Evidence for in vivo transport of bioactive nitric oxide in human plasma

Abstract

Although hitherto considered as a strictly locally acting vasodilator, results from recent clinical studies with inhaled nitric oxide (NO) indicate that NO can exert effects beyond the pulmonary circulation. We therefore sought to investigate potential remote vascular effects of intra-arterially applied aqueous NO solution and to identify the mechanisms involved. On bolus application of NO into the brachial artery of 32 healthy volunteers, both diameter of the downstream radial artery and forearm blood flow increased in a dose-dependent manner. Maximum dilator responses were comparable to those after stimulation of endogenous NO formation with acetylcholine and bradykinin. Response kinetics and pattern of NO decomposition suggested that despite the presence of hemoglobin-containing erythrocytes, a significant portion of NO was transported in its unbound form. Infusion of NO (36 micromol/min) into the brachial artery increased levels of plasma nitroso species, nitrite, and nitrate in the draining antecubital vein (by < 2-fold, 30-fold, and 4-fold, respectively), indicative of oxidative and nitrosative chemistry. Infused N-oxides were inactive as vasodilators whereas S-nitrosoglutathione dilated conduit and resistance arteries. Our results suggest that NO can be transported in bioactive form for significant distances along the vascular bed. Both free NO and plasma nitroso species contribute to the dilation of the downstream vasculature.

Figure 1

Dose-response curve for changes in FBF following application of increasing doses of NO applied into the brachial artery (n = 15). *Significant differences from base line. C, control.

Figure 2

Comparison of the maximal dilator effects of endogenous and exogenous NO in the human forearm. The endothelium-dependent dilators ACH and BK, the NO donor SNP, and authentic NO (NO) were applied in random order at maximally effective doses (n = 8). *Significant differences from base line. C, control.

Figure 3

Comparison of the dilator kinetics of endogenous and exogenous NO in human forearm resistance vessels. Authentic NO (NO), the endothelium-dependent dilators ACH and BK, and the NO donor SNP were infused for 1 minute at maximally effective doses. Each line represents the average time course of changes of FBF of three individuals. Note the sustained flow response to authentic NO.

Figure 4

Dilator response to aqueous NO in conduit (a) and resistance (b) arteries as compared with the formation of the NO-related metabolites nitrite (c), RSNO (d), and nitrate (e). Radial artery diameter and FBF during 1-minute infusion of 36 μmol NO into the brachial artery were measured by high-resolution ultrasound and plethysmography (n = 3). In parallel, plasma concentration of the respective NO metabolites was determined from blood drawn from the antecubital vein. The bar chart (f) indicates the relative proportion (0 to 100%) of recovered N-oxides (nitrite, nitrate, and RSNO) formed after application of 36 μmol NO, assuming that steady-state conditions were reached after 30 seconds.

Figure 5

Dilator response of conduit and resistive arteries in the human forearm circulation following infusion of NO-related metabolites. Changes in radial artery diameter and FBF were recorded simultaneously using high-resolution ultrasound and plethysmography in three individuals during application of (a) nitrite (36 μmol/min), (b) GSNO (5 nmol/min), or (c) nitrate (NO₃^–; 36 μmol/min).