| Organic nitrates such as nitroglycerin (NTG) are potent and safe cardiovascular drugs. Although these agents are generally considered as prodrugs of nitric oxide (NO), they differ from other NO donors in their pharmacological action as well as the development of nitrate tolerance. A number of hypotheses have been proposed as the underlying mechanism(s) of nitrate tolerance, but a unifying theory is not yet available. This thesis work, therefore, focused on generating an understanding of the biochemical reaction of NTG, a prototype organic nitrate, in the context of NTG action and vascular tolerance. We demonstrated that NTG reacted with various biochemical target sites in model biomolecules and vascular tissue, consistent with its diverse pharmacological action. Our results also indicated that NTG acts as a chemical/biochemical oxidant in reacting with the biochemical target sites. Thus, NTG induced inactivation of glutathione S-transferase (GST), its own metabolizing enzyme via oxidative reaction. This oxidative reaction appeared to involve the formation of oxidized GST dimer, possibly via −SH oxidation to form disulfide and disulfide S-oxidized product(s). The chemical reaction of NTG with glutathione also provided supportive evidence for the formation of such oxidation products including disulfide with additional oxygen atoms. Consistent with the oxidative property of NTG, the in vitro nitrate vascular tolerance appeared to involve enhanced oxidative stress. Using DNA microarray, we showed that NTG can induce widespread changes in the expression of many genes including those involved in metabolism, cellular signaling and redox regulation. We also observed that NTG can stimulate the vascular release of a neuropeptide, calcitonin gene-related peptide, offering a novel neurogenic mechanism of NTG action. This stimulatory effect of NTG appeared to involve the activation of endothelial NO synthase (eNOS), suggesting that NTG may exert its action via an eNOS-dependent mechanism. This neurogenic component of NTG action appeared to contribute to the anti-aggregatory and cardioprotective effects of NTG. These findings may contribute to a better understanding of NTG pharmacology, and thereby better use of this important drug in the future. |
