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Sources of Intra-Oesophageal Nitric Oxide Production Following Intraluminal Acid Exposure

Background: The aim of the present study was to assess luminal nitric oxide (NO) levels in the oesophagus during baseline and acidic conditions and to clarify the sources of such oesophageal NO formation. Methods: Healthy volunteers received an intra-oesophageal infusion of either HCl (100 mM) or NaCl (150 mM) on two separate study days. After a low nitrate diet, nitrate load or no dietary restrictions/pretreatment, direct intraluminal measurements of NO formation were performed using a tonometric technique. Endoscopy was performed and mucosal biopsies were taken and analysed by means of immunohistochemistry, Western blot and RT-PCR. Results: No intra-oesophageal NO was detected during baseline conditions with pH neutrality. During the infusion of HCl the NO levels rose dramatically to around 12000 ppb. This high rate of NO formation fell by 95% following deviation of saliva. NO formation after an acute nitrate load was almost doubled during acid perfusion compared to control. Immunohistochemistry demonstrated distinct staining for iNOS in the oesophageal squamous epithelial cells, and Western blot and RT-PCR confirmed the presence of iNOS. Conclusion: Two sources exist for intra-oesophageal NO formation, both dependent on the luminal acidity: 1) chemical reduction of salivary nitrite, a mechanism related to dietary intake of nitrate, and 2) NO formation within the oesophageal mucosal epithelium by enzymatic degradation of L-arginine. In the latter case, the NO synthase has antigenic characteristics, indicating the inducible isoform, although a functional behaviour suggests an unconventional subtype.