Tissue-specific regulation of sodium/proton exchanger isoform 3 activity in Na⁺/H⁺ Exchanger Regulatory Factor 1 (NHERF1) null mice: cAMP inhibition is differentially dependent on NHERF1 and exchange protein directly activated by cAMP in ileum versus proximal tubule

The multi-PDZ domain containing protein Na⁺/H⁺ Exchanger Regulatory Factor 1 (NHERF1) binds to Na⁺/H⁺ exchanger 3 (NHE3) and is associated with the brush border (BB) membrane of murine kidney and small intestine. Although studies in BB isolated from kidney cortex of wild type and NHERF1^-/- mice have shown that NHERF1 is necessary for cAMP inhibition of NHE3 activity, a role of NHERF1 in NHE3 regulation in small intestine and in intact kidney has not been established. Here a method using multi-photon microscopy with the pH-sensitive dye SNARF-4F (carboxyse-minaphthorhodafluors-4F) to measure BB NHE3 activity in intact murine tissue and use it to examine the role of NHERF1 in regulation of NHE3 activity. NHE3 activity in wild type and NHERF1^-/- ileum and wild type kidney cortex were inhibited by cAMP, whereas the cAMP effect was abolished in kidney cortex of NHERF1^-/- mice. cAMP inhibition of NHE3 activity in these two tissues is mediated by different mechanisms. In ileum, a protein kinase A (PKA)-dependent mechanism accounts for all cAMP inhibition of NHE3 activity since the PKA antagonist H-89 abolished the inhibitory effect of cAMP. In kidney, both PKA-dependent and non-PKA-dependent mechanisms were involved, with the latter reproduced by the effect on an EPAC (exchange protein directly activated by cAMP) agonist (8-(4-chlorophenylthio)-2′O-Me-cAMP). In contrast, the EPAC agonist had no effect in proximal tubules in NHERF1 ^-/- mice. These data suggest that in proximal tubule, NHERF1 is required for all cAMP inhibition of NHE3, which occurs through both EPAC-dependent and PKA-dependent mechanisms; in contrast, cAMP inhibits ileal NHE3 only by a PKA-dependent pathway, which is independent of NHERF1 and EPAC.