Lawson D L, Haught W H, Mehta P, Mehta J L
Department of Medicine, University of Florida College of Medicine, Gainesville 32610-0277, USA.
J Cardiovasc Pharmacol. 1996 Sep;28(3):418-24. doi: 10.1097/00005344-199609000-00011.
Development of vascular tolerance to nitroglycerin (NTG) has been attributed to sulfhydryl (SH) depletion, guanylate cyclase desensitization, or both. Controversy regarding the precise contribution of these mechanisms may be due to variations in experimental design. To examine further the biochemical basis of NTG tolerance, norepinephrine (NE)-precontracted rat aortic rings were exposed to NTG (10(-5)M), which resulted in 84 +/- 6% relaxation. Other rings were first superfused with NTG (10(-6)M) and then contracted with NE. These rings showed a marked tolerance 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 acetylcholine (ACh 10(-6)M), indicating cross-tolerance between ACh and NTG. Treatment of NTG-tolerant rings with N-acetylcysteine (NAC) (10(-5)M) did not restore vascular smooth muscle (VSM) relaxation in response to NTG (maximal relaxation 23 +/- 5%, n = 8), suggesting that SH depletion may not be the basis of NTG tolerance in these experiments. Parallel sets of NTG-tolerant aortic rings were contracted with endothelin-1 (ET-1, n = 5) or the endothelium-derived relaxing factor (EDRF) synthase inhibitor NG-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). Measurement of cyclic GMP 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-1, but not NE, accumulated cyclic GMP when exposed to NTG (280 +/- 20 fmol/mg). These data indicate that NTG tolerance develops on exposure of vascular rings superfused with NTG or ACh and is probably not related to tissue SH depletion. Contraction of NTG-tolerant rings with ET-1 or L-NMMA restores NTG-mediated relaxation.
血管对硝酸甘油（NTG）耐受性的产生归因于巯基（SH）耗竭、鸟苷酸环化酶脱敏或两者兼而有之。关于这些机制的确切作用存在争议，这可能是由于实验设计的差异所致。为了进一步研究NTG耐受性的生化基础，将去甲肾上腺素（NE）预收缩的大鼠主动脉环暴露于NTG（10⁻⁵M），可导致84±6%的舒张。其他环先灌注NTG（10⁻⁶M），然后用NE收缩。这些环对NTG的血管舒张作用表现出明显的耐受性（最大舒张率为20±5%，n = 15，与对照环相比，p < 0.001）。当血管环先灌注乙酰胆碱（ACh 10⁻⁶M）时，观察到对NTG有类似的耐受性，表明ACh和NTG之间存在交叉耐受性。用N - 乙酰半胱氨酸（NAC）（10⁻⁵M）处理NTG耐受性环并不能恢复血管平滑肌（VSM）对NTG的舒张反应（最大舒张率为23±5%，n = 8），这表明在这些实验中，SH耗竭可能不是NTG耐受性的基础。用内皮素 - 1（ET - 1，n = 5）或内皮衍生舒张因子（EDRF）合成酶抑制剂N - 单甲基L - 精氨酸（L - NMMA，10⁻⁴M，n = 8）收缩平行的NTG耐受性主动脉环。在ET - 1和L - NMMA收缩的环中，对NTG的血管舒张反应均得以保留（分别为80±6%和88±8%的舒张率）。对主动脉环中环磷酸鸟苷（cGMP）的测量显示，在组织最初暴露于NTG时cGMP有显著积累（310±10 fmol/mg），而NTG耐受性环中cGMP积累则少得多（48±29 fmol/mg）。用L - NMMA或ET - 1收缩但不用NE收缩的环在暴露于NTG时积累cGMP（280±20 fmol/mg）。这些数据表明，在灌注NTG或ACh的血管环暴露时会产生NTG耐受性，并且可能与组织SH耗竭无关。用ET - 1或L - NMMA收缩NTG耐受性环可恢复NTG介导的舒张。
