Sustained pulmonary vasodilation after inhaled nitric oxide for hypoxic pulmonary hypertension in swine

Generation of purified nitric oxide from liquid N2O4 for the treatment of pulmonary hypertension in hypoxemic swine

Inhaled nitric oxide (NO) selectively dilates pulmonary blood vessels, reduces pulmonary vascular resistance (PVR), and enhances ventilation-perfusion matching. However, existing modes of delivery for the treatment of chronic pulmonary hypertension are limited due to the bulk and heft of large tanks of compressed gas. We present a novel system for the generation of inhaled NO that is based on the initial heat-induced evaporation of liquid N2O4 into gas phase NO2 followed by the room temperature reduction to NO by an antioxidant, ascorbic acid cartridge just prior to inhalation. The biologic effects of NO generated from liquid N2O4 were compared with the effects of NO gas, on increased mean pulmonary artery pressure (mPAP) and PVR in a hypoxemic (FiO2 15%) swine model of pulmonary hypertension. We showed that NO concentration varied directly with the fixed cross sectional flow of the outflow aperture when studied at temperatures of 45, 47.5 and 50°C and was independent of the rate of heating. Liquid N2O4-sourced NO at 1, 5, and 20 ppm significantly reduced the elevated mPAP and PVR induced by experimental hypoxemia and was biologically indistinguishable from gas source NO in this model. These experiments show that it is feasible to generate highly purified NO gas from small volumes of liquid N2O4 at concentrations sufficient to lower mPAP and PVR in hypoxemic swine, and suggest that a miniaturized ambulatory system designed to generate biologically active NO from liquid N2O4 is achievable.

Role of nitric oxide in the pathogenesis of chronic pulmonary hypertension

Chronic pulmonary hypertension is a serious complication of a number of chronic lung and heart diseases. In addition to vasoconstriction, its pathogenesis includes injury to the peripheral pulmonary arteries leading to their structural remodeling. Increased pulmonary vascular synthesis of an endogenous vasodilator, nitric oxide (NO), opposes excessive increases of intravascular pressure during acute pulmonary vasoconstriction and chronic pulmonary hypertension, although evidence for reduced NO activity in pulmonary hypertension has also been presented. NO can modulate the degree of vascular injury and subsequent fibroproduction, which both underlie the development of chronic pulmonary hypertension. On one hand, NO can interrupt vascular wall injury by oxygen radicals produced in increased amounts in pulmonary hypertension. NO can also inhibit pulmonary vascular smooth muscle and fibroblast proliferative response to the injury. On the other hand, NO may combine with oxygen radicals to yield peroxynitrite and other related, highly reactive compounds. The oxidants formed in this manner may exert cytotoxic and collagenolytic effects and, therefore, promote the process of reparative vascular remodeling. The balance between the protective and adverse effects of NO is determined by the relative amounts of NO and reactive oxygen species. We speculate that this balance may be shifted toward more severe injury especially during exacerbations of chronic diseases associated with pulmonary hypertension. Targeting these adverse effects of NO-derived radicals on vascular structure represents a potential novel therapeutic approach to pulmonary hypertension in chronic lung diseases.

Combined therapy with zaprinast and inhaled nitric oxide abolishes hypoxic pulmonary hypertension

OBJECTIVE

To determine whether the combination of the phosphodiesterase 5 inhibitor zaprinast and inhaled nitric oxide (NO) decreases hypoxic pulmonary hypertension in the rat.

DESIGN

Prospective, experimental study.

SETTING

Animal laboratory of a university medical center.

SUBJECTS

Male Sprague-Dawley rats.

INTERVENTIONS

Anesthetized rats were mechanically ventilated and instrumented for measurement of mean systemic arterial pressure, pulmonary arterial pressure, and cardiac output. In group 1, four acute hypoxic challenges (FIO2 = 0.17 for 5 mins) were performed: initial, during 40 ppm inhaled NO, immediately after discontinuation of 5 mins of inhaled NO, and final. In group 2 rats, an initial hypoxic challenge was performed and rats then received zaprinast (3 mg/kg bolus followed by 0.3 mg/kg/min infusion). Four hypoxic challenges analogous to group 1 were then performed during zaprinast administration.

MEASUREMENTS AND MAIN RESULTS

Initial hypoxic challenge produced similar increases in pulmonary arterial pressure in both groups. In group 1, inhaled NO either only before or only during hypoxia decreased the pulmonary hypertensive response to hypoxia. In group 2, zaprinast administration did not alter hemodynamics. Zaprinast alone decreased the pulmonary hypertensive response to hypoxia. The combination of zaprinast and inhaled NO (either before or during hypoxia) abolished the pulmonary hypertensive response to hypoxia.

