Role of EDRF in pulmonary circulation during sustained hypoxia

Safety, efficacy, and clinical utility of macitentan in the treatment of pulmonary arterial hypertension

Pulmonary arterial hypertension is a progressive, debilitating disease caused by a dysregulation of the pulmonary vascular tone that inevitably leads to right heart failure and death without treatment. Until relatively recently, the treatment options for those afflicted by pulmonary arterial hypertension were limited; today, a greater understanding of the pathophysiology behind this disease has led to several evidence-based therapies that can improve pulmonary function and quality of life for these patients. One of the primary mediators of pulmonary vascular tone is endothelin-1, which is a potent and long-lasting vasoconstrictor. Macitentan is a second-generation endothelin receptor antagonist that acts selectively as a pulmonary vasodilator without the significant side effects noted with previous endothelin receptor antagonists. This review focuses on the mechanism of action and pharmacokinetics of macitentan, as well as the adverse effects, efficacy, and clinical uses of macitentan in the clinical trials to date. In addition, the authors briefly review clinical trials currently underway to illustrate possible future directions for the use of macitentan.

Role of nitric oxide in pathophysiology and treatment of pulmonary hypertension

Pulmonary hypertension is a condition characterized by vasoconstriction, vascular cell proliferation, inflammation, microthrombosis, and vessel wall remodelation. Pulmonary endothelial cells produce vasoactive substances with vasoconstrictive as well as vasodilatative effects. The imbalance of these endothelium-derived vasoactive substances induced by endothelial dysfunction is very important in the pathogenesis of PH. One of most important substances with vasodilatative effect is nitric oxide. We provide a comprehensive insight into role of NO in the pathgenesis of PH and discuss perspectives and challenges in PH therapy based on NO administration.

Analysis of responses to the Rho-kinase inhibitor Y-27632 in the pulmonary and systemic vascular bed of the rat

Responses to the Rho kinase inhibitor Y-27632 were investigated in the anesthetized rat. Under baseline conditions intravenous injections of Y-27632 decreased pulmonary and systemic arterial pressures and increased cardiac output. The decreases in pulmonary arterial pressures were enhanced when baseline tone was increased with U-46619, and under elevated tone conditions Y-27632 produced similar percent decreases in pulmonary and systemic arterial pressures. Injections of Y-27632 prevented and reversed the hypoxic pulmonary vasoconstrictor response. The increase in pulmonary arterial pressure in response to ventilation with a 10% O(2)-90% N(2) gas mixture was not well maintained during the period of hypoxic exposure. Treatment with the nitric oxide (NO) synthase (NOS) inhibitor nitro-l-arginine methyl ester (l-NAME) increased pulmonary arterial pressure and prevented the decline or fade in the hypoxic pulmonary vasoconstrictor response. The hypoxic pulmonary vasoconstrictor response was reversed by Y-27632 in control and in l-NAME-treated animals. The Rho kinase inhibitor attenuated increases in pulmonary arterial pressures in response to intravenous injections of serotonin, angiotensin II, and Bay K 8644. Y-27632, sodium nitrite, and BAY 41-8543, a guanylate cyclase stimulator, decreased pulmonary and systemic arterial pressures and vascular resistances in monocrotaline-treated rats. These data suggest that Rho kinase is involved in the regulation of baseline tone and in the mediation of pulmonary vasoconstrictor responses. The present data suggest that the hypoxic pulmonary vasoconstrictor response is modulated by the release of NO that mediates the nonsustained component of the response in the anesthetized rat. These data suggest that Rho kinase and NOS play important roles in the regulation of vasoconstrictor tone in physiological and pathophysiological states and that monocrotaline-induced pulmonary hypertension can be reversed by agents that inhibit Rho kinase, generate NO, or stimulate soluble guanylate cyclase.

