N-Acetylcysteine Does Not Modify Nitroglycerin-Induced Tolerance in Canine Vascular Rings

The mechanism whereby nitroglycerin initiates relaxation in vascular smooth muscle remains unclear. One hypothesis states that nitroglycerin oxidizes critical sulfhydryl groups in smooth muscle to initiate relaxation, and that repeated exposure to nitroglycerin results in tolerance. In the current study, N-acetylcysteine, a sulfhydryl-reducing agent, was used to explore the sulfhydryl hypothesis by assessing whether or not tolerance to nitroglycerin was reversed by N-acetylcysteine in canine dorsal pedal artery rings. Two nitroglycerin dose-response curves were performed (n = 18)–one before (1st dose-response curve, from 10−9 to 1.1 x 10−5M nitroglycerin) and one after (2nd dose-response curve, from 10−9 to 5 x 10−7M nitroglycerin) incubation with 10−5M nitroglycerin for 105 min. At 5 x 10−7M nitroglycerin there was 50.7 ± 10.0% relaxation during the first dose-response curve. During the second dose-response curve, tolerance to nitroglycerin was evident, as demonstrated by a 6.8 ± 4.8% relaxation (p < 0.001) at 5 x 10−7M nitroglycerin. A 10-min treatment with 10−3M N-acetylcysteine (n = 10) during the second nitroglycerin dose-response curve was performed after the 5 x 10−7M concentration of nitroglycerin; the second dose-response curve was then completed up to 1.1 x 10−5M nitroglycerin. The dose of 10−3M N-acetylcysteine was chosen since higher concentrations (i.e., 1.3 x 10−2 and 1.2 x 10− M N-acetylcysteine) produced 20.3 ± 8.4 and 43.6 ± 11.6% relaxation in vascular rings (n = 5). At 10−5M nitroglycerin in the second dose-response curve, relaxation was attenuated from 60.2 ± 11.9% (1st dose-response curve) to 19.0 ± 9.6% (p < 0.001); at 1.1 x 10−5M nitroglycerin, relaxation was reduced from 96.5 ± 9.0 to 45.9 ± 10.9% (p < 0.001). In the other eight rings, N-acetylcysteine vehicle was studied in an identical fashion and similar results were obtained. These data show that N-acetylcysteine has direct relaxant effects on vascular smooth muscle and does not modify nitroglycerin-induced tolerance.