Contribution of endothelin to pulmonary vascular tone under normoxic and hypoxic conditions

Regulation, signalling and functions of hormonal peptides in pulmonary vascular remodelling during hypoxia

Hypoxic state affects organism primarily by decreasing the amount of oxygen reaching the cells and tissues. To adjust with changing environment organism undergoes mechanisms which are necessary for acclimatization to hypoxic stress. Pulmonary vascular remodelling is one such mechanism controlled by hormonal peptides present in blood circulation for acclimatization. Activation of peptides regulates constriction and relaxation of blood vessels of pulmonary and systemic circulation. Thus, understanding of vascular tone maintenance and hypoxic pulmonary vasoconstriction like pathophysiological condition during hypoxia is of prime importance. Endothelin-1 (ET-1), atrial natriuretic peptide (ANP), and renin angiotensin system (RAS) function, their receptor functioning and signalling during hypoxia in different body parts point them as disease markers. In vivo and in vitro studies have helped understanding the mechanism of hormonal peptides for better acclimatization to hypoxic stress and interventions for better management of vascular remodelling in different models like cell, rat, and human is discussed in this review.

The principal pathways involved in the in vivo modulation of hypoxic pulmonary vasoconstriction, pulmonary arterial remodelling and pulmonary hypertension

Hypoxic pulmonary vasoconstriction (HPV) serves to optimize ventilation-perfusion matching in focal hypoxia and thereby enhances pulmonary gas exchange. During global hypoxia, however, HPV induces general pulmonary vasoconstriction, which may lead to pulmonary hypertension (PH), impaired exercise capacity, right-heart failure and pulmonary oedema at high altitude. In chronic hypoxia, generalized HPV together with hypoxic pulmonary arterial remodelling, contribute to the development of PH. The present article reviews the principal pathways in the in vivo modulation of HPV, hypoxic pulmonary arterial remodelling and PH with primary focus on the endothelin-1, nitric oxide, cyclooxygenase and adenine nucleotide pathways. In summary, endothelin-1 and thromboxane A₂ may enhance, whereas nitric oxide and prostacyclin may moderate, HPV as well as hypoxic pulmonary arterial remodelling and PH. The production of prostacyclin seems to be coupled primarily to cyclooxygenase-1 in acute hypoxia, but to cyclooxygenase-2 in chronic hypoxia. The potential role of adenine nucleotides in modulating HPV is unclear, but warrants further study. Additional modulators of the pulmonary vascular responses to hypoxia may include angiotensin II, histamine, serotonin/5-hydroxytryptamine, leukotrienes and epoxyeicosatrienoic acids. Drugs targeting these pathways may reduce acute and/or chronic hypoxic PH. Endothelin receptor antagonists and phosphodiesterase-5 inhibitors may additionally improve exercise capacity in hypoxia. Importantly, the modulation of the pulmonary vascular responses to hypoxia varies between species and individuals, with hypoxic duration and age. The review also define how drugs targeting the endothelin-1, nitric oxide, cyclooxygenase and adenine nucleotide pathways may improve pulmonary haemodynamics, but also impair pulmonary gas exchange by interference with HPV in chronic lung diseases.

Lung Capillary Stress Failure and Arteriolar Remodelling in Pulmonary Hypertension Associated with Left Heart Disease (Group 2 PH)

Left heart diseases (LHD) represent the most prevalent cause of pulmonary hypertension (PH), yet there are still no approved therapies that selectively target the pulmonary circulation in LHD. The increase in pulmonary capillary pressure due to LHD is a triggering event leading to physical and biological alterations of the pulmonary circulation. Acutely, mechanosensitive endothelial dysfunction and increased capillary permeability combined with reduced fluid resorption lead to the development of interstitial and alveolar oedema. From repeated cycles of such capillary stress failure originate more profound changes with pulmonary endothelial dysfunction causing increased basal and reactive pulmonary vascular tone. This contributes to pulmonary vascular remodelling with increased arterial wall thickness, but most prominently, to alveolar wall remodelling characterized by myofibroblasts proliferation with collagen and interstitial matrix deposition. Although protective against acute pulmonary oedema, alveolar wall thickening becomes maladaptive and is responsible for the development of a restrictive lung syndrome and impaired gas exchanges contributing to shortness of breath and PH. Increasing awareness of these processes is unraveling novel pathophysiologic processes that could represent selective therapeutic targets. Thus, the roles of caveolins, of the intermediate myofilament nestin and of endothelial calcium dyshomeostasis were recently evaluated in pre-clinical models. The pathophysiology of PH due to LHD (group II PH) is distinctive from other groups of PH. Therefore, therapies targeting PH due to LHD must be evaluated in that context.