Rosuvastatin ameliorates the development of pulmonary arterial hypertension in the transgenic (mRen2)27 rat

We have recently reported that transgenic (mRen2)27 rats (Ren2 rats) exhibit pulmonary arterial hypertension (PAH), which is, in part, mediated by oxidative stress. Since 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors (statins) exhibit beneficial vascular effects independent of cholesterol synthesis, we hypothesized that rosuvastatin (RSV) treatment ameliorates PAH and pulmonary vascular remodeling in Ren2 rats, in part, by reducing oxidative stress. Six-week-old male Ren2 and Sprague-Dawley rats received RSV (10 mg x kg(-1) x day(-)1 ip) or vehicle for 3 wk. After treatment, right ventricular systolic pressure (RVSP) and mean arterial pressure (MAP) were measured. To evaluate treatment effects on pulmonary arteriole remodeling, morphometric analyses were performed to quantitate medial thickening and cell proliferation, whereas whole lung samples were used to quantitate the levels of 3-nitrotyrosine, superoxide, stable nitric oxide (NO) metabolites [nitrates and nitrites (NO(x))], and expression of NO synthase isoforms. In the Ren2 rat, RVSP is normal at 5 wk of age, PAH develops between 5 and 7 wk of age, and the elevated pressure is maintained with little variation through 13 wk. At 8 wk of age, left ventricular function and blood gases were normal in the Ren2 rat. Ren2 rats exhibited elevations in medial hypertrophy due to smooth muscle cell proliferation, 3-nitrotyrosine, NO(x), NADPH oxidase activity, and endothelial NO synthase expression compared with Sprague-Dawley rats. RSV significantly blunted the increase in RVSP but did not reduce MAP in the Ren2 rat; additionally, RSV significantly attenuated the elevated parameters examined in the Ren2 rat. These data suggest that statins may be a clinically viable adjunct treatment of PAH through reducing peroxynitrite formation.

Role of NO and prostaglandins in acute hypoxic vasoconstriction in sheep pulmonary veins

The aim of this study was to investigate the effect of hypoxia on and the role of nitric oxide (NO) and cyclooxgenase inhibition in hypoxia-induced vasoconstriction in sheep isolated pulmonary veins. We used the potent pulmonary vasoconstrictor U46619, a thromboxane analog, as a precontractile agent. Our results showed that hypoxia caused a vasoconstriction both under resting tone and in U46619 (10(-6) mol/l) precontracted pulmonary veins. In the presence of the nonselective NO synthase inhibitior Nomega-nitro-L-arginine methyl ester (L-NAME; 3 x 10(-5) mol/l), the hypoxic pulmonary vasoconstriction (HPV) was significantly increased in veins under resting force. However, there was a decrease in HPV in pulmonary veins precontracted with U46619 in the presence of L-NAME. Moreover, L-NAME markedly augmented the U46619-induced pulmonary contractions under normoxic conditions. Cyclooxygenase inhibition with indomethacin (10(-5) mol/l) significantly reduced the HPV both under resting tone and in precontracted veins. Indomethacin also significantly decreased the U46619-induced pulmonary contractions prior to the induction of hypoxia. Our findings suggest that NO and prostaglandins can act as a modulators of the hypoxic vasoconstriction in isolated pulmonary veins.

Pulmonary vascular iNOS induction participates in the onset of chronic hypoxic pulmonary hypertension

Pathogenesis of hypoxic pulmonary hypertension is initiated by oxidative injury to the pulmonary vascular wall. Because nitric oxide (NO) can contribute to oxidative stress and because the inducible isoform of NO synthase (iNOS) is often upregulated in association with tissue injury, we hypothesized that iNOS-derived NO participates in the pulmonary vascular wall injury at the onset of hypoxic pulmonary hypertension. An effective and selective dose of an iNOS inhibitor, L-N6-(1-iminoethyl)lysine (L-NIL), for chronic peroral treatment was first determined (8 mg/l in drinking water) by measuring exhaled NO concentration and systemic arterial pressure after LPS injection under ketamine+xylazine anesthesia. A separate batch of rats was then exposed to hypoxia (10% O2) and given L-NIL or a nonselective inhibitor of all NO synthases, N(G)-nitro-L-arginine methyl ester (L-NAME, 500 mg/l), in drinking water. Both inhibitors, applied just before and during 1-wk hypoxia, equally reduced pulmonary arterial pressure (PAP) measured under ketamine+xylazine anesthesia. If hypoxia continued for 2 more wk after L-NIL treatment was discontinued, PAP was still lower than in untreated hypoxic controls. Immunostaining of lung vessels showed negligible iNOS presence in control rats, striking iNOS expression after 4 days of hypoxia, and return of iNOS immunostaining toward normally low levels after 20 days of hypoxia. Lung NO production, measured as NO concentration in exhaled air, was markedly elevated as early as on the first day of hypoxia. We conclude that transient iNOS induction in the pulmonary vascular wall at the beginning of chronic hypoxia participates in the pathogenesis of pulmonary hypertension.

