Vascular Effects Of Nitric Oxide Inhalation In Humans

一氧化氮(NO)是由血管内皮细胞持续合成的，通过激活血管平滑肌中的鸟苷酸环化酶，导致血管松弛，从而对冠脉和全身循环的血管扩张起重要作用。虽然内皮细胞局部合成NO有助于局部血管扩张张力，但Stamler和他的同事提出，由于NO与反应性硫醇基团的结合增强，NO也可能通过血红蛋白和其他蛋白质从肺输送到远处的血管部位来调节局部血管张力。为了确定血液转运的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(男8名，女8名，平均年龄33岁)在静息状态和局部缺氧与前臂运动应激状态下的前臂血流量和阻力(平均血压/流量)，并在阻断前臂内皮细胞NO合成前后进行了测量。为此，采用应变式静脉阻塞体积描记仪测定前臂静脉血流量，观察前臂静脉注射与L精氨酸竞争NO合酶底物但不能氧化生成NO的NG-单甲基-L-精氨酸(L-NMMA；8mol/min)抑制前臂NO合成前后的血流变化。将此剂量的L-NMMA注入臂动脉可避免这种一氧化氮合酶抑制剂的全身效应，后者可能反过来对局部血管张力产生反射性次级效应。在呼吸室内空气的情况下，动脉内注射L-NMMA 5min，前臂血流量从2.6m l/m in/100g组织减少到1.8m l/m in/100g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%(34.1+/-3.1至48.3+/-3.9阻力单位，P=0.001)、肱动脉-臂静脉(A-V)pH差扩大(从0.029+/-0.008至0.049+/-0.010 pH单位，P=0.0 3)和A-VPO2差(从59+/-4至+/-4 mm Hg，P=0.0 4)有关。在持续输注L-NMMA期间进行重复握手练习5分钟后，前臂血流量增加到19.9±2.2ml/分钟/100g组织(与基线相比P&lt；0.001)。前臂血流量的742%的平均增加与前臂血管阻力降低84%(与基线相比，P&lt；0.001)和A-V pH差(与基线相比，P&lt；0.001进一步扩大至0.124+/-0.012 pH单位)和A-V PO2差(与基线相比，P&lt；0.001与基线相比，降至79+/-2 mm Hg，P&lt；0.001)有关。运动结束后，停止输注L-NMMA，开始吸入80ppm的NO，并进行在线监测，以确保21%的持续供氧量。一小时后，随着持续吸入NO，重复上述对房间空气的测量序列。吸入NO期间，动脉内注入相同剂量的L-NMMA不再引起前臂血流量的显著下降(从2.2±-0.2降至2.0±0.2ml/min/100g组织，P=0.0 8)。在注射L-NMMA期间，前臂血流量平均减少7%，显著低于在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)。在重复握手运动中，持续动脉内注入L-NMMA和吸入NO，前臂血流量增加到21.0+/-2.0ml/分钟/百组织。前臂血流量平均增加914%，明显大于室内空气流量742|%的增幅(P=0.09)，前臂血管阻力下降88%，明显大于室内空气阻力下降84%(P=0.01)。吸入NO时运动时的A-V pH差显著小于室内空气运动值(0.078+/-0.015对0.124+/-0.012个pH单位，P&t；0.001)，A-V PO2差(76+/-2对79+/-2 mm Hg，P=0.08)。在未接受L-NMMA动脉内输注的对侧手臂，作为对照手臂，吸入NO没有改变基础(2.6ml/-0.3vs.2.7+/-0.4ml/min/100g组织，P=0.875)或运动(21.8+/-2.0vs.20.7+/-1.8ml/min/100g组织，P=0.953)前臂血流量，也没有影响吸入NO对A-V pH或PO2与室内空气测量的差异。为了确定无呼吸状态下血中生物活性NO的释放机制，我们测量了前臂动脉血和静脉血中已知的所有NO物种。呼吸停止时，S-亚硝化血红蛋白和血浆S-亚硫醇无明显变化。动脉亚硝酸盐水平增加了11%，动脉亚硝基(血红素)血红蛋白增加了4倍，达到微摩尔范围，这两个指标在动脉血中一直高于静脉血。我们的结论是：1)局部抑制NO合成导致局部血流显著减少，表明局部NO合成在调节局部血管张力中的重要性；2)吸入NO显著减弱局部阻断NO合成后前臂血流的减少，但对组织氧合的影响较小；b)显著增强运动应激所致的前臂血流反应，减少组织酸中毒；但c)在静息或运动时局部NO合成正常的情况下，对前臂血流没有影响；3)亚硝基(血红素)血红蛋白和亚硝酸盐是生物活性NO向血管组织的潜在转运体。这些发现可能与理解血红蛋白转运的NO在调节血管扩张张力中的生理作用和治疗潜力有关，这些疾病和条件与局部内皮功能障碍和内皮NO生物活性降低有关。

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.