Validation of microdialysis probes for studying nitrosative chemistry within localized regions of the human upper gastrointestinal tract

Background: We have examined the suitability of microdialysis probes for examining nitrosative chemistry within localized regions of the lumen of the gastrointestinal tract. Chemical nitrosation occurs maximally at pH 2.5 in the presence of nitrite and thiocyanate and absence of ascorbic acid. Nitrite and thiocyanate are delivered into the stomach in saliva and ascorbic acid is secreted in gastric juice. Methods: We used a benchtop model to reproduce the nitrosative chemistry occurring in the human upper GI tract and assessed the ability of the microdialysis probes to measure it. Results: The microdialysis probes were reliable at measuring nitrite ascorbic acid, total vitamin C and thiocyanate in both aqueous solutions and human gastric juice over the full range of intragastric pH, i.e. 1.5, 2.5, 3.5, 5.0 and 7.0. The probes were also reliable at measuring these chemicals under conditions simulating the active interaction between nitrite and ascorbic acid. Under such conditions with ascorbic acid in excess the probes gave a more accurate assessment of the nitrite level than that obtained by directly sampling the gastric juice. This was due to the probes not being subject to artefactual measurement of nitric oxide as nitrite. This was prevented by the rapid diffusion of nitric oxide through the probe collecting tube. Conclusion: Microdialysis probes provide a reliable means of examining nitrosative chemistry within the lumen of the upper GI tract. In addition, they have the advantage of measuring this chemistry in very local regions and of simultaneously comparing the chemistry in different regions of the upper GI tract.