STUDIES OF VASCULAR TOLERANCE TO NITROGLYCERIN - EFFECTS OF N-ACETYLCYSTEINE, N-G-MONOMETHYL L-ARGININE, AND ENDOTHELIN-1

Development of vascular tolerance to nitroglycerin (NTG) has been attr ibuted to sulfhydryl (SH) depletion, guanylate cyclase desensitization , or both. Controversy regarding the precise contribution of these mec hanisms may be due to variations in experimental design. To examine fu rther the biochemical basis of NTG tolerance, norepinephrine (NE)-prec ontracted rat aortic rings were exposed to NTG(10(-5)M), which resulte d in 84 +/- 6% relaxation. Other rings were first superfused with NTG (10(-6)M) and then contracted with NE. These rings showed a marked tol erance to the vasorelaxant effects of NTG (maximal relaxation 20 +/- 5 %, n = 15, p < 0.001 vs. control rings). Similar tolerance to NTG was observed when the vascular rings were first superfused with acetylchol ine (ACh 10(-6)M), indicating cross-tolerance between ACh and NTG. Tre atment of NTG-tolerant rings with N-acetylcysteine (NAG) (10(-5)M) did not restore vascular smooth muscle (VSM) relaxation in response to NT G (maximal relaxation 23 +/- 5%, n = 8), suggesting that SH depletion may not be the basis of NTG tolerance in these experiments. Parallel s ets of NTG-tolerant aortic rings were contracted with endothelin-l (ET -I, n = 5) or the endothelium-derived relaxing factor (EDRF) synthase inhibitor N-G-monomethyl L-arginine (L-NMMA, 10(-4)M, n = 8). In both ET-1- and L-NMMA-contracted rings, vascular relaxation in response to NTG was preserved (80 +/- 6 and 88 +/- 8% relaxation, respectively). M easurement of cyclic OMP in aortic rings showed marked accumulation on initial exposure of tissues to NTG (310 +/- 10 fmol/mg), whereas the NTG-tolerant rings showed much less cyclic GMP accumulation (48 +/- 29 fmol/mg). Rings contracted with L-NMMA or ET-I, but not NE, accumulat ed cyclic GMP when exposed to NTG (280 +/- 20 fmol/mg). These data ind icate that NTG tolerance develops on exposure of vascular rings superf used with NTG or ACh and is probably not related to tissue SH depletio n. Contraction of NTG-tolerant rings with ET-I or L-NMMA restores NTG- mediated relaxation.