CONCLUSIONS

Treatment with inhaled NO for 5 mins before but not during hypoxia is as effective as inhaled NO during hypoxia. Inhaled NO and zaprinast both decrease the pulmonary hypertensive response to hypoxia, and the combination abolishes the response. The combination of a phosphodiesterase 5 inhibitor and inhaled NO may have clinical applicability in the treatment of pulmonary hypertension.

Superoxide and endothelium-dependent constriction to flow in porcine small pulmonary arteries

1. The aim of this study was to determine the response of porcine small pulmonary arteries to intralumenal flow and to identify the cellular mechanisms and potential mediators involved in the response. 2. Porcine small pulmonary arteries were isolated from a branch of the main intrapulmonary artery of the lower lung lobe and studied in a perfusion myograph system that allowed independent control of transmural pressure and intralumenal flow. At a transmural pressure of 20 mmHg, the baseline internal diameter (BID) of the arteries was 251.2+/-16.1 microm (n=16). 3. Under quiescent conditions or during constriction with U46619 to approximately 60% of BID, intralumenal flow caused reversible constriction in arteries with endothelium (in the presence of U46619, flow decreased diameter from 60.0+/-2.5% to 49.5+/-3.0% BID at 10 microl min(-1), n=16, P<0.05) but no change in diameter of arteries without endothelium. 4. In the presence of superoxide dismutase (SOD, 150 u ml(-1)), the response to flow was converted from constriction to vasodilatation (in presence of U46619 and SOD, flow increased diameter from 54.2+/-3.4% to 76.7+/-4.5% BID at 10 microl min(-1), n=10, P<0.05). Inhibition of NO synthase with L-NAME (3 x 10(-5) M) abolished the flow-induced vasodilatation occurring in the presence of SOD and the flow-induced constriction occurring in the absence of SOD. In arteries with endothelium, L-NAME (3 x 10(-5) M) caused significant vasoconstriction, whereas SOD did not alter vasomotor tone. 5. Acetylcholine (10(-8) to 10(-6) M) caused endothelium-dependent relaxation of small pulmonary arteries that was not significantly affected by SOD (150 u ml(-1)) but was inhibited by L-NAME (3 x 10(-5) M). 6. These results suggest that in small, porcine, isolated pulmonary arteries, intralumenal flow increases the production of NO but this is obscured by the generation of superoxide which causes vasoconstriction.

New developments in nitrosovasodilator therapy

Under basal conditions, nitric oxide (NO) modulates vascular tone, serves as an antithrombotic agent, and inhibits vascular smooth muscle cell proliferation. NO deficiency has been implicated in the pathophysiology of several vascular disorders, including hypertension, atherosclerosis, and restenosis, and provides a plausible biologic basis for the use of NO replacement therapy in these conditions. Treatment with conventional nitrate preparations is limited by a short therapeutic half-life, systemic absorption with potentially adverse hemodynamic effects, and drug tolerance. To overcome these limitations, novel delivery systems and novel NO donors have been developed that offer selective effects, a prolonged half-life, and a reduced incidence of tolerance.

Nitric oxide dose response during moderate and severe hypoxia in swine

BACKGROUND

Elucidation of factors that influence the dose response to inhaled nitric oxide is crucial to optimizing its therapeutic benefit. We investigated whether severity of hypoxia is one such factor.

METHODS

Seven Yorkshire swine underwent 14 triphasic experiments: (1) control period of mechanical ventilation (fractional concentration of oxygen = 0.3); (2) induction of hypoxic pulmonary hypertension (fractional concentration of oxygen = 0.1 to 0.15); and (3) inhaled nitric oxide at 5, 10, 20, 40, and 80 parts per million (ppm). Hemodynamics and arterial blood gases were measured by pulmonary and systemic arterial catheters.

RESULTS

Experiments were divided into two groups of seven based on hypoxia severity: severe (arterial oxygen tension, 25 to 40 mm Hg) and moderate (arterial oxygen tension, 41 to 60 mm Hg). The percent changes in mean pulmonary artery pressure after each dose were compared within each group by repeated measure analysis of variance and each dose was compared between the two groups by Student's t test. A statistically significant dose response existed in both groups (p < 0.02). Low doses resulted in significantly less vasodilation in the severe versus the moderate hypoxia group (5 ppm, 59% +/- 6% versus 94% +/- 7%, p = 0.003; 10 ppm, 69% +/- 8% versus 99% +/- 8%, p = 0.017).

CONCLUSIONS

Lower doses are significantly less effective in achieving maximal pulmonary vasodilation during severe hypoxia. Therefore, the degree of hypoxia is a determinant of the inhaled nitric oxide dose response.