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

Abstract

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 o) of 8.0 than at pH o 6.4. Increasing pH o 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 HCO 3 − and in the presence of 1 mM 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS) or 10 μM 5-( N-ethyl- N-isopropyl)amiloride (EIPA) or 1 μM 5-( N-methyl- N-isobutyl)amiloride (MIA). These data indicate that Na +-H + exchange, Cl −-HCO 3 − exchange, and electrogenic Na +-(HCO 3 −) n cotransport are not involved in cAMP-induced increase in I sc. Apical exposure to 2 mM Cd 2+ or Zn 2+ depolarized V a, decreased FR a, increased I sc and increased pH i. In HCO 3 −-free solutions containing DIDS, unilateral replacement of apical Cl − by NO 3 − 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.