Nitric Oxide Clamp Research Articles

Coexistence of functional angiotensin II type 2 receptors mediating both vasoconstriction and vasodilation in humans

Angiotensin (Ang) II type 1 (AT₁) receptors mediate the majority of cardiovascular effects of Ang II, whereas the role of type 2 (AT₂) receptors is still controversial. The present study, therefore, investigated regional hemodynamic responses mediated by AT₂ receptors in humans. Studies were performed in 20 healthy individuals (eight men; mean age 37 ± 3 years) through intra-arterial infusion of agonists and antagonists of Ang II receptors by use of strain-gauge plethysmography. Selective blockade of either AT₁ or AT₂ receptors by telmisartan or PD 123319, respectively, resulted in mild forearm blood flow increase (15 ± 5% and 10 ± 4, respectively; both P > 0.05); combined AT₁ and AT₂ receptor antagonism, however, was associated with greater vasodilator response (25 ± 5%; P = 0.02). This effect was unrelated to increased nitric oxide activity, being unaffected (27 ± 5%; P = 0.65) by a ' nitric oxide clamp' (coinfusion of the nitric oxide synthase inhibitor L-NMMA and the nitric oxide donor sodium nitroprusside). Graded doses of Ang II induced a progressive vasoconstrictor response that was blunted not only by telmisartan, but also by PD 123319 and by the combination of PD 123319 and telmisartan (all P < 0.001 vs. Ang II alone). AT₂ receptor stimulation by CGP 42112A resulted in a dose-dependent vasodilation (P < 0.001 vs. baseline), that was abolished by the nitric oxide clamp and by PD 123319 (both P < 0.001 vs. CGP 42112A alone). In the human forearm, vasoconstrictor AT₂ receptors coexist with AT₂ receptors mediating nitric oxide-dependent vasodilation. These findings suggest that imbalance between these opposing hemodynamic actions may affect vascular homeostasis and foster further investigation about the role of AT₂ receptors in human disease.

Morphine is an arteriolar vasodilator in man

The mechanisms of action of morphine on the arterial system are not well understood. The aim was to report forearm vascular responses, and their mediation, to intra-arterial morphine in healthy subjects. Three separate protocols were performed: (i) dose ranging; (ii) acute tolerance; (iii) randomized crossover mechanistic study on forearm blood flow (FBF) responses to intrabrachial infusion of morphine using venous occlusion plethysmography. Morphine was infused either alone (study 1 and 2), or with an antagonist: naloxone, combined histamine-1 and histamine-2 receptor blockade or during a nitric oxide clamp. Morphine caused an increase in FBF at doses of 30 microg min(-1)[3.25 (0.26) ml min(-1) 100 ml(-1)][mean (SEM)] doubling at 100 microg min(-1) to 5.23 (0.53) ml min(-1) 100 ml(-1). Acute tolerance was not seen to 50 microg min(-1) morphine, with increased FBF [3.96 (0.35) ml min(-1) 100 ml(-1)] (P = 0.003), throughout the 30-min infusion period. Vasodilatation was abolished by pretreatment with antihistamines (P = 0.008) and the nitric oxide clamp (P < 0.001), but not affected by naloxone. The maximum FBF with pretreatment with combined H1/H2 blockade was 3.06 (0.48) and 2.90 (0.17) ml min(-1) 100 ml(-1) after 30 min, whereas with morphine alone it reached 4.3 (0.89) ml min(-1) 100 ml(-1). Intra-arterial infusion of morphine into the forearm circulation causes vasodilatation through local histamine-modulated nitric oxide release. Opioid receptor mechanisms need further exploration.

