Vascular pharmacology of nitric oxide synthase inhibitors

This dissertation examined the effects of nitric oxide synthase (NOS) inhibitors, NGsubstituted arginine analogues (NSAAs) including NGnitroarginine (NNA) and its methyl ester (NAME), as well as diphenyleneiodonium (DPI), on endothelium-dependent vasodilatation and pressor response in rats. All NSAAs caused complete, prolonged and “noncompetitive” inhibition of acetyicholine (ACh)-, the calcium ionophore A 23187- and/or bradykinin-induced relaxations in vitro and ex vivo, with lower potencies for the D-enantiomers. The Hill coefficient (n)s for the inhibition of NSAAs, were not significantly different from 1. The inhibition of NSAAs was antagonized by either pre- or post-treatment with L-arginine (L-Arg). L-NAME partially inhibited ACh-induced depressor response in vivo. NSAAs caused long-lasting pressor responses in conscious rats. The ns of the dose-pressor response curves of L-NNA, L-NAME and D-NAME were 2 or more. The pressor response to L-NNA or L-NAME was not attenuated by pithing, or treatment with mecamylamine, reserpine, phentolamine, captopril, or indomethacin, while those to NSAAs were attenuated by L-Arg; L-Arg competitively antagonized the pressor response to L-NAME at n=2. DPI caused complete,Iong-Iasting and “noncompetitive” inhibition of ACh- and A 23187-induced vasodilatation in vitro and partial inhibition of ACh-induced vasodilatation in vivo. The inhibition was prevented by pretreatments with nicotinamide adenine dinucleotide (NADPH) and flavin adenine dinucleotide (FAD), but was slightly reversed by post-treatment with NADPH. DPI caused transient pressor response with n of 3 or 4, as well as raised plasma catecholamines in rats. The pressor response to DPI was attenuated by pithing and spinal cord transection, as well as treatment with tetrodotoxin, reserpine, mecamylamine, guanethidine or phentolamine. Therefore, both NSAA5 and DPI inhibit endothelium-dependent relaxations, the inhibiton of NSAAs is reversible whereas that of DPI is irreversible. The L configuration is preferred but not essential for NSAA5 to inhibit endothelium dependent relaxation. The mechanism of NSAAs involves antagonizing the NOS substrate L-Arg, whereas that of DPI involves interfering with the NOS cofactors FAD and NADPH. The pressor responses to NSAAs are competitively antagonized by L-Arg and are not dependent on the integrity of the central and autonomic nervous, angiotensin or prostaglandin system. The pressor response to DPI is due to sympathetic activation.Medicine, Faculty ofAnesthesiology, Pharmacology and Therapeutics, Department ofGraduat