Vascular reactivity and biomarkers of endothelial function in healthy subjects exposed to acute hypobaric hypoxia

AIMS

The aim of this study was to evaluate the effects of acute hypobaric hypoxia (HH) on vascular reactivity and biochemical markers associated with endothelial function (EF).

MAIN METHODS

Ten healthy subjects were exposed to a simulated altitude of 4,000 meters above sea level for 4 hours in a hypobaric chamber. Vascular reactivity was measured by the flow-mediated vasodilatation (FMVD) test. Endothelin-1, high sensitive-C reactive protein (hsCRP), vascular cell adhesion molecule 1, interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), paraoxonase and adiponectin levels, and FMVD were evaluated before and after the exposure.

KEY FINDINGS

Subjects were young (age: 32±6 years), lean [body mass index: 23.9±2.0kg/m(2), waist circumference: 77(IQR: 72-80) cm], and presented normal clinical and biochemical parameters. No significant changes were evidenced in FMVD in response to HH (pre: 0.45 (0.20-0.70) vs. during: 0.50 (0.20-1.22) mm; p=0.594). On the other hand, endothelin-1 (+54%, p<0.05), hsCRP (+37%, p<0.001), IL-6 (+75%, p<0.05), TNF-α (+75%, p<0.05), and adiponectin (-39%, p<0.01) levels were significantly altered post-HH. FMVD was increased in 7 subjects, and it was decreased in 3 individuals during HH exposure. Interestingly, when EF biomarkers were compared between these two subgroups of subjects, only post exposure-adiponectin levels were significantly different (49±5 vs. 38±6μg/ml, respectively, p<0.05).

SIGNIFICANCE

HH exposure had an effect on endothelin-1, adiponectin, hsCRP, IL-6, and TNF-α concentration. However, adiponectin was the only biomarker associated with an altered vascular reactivity.

Reduced contribution of endothelin to the regulation of systemic and pulmonary vascular tone in severe familial hypercholesterolaemia

Vascular dysfunction has been associated with familial hypercholesterolaemia (FH), a severe form of hyperlipidaemia. We recently demonstrated that swine with FH exhibit reduced exercise-induced systemic, but not pulmonary, vasodilatation involving reduced nitric oxide (NO) bioavailability. Since NO normally limits endothelin (ET) action, we examined the hypothesis that reduced systemic vasodilatation during exercise in FH swine results from increased ET-mediated vasoconstriction. Systemic and pulmonary vascular responses to exercise were examined in chronically instrumented normal and FH swine in the absence and presence of the ETA/B receptor antagonist tezosentan. Intrinsic reactivity to ET was further assessed in skeletal muscle arterioles. FH swine exhibited ∼9-fold elevation in total plasma cholesterol versus normal swine. Similar to our recent findings, systemic, not pulmonary, vasodilatation during exercise was reduced in FH swine. Blockade of ET receptors caused marked systemic vasodilatation at rest and during exercise in normal swine that was significantly reduced in FH swine. The reduced role of ET in FH swine in vivo was not the result of decreased arteriolar ET responsiveness, as responsiveness was increased in isolated arterioles. Smooth muscle ET receptor protein content was unaltered by FH. However, circulating plasma ET levels were reduced in FH swine. ET receptor antagonism caused pulmonary vasodilatation at rest and during exercise in normal, but not FH, swine. Therefore, contrary to our hypothesis, FH swine exhibit a generalised reduction in the role of ET in regulating vascular tone in vivo probably resulting from reduced ET production. This may represent a unique vascular consequence of severe familial hypercholesterolaemia.

