Inhaled nitric oxide: diameter response patterns in feline small pulmonary arteries and veins

Inducible nitric oxide synthase inhibition reverses pulmonary arterial dysfunction in lung transplantation

BACKGROUND

Ischemia-reperfusion injury (IRI) after lung transplantation remains a significant cause of morbidity and mortality. Lung IRI induces nitric oxide synthesis (iNOS) and reactive nitrogen species, decreasing nitric oxide bioavailability. We hypothesized that ischemia-induced iNOS intensifies with reperfusion and contributes to IRI-induced pulmonary arterial regulatory dysfunction, which may lead to early graft failure and cause pulmonary edema. The aim of this study was to determine whether ischemia-reperfusion alters inducible and endothelial nitric oxide synthase expression, potentially affecting pulmonary perfusion. We further evaluated the role of iNOS in post-transplantation pulmonary arterial disorder.

METHODS

We randomized 32 Sprague-Dawley rats into two groups. The control group was given a sham operation whilst the experimental group received orthotropic lung transplants with a modified three-cuff technique. Changes in lung iNOS, and endothelial nitric oxide synthase expression were measured after lung transplantation by enzyme-linked immunosorbent assay (ELISA). Vasoconstriction in response to exogenous phenylephrine and vasodilation in response to exogenous acetylcholine of pulmonary arterial rings were measured in vitro as a measure of vascular dysfunction. To elucidate the roles of iNOS in regulating vascular function, an iNOS activity inhibitor (N6-(1-iminoethyl)-L-lysine, L-NIL) was used to treat isolated arterial rings. In order to test whether iNOS inhibition has a therapeutic effect, we further used L-NIL to pre-treat transplanted lungs and then measured post-transplantation arterial responses.

RESULTS

Lung transplantation caused upregulation of iNOS expression. This was also accompanied by suppression of both vasoconstriction and vasodilation of arterial rings from transplanted lungs. Removal of endothelium did not interfere with the contraction of pulmonary arterial rings from transplanted lungs. In contrast, iNOS inhibition rescued the vasoconstriction response to exogenous phenylephrine of pulmonary arterial rings from transplanted lungs. In addition, lung transplantation led to suppression of PaO2/FiO2 ratio, increased intrapulmonary shunt (Q s/Q t), and increase of lung wet to dry ratio (W/D), malondialdehyde and myeloperoxidase levels, all of which were reversed upon iNOS inhibition. Furthermore, inhibition of iNOS significantly rescued vascular function and alleviated edema and inflammatory cell infiltration in the transplanted lung.

CONCLUSIONS

Our data suggest that lung transplantation causes upregulation of iNOS expression, and pulmonary vascular dysfunction. iNOS inhibition reverses the post-transplantational pulmonary vascular dysfunction.

Changes in macrovessel pulmonary blood flow distribution following chronic hypoxia: assessed using synchrotron radiation microangiography

Structural and functional changes of the pulmonary circulation, particularly during the pathogenesis of pulmonary arterial hypertension (PAH), remain to be fully elucidated. In this study, we utilized monochromatic synchrotron radiation (SR) microangiography to assess changes in pulmonary arteriole blood flow in the intact-chest rat after 4 wk of chronic hypoxia. Sprague-Dawley rats were exposed to normoxia (N-rats) or chronic hypoxia (10% O2; CH-rats) for 28 days. Rats were anesthetized, and microangiography was performed on the left lung to assess 1) the branching distribution of pulmonary arteriole blood flow (internal diameter >80 μm) and 2) dynamic changes in vessel lumen diameter during acute hypoxic (8% O2 for 4 min) pulmonary vasoconstriction (HPV) before and after β-adrenoceptor blockade (2 mg/kg iv propranolol). Using SR angiography, we observed that the number of opaque third- and fourth-generation vessels (100–300 μm) for CH-rats was significantly fewer than the number for N-rats. The magnitude of HPV was not different between CH-rats and N-rats. β-Adrenoceptor blockade accentuated the HPV in 200- to 300-μm vessels for CH-rats, but even more so in N-rats. However, in CH-rats, β-adrenoceptor blockade also accentuated the HPV in 100- to 200-μm vessels. In summary, we utilized SR to assess gross blood flow changes and functional changes (i.e., HPV) of the pulmonary circulation in PAH. These results highlight the benefits of SR for assessing pulmonary circulatory pathology. Of particular importance, future use of SR will provide an effective method for assessing potential therapeutic treatments for PAH.