Endogenous nitric oxide and pulmonary circulation response to hypoxia in high-altitude adapted Tibetan sheep

Nitric oxide (NO) is important for the pulmonary circulation response to acute and chronic hypoxia. We examined effects of endogenous nitric oxide synthase (NOS) inhibition on pulmonary vascular tone in response to hypoxia in Tibetan sheep dwelling at 3,000 m above sea level using a pressure chamber. Unanaesthetized male sheep living at 2,300 m above sea level ( n=7) were prepared for vascular monitoring. Pulmonary artery ( P(pa)), pulmonary artery wedge ( P(cwp)) and systemic artery pressures together with cardiac output (CO) were measured, and pulmonary vascular resistance (PVR) was calculated as ( P(pa)- P(cwp))/CO. A non-selective NOS inhibitor, N(omega)-nitro- l-arginine (NLA; 20 mg kg(-1)), and a selective NOS inhibitor, ONO-1714 (0.1 mg kg(-1)), were used and measurements were made at 0 m, 2,300 m, and 4,500 m, with and without the NOS inhibitors. After NLA, P(pa) increased slightly and CO decreased in animals at baseline (2,300 m). The increased PVR after NLA at 4,500 m was greater than that at 2,300 m ( P<0.05). Selective NOS inhibition increased PVR at baseline, but not at 4,500 m. The enhanced pulmonary vasoconstriction after NO inhibition at basal and hypoxic conditions suggests a modulating role of NOS bioactivity in the pulmonary circulation and that augmented endothelial NOS plays a counterregulatory role in the pulmonary vasoconstrictor response to acute hypoxia in high-altitude adapted Tibetan sheep.

Contribution of nitric oxide to adaptation of tibetan sheep to high altitude

We examined the effects of endogenous nitric oxide synthase (NOS) inhibition on pulmonary hemodynamics in awake sheep living at low and high altitudes to evaluate the role of NO in adaptation to an hypoxic environment. Unanaesthetized male sheep in three places--Matsumoto, Japan (680 m above sea level), Xing, China (2300 m) and Maxin, China (3750 m)--were prepared for measurements of pulmonary artery (Ppa) and pulmonary vascular resistance (PVR) before and after the NOS inhibition. The non-selective NOS inhibitor, Nw-nitro-l-argine (NLA, 20 mg/kg) was used. Baseline Ppa became elevated with an increase in altitude. After NLA administration, PVR significantly increased in animals of all groups. However, the increase in PVR after NLA in tibetan sheep at 3750 m was significantly higher than those in other groups. We conclude that augmented endogenous NO production may contribute to regulating the pulmonary vascular tone in tibetan sheep (3750 m) adapted to high altitude.

Augmented Pulmonary Vascular and Venous Constrictions to NG-Nitro-L-Arginine Methyl Ester in Rats with Monocrotaline-Induced Pulmonary Hypertension

The hemodynamic effects of N(G)-nitro-L-arginine methyl ester (L-NAME, inhibitor of nitric oxide (NO) synthase) were examined in thiobutabarbital-anesthetized control-rats and rats with monocrotaline-induced pulmonary hypertension. L-NAME (1-16 mg/kg i.v.) increased mean arterial pressure, systemic vascular resistance, venous resistance and pulmonary vascular resistance, and decreased cardiac output in both the control and pulmonary hypertensive rats. Relative to the controls, L-NAME (16 mg/kg) caused a smaller increase (approximately 50% of control) in mean arterial pressure in the pulmonary hypertensive rats, but greater increases in venous (approximately 200%) as well as pulmonary vascular (approximately 400%) resistances and a greater decrease in cardiac output (approximately 140%). The results show that NO is an important dilator within the arterial, venous and pulmonary circulation; its pulmonary and venous dilator roles are augmented in pulmonary hypertension.

