Vascular Effects Of Nitric Oxide Inhalation In Humans

一氧化氮（NO）由内皮细胞持续合成，并通过激活血管平滑肌中的鸟苷酸环化酶而对冠状动脉和体循环的血管舒张张力做出重要贡献，从而引起舒张。虽然内皮细胞的NO局部合成有助于局部血管舒张张力，但Stamler和同事提出，局部血管张力也可能受到通过血红蛋白和其他蛋白质从肺转运到远处血管部位的NO的调节，这是NO与反应性巯基结合增强的结果。为了确定血液运输的NO对人体局部血管张力的贡献，我们测量了16名健康受试者（8名男性，8名女性，平均年龄33岁）在休息时和与前臂运动应激相关的局部缺氧期间的前臂血流量并计算阻力（平均血压/流量），在阻断前臂内皮NO合成之前和之后进行测量。为此，通过应变计静脉闭塞体积描记法测量前臂血流量，在通过输注NG-单甲基-L-精氨酸抑制前臂NO合成之前和之后（L-NMMA; 8 mol/min），其与作为NO合酶底物的L-精氨酸竞争，但不能被氧化形成NO。将该剂量的L-NMMA输注到肱动脉中避免了该NO合酶抑制剂的全身作用，这可能反过来对局部血管张力具有反射性的次级效应。当呼吸室内空气时，动脉内L-NMMA 5分钟将前臂血流量从2.6+/-0.2（平均值+/-SEM）降低至1.8+/-0.1 ml/min/100 g组织（P=0.001）。前臂血流量平均减少25%与前臂血管阻力增加50%有关（34.1+/-3.1至48.3+/-3.9阻力单位，P=0.001）和肱动脉-肱静脉（A-V）pH差增宽（从0.029+/-0.008至0.049+/-0.010 pH单位，P=0.03）和A-V pO 2差异扩大（从59+/-4至64+/-4 mmHg，P=0.04）。在持续L-NMMA输注期间进行5分钟的重复握力运动后，前臂血流量增加至19.9+/-2.2 ml/min/100 g组织（与基线相比，P<0.001）。前臂血流平均增加742%，同时前臂血管阻力降低84（至5.0+/-0.6阻力单位，与基线相比P<0.001），并进一步扩大A-V pH差异（至0.124+/-0.012 pH单位，P<0.001 vs基线）和A-V pO 2差异（至79+/-2 mmHg，P<0.001 vs基线）。运动结束后，停止L-NMMA输注，并开始吸入80 ppm的NO，在线监测以确保21%的持续氧气输送。一小时后，继续吸入NO，重复上述对室内空气的测量顺序。在吸入NO期间，动脉内输注相同剂量的L-NMMA不再引起前臂血流量的显著减少（从2.2+/-0.2至2.0+/-0.2 ml/min/100 g组织，P=0.08）。L-NMMA输注期间前臂血流量平均减少7%，显著低于室内空气中前臂血流量减少25%（P=0.04）。同样，前臂血管阻力平均增加11%，显著低于室内空气中阻力增加50%（P=0.018）。在持续动脉内L-NMMA输注和NO吸入的重复握力运动期间，前臂血流量增加至21.0+/-2.0 ml/min/100个组织。前臂血流量平均增加914%，|在室内空气条件下，血流增加%（P=0.09）;前臂血管阻力降低88%显著大于室内空气条件下阻力降低84%（P=0.01）。此外，在L-NMMA输注并吸入NO期间，运动期间的A-V pH差异显著小于室内空气运动值（0.078+/-0.015 vs. 0.124+/-0.012 pH单位，P<0.001），A-V pO 2差异无显著性降低（76+/-2 vs. 79+/-2 mmHg，P=0.08）。在对侧臂，没有接受动脉内L-NMMA输注，因此作为对照组，NO吸入没有改变基础（2.6 ± 0.3 vs. 2.7 ± 0.4 ml/min/100 g组织，P=0.875）或运动（21.8+/-2.0 vs. 20.7+/-1.8 ml/min/100 g组织，P=0.953）前臂血流量与室内空气值相比，与室内空气测量值相比，NO吸入对A-V pH或pO 2差异也没有任何影响。为了确定NO呼吸过程中血液中生物活性NO的传递机制，我们测量了前臂动脉和静脉血中所有已知的NO物质。S-亚硝基血红蛋白和血浆S-亚硝基硫醇没有改变与NO呼吸。动脉亚硝酸盐水平增加了11%，动脉亚硝酰（血红素）血红蛋白增加了4倍，达到微摩尔范围，这两项措施在动脉中始终高于静脉血。我们的结论是：1）NO合成的区域抑制导致局部血流量的显著减少，表明NO的区域合成在调节局部血管紧张度中的重要性，2）NO吸入a）显著减弱了用NO合成的区域阻断观察到的前臂血流量的减少，但对组织氧合的影响较小，B）显著增强前臂血流对运动应激的反应，具有较少的组织酸中毒，但c）在存在正常的区域NO合成的情况下，对静息时或运动期间的前臂血流没有影响，3）亚硝酰血红蛋白和亚硝酸盐是活性NO向血管组织的潜在转运体。这些研究结果可能是相关的了解血红蛋白转运的NO在调节血管舒张张力的疾病和条件与区域内皮功能障碍和内皮NO生物活性降低的生理贡献和治疗潜力。

Nitric oxide (NO) is continuously synthesized by the endothelium, and contributes importantly to vasodilator tone of the coronary and systemic circulations by activating guanylyl cyclase in vascular smooth muscle, causing relaxation. Although regional synthesis of NO by the endothelium contributes to local vasodilator tone, Stamler and co-workers have proposed that regional vascular tone may also be regulated by NO transported from the lungs to distant vascular sites by hemoglobin and other proteins as a consequence of enhanced binding of NO to reactive thiol groups. In order to determine the contribution of blood-transported NO to regional vascular tone in humans, we measured forearm blood flow and calculated resistance (mean blood pressure/flow) in 16 healthy subjects (8 men, 8 women, average age 33 years) at rest and during regional hypoxia associated with forearm exercise stress, with measurements made before and after blockade of forearm endothelial NO synthesis. To this end, forearm blood flow was measured by strain gauge venous occlusion plethysmography, before and after inhibition of NO synthesis in the forearm with infusion of NG-monomethyl-L-arginine (L-NMMA; 8 mol/min), which competes with L-arginine as the substrate for NO synthase but cannot be oxidized to form NO. Infusion of L-NMMA at this dosage into the brachial artery avoids systemic effects of this NO synthase inhibitor, which might in turn have reflexive secondary effects on regional vascular tone. While breathing room air, intra-arterial L-NMMA for 5 minutes reduced forearm blood flow from 2.6+/-0.2 (mean+/-SEM) to 1.8+/-0.1 ml/min/100 g tissue (P=0.001). This 25% average reduction in forearm blood flow was associated with a 50% increase in forearm vascular resistance (34.1+/-3.1 to 48.3+/-3.9 resistance units, P=0.001) and with widening of the brachial artery minus brachial vein (A-V) pH difference (from 0.029+/-0.008 to 