Vascular Effects of Apelin In Vivo in Man

This study was designed to establish the direct vascular effects of apelin in vivo in man. Apelin is the endogenous ligand for the previously orphaned G-protein-coupled receptor, APJ. This novel pathway is widely expressed in the cardiovascular system and is emerging as an important mediator of cardiovascular homeostasis. In pre-clinical models, apelin causes venous and arterial vasodilation. Vascular effects of apelin were assessed in 24 healthy volunteers. Dorsal hand vein diameter was measured by the Aellig technique during local intravenous infusions (0.1 to 3 nmol/min) of apelin-36, (Pyr(1))apelin-13, and sodium nitroprusside (0.6 nmol/min). Forearm blood flow was measured by venous occlusion plethysmography during intrabrachial infusions of apelin-36 and (Pyr(1))apelin-13 (0.1 to 30 nmol/min) and subsequently in the presence or absence of a "nitric oxide clamp" (nitric oxide synthase inhibitor, L-N(G)-monomethylarginine [8 mumol/min], coinfused with nitric oxide donor, sodium nitroprusside [90 to 900 ng/min]), or a single oral dose of aspirin (600 mg) or matched placebo. Although sodium nitroprusside caused venodilation (p < 0.0001), apelin-36 and (Pyr(1))apelin-13 had no effect on dorsal hand vein diameter (p = 0.2). Both apelin isoforms caused reproducible vasodilation in forearm resistance vessels (p < 0.0001). (Pyr(1))apelin-13-mediated vasodilation was attenuated by the nitric oxide clamp (p = 0.004) but unaffected by aspirin (p = 0.7). Although having no apparent effect on venous tone, apelin causes nitric oxide-dependent arterial vasodilation in vivo in man. The apelin-APJ system merits further clinical investigation to determine its role in cardiovascular homeostasis.

The vascular effects of rotigaptide in vivo in man

Endothelium-derived hyperpolarising factor (EDHF) causes vasorelaxation and may contribute to the release of the endogenous fibrinolytic factor, tissue-plasminogen activator (t-PA). Rotigaptide enhances communication via the connexin 43 gap junction subunit and may potentiate the vascular actions of EDHF. The aims of the present study were therefore to determine whether rotigaptide influences basal and stimulated endothelium-dependent vasodilatation and t-PA release in vivo in man. Using venous occlusion plethysmography, forearm blood flow was measured in 27 healthy volunteers during intra-brachial infusions of rotigaptide (0.25–25 nmol/min) alone, or co-administered with endothelium-dependent (acetylcholine [5–20 μg/min] and bradykinin [30–300 pmol/min]) and independent (sodium nitroprusside [2–8 μg/min]) vasodilators in the presence or absence of aspirin and the ‘nitric oxide clamp’. The ‘nitric oxide clamp’ inhibits endogenous nitric oxide synthesis with L-N-monomethylarginine and restores resting blood flow with the exogenous nitric oxide donor, sodium nitroprusside. Basal blood flow was unaffected by rotigaptide ( P = NS). Acetylcholine, bradykinin and sodium nitroprusside all caused dose-dependent vasodilatation in the presence and absence of aspirin and the ‘nitric oxide clamp’ ( P ≤ 0.005 for all). These responses were unaffected by rotigaptide ( P = NS). Bradykinin caused t-PA antigen and activity release ( P = 0.04, P < 0.0001, respectively) that was unaffected by rotigaptide. Augmentation of connexin 43 communication has no effect on basal vascular tone and does not enhance endothelium-dependent or independent vasodilatation, or t-PA release in the forearm arterial circulation of healthy men. It remains to be established whether augmentation of connexin 43 communication improves endothelial function in patients with vascular disease.

Methodological validity and feasibility of the nitric oxide clamp technique for nitric oxide research in human resistant vessels.

