Inhalation of the ETAreceptor antagonist LU-135252 selectively attenuates hypoxic pulmonary vasoconstriction

Impairment of hypoxic pulmonary vasoconstriction in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a serious complication of severe systemic or local pulmonary inflammation, such as caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. ARDS is characterised by diffuse alveolar damage that leads to protein-rich pulmonary oedema, local alveolar hypoventilation and atelectasis. Inadequate perfusion of these areas is the main cause of hypoxaemia in ARDS. High perfusion in relation to ventilation (V/Q<1) and shunting (V/Q=0) is not only caused by impaired hypoxic pulmonary vasoconstriction but also redistribution of perfusion from obstructed lung vessels. Rebalancing the pulmonary vascular tone is a therapeutic challenge. Previous clinical trials on inhaled vasodilators (nitric oxide and prostacyclin) to enhance perfusion to high V/Q areas showed beneficial effects on hypoxaemia but not on mortality. However, specific patient populations with pulmonary hypertension may profit from treatment with inhaled vasodilators. Novel treatment targets to decrease perfusion in low V/Q areas include epoxyeicosatrienoic acids and specific leukotriene receptors. Still, lung protective ventilation and prone positioning are the best available standard of care. This review focuses on disturbed perfusion in ARDS and aims to provide basic scientists and clinicians with an overview of the vascular alterations and mechanisms of V/Q mismatch, current therapeutic strategies, and experimental approaches.

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.

sGC stimulation totally reverses hypoxia-induced pulmonary vasoconstriction alone and combined with dual endothelin-receptor blockade in a porcine model

AIM

Stimulation of soluble guanylate cyclase (sGC) with BAY 41-8543 was hypothesized to attenuate acute hypoxic pulmonary vasoconstriction alone and combined with dual endothelin (ET)-receptor antagonist tezosentan.

METHODS

Measurements were taken in 18 anaesthetized pigs with a mean ± SEM weight of 31.1 ± 0.4 kg, in normoxia (FiO(2)~0.21) and hypoxia (FiO(2)~0.10) without (control protocol, n = 6), and with right atrial infusion of BAY 41-8543 at 1, 3, 6, 9 and 12 μg min(-1) per kg (protocol 2, n = 6) or tezosentan at 5 mg kg(-1) followed by BAY 41-8543 at 1, 3 and 6 μg min(-1) per kg (protocol 3, n = 6).

RESULTS

Hypoxia (n = 18) increased (P < 0.001) mean pulmonary artery pressure (MPAP) and pulmonary vascular resistance (PVR) by 14.2 ± 0.6 mmHg and 2.8 ± 0.3 WU respectively. During sustained hypoxia without treatment, MPAP and PVR remained stable. BAY 41-8543 (n = 6) dose-dependently decreased (P < 0.001) MPAP and PVR by 15.0 ± 1.2 mmHg and 4.7 ± 0.7 WU respectively. Tezosentan (n = 6) decreased (P < 0.001) MPAP and PVR by 11.8 ± 1.2 mmHg and 2.0 ± 0.2 WU, respectively, whereafter BAY 41-8543 (n = 6) further decreased (P < 0.001) MPAP and PVR by 6.6 ± 0.9 mmHg and 1.9 ± 0.4 WU respectively. Both BAY 41-8543 and tezosentan decreased (P < 0.001) systemic arterial pressure and systemic vascular resistance. Blood-O(2) consumption remained unaltered (P = ns) during all interventions.

CONCLUSION

BAY 41-8543 totally reverses the effects of acute hypoxia-induced pulmonary vasoconstriction, and enhances the attenuating effects of tezosentan, without affecting oxygenation. Thus, sGC stimulation, alone or combined with dual ET-receptor blockade, could offer a means to treat pulmonary hypertension related to hypoxia and potentially other causes.

Dual endothelin receptor blockade with tezosentan markedly attenuates hypoxia-induced pulmonary vasoconstriction in a porcine model

AIM

Our aim was to test the hypothesis that dual endothelin receptor blockade with tezosentan attenuates hypoxia-induced pulmonary vasoconstriction.

METHODS

Fourteen anaesthetized, ventilated pigs, with a mean ± SEM weight of 30.5 ± 0.6 kg, were studied, in normoxia (FiO(2) 0.21) and with tezosentan (5 mg kg(-1)) infusion during (n = 7) or before (n = 7) hypoxia (FiO(2) 0.10).