0.049+/-0.010 pH units, P=0.03) and widening of the A-V pO2 difference (from 59+/-4 to 64+/-4 mm Hg, P=0.04). After 5 minutes of repetitive hand-grip exercise during continued L-NMMA infusion, forearm blood flow increased to 19.9+/-2.2 ml/min/100 g tissue (P<0.001 vs. baseline). This 742% average increase in forearm blood flow was associated with an 84% reduction in forearm vascular resistance (to 5.0+/-0.6 resistance units, P<0.001 vs. baseline) and further widening of the A-V pH difference (to 0.124+/-0.012 pH units, P<0.001 vs. baseline) and the A-V pO2 difference (to 79+/-2 mm Hg, P<0.001 vs. baseline). Following termination of exercise, L-NMMA infusion was discontinued and inhalation of NO at 80 ppm was initiated, with on-line monitoring to assure continuous oxygen delivery at 21%. One hour later and with continuation of NO inhalation, the sequence of measurements described above on room air was repeated. During NO inhalation, intra-arterial infusion of L-NMMA at the same dosage no longer caused significant reduction in forearm blood flow (from 2.2+/-0.2 to 2.0+/-0.2 ml/min/100 g tissue, P=0.08). This 7% average reduction in forearm blood flow during L-NMMA infusion was significantly less than the 25% reduction in forearm blood flow achieved on room air (P=0.04). Similarly, the 11% average increase in forearm vascular resistance was significantly less than the 50% increase in resistance achieved on room air (P=0.018).During repetitive hand-grip exercise with continuation of intra-arterial L-NMMA infusion and NO inhalation, forearm blood flow increased to 21.0+/-2.0 ml/min/100 tissue. This 914% average increase in forearm blood flow tended to be greater than the 742|% increase in flow on room air (P=0.09); the 88% decrease in forearm vascular resistance was significantly greater than the 84% decrease in resistance on room air (P=0.01). Additionally, the A-V pH difference during exercise during L-NMMA infusion with NO inhalation was significantly less than room air exercise values (0.078+/-0.015 vs. 0.124+/-0.012 pH units, P<0.001), with nonsignificant reduction in the A-V pO2 difference (76+/-2 vs.79+/-2 mm Hg, P=0.08). In the contralateral arm, which did not receive intra-arterial L-NMMA infusion and thus served as the control arm, NO inhalation did not alter basal (2.6+/-0.3 vs. 2.7+/-0.4 ml/min/100 g tissue, P=0.875) or exercise (21.8+/-2.0 vs. 20.7+/-1.8 ml/min/100 g tissue, P=0.953) forearm blood flow compared with room air values, nor was there any effect of NO inhalation on A-V pH or pO2 differences compared with room air measurements. In order to determine the mechanism of bioactive NO delivery in blood during NO breathing, we measured all known NO species in arterial and venous blood of the forearm. S-nitrosohemoglobin and plasma S-nitrosothiols did not change with NO breathing. Arterial nitrite levels increased by 11% and arterial nitrosyl(heme)hemoglobin increased 4-fold to the micromolar range, and both measures were consistently higher in the arterial than in venous blood. We conclude that 1) regional inhibition of NO synthesis causes significant reduction in regional blood flow, indicating the importance of regional synthesis of NO in regulating local vascular tone, 2) NO inhalation a) significantly attenuates the reduction in forearm blood flow seen with regional blockade of NO synthesis, but with lesser effect on tissue oxygenation, b) significantly enhances the forearm blood flow response to exercise stress with less tissue acidosis, but c) has no effect on forearm blood flow at rest or during exercise in the presence of normal regional NO synthesis, and 3) nitrosyl(heme)hemoglobin and nitrite are potential transporters of bioactive NO to vascular tissue. These findings may be relevant to understanding the physiological contribution and therapeutic potential of hemoglobin-transported NO in the regulation of vasodilator tone in diseases and conditions associated with regional endothelial dysfunction and reduced endothelial NO bioactivity.