The effect of bunazosin on monocrotaline-induced pulmonary hypertension in rats

Neurohormonal modulation as therapeutic avenue for right ventricular dysfunction in pulmonary artery hypertension: till the dawn, waiting

Neuro-hormonal activation may lead to or be associated with pulmonary arterial hypertension (PAH) and right ventricular dysfunction. Notwithstanding whether it is the cause or the consequence of PAH-related right ventricle (RV) dysfunction neurohormonal activation contributes to significant morbidity and mortality in patients with PAH and the progression of RV dysfunction. Experimental data regarding the use of beta adrenergic blockade and renin-angiotensin aldosterone system modulation are encouraging. However, clinical studies have largely been negative or neutral; and, neuro-hormonal modulation is discouraged in patients with PAH related RV dysfunction for fear of systemic hypotension. Herein, we summarize the pathophysiological background that supports the potential role of neuro-hormonal modulation in the management of PAH related RV dysfunction; also present current clinical experience; and, discuss the need for controlled studies to move forward. Lastly, we review potential non- pharmacological modalities for neuro-hormonal modulations in PAH patients with RV dysfunction.

Search for an animal model to investigate selective pulmonary vasodilation

Pulmonary arterial hypertension is a life-threatening disease with a poor prognosis. Oral treatment with vasodilators is often limited by systemic hypotension. Inhalation of vasodilators offers the opportunity for selective pulmonary vasodilation. Testing selective pulmonary vasodilation by inhaled nitric oxide or alternative substances in animal models requires an increased pulmonary vascular tone. The aim of this study was to identify animal models that are suitable for investigating selective pulmonary vasodilation. To do so, a haemodynamic stable pulmonary hypertension was initiated, with a 30 min duration deemed to be a sufficient time interval before and after a possible intervention. In anaesthetized and mechanically-ventilated Sprague-Dawley rats pulmonary hypertension was induced either by acute hypoxia due to reduction of the inspired oxygen fraction from 0.21 to 0.1 ( n = 6), a fixed infusion rate of the thromboxane analogue U46619 (240 ng/min; n = 6) or a monocrotaline injection (MCT; 60 mg/kg applied 23 days before the investigation; n = 7). The animals were instrumented to measure right ventricular and systemic arterial pressures. Acute hypoxia caused a short, and only transient, increase of pulmonary artery pressure as well as profound systemic hypotension which suggested haemodynamic instability. U46619 infusion induced variable changes in the pulmonary and systemic vascular tone without sufficient stabilization within 30 min. MCT provoked sustained pulmonary hypertension with normal systemic pressure values and inhalation of nitric oxide caused selective pulmonary vasodilation. In conclusion, out of the three examined rat animal models only MCT-induced pulmonary hypertension is a solid and reliable model for investigating selective pulmonary vasodilation.

α1-adrenoceptor subtypes mediating noradrenaline-induced contraction of pulmonary artery from pulmonary hypertensive rats

The effect of monocrotaline-induced pulmonary hypertension on alpha(1)-adrenoceptor-mediated contractions of pulmonary artery segments was studied. In control and monocrotaline-treated rats, noradrenaline evoked concentration-dependent contractions of the pulmonary artery. There was no change in the potency and affinity of noradrenaline but the maximum response and receptor reserve were significantly reduced. Noradrenaline-induced contractions were competitively antagonized by prazosin, 2-(2-6dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride (WB 4101) and 8-[2-[4-(2methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4,5]decane-7,9 dione dihydrochloride (BMY 7378) with pA(2) values of 9.64+/-0.16, 9.45+/-0.10 and 8.30+/-0.14, respectively. These antagonists also competitively antagonized noradrenaline-induced contractions of pulmonary artery segments isolated from rats with monocrotaline-induced pulmonary hypertension. The pA(2) values were 9.66+/-0.11 (prazosin), 9.62+/-0.09 (WB 4101) and 8.47+/-0.15 (BMY 7378). Chloroethylclonidine (CEC) shifted noradrenaline concentration-response curve to the right and depressed the maximum response. There was no difference between the effects of CEC in both groups. It was therefore concluded that pulmonary hypertension significantly reduced noradrenaline-induced contractions of the rat pulmonary artery without affecting the sensitivity. Studies with receptor-selective antagonists confirmed that alpha(1)D-adrenoceptor subtype is the predominant receptor subtype in the pulmonary artery and this was maintained in this disease state.

Alpha1-adrenergic hypothesis for pulmonary hypertension

Pulmonary hypertension (PH) is a chronic and disabling condition that affects the pulmonary vasculature. Once PH is diagnosed, the prognosis is generally poor with a rapid downhill course. PH management is largely empirical because the underlying pathophysiologic mechanisms that are responsible for the excessive vasoconstrictor and vascular smooth muscle proliferative responses are poorly understood. Based on new information concerning the role of adrenergic receptors in regulating various cellular functions, a new perspective on the genesis of PH has emerged, along with a unifying hypothesis for the role of alpha1-adrenergic receptors present in the pulmonary vasculature as the major contributor to the pathophysiologic changes associated with PH. Adrenergic receptors that are present on vascular smooth muscle cells regulate vascular tone and growth. The alpha1-adrenergic receptors that are present on the small- and medium-sized pulmonary arteries have a unique and greatly enhanced affinity and activity to alpha1-adrenergic agonists. Under physiologic conditions, this helps in regulating vascular tone and maintains an adequate ventilation/perfusion matching. However, the excessive stimulation of alpha1-adrenergic receptors produces not only smooth muscle contraction but also proliferation and growth. The conditions that produce an increase in alpha1-adrenoreceptor gene synthesis, density, and activity (such as hypoxia or changes in vessel wall pressure) or increase the levels of its agonists (such as norepinephrine, appetite suppressants, or cocaine) greatly enhance pulmonary vascular smooth muscle contractile and proliferative responses and lead to the development of PH. An understanding of the role played by these receptors in the pathophysiology of PH would not only help to avoid the use of alpha1-agonists for appetite suppression and other disease states, but also would help in developing new drugs to block these receptors. A further understanding of the alpha1-adrenoreceptor subtypes present in the pulmonary vasculature, the factors that regulate their expression, and their intracellular signaling pathways would help researchers to devise newer therapeutic strategies and, hopefully, to find a cure for this crippling condition.

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.