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

Initiation of intestinal Na+-glucose cotransport results in transient cell swelling and sustained increases in tight junction permeability. Since Na+/H+ exchange has been implicated in volume regulation after physiological cell swelling, we hypothesized that Na+/H+ exchange might also be required for Na+-glucose cotransport-dependent tight junction regulation. In Caco-2 monolayers with active Na+-glucose cotransport, inhibition of Na+/H+ exchange with 200 muM 5-(N,N-dimethyl)amiloride induced 36 +/- 2% increases in transepithelial resistance (TER). Evaluation using multiple Na+/H+ exchange inhibitors showed that inhibition of the Na+/H+ exchanger 3 (NHE3) isoform was most closely related to TER increases. TER increases due to NHE3 inhibition were related to cytoplasmic acidification because cytoplasmic alkalinization with 5 mM NH4Cl prevented both cytoplasmic acidification and TER increases. However, NHE3 inhibition did not affect TER when Na+-glucose cotransport was inhibited. Myosin II regulatory light chain (MLC) phosphorylation decreased up to 43 +/- 5% after inhibition of Na+/H+ exchange, similar to previous studies that associate decreased MLC phosphorylation with increased TER after inhibition of Na+-glucose cotransport. However, NHE3 inhibitors did not diminish Na+-glucose cotransport. These data demonstrate that inhibition of NHE3 results in decreased MLC phosphorylation and increased TER and suggest that NHE3 may participate in the signaling pathway of Na+-glucose cotransport-dependent tight junction regulation.