Airway hyperresponsiveness to bronchoconstrictors has been found in asthma and related to the severity of the disease. The factors which result in hyperresponsiveness are not completely understood. A possible mechanism is an imbalance between endogenous bronchoconstrictors and dilators. NO is known to relax tracheal smooth muscle by activating soluble guanylate cyclase and increasing the level of intracellular cyclic guanosine monophosphate (GMP). The first hypothesis tested was that the NO-cyclic GMP-relaxant pathway is involved in the regulation of airway responsiveness. Inhibition of endogenous nitric oxide by N$ sp{ omega}$-nitro-L-arginine (L-NNA) significantly increased airway responsiveness to inhaled methacholine in normoresponsive Lewis rats but less so in hyperresponsiveness Fisher rats. In addition, carbachol increased cyclic GMP levels in tracheal tissues from both strains; this cyclic GMP accumulation in tracheal tissues was also less in Fisher than in Lewis rats and abolished by L-NNA in both strains, indicating that it was mediated by a NO-dependent mechanism. These results suggest that endogenous NO plays a role in regulation of airway responsiveness and contributes to the strain-related difference in airway responsiveness in rats. To investigate the NO-cyclic GMP-relaxant pathway in rat airway, the effect of sodium nitroprusside (SNP, a NO donor) on airway responsiveness to a cholinergic agonist was measured in hyperresponsive Fisher rats and compared with the less responsive Lewis strain. Fisher rats were resistant to SNP as evidenced by less relaxation of carbachol contracted tracheal rings by SNP and less cyclic GMP accumulation induced by SNP in cultured airway smooth muscle cells in Fisher rats compared with Lewis rats, indicating an impaired response to SNP in Fisher airways. / NO is known to be synthesized from L-arginine in a reaction catalyzed by NO synthase (NOS). Liver cytochrome P450 also catalyzes the oxidative cleavage of C=N bonds of compounds containing a -C(NH$ sb2$)NOH function, producing NO in vitro. We hypothesized that the biosynthesis of NO in airway smooth muscle cells could result from P450 enzymes acting on appropriate substrates. NO can be synthesized in a number of lung cell types. However, to date, no constitutive form of NOS activity has been found in airway smooth muscle cells. We next examined the possibility that airway smooth muscle itself might be able to synthesize NO. Formamidoxime, a compound containing the -C(NH$ sb2$)NOH function, was found to produce NO in cultured airway smooth muscle cells. As well, formamidoxime relaxed pre-contracted trachealis and cyclic GMP accumulation in airway smooth muscle cells in culture. These effects were inhibited by P450 inhibitors but not by NOS inhibitors. Thus, an L-arginine-independent pathway for production of NO was demonstrable in airway smooth muscle cells. This NO production was catalyzed by P450 but not by NOS. / In conclusion, my studies have demonstrated an important role for endogenous NO production in determining the airway responsiveness of normal rats to inhaled cholinergic agonists. This mechanism contributes to strain-related differences in airway responsiveness in the rat.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.29052 |
| Date | January 1995 |
| Creators | Jia, Yanlin |
| Contributors | Martin, James G. (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Division of Experimental Medicine.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001467465, proquestno: NN08116, Theses scanned by UMI/ProQuest. |
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