N(G)-methyl-L-arginine (L-NMMA) has been widely used for nitric oxide (NO) research, particularly for the assessment of NO-dependent vasodilatation evoked by agonists. However, such experiments may not be straightforward because L-NMMA causes vasoconstriction, which itself must non-specifically affect responses to any vasoactive agents. Therefore, in order to more accurately estimate the roles of NO in human vessels in vivo, we developed an NO clamp technique that uses co-infusion of an NO donor with L-NMMA. To assess the validity and feasibility of this technique, we compared the effects of intra-arterial infusion of L-NMMA on the forearm blood flow responses to vasodilators with and without the NO clamp technique in healthy males. All drugs were intra-arterially infused and changes in forearm blood flow (FBF) were measured by strain-gauge plethysmography. Vasodilatation evoked by atrial natriuretic peptide was significantly attenuated by L-NMMA alone (p = 0.001) but not by the NO clamp technique. L-NMMA significantly attenuated the responses to acetylcholine either with or without the NO clamp technique. However, the ratio of the area under the curve (AUC) of acetylcholine with L-NMMA to that without L-NMMA was significantly higher when the NO clamp technique was not used (AUC ratio: 0.62 +/- 0.13 vs. 0.48 +/- 0.14, respectively; p = 0.031). The contribution of NO to the FBF responses to vasodilators may be more properly assessed by the co-infusion of L-NMMA with the NO clamp technique than by L-NMMA alone. Our NO clamp technique thus appears to be valid and feasible for human NO research.

An in vivo nitric oxide clamp to investigate the influence of nitric oxide on continuous umbilical blood flow during acute hypoxaemia in the sheep fetus.

1. The aims of this study in the ovine fetus were to (1) characterise continuous changes in umbilical blood flow and vascular conductance during acute hypoxaemia and (2) determine the effects of nitric oxide blockade on umbilical blood flow and vascular conductance during normoxic and hypoxaemic conditions using a novel in vivo 'nitric oxide clamp'. 2. Under 1-2% halothane anaesthesia, seven ovine fetuses were instrumented between 118 and 125 days of gestation (term is ca 145 days) with vascular and amniotic catheters and a flow probe around an umbilical artery. At least 5 days after surgery, all fetuses were subjected to a 3 h protocol: 1 h of normoxia, 1 h of hypoxaemia and 1 h of recovery during fetal I.V. infusion with saline or, 1-2 days later, during combined fetal treatment with the nitric oxide (NO) inhibitor N (G)-nitro-L-arginine methyl ester (L-NAME, 100 mg x kg(-1)) and the NO donor sodium nitroprusside (NP, 5.1 +/- 2.0 microg x kg(-1) x min(-1), the 'nitric oxide clamp'). Following the end of the 3 h experimental protocol, the infusion of NP was withdrawn to unmask any persisting effects of fetal treatment with L-NAME alone. 3. During acute hypoxaemia, the reduction in arterial partial pressure of O2 (Pa,O2) was similar in fetuses infused with saline or treated with the nitric oxide clamp. In all fetuses, acute hypoxaemia led to a progressive increase in mean arterial blood pressure and a fall in heart rate. In saline-infused fetuses, acute hypoxaemia led to a rapid, but transient, decrement in umbilical vascular conductance. Thereafter, umbilical vascular conductance was maintained and a significant increase in umbilical blood flow occurred, which remained elevated until the end of the hypoxaemic challenge. In contrast, while the initial decrement in umbilical vascular conductance was prevented in fetuses treated with the nitric oxide clamp, the increase in umbilical blood flow during hypoxaemia was similar to that in fetuses infused with saline. After the 1 h recovery period of the acute hypoxaemia protocol, withdrawal of the sodium nitroprusside infusion from fetuses undergoing the nitric oxide clamp led to a significant, but transient, hypertension and a sustained umbilical vasoconstriction. 4. In conclusion, the data reported in this study of unanaesthetised fetal sheep (1) show that minute-by-minute analyses of haemodynamic changes in the umbilical vascular bed reveal an initial decrease in umbilical vascular conductance at the onset of hypoxaemia followed by a sustained increase in umbilical blood flow for the duration of the hypoxaemic challenge, (2) confirm that the increase in umbilical blood flow after 15 min hypoxaemia is predominantly pressure driven, and (3) demonstrate that nitric oxide plays a major role in the maintenance of umbilical blood flow under basal, but not under acute hypoxaemic, conditions.