Imaging of the pulmonary circulation in the closed-chest rat using synchrotron radiation microangiography

Structural changes of the pulmonary circulation during the pathogenesis of pulmonary arterial hypertension remain to be fully elucidated. Although angiography has been used for visualizing the pulmonary circulation, conventional angiography systems have considerable limitations for visualizing small microvessels (diameters < 200 μm), particularly within a closed-chest animal model. In this study we assess the effectiveness of monochromatic synchrotron radiation (SR) for microangiography of the pulmonary circulation in the intact-chest rat. Male adult Sprague-Dawley rats were anesthetized, and a catheter was positioned within the right ventricle, for administering iodinated contrast agent (Iomeron 350). Subsequently, microangiography of pulmonary arterial branches within the left lung was performed using monochromatic SR. Additionally, we assessed dynamic changes in vessel diameter during acute hypoxic (10% and 8% O2 for 4 min each) pulmonary vasoconstriction (HPV). Using SR we were able to visualize pulmonary microvessels with a diameter of <100 μm (the 4th generation of branching from the left axial artery). Acute hypoxia caused a significant decrease in the diameter of all vessels less than 500 μm. The greatest degree of pulmonary vasoconstriction was observed in vessels with a diameter between 200 and 300 μm. These results demonstrate the effectiveness of SR for visualizing pulmonary vessels in a closed-chest rat model and for assessing dynamic changes associated with HPV. More importantly, these observations implicate SR as an effective tool in future research for assessing gross structural changes associated with the pathogenesis of pulmonary arterial hypertension.

Pulmonary arterial dilation by inhaled NO: arterial diameter, NO concentration relationship

The objective of this study was to determine the nitric oxide (NO) concentration and vessel diameter dependence of the pulmonary arterial dilation induced by inhaled NO. Isolated dog lung lobes were situated between a microfocal X-ray source and X-ray detector and perfused with either blood or plasma. Boluses of radiopaque contrast medium were injected into the lobar artery under control conditions, when the pulmonary arteries were constricted by infusion of serotonin and when the serotonin infusion was accompanied by inhalation of from 30 to 960 parts/million NO. Arterial diameter measurements were obtained from X-ray images of vessels having control diameters in the 300- to 3,400-microm range. Serotonin constricted the vessels throughout the size range studied, with an average decrease in diameter of approximately 20%. The fractional reversal of the serotonin-induced constriction by inhaled NO was directly proportional to inhaled NO concentration, inversely proportional to vessel size, and greater with plasma than with blood perfusion in vessels as large as 3 mm in diameter. The latter indicates that intravascular hemoglobin affected the bronchoalveolar-to-arterial luminal NO concentration gradient in fairly large pulmonary arteries. The data provide information regarding pulmonary arterial smooth muscle accessibility to intrapulmonary gas that should be useful as part of the database for modeling the communication between intrapulmonary gas and pulmonary arterial smooth muscle cells in future studies.

Helium inhalation enhances vasodilator effect of inhaled nitric oxide on pulmonary vessels in hypoxic dogs

