Measurements of intracellular pH (pHi) and of apical cell membrane potential (Va) were made in principal cells of frog skin (Rana pipiens) with double-barrel microelectrodes under open-circuit conditions. The tissues were pretreated with stilbenes (10(-3) M) and bathed in HCO3- -free NaCl Ringer solution that was buffered with 6 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (pH 7.8). Substitution of extracellular Na+ on both sides of the epithelium with N-methyl-D-glucamine caused intracellular acidification by 0.27 pH units. Restoration of Na+ on the apical side alone or on both sides caused a pHi recovery of 0.24 and 0.28 pH units, respectively, whereas return of Na+ on the basolateral side caused no recovery. Recovery of pHi on restoration of Na+ to the apical side was prevented by 10(-5) M 5-(N-ethyl-N-isopropyl)-amiloride. In individual preparations there was no correlation between pHi recovery due to return of apical Na+ and changes in Va. The average change in pHi was several times greater than the one expected from voltage clamp-induced changes in Va at constant extracellular Na+. The results suggest the presence of a Na+-H+ exchange on the apical side of principal cells. Such a process could be part of a negative feedback mechanism for regulation of Na+ entry via apical Na+ channels into principal cells.
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