Helicobacter pylori interference with the L-arginine/NO pathway in the upper gut

Abstract

Helicobacter pylori causes chronic gastritis and sometimes peptic ulcers but the pathogenesis is not fully understood. H. pylori-infected individuals with duodenal ulcer have an impaired duodenal mucosal bicarbonate secretion (DMBS) in response to acid. NO is an important signalling molecule in the body. It is also a toxic substance when released in high doses by the host defence system against invading microorganisms. In addition, NO has been suggested to be a mediator of the acid-induced DMBS. NO is generated from L-arginine by NO synthase (NOS). Reduction of swallowed nitrite can also generate NO in the presence of acid. The general aim of this thesis was to investigate if H. pylori interferes with the L-arginine/NO synthesis. An inhibition of the NO synthesis could be a mechanism to avoid the host defence and, moreover, cause an impairment of the mucosal response to duodenal acid. The specific aims were to develop a bioassay for investigation of the effects on the L-arginine/NO pathway of different H. pylori strains and to compare the NO synthesis in H. pylori-infected to the syntesis in non-infected humans. Effects of H. pylori protein extracts on DMBS were explored in rat and found to be inhibiting in an L-arginine-dependent manner. The concentrations of the NOS inhibitor asymmetrical dimethyl arginine (ADMA) were assessed by means of an HPLC technique and found to be increased in the extract-exposed duodenal tissue. A bioassay was developed and evaluated. Murine macrophages were grown and stimulated by lipopolysaccharide and interferon gamma to produce NO. The effect of H. pylori on the NO synthesis was measured by means of chemiluminescence technique. Both a water extract and a whole-cell suspension inhibited the NO production but the expression of inducible NOS (iNOS) was not affected, as assessed by western blotting. NO production, ADMA concentration and iNOS expression were studied in human stomach. Nonenzymatic NO synthesis was characterised in order to avoid it when assessing the NOS-derived NO production. Infected individuals had increased ADMA concentrations and increased levels of iNOS in antral biopsy specimens compared to controls. The basal NO production did not differ significantly from controls but topical administration of the NOS-substrate L-arginine doubled the production in infected and had no effect in non-infected individuals. It is suggested that H. pylori infection induces inhibition of the mucosal NO production, probably to escape the toxic effects of NO produced by the host defence. In addition, the regulation of DMBS is impaired which may contribute to the development of ulcer disease.