Transepithelial resistance can be regulated by the intestinal brush-border Na(+)/H(+) exchanger NHE3

Initiation of intestinal Na⁺-glucose cotransport results intransient cell swelling and sustained increases in tight junction permeability. Since Na⁺/H⁺ exchange has beenimplicated in volume regulation after physiological cell swelling, wehypothesized that Na⁺/H⁺ exchange might also berequired for Na⁺-glucose cotransport-dependent tightjunction regulation. In Caco-2 monolayers with activeNa⁺-glucose cotransport, inhibition ofNa⁺/H⁺ exchange with 200 µM5-(N,N-dimethyl)- amiloride induced 36 ± 2% increases in transepithelial resistance (TER). Evaluation using multiple Na⁺/H⁺ exchange inhibitors showed thatinhibition of the Na⁺/H⁺ exchanger 3 (NHE3)isoform was most closely related to TER increases. TER increases due toNHE3 inhibition were related to cytoplasmic acidification becausecytoplasmic alkalinization with 5 mM NH₄Cl prevented bothcytoplasmic acidification and TER increases. However, NHE3 inhibitiondid not affect TER when Na⁺-glucose cotransport wasinhibited. Myosin II regulatory light chain (MLC) phosphorylationdecreased up to 43 ± 5% after inhibition ofNa⁺/H⁺ exchange, similar to previous studiesthat associate decreased MLC phosphorylation with increased TER afterinhibition of Na⁺-glucose cotransport. However, NHE3inhibitors did not diminish Na⁺-glucose cotransport. Thesedata demonstrate that inhibition of NHE3 results in decreased MLCphosphorylation and increased TER and suggest that NHE3 may participatein the signaling pathway of Na⁺-glucosecotransport-dependent tight junction regulation.