RESULTS

Compared to normoxia, hypoxia increased (P < 0.05) pulmonary vascular resistance (PVR) by 3.4 ± 0.7 WU, mean pulmonary artery pressure by 13.7 ± 1.3 mmHg, mean right atrial pressure by 1.9 ± 0.4 mmHg and decreased (P < 0.02) systemic vascular resistance (SVR) by 5.2 ± 2.1 WU. Pulmonary capillary wedge pressure (PCWP), mean aortic blood pressure, heart rate, cardiac output, stroke volume and blood-O(2)-consumption were unaltered (P = ns). Tezosentan infused during hypoxia, normalized PVR, decreased (P < 0.05) maximally mean pulmonary artery pressure by 7.5 ± 0.8 mmHg, SVR by 5.8 ± 0.7 WU, mean aortic blood pressure by 10.8 ± 3.0 mmHg and increased (P < 0.04) stroke volume by 8.5 ± 1.8 mL. Mean right atrial pressure, PCWP, heart rate, cardiac output and blood-O(2) -consumption were unaltered (P = ns). Tezosentan infused before hypoxia additionally attenuated approx. 70% of the initial mean pulmonary artery pressure increase and abolished the PVR increase, without additionally affecting the other parameters.

CONCLUSION

Dual endothelin receptor blockade during hypoxia attenuates the 'sustained' acute pulmonary vasoconstrictor response by reducing the mean pulmonary artery pressure increase by approx. 62% and by normalizing PVR. Pre-treatment with tezosentan before hypoxia, additionally attenuates the initial hypoxia-induced mean pulmonary artery pressure rise by approx. 70% and abolishes the PVR increase, during stable circulatory conditions, without affecting oxygenation.

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.

Prolonged hypoxemia and acidemia in anesthetized pigs: a model for research on extracorporeal organ support in an intensive care setting

PURPOSE

Hypoxemia and acidemia (hypoxemia/acidemia) are serious complications in the critically ill and often occur in unstable patients exposed to extracorporeal organ support. Still, little is known about the biocompatibility interactions of hypoxemia/acidemia with extracorporeal circuits (ECC). Existing animal models often include the release of mediator cascades (sepsis-, lung injury models) or are based on small laboratory animals. We established a porcine model of hypoxemia/acidemia without an underlying disease and further challenged the situation with an extracorporeal circuit (ECC).

METHODS

Hypoxemia/acidemia were induced (3.5 h) and maintained (3 h) in anesthetized pigs (40 kg) by a stepwise reduction in oxygenation, infusion of 0.4 mol.l⁻¹ lactic and hydrochloric acid and by low tidal volume ventilation, targeting an PaO₂ < 70 mmHg, SvO₂ < 65%, pH ~ 7.2. Venovenous hemofiltration (CVVH) operated in recirculation mode without volume exchange was chosen to prove the suitability of the model for studies on ECCs under clinical conditions (ECC group, n=6). Another 6 animals underwent the same protocol except for the CVVH (reference group, n=6).

RESULTS

The median PaO₂ during hypoxemia/acidemia was 62 mmHg, the median SvO₂ was 38%, and the median pH was 7.22. Hypoxemia/acidemia was successfully induced and maintained for 6.5 h in all pigs. CVVH could be performed for 3 h with blood flow rates up to 300 ml.min⁻¹ and filtrate rates up to 60 ml.min⁻¹.

CONCLUSIONS

Our model provides hypoxemia/acidemia with blood gas values comparable to critically ill adult patients for several hours, during which it is possible to perform CVVH. Thus, it enables research on the biocompatibility reactions of extracorporeal circuits under intensive care conditions.

Inhaled tezosentan reduces pulmonary hypertension in endotoxin-induced lung injury

