PH MODULATES CAMP-INDUCED INCREASE IN NA+ TRANSPORT ACROSS FROG-SKIN EPITHELIUM

Apical membrane potential (V-a), fractional apical membrane resistance (FR(a)), and/or intracellular pH (pH(i)) 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 V-a, decreased FR(a) and increased short-circuit current (I-sc). cAMP-induced 50% larger effects on V-a and I-sc at external pH (pH(0)) of 8.0 than at pH(0) 6.4. Increasing pH(0) from 6.4 to 8.0 in presence of cAMP further depolarized V-a and increased I-sc. cAMP-induced effects on V-a and I-sc 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 mu M 5-(N-ethyl-N-isopropyl)amiloride (EIPA) or 1 mu M 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 I-sc. Apical exposure to 2 mM Cd2+ or Zn2+ depolarized V-a, decreased FR(a), increased I-sc and increased pH(i). In HCO3- -free solutions containing DIDS, unilateral 3 replacement of apical Cl- by NO3- induced a fast transient depolarization of V-a and an increase in I-sc. These data suggest that potential-dependent changes in pH(i) are involved in increases in I-sc. However, when changes in V-a were minimized by pretreating the basolateral membrane with 25 or 75 mM K+, the cAMP-induced increase in I-sc was not blocked. These data indicate that changes in pH(i), do not play a strict regulatory role but are only permissive in cAMP-induced effects on I-sc.