Evidence for enhanced vascular superoxide anion production in nitrate tolerance. A novel mechanism underlying tolerance and cross-tolerance.
We sought to examine mechanisms underlying nitroglycerin (NTG) tolerance and "cross-tolerance" to other nitrovasodilators. Rabbits were treated for 3 d with NTG patches (0.4 mg/h) and their aortic segments studied in organ chambers. Relaxations were examined after preconstriction with phenylephrine. In NTG tolerant rabbit aorta, relaxations to cGMP-dependent vasodilators such as NTG (45 +/- 6%), SIN-1 (69 +/- 7%), and acetylcholine (ACh, 64 +/- 5%) were attenuated vs. controls, (90 +/- 2, 94 +/- 3, and 89 +/- 2% respectively, P < 0.05 for all), while responses to the cAMP-dependent vasodilator forskolin remained unchanged. In tolerant aorta, endothelial removal markedly enhanced relaxations to NTG and SIN-1 (82 +/- 4 and 95 +/- 3%, respectively). Other studies were performed to determine how the endothelium enhances tolerance. Vascular steady state .-O2 levels (assessed by lucigenin chemiluminescence) was increased twofold in tolerant vs. control vessels with endothelium (0.31 +/- 0.01 vs. 0.61 +/- 0.01 nmol/mg per minute). This difference was less in vessels after denudation of the endothelium. Diphenylene iodonium, an inhibitor of flavoprotein containing oxidases, and Tiron a direct .-O2 scavenger normalized .-O2 levels. In contrast, oxypurinol (1 mM) an inhibitor of xanthine oxidase, rotenone (50 microM) an inhibitor of mitochondrial electron transport and NG-nitro-L-arginine (100 microM) an inhibitor of nitric oxide synthase did not affect the chemiluminescence signals from NTG-tolerant aortas. Pretreatment of tolerant aorta with liposome-entrapped, pH sensitive superoxide dismutase (600 U/ml) significantly enhanced maximal relaxation in response to NTG, SIN-1, and ACh, and effectively reduced chemiluminescence signals. These studies show that continuous NTG treatment is associated with increased vascular .-O2-production and consequent inhibition of NO. mediated vasorelaxation produced by both exogenous and endogenous nitrovasodilators.
Münzel
T
T
Department of Medicine, Emory University School of Medicine, Atlanta Veterans Administration Medical Center, Georgia 30322.
Sayegh
H
H
Freeman
B A
BA
Tarpey
M M
MM
Harrison
D G
DG
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D001011:Aorta / Q000378:metabolism*; D001713:Biphenyl Compounds / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004361:Drug Tolerance; D004730:Endothelium, Vascular / Q000378:metabolism; D005260:Female; D066298:In Vitro Techniques; D008081:Liposomes / Q000494:pharmacology; D008297:Male; D008981:Molsidomine / Q000031:analogs & derivatives / Q000494:pharmacology; D009574:Nitro Compounds / Q000494:pharmacology*; D005996:Nitroglycerin / Q000494:pharmacology*; D009861:Onium Compounds / Q000494:pharmacology; D011817:Rabbits; D013482:Superoxide Dismutase; D013481:Superoxides / Q000378:metabolism*; D014664:Vasodilation / Q000187:drug effects; D014665:Vasodilator Agents / Q000494:pharmacology*
D001713:Biphenyl Compounds; D008081:Liposomes; D009574:Nitro Compounds; D009861:Onium Compounds; D014665:Vasodilator Agents; C031291:diphenyliodonium; D013481:Superoxides; D005576:Colforsin; C002385:linsidomine; D008981:Molsidomine; D013482:Superoxide Dismutase; D005996:Nitroglycerin; D000109:Acetylcholine