductances in toad skin (Bufo viridis)

The effect of aldosterone (1 mol/l for 4–6 h) on Na⁺ transport across toad skin (Bufo viridis) was studied in skins preincubated in vitro. Short-circuit current (I_sc) was consistently and reproducibly elevated in skins from NaCl(100 and 200 mmol/l)-acclimated toads, where the baseline I_sc was greatly reduced. The effect of aldosterone was tested in NaCl and NaNO₃ Ringer's and also after oxytocin (50 mU/ml) in the latter conditions. Apical membrane conductance of the principal cells increased consistently after aldosterone in all skins and was linearly correlated with the I_sc under all conditions. This confirms that the stimulation of Na⁺ transport originates from the effect of the aldosterone on apical Na⁺ channels. Basolateral membrane conductance was also significantly elevated compared with control pieces in those tissues that were preincubated with aldosterone for 4–6 h. The increase, however, did not correlate with the magnitude of the I_sc. It is therefore concluded that aldosterone specifically stimulates, in addition to the apical effect, the basolateral membrane conductance. This stimulation appears to be direct and not a secondary response to the elevation of transepithelial transport rate.