Peripheral circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulations.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Exogenous ghrelin improves blood flow distribution in pulmonary hypertension-assessed using synchrotron radiation microangiography

Ghrelin has cardioprotective properties and, recently, has been shown to improve endothelial function and reduce endothelin-1 (ET-1)-mediated vasoconstriction in peripheral vascular disease. Recently, we reported that ghrelin attenuates pulmonary hypertension (PH) caused by chronic hypoxia (CH), which we hypothesized in this study may be via suppression of the ET-1 pathway. We also aimed to determine whether ghrelin's ability to prevent alterations of the ET-1 pathway also prevented adverse changes in pulmonary blood flow distribution associated with PH. Sprague-Dawley rats were exposed to CH (10% O(2) for 2 weeks) with daily subcutaneous injections of ghrelin (150 μg/kg) or saline. Utilizing synchrotron radiation microangiography, we assessed pulmonary vessel branching structure, which is indicative of blood flow distribution, and dynamic changes in vascular responsiveness to (1) ET-1 (1 nmol/kg), (2) the ET-1(A) receptor antagonist, BQ-123 (1 mg/kg), and (3) ACh (3.0 μg kg⁻¹ min⁻¹). CH impaired blood flow distribution throughout the lung. However, this vessel "rarefaction" was attenuated in ghrelin-treated CH-rats. Moreover, ghrelin (1) reduced the magnitude of endothelial dysfunction, (2) prevented an increase in ET-1-mediated vasoconstriction, and (3) reduced pulmonary vascular remodeling and right ventricular hypertrophy-all adverse consequences associated with CH. These results highlight the beneficial effects of ghrelin for maintaining optimal lung perfusion in the face of a hypoxic insult. Further research is now required to establish whether ghrelin is also an effective therapy for restoring normal pulmonary hemodynamics in patients that already have established PH.

Exogenous ghrelin attenuates the progression of chronic hypoxia-induced pulmonary hypertension in conscious rats

Chronic exposure to hypoxia, a common adverse consequence of most pulmonary disorders, can lead to a sustained increase in pulmonary arterial pressure (PAP), right ventricular hypertrophy, and is, therefore, closely associated with heart failure and increased mortality. Ghrelin, originally identified as an endogenous GH secretagogue, has recently been shown to possess potent vasodilator properties, likely involving modulation of the vascular endothelium and its associated vasoactive peptides. In this study we hypothesized that ghrelin would impede the pathogenesis of pulmonary arterial hypertension during chronic hypoxia (CH). PAP was continuously measured using radiotelemetry, in conscious male Sprague Dawley rats, in normoxia and during 2-wk CH (10% O(2)). During this hypoxic period, rats received a daily sc injection of either saline or ghrelin (150 microg/kg). Subsequently, heart and lung samples were collected for morphological, histological, and molecular analyses. CH significantly elevated PAP in saline-treated rats, increased wall thickness of peripheral pulmonary arteries, and, consequently, induced right ventricular hypertrophy. In these rats, CH also led to the overexpression of endothelial nitric oxide synthase mRNA and protein, as well as endothelin-1 mRNA within the lung. Exogenous ghrelin administration attenuated the CH-induced overexpression of endothelial nitric oxide synthase mRNA and protein, as well as endothelin-1 mRNA. Consequently, ghrelin significantly attenuated the development of pulmonary arterial hypertension, pulmonary vascular remodeling, and right ventricular hypertrophy. These results demonstrate the therapeutic benefits of ghrelin for impeding the pathogenesis of pulmonary hypertension and right ventricular hypertrophy, particularly in subjects prone to CH (e.g. pulmonary disorders).

Effects of endothelin receptor antagonism on acute lung injury induced by chlorine gas

OBJECTIVE

To test the hypothesis that the endothelin system is involved in chlorine gas-induced lung injury.

DESIGN

Experimental study.

SETTING

Academic research laboratory.

SUBJECTS

Twenty-four domestic juvenile pigs.

INTERVENTIONS

Anesthetized, ventilated pigs were exposed to chlorine gas (400 parts per million in air) for 20 mins and then randomly allocated to four groups (n=6 in each group). The tezosentan pretreatment group received the dual endothelin receptor antagonist tezosentan 20 mins before and hyperoxic gas (Fio2 0.6) after chlorine gas exposure. The tezosentan postinjury treatment group received hyperoxic gas after chlorine gas exposure and tezosentan 60 mins later. Animals in the oxygen group received hyperoxic gas after chlorine gas exposure. Pigs in the fourth group (air) were ventilated with room air (Fio2 0.21) throughout the experiment.