There are theoretical and experimental indications that the presence of He as a balance gas markedly increase the diffusion velocity of other gases contained in a gas mixture. We allowed dogs with pulmonary vasoconstriction induced by hypoxia to inhale a mixture of 5 parts per million (ppm) of nitric oxide (NO) and O(2) balanced with He (NO in He) instead of N(2) (NO in N(2)). The dilating effect of NO in He and NO in N(2) on the pulmonary artery was evaluated by determining conventional pulmonary hemodynamic parameters, mean pulmonary artery (PA) pressure (MPAP), and pulmonary vascular resistance indexed to body surface area (PVRI), pulmonary impedance (Z), and the recently developed hemodynamic index, time-corrected wave intensity (WI). The main findings in this study were as follows: 1) hypoxia increased MPAP, PVRI, Z at 0 Hz (Z(0)), Z at the first harmonics, characteristic impedance (Z(c)), the reflection coefficient (Gamma), and the first peak of WI; 2) NO in N(2) reduced Z(0) and Gamma; and 3) NO in He reduced the first peak of WI and reduced Z(0) and Gamma more than NO in N(2). The enhanced vasodilatory effect of NO in He might be associated with facilitated diffusion of NO diluted in the gas mixture with He. In conclusion, increased efficacy of NO in He offers the possibility to reduce the inhaled NO concentration.

Hypoxic vasoconstriction in intact lungs: a role for NADPH oxidase-derived H(2)O(2)?

Hypoxic pulmonary vasoconstriction (HPV) matches lung perfusion with ventilation. Controversy exists whether decreased or increased reactive oxygen species may elicit HPV and from which source such oxygen metabolites are derived. In rabbit lungs, we detected transcripts of a nonphagocytic NADPH oxidase subunit homologous to mitogenic oxidase-1 (Mox1) or NADPH oxidase homolog 1 (NOH-1L). In perfused rabbit lungs, we employed 1) a new NADPH oxidase inhibitor [4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF; 100-600 microM)] and 2) the superoxide dismutase (SOD) inhibitors diethyldithiocarbamic acid (DETC; 100 microM to 10 mM) and triethylenetetramine (TETA; 1-25 mM). Specificity of these agents for HPV was investigated by comparison with U-46619-induced vasoconstrictions. AEBSF induced a transient increase in pulmonary arterial pressure with increased strength of HPV. Subsequent to this initial response, normoxic pulmonary arterial pressure was not affected and HPV was specifically suppressed. Whereas DETC turned out to act in a nonspecific fashion, TETA suppressed HPV specifically. These findings provide evidence of a role for a nonphagocytic NAD(P)H oxidase with superoxide and SOD-related hydrogen peroxide formation in HPV. Because HPV was inhibited but not mimicked by the inhibitors, increased rather than decreased superoxide and/or hydrogen peroxide formation is suggested as the hypoxia-provoked signaling event.

Pulmonary microvascular changes during sepsis: evaluation using intravital videomicroscopy

A variety of pulmonary microvascular changes occur during sepsis. These include abnormal vascular reactivity, leukocyte sequestration, and leakage of protein into the alveoli. Based on intravital videomicroscopy we have developed a method to directly assess in vivo the changes that occur in the pulmonary microcirculation in a rat model of sepsis. Male Sprague-Dawley rats were assigned to control or sepsis groups. Sepsis was induced by cecal ligation and perforation. Twenty four hours later, rats were anesthetized, mechanically ventilated, and their lung prepared for intravital videomicroscopy. A specially designed transparent thoracic window was inserted into the chest wall. The dependent surface of the lung was superfused with saline solution and visualized with an inverted microscope. Vascular contractility, to phenylephrine, (PE) and hypoxia of small (15-25 microm in diameter) and medium (40-50 microm) arterioles was examined. Leukocyte traffic in the pulmonary microcirculation was studied after in vivo labeling of leukocytes with Rhodamine and visualized with fluorescence microscopy. Leak of albumin into the alveolar space was measured with FITC-labeled albumin and fluorescence microscopy. Both small and medium sized pulmonary arterioles in septic animals exhibited attenuated vascular contractility to phenylephrine, but only medium-sized arterioles displayed hypocontractility to hypoxia. Further, in septic animals there was an increase in both the number of stationary leukocytes in the pulmonary microcirculation and an increase in alveolar capillary protein leak. We conclude: (1) direct visualization of the pulmonary microvascular pressor response to hypoxia and PE in the rat is possible using this technique, (2) similar to previous in vitro studies with larger vessels, pulmonary arterioles have an attenuated contractile response to PE and hypoxia in sepsis, and (3) there is an increase in both the number of stationary leukocytes and protein leak into the alveolus in the lungs of septic animals.