Chronic hypoxia attenuates nitric oxide-dependent hemodynamic responses to acute hypoxia

Alterations in the nitric oxide (NO) pathway have been implicated in the pathogenesis of chronic hypoxia-induced pulmonary hypertension. Chronic hypoxia can either suppress the NO pathway, causing pulmonary hypertension, or increase NO release in order to counteract elevated pulmonary arterial pressure. We determined the effect of NO synthase inhibitor on hemodynamic responses to acute hypoxia (10% O(2)) in anesthetized rats following chronic exposure to hypobaric hypoxia (0.5 atm, air). In rats raised under normoxic conditions, acute hypoxia caused profound systemic hypotension and slight pulmonary hypertension without altering cardiac output. The total systemic vascular resistance (SVR) decreased by 41 +/- 5%, whereas the pulmonary vascular resistance (PVR) increased by 25 +/- 6% during acute hypoxia. Pretreatment with N(omega)-nitro-L-arginine methyl ester (L-NAME; 25 mg/kg) attenuated systemic vasodilatation and enhanced pulmonary vasoconstriction. In rats with prior exposure to chronic hypobaric hypoxia, the baseline values of mean pulmonary and systemic arterial pressure were significantly higher than those in the normoxic group. Chronic hypoxia caused right ventricular hypertrophy, as evidenced by a greater weight ratio of the right ventricle to the left ventricle and the interventricular septum compared to the normoxic group (46 +/- 4 vs. 28 +/- 3%). In rats which were previously exposed to chronic hypoxia (half room air for 15 days), acute hypoxia reduced SVR by 14 +/- 6% and increased PVR by 17 +/- 4%. Pretreatment with L-NAME further inhibited the systemic vasodilatation effect of acute hypoxia, but did not enhance pulmonary vasoconstriction. Our results suggest that the release of NO counteracts pulmonary vasoconstriction but lowers systemic vasodilatation on exposure to acute hypoxia, and these responses are attenuated following adaptation to chronic hypoxia.

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.

The effect of the nitric oxide synthase inhibitor N-gamma-nitro-L-argine methyl ester on hypoxic pulmonary vasoconstriction

We studied the role of nitric oxide in the regulation of pulmonary arterial tone and hypoxic pulmonary vasoconstriction. Rat pulmonary arteries (n=65, diameter=440+/-12 microm) were loaded to 17.5 mm Hg in a wire myograph and incubated with the nitric oxide synthase inhibitor N-gamma-nitro-L-argine methyl ester (L-NAME; 1, 10 or 100 microM) or distilled water (50 microl) prior to preconstriction with either 100 microM prostaglandin F(2 alpha) followed by acetylcholine (0.1-100 microM) or 5 microM prostaglandin F(2 alpha) followed by hypoxia. Concentrations of L-NAME (10 and 100 microM) which attenuated acetylcholine dilatation, elevated basal tone from 0. 2+/-0.5% to 9.4+/-2.1% (P<0.01) and 18.3+/-3.2% (P<0.001), respectively, potentiated contraction to 5 microM prostaglandin F(2 alpha) from 35.9+/-3.1% to 56.2+/-6.8% (P<0.05) and 66.4+/-5.8% (P<0.001), respectively, but had no significant effect on hypoxic pulmonary vasoconstriction. This suggests basal pulmonary nitric oxide release occurs, as well as in response to agonist-induced contraction, but not hypoxic pulmonary vasoconstriction.

Hypoxia causes leukocyte adherence to mesenteric venules in nonacclimatized, but not in acclimatized, rats

Although the effects of ischemia-reperfusion have received considerable attention, few studies have directly evaluated the microcirculatory response to systemic hypoxia. The overall objective of this study was to assess the effect of environmental hypoxia on adhesive interactions of circulating leukocytes with rat mesenteric venules by using intravital microscopy. Experiments were designed to 1) characterize the adhesive interactions of circulating leukocytes to venules during acute hypoxia produced by a reduction in inspired PO(2), 2) evaluate the role of nitric oxide in these adhesive interactions, 3) determine whether the effect of hypoxia on leukocyte adhesive interactions differs between acclimatized and nonacclimatized rats, and 4) assess whether compensatory changes in nitric oxide formation contribute to this difference. The results showed that acute hypoxia promotes leukocyte-endothelial adherence in mesenteric venules of nonacclimatized rats. The mechanism of this response is consistent with depletion of nitric oxide within the microcirculation. In contrast, no leukocyte-endothelial adherence occurred during hypoxia in rats acclimatized to hypobaric hypoxia. The results are consistent with increased nitric oxide formation due to expression of inducible nitric oxide synthase during the acclimatization period. Further studies are needed to establish the cause of nitric oxide depletion during acute hypoxia as well as to define the compensatory responses that attenuate hypoxia-induced leukocyte-endothelial adherence in the microvasculature of acclimatized rats.