The vasoconstrictive and proinflammatory peptide endothelin 1 (ET-1) is highly involved in the pathogenesis of sepsis and associated lung injury. Systemic administration of ET-receptor antagonists has been beneficial in experimental pulmonary hypertension. We wanted to study the effects of inhaled tezosentan, a dual endothelin-receptor antagonist on endotoxin-induced pulmonary hypertension, deterioration of gas exchange, and edema formation. After 2 h of endotoxemia, 28 anesthetized, mechanically ventilated pigs were randomized to either inhaled tezosentan 0.5 mg x kg (TEZO(0.5), n = 7), 0.05 mg x kg (TEZO(0.05), n = 7), intravenous 0.5 mg x kg (TEZO(iv), n = 7), or control (n = 7). Cardiopulmonary hemodynamics and gas-exchange parameters were recorded as well as extravascular lung water and pulmonary capillary pressure. In addition, plasma levels of tezosentan and ET-1 were analyzed. The protocol lasted for 5 h. Endotoxin-induced pulmonary hypertension (mean pulmonary artery pressure) was efficiently reduced by all treatments (TEZO(0.5) 24 +/- 2, TEZO(0.05) 27 +/- 2, TEZO(iv) 26 +/- 1, and control 37 +/- 2 mmHg at 4 h). TEZO(0.5) and TEZO(iv) also reduced pulmonary capillary pressure. All treatments led to a modest reduction in extravascular lung water, whereas no effects were noted on oxygenation or systemic circulation. Despite similar effects on pulmonary hypertension systemic treatment resulted in significantly higher plasma levels of ET-1 (twofold) and tezosentan (10- to 100-fold). Inhalation of the dual ET-receptor antagonist tezosentan was feasible and efficiently counteracted endotoxin-induced pulmonary hypertension. These effects were obtained with only minor systemic uptake of tezosentan and without affecting circulating levels of plasma ET-1 as compared with intravenous administration.

Inhibition of cGMP phosphodiesterase 5 suppresses matrix metalloproteinase-2 production in pulmonary artery smooth muscles cells

1. It has been shown that the beneficial effects of phosphodiesterase (PDE) 5 inhibition on pulmonary hypertension (PH) are associated with the induction of vascular relaxation and suppression of the proliferation of pulmonary artery smooth muscle cells (PASMC). In the present study, we investigated whether PDE5 inhibition affects the production and/or secretion of matrix metalloproteinases (MMPs) in PASMC, resulting in extracellular matrix remodelling in the pulmonary vasculature and, thus, the development of PH. 2. Primary cultured PASMC were stimulated with endothelin (ET)-1 and MMP-2 production and RhoA activation were then determinded using gelatin zymography and a GTP-bound RhoA assay, respectively. The effects of the selective PDE5 inhibitor sildenafil and subsequent protein kinase G-specific inhibitor Rp-8Br-cGMPs on MMP-2 production and RhoA activation were further exmamined. 3. Endothelin-1 (1-1000 nmol/L) concentration-dependently stimulated MMP-2 production and/or secretion in primary cultured PASMC, with 100 nmol/L ET-1 causing a 2.41-fold increase in MMP-2 production compared with control (P < 0.01). This increase in MMP-2 production was accompanied by RhoA activation, which was abolished by preincubation of cells with 10 micromol/L Y27632, an inhibitor of Rho-associated kinase (ROCK). Furthermore, 10 micromol/L Y27632 abolished the ET-1-induced production of MMP-2. 4. The selective PDE5 inhibitor sildenafil (0.1-1 micromol/L) concentration-dependently reduced the increased MMP-2 production induced by 100 nmol/L ET-1. Specifically, in the presence of 1 micromol/L sildenafil, the 100 nmol/L ET-1-induced increase in MMP-2 production was only increased 1.3-fold over that of the control (P < 0.01 vs 100 nmol/L ET-1-stimulated cells). 5. Suppression of RhoA activation was found to mediate the inhibitory effect of sildenafil on ET-1-induced increases in MMP-2 production. Furthermore, the protein kinase G-specific inhibitor Rp-8Br-cGMPs reversed the inhibitory effects of sildenafil on RhoA activation and MMP-2 production. 6. The results of the present study indicate that PDE5 inhibition suppresses RhoA/ROCK-mediated MMP-2 production by PASMC, which may contribute to the regulation of pulmonary vascular remodelling. Thus, PDE5 inhibition may benefit patients with PH through multiple mechanisms of action.

Protective effects of vasonatrin peptide against hypobaric hypoxia-induced pulmonary hypertension in rats

1. The aim of the present study was to investigate the in vivo effects of vasonatrin peptide (VNP) on hypoxia-induced pulmonary hypertension (HPH). 2. The HPH model was developed by subjecting rats to hypobaric hypoxia. The HPH rats were then treated with either VNP (50 microg/kg per day, i.p.) or saline (0.5 mL, i.p.) every day for 7 days. Haemodynamic indices, right ventricular hypertrophy (RVH) and remodelling of the pulmonary arteries were evaluated. In addition, plasma levels of atrial natriuretic peptide (ANP), endothelin (ET)-1 and angiotensin II (AngII) were determined, as was natriuretic peptide receptor-C (NPR-C) mRNA expression in the right ventricle. 3. Hypobaric hypoxia induced severe HPH compared with the normoxic control group. Treatment of HPH rats with VNP for 1 week significantly reduced mean pulmonary arterial pressