L-arginine-related responses to pressure and vasoactive agents in monocrotaline-treated rat pulmonary arteries

The nitric oxide/cGMP signaling pathway in pulmonary hypertension

This article briefly reviews the background of endothelium-dependent vasorelaxation, describes the nitric oxide/cGMP/protein kinase pathway and its role in modulating pulmonary vascular tone and remodeling, and describes three approaches that target the nitric oxide/cGMP pathway in the treatment of patients with pulmonary arterial hypertension.

Training does not affect the alteration in pulmonary artery vasoreactivity in pulmonary hypertensive rats

This study examined the effects of training on intrinsic vasorelaxation and vasoconstriction properties of pulmonary hypertensive rat arteries. Fifty seven male Wistar rats were randomly assigned to 4 groups: normotensive sedentary (n = 14), normotensive trained (n = 15), pulmonary hypertensive sedentary (n = 15) and pulmonary hypertensive trained (n = 13). Pulmonary hypertension was obtained using a chronic hypoxia exposure model. Endothelium-dependent vasorelaxation to acetylcholine (10(-8)-10(-4) M), endothelium-independent vasorelaxation to sodium nitro-prusside (10(-8)-10(-4) M), and vasoconstriction to epinephrine (10(-9)-10(-4) M) and endothelin-1 (10(-12)-10(-7) M) were assessed on isolated rings of large pulmonary arteries. Alterations in endothelium-dependent and -independent vasorelaxation properties as well as enhanced vasoconstrictor responses were obtained in pulmonary hypertensive rats. Chronic exercise did not affect those pulmonary vasoreactivity alterations. A predominant effect of chronic hypoxia over training seems to be partially responsible for this phenomenon, probably through impairment in nitric oxide bioavailability and vascular smooth muscle sensitivity.

Attenuated Noradrenaline-Induced Contraction of Pulmonary Arteries from Rats Treated with Monocrotaline: Role of Rho Kinase

Noradrenaline-induced pulmonary artery contraction was reduced in monocrotaline-treated rats. The possibility that this could be due to alterations in the rho kinase pathway was examined in this study. A combination of nifedipine (10(-6) M) and thapsigargin (10(-6) M) attenuated noradrenaline-induced contraction significantly more in artery segments from monocrotaline-treated rats than in artery segments from control rats indicating a reduced role for calcium sensitization in artery segments from monocrotaline-treated rats. In artery segments permeabilized with ionomycin, CaCl(2) (1.25 mmol/l) produced significantly greater contraction in monocrotaline treated rats compared with control rats. Addition of noradrenaline (10(-5) M) to the bath produced further contractions in both groups. However, noradrenaline-induced contraction was less in monocrotaline-treated rats compared with controls. Y-27632 concentration dependently relaxed ring segments of pulmonary artery pre-contracted with noradrenaline (10(-5)M). The pIC(50) values were 6.46+/- 0.09 (n=5) 5.81+/- 0.06 (n=5) in control and pulmonary hypertensive rings, respectively. The maximum relaxation to Y-27632 was significantly higher in monocrotaline-treated rats. ROCK II was the predominant isoform of rho kinase expressed in the pulmonary artery. The level of expression was increased in rats treated with monocrotaline. These results would suggest that while basal rho kinase activity was elevated in monocrotaline-induced pulmonary hypertension, noradrenaline-induced contraction was attenuated, suggesting poor coupling of the receptor activation to rho kinase activation.

Nitric oxide formation and plasma l-arginine levels in pulmonary hypertensive rats

The elevation of plasma L-arginine levels stimulates nitric oxide (NO) synthesis, but the underlying mechanisms are not yet understood. We examined the role of physiological changes in pulmonary arteries on endogenous NO production. Male Wistar rats were divided into following groups: (1) control rats receiving normal water orally, (2) ARG rats receiving L-arginine water orally, (3) MCT rats injected with monocrotaline (MCT) on day 0 and receiving normal water orally, and (4) MCT+ARG rats injected with MCT on day 0 and receiving L-arginine water orally. The rats were studied after 23 days of dietary intervention. In MCT+ARG rats, supplemental L-arginine exhibited a significant pulmonary vasodilatory effect, as shown by a decreased pulmonary arterial pressure (PAP) (P<0.001), decreased right ventricular hypertrophy (P<0.01), and improved endothelium-dependent relaxation (P<0.01). Also L-arginine inhibited the elevation of plasma endothelin-1 (P<0.01). Oral L-arginine administration increased plasma L-arginine levels about twofold, but in only MCT+ARG rats (i.e., not in ARG rats) did the urinary nitrate excretion significantly increase (P<0.05), which is an indicator of endogenous NO formation. Oral administration of L-arginine is effective against pulmonary vascular remodeling. The data also suggest that the initial elevation of PAP is important for the induction of endogenous NO synthesis.

Defects in caveolin-1 cause dilated cardiomyopathy and pulmonary hypertension in knockout mice

Caveolins are important components of caveolae, which have been implicated in vesicular trafficking and signal transduction. To investigate the in vivo significance of Caveolins in mammals, we generated mice deficient in the caveolin-1 (cav-1) gene and have shown that, in the absence of Cav-1, no caveolae structures were observed in several nonmuscle cell types. Although cav-1(-/-) mice are viable, histological examination and echocardiography identified a spectrum of characteristics of dilated cardiomyopathy in the left ventricular chamber of the cav-1-deficient hearts, including an enlarged ventricular chamber diameter, thin posterior wall, and decreased contractility. These animals also have marked right ventricular hypertrophy, suggesting a chronic increase in pulmonary artery pressure. Direct measurement of pulmonary artery pressure and histological analysis revealed that the cav-1(-/-) mice exhibit pulmonary hypertension, which may contribute to the right ventricle hypertrophy. In addition, the loss of Cav-1 leads to a dramatic increase in systemic NO levels. Our studies provided in vivo evidence that cav-1 is essential for the control of systemic NO levels and normal cardiopulmonary function.

Pulmonary arterial wall distensibility assessed by intravascular ultrasound in children with congenital heart disease: an indicator for pulmonary vascular disease?

BACKGROUND

Both pulmonary hypertension and pulmonary overflow are associated with functional and structural changes of the pulmonary arterial wall. Current techniques to evaluate the pulmonary vasculature neglect the pulsatile nature of pulmonary flow.

STUDY OBJECTIVES

To determine whether the dynamic properties of the pulmonary arterial wall are altered in patients with abnormal pulmonary hemodynamics due to congenital heart defects, and whether these changes are associated with the progression of pulmonary vascular disease (PVD).

PATIENTS AND METHODS

In 43 children with PVD due to congenital heart defects and 12 control subjects, pulmonary arterial pulsatility (the relative increase in vessel area during the cardiac cycle) and distensibility (the inverse of the stress/strain elastic modulus) were determined with intravascular ultrasound. Results were correlated with clinical and hemodynamic parameters.

RESULTS

Pulsatility correlated with pulmonary pulse pressure (p < 0.001), pulmonary-to-systemic vascular resistance ratio (PVR/SVR) [p = 0.001], and hemoglobin concentration (p = 0.01). However, when corrected for these variables, pulsatility did not differ between patients and control subjects. In contrast, arterial wall distensibility decreased with the severity of PVD and correlated independently with pulmonary-to-systemic arterial pressure ratio (p < 0.001) and PVR/SVR (p = 0.03), and with hemoglobin concentration (p < 0.01). Adjusted for hemodynamic variables, distensibility was still decreased in patients with PVD compared to control subjects.

CONCLUSIONS

These results demonstrate that pulmonary arterial wall distensibility is progressively decreased in PVD; moreover, this decreased distensibility is, in part, related to increased distending pressure as a result of pulmonary hypertension but also, in part, to stiffening of the arterial wall during the disease process. Arterial wall distensibility may be of additional value in the evaluation of pulmonary vasculature and ventricular workload.

Pulmonary hypertension secondary to left ventricular dysfunction: the role of nitric oxide and endothelin-1 in the control of pulmonary vascular tone

1. Using an in vivo model of pulmonary hypertension (PHT) secondary to left ventricular dysfunction (LVD), the pulmonary arterial response to the nitric oxide synthase (NOS) blocker L-NAME (30 micromol.min(-1) i.v.) and the subsequent responses to cumulatively administered endothelin-1 (ET-1) (0.001 -- 4 nmol.kg(-1) i.v.) or big ET-1 (0.1 -- 2.0 nmol.kg(-1) i.v.) were studied. Additionally, the effect of the non-selective ET-1 receptor antagonist, SB209670, was investigated. 2. Eight weeks after coronary artery ligation or sham operation, rabbits demonstrated increased mean pulmonary arterial pressure (PAP) accompanied by right ventricular hypertrophy. 3. Blockade of NOS caused a greater increase in basal PAP (increased by 7.7 +/- 1.1 mmHg c.f. 3.8 +/- 1.0 mmHg in controls, P<0.05) and uncovered a greater pulmonary pressor response to exogenous ET-1 in rabbits with PHT (increased by 10.2 +/- 2.3 mmHg c.f. 4.9 +/- 1.0 mmHg in controls, P<0.05). 4. Big ET-1 evoked a pulmonary pressor effect, in both groups of rabbits, that was increased following blockade of NOS and was more potent in rabbits with PHT. 5. The non-selective ET-1 receptor antagonist, SB209670, reduced basal PAP (from 16.9 mmHg to 15.9 mmHg, P < 0.05) in rabbits with PHT and blocked the response to ET-1 in the presence of L-NAME. 6. In conclusion, the results demonstrate that basal NO activity masks a pulmonary pressor response to exogenously administered ET-1. An increased responsiveness to ET-1 was shown in the pulmonary arterial bed of rabbits with PHT secondary to LVD, implicating a pathophysiological role for ET-1 in this model.

Variable expression of endothelial NO synthase in three forms of rat pulmonary hypertension

Endothelial nitric oxide (NO) synthase (eNOS) mRNA and protein and NO production are increased in hypoxia-induced hypertensive rat lungs, but it is uncertain whether eNOS gene expression and activity are increased in other forms of rat pulmonary hypertension. To investigate these questions, we measured eNOS mRNA and protein, eNOS immunohistochemical localization, perfusate NO product levels, and NO-mediated suppression of resting vascular tone in chronically hypoxic (3-4 wk at barometric pressure of 410 mmHg), monocrotaline-treated (4 wk after 60 mg/kg), and fawn-hooded (6-9 mo old) rats. eNOS mRNA levels (Northern blot) were greater in hypoxic and monocrotaline-treated lungs (130 and 125% of control lungs, respectively; P < 0.05) but not in fawn-hooded lungs. Western blotting indicated that eNOS protein levels increased to 300 +/- 46% of control levels in hypoxic lungs (P < 0.05) but were decreased by 50 +/- 5 and 60 +/- 11%, respectively, in monocrotaline-treated and fawn-hooded lungs (P < 0.05). Immunostaining showed prominent eNOS expression in small neomuscularized arterioles in all groups, whereas perfusate NO product levels increased in chronically hypoxic lungs (3.4 +/- 1.4 microM; P < 0.05) but not in either monocrotaline-treated (0.7 +/- 0.3 microM) or fawn-hooded (0.45 +/- 0.1 microM) lungs vs. normotensive lungs (0.12 +/- 0.07 microM). All hypertensive lungs had increased baseline perfusion pressure in response to nitro-L-arginine but not to the inducible NOS inhibitor aminoguanidine. These results indicate that even though NO activity suppresses resting vascular tone in pulmonary hypertension, there are differences among the groups regarding eNOS gene expression and NO production. A better understanding of eNOS gene expression and activity in these models may provide insights into the regulation of this vasodilator system in various forms of human pulmonary hypertension.

Endothelin-1 is elevated in monocrotaline pulmonary hypertension

These studies document striking pulmonary vasoconstrictor response to nitric oxide synthase (NOS) inhibition in monocrotaline (MCT) pulmonary hypertension in rats. This constriction is caused by elevated endothelin (ET)-1 production acting on ETA receptors. Isolated, red blood cell plus buffer-perfused lungs from rats were studied 3 wk after MCT (60 mg/kg) or saline injection. MCT-injected rats developed pulmonary hypertension, right ventricular hypertrophy, and heightened pulmonary vasoconstriction to ANG II and the NOS inhibitor NG-monomethyl-L-arginine (L-NMMA). In MCT-injected lungs, the magnitude of the pulmonary pressor response to NOS inhibition correlated strongly with the extent of pulmonary hypertension. Pretreatment of isolated MCT-injected lungs with combined ETA (BQ-123) plus ETB (BQ-788) antagonists or ETA antagonist alone prevented the L-NMMA-induced constriction. Addition of ETA antagonist reversed established L-NMMA-induced constriction; ETB antagonist did not. ET-1 concentrations were elevated in MCT-injected lung perfusate compared with sham-injected lung perfusate, but ET-1 levels did not differ before and after NOS inhibition. NOS inhibition enhanced hypoxic pulmonary vasoconstriction in both sham- and MCT-injected lungs, but the enhancement was greater in MCT-injected lungs. Results suggest that in MCT pulmonary hypertension, elevated endogenous ET-1 production acting through ETA receptors causes pulmonary vasoconstriction that is normally masked by endogenous NO production.

Chronic pulmonary hypertension--the monocrotaline model and involvement of the hemostatic system

Monocrotaline (MCT) is a toxic pyrrolizidine alkaloid of plant origin. Administration of small doses of MCT or its active metabolite, monocrotaline pyrrole (MCTP), to rats causes delayed and progressive lung injury characterized by pulmonary vascular remodeling, pulmonary hypertension, and compensatory right heart hypertrophy. The lesions induced by MCT(P) administration in rats are similar to those observed in certain chronic pulmonary vascular diseases of people. This review begins with a synopsis of the hemostatic system, emphasizing the role of endothelium since endothelial cell dysfunction likely underlies the pathogenesis of MCT(P)-induced pneumotoxicity. MCT toxicology is discussed, focusing on morphologic, pulmonary mechanical, hemodynamic, and biochemical and molecular alterations that occur after toxicant exposure. Fibrin and platelet thrombosis of the pulmonary microvasculature occurs after administration of MCT(P) to rats, and several investigators have hypothesized that thrombi contribute to the lung injury and pulmonary hypertension. The evidence for involvement of the various components of the hemostatic system in MCT(P)-induced vascular injury and remodeling is reviewed. Current evidence is consistent with involvement of platelets and an altered fibrinolytic system, yet much remains to be learned about specific events and signals in the vascular pathogenesis.

EndothelinA receptor blockade improves nitric oxide-mediated vasodilation in monocrotaline-induced pulmonary hypertension

BACKGROUND

Nitric oxide (NO) and endothelin (ET) have been implicated in the pathogenesis of pulmonary hypertension (PH). Chronic ETA antagonist therapy reduces PH in monocrotaline (MCT)-treated rats. Interactions between the L-arginine-NO pathway and the ET system have been described. We therefore studied the effect of long-term treatment with an oral ETA antagonist (LU 135252) on NO-related vasodilation in isolated lungs from control rats and rats with MCT-induced PH.

METHODS AND RESULTS

Three weeks after MCT injection, PH was associated with an increase in right ventricular pressure (from 27.4 +/- 0.9 to 66.6 +/- 4.1 mm Hg) and a decrease in endothelium-independent vasodilation in response to sodium nitroprusside (10(-10) to 10(-5) mol/L; delta Emax, from 11.1 +/- 0.9 to 2.7 +/- 0.3 mm Hg). Endothelium-dependent vasodilation in response to acetylcholine (10(-9) to 10(-4) mol/L) and the calcium ionophore A23187 (10(-9) to 10(-7) mol/L) remained unaffected. Treatment with LU 135252 did not significantly affect the endothelium-dependent and -independent vasodilations in control rats. However, in MCT-treated rats, LU 135252 therapy significantly reduced right ventricular pressure (39.7 +/- 2.1 mm Hg), potentiated acetylcholine-induced vasodilatation (delta Emax, from 1.6 +/- 0.2 to 3.7 +/- 0.4 mm Hg), and improved the responses to sodium nitroprusside (delta Emax, from 2.7 +/- 0.3 to 5.6 +/- 0.6 mm Hg). LU 135252 did not significantly alter the non-receptor-mediated endothelium-dependent vasodilation to A23187 or pulmonary constitutive NO synthase activity.

CONCLUSIONS

MCT PH is associated with a reduced smooth muscle responsiveness to NO but a maintained endothelium-dependent vasodilatory potency. Long-term ETA antagonist therapy not only restores smooth muscle responsiveness to NO but also increases endothelium-dependent dilation in response to acetylcholine. This mechanism may contribute to the therapeutic benefit of ETA antagonists in PH.

High vascular and airway pressures increase interstitial protein mRNA expression in isolated rat lungs

We hypothesized that wall stresses produced by high peak airway (Paw) and venous (Ppv) pressures would increase mRNA levels for structural proteins of the interstitial matrix in isolated rat lungs. Groups of lungs (n = 6) were perfused for 4 h at a peak Paw of 35 cmH2O (HiPaw), cyclical peak Ppv of 28 cmH2O (HiPv), or baseline vascular and airway pressures (LoPress). In two separate groups, comparable peak pressures increased capillary filtration coefficient fourfold in each group. Northern blots were probed for mRNA of alpha 1(I), alpha 1(III), and alpha 2(IV) procollagen chains, laminin B chain, fibronectin, and transforming growth factor-beta 1, and densities were normalized to 18S rRNA. mRNA was significantly higher in the HiPv group for type I (4.3-fold) and type III (3.8-fold) procollagen and laminin B chain (4.8-fold) and in the HiPaw group for type I (2.4-fold) and type IV (4.5-fold) procollagen and laminin B chain (2.3-fold) than in the LoPress group. Only fibronectin mRNA was significantly increased (3.9-fold) in the LoPress group relative to unperfused lungs. Estimated wall stresses were highest for alveolar septa in the HiPaw group and for capillaries in the HiPv group. The different patterns of mRNA expression are attributed to different regional stresses or extent of injury.