pH modulates cAMP-induced increase in Na+ transport across frog skin epithelium

Apical membrane potential (Va), fractional apical membrane resistance (FRa), and/or intracellular pH (pHi) were measured in principal cells of isolated frog (Rana pipiens) skin with microelectrodes under short-circuit conditions. Apical exposure to 0.33 mM 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (cAMP) depolarized Va, decreased FRa and increased short-circuit current (Isc). cAMP-induced 50% larger effects on Va and Isc at external pH (pHo) of 8.0 than at pHo 6.4. Increasing pHo from 6.4 to 8.0 in presence of cAMP further depolarized Va and increased Isc. cAMP-induced effects on Va and Isc were observed in the absence of Cl- and HCO3- and in the presence of 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) or 10 microM 5-(N-ethyl-N-isopropyl)amiloride (EIPA) or 1 microM 5-(N-methyl-N-isobutyl)amiloride (MIA). These data indicate that Na(+)-H+ exchange, Cl(-)-HCO3- exchange, and electrogenic Na(+)-(HCO3-)n cotransport are not involved in cAMP-induced increase in Isc. Apical exposure to 2 mM Cd2+ or Zn2+ depolarized Va, decreased FRa, increased Isc and increased pHi. In HCO(3-)-free solutions containing DIDS, unilateral replacement of apical Cl- by NO3- induced a fast transient depolarization of Va and an increase in Isc. These data suggest that potential-dependent changes in pHi are involved in increases in Isc. However, when changes in Va were minimized by pretreating the basolateral membrane with 25 or 75 mM K+, the cAMP-induced increase in Isc was not blocked. These data indicate that changes in pHi do not play a strict regulatory role but are only permissive in cAMP-induced effects on Isc.