MEASUREMENTS AND MAIN RESULTS

Hemodynamics, gas exchange, lung mechanics, and plasma endothelin-1 were evaluated for 6 hrs. Chlorine gas exposure induced an increase in circulating endothelin-1 by 90% (p<.05). The acute chlorine gas-induced rise in pulmonary vascular resistance was partly blocked by tezosentan pretreatment (p<.001). Tezosentan postinjury treatment also decreased pulmonary vascular resistance to levels significantly lower than in the air and oxygen groups (p<.001). Recovery of peak airway pressure was better in the tezosentan-treated groups than in the air group. There were significant linear relationships between circulating endothelin-1 and pulmonary vascular resistance (r=.47, p<.001) and endothelin-1 and peak airway pressure (r=.41, p<.001). These relationships were modified by tezosentan.

CONCLUSIONS

Tezosentan modified chlorine gas-induced pulmonary dysfunction, indicating that the endothelin system is involved in this mode of acute lung injury.

Hypoxia Impairs Systemic Endothelial Function in Individuals Prone to High-Altitude Pulmonary Edema

RATIONALE

High-altitude pulmonary edema (HAPE) is characterized by excessive pulmonary vasoconstriction and is associated with decreased concentrations of nitric oxide (NO) in the lung.

OBJECTIVES

We hypothesized that individuals susceptible to HAPE (HAPE-S) would also have dysfunction of the vascular NO vasodilator pathway during hypoxia in the systemic vasculature.

METHODS

During normoxia (FI(O(2)) = 0.21) and 4 hours of normobaric hypoxia (FI(O(2)) = 0.12, corresponding to an altitude of 4,500 m above sea level) endothelium-dependent and endothelium-independent vasodilator responses to intraarterial infusion of acetylcholine (ACh) and sodium nitroprusside, respectively, were measured by forearm venous occlusion plethysmography in nine HAPE-S subjects and in nine HAPE-resistant control subjects.

MAIN RESULTS

Pulmonary artery systolic pressure increased from 22 +/- 3 to 33 +/- 6 mm Hg (p < 0.001) during hypoxia in control subjects, and from 25 +/- 4 to 50 +/- 9 mm Hg in HAPE-S subjects (p < 0.001). Despite similar responses during normoxia in both groups, ACh-induced changes in forearm blood flow markedly decreased during hypoxia in HAPE-S subjects (p = 0.01) but not in control subjects. The attenuated vascular response to ACh infusion during hypoxia inversely correlated with increased pulmonary artery systolic pressure (p = 0.04) and decreased plasma nitrite correlated with attenuated ACh-induced vasodilation in HAPE-S subjects (p = 0.02).

CONCLUSIONS

Hypoxia markedly impairs vascular endothelial function in the systemic circulation in HAPE-S subjects due to a decreased bioavailability of NO. Impairment of the NO pathway could contribute to the enhanced hypoxic pulmonary vasoconstriction that is central to the pathogenesis of HAPE.

Hypoxic pulmonary vasoconstriction

Humans encounter hypoxia throughout their lives. This occurs by destiny in utero, through disease, and by desire, in our quest for altitude. Hypoxic pulmonary vasoconstriction (HPV) is a widely conserved, homeostatic, vasomotor response of resistance pulmonary arteries to alveolar hypoxia. HPV mediates ventilation-perfusion matching and, by reducing shunt fraction, optimizes systemic Po(2). HPV is intrinsic to the lung, and, although modulated by the endothelium, the core mechanism is in the smooth muscle cell (SMC). The Redox Theory for the mechanism of HPV proposes the coordinated action of a redox sensor (the proximal mitochondrial electron transport chain) that generates a diffusible mediator [a reactive O(2) species (ROS)] that regulates an effector protein [voltage-gated potassium (K(v)) and calcium channels]. A similar mechanism for regulating O(2) uptake/distribution is partially recapitulated in simpler organisms and in the other specialized mammalian O(2)-sensitive tissues, including the carotid body and ductus arteriosus. Inhibition of O(2)-sensitive K(v) channels, particularly K(v)1.5 and K(v)2.1, depolarizes pulmonary artery SMCs, activating voltage-gated Ca(2+) channels and causing Ca(2+) influx and vasoconstriction. Downstream of this pathway, there is important regulation of the contractile apparatus' sensitivity to calcium by rho kinase. Controversy remains as to whether hypoxia decreases or increases ROS and which electron transport chain complex generates the ROS (I and/or III). Possible roles for cyclic adenosine diphosphate ribose and an unidentified endothelial constricting factor are also proposed by some groups. Modulation of HPV has therapeutic relevance to cor pulmonale, high-altitude pulmonary edema, and sleep apnea. HPV is clinically exploited in single-lung anesthesia, and its mechanisms intersect with those of pulmonary arterial hypertension.