['Nitric oxide (NO.) increases the permeability of cultured intestinal epithelial monolayers. NO. reacts with superoxide anion to form peroxynitrite anion, which can be protonated under mildly acidic conditions to form the potent and versatile oxidizing agent, peroxynitrous acid. We hypothesized that intracellular acidosis induced by the Na(+)-H+ antiport blocker, amiloride, would favor the formation of peroxynitrous acid and thereby augment hyperpermeability induced by the NO. donor, SIN-1.', 'Caco-2BBe human intestinal epithelial monolayers were grown on permeable supports in bicameral chambers. The permeability of monolayers was assessed by measuring the transepithelial flux of fluorescein disulfonic acid (FS).', 'Incubation of monolayers with SIN-1 increased permeability to FS. Adding amiloride augmented SIN-1-induced hyperpermeability. SIN-1 plus amiloride also decreased cellular adenosine triphosphate content and caused derangements of the actin-based cytoskeleton as demonstrated by fluorescence microscopy. Coincubation of monolayers with several free-radical or peroxynitrous acid scavengers (deferoxamine, mannitol, dimethyl sulfoxide, or ascorbate) ameliorated hyperpermeability induced by SIN-1 plus amiloride.'] Is nitric oxide-induced hyperpermeability of human intestinal epithelial monolayers augmented by inhibition of the amiloride-sensitive Na ( + ) -H+ antiport : potential role of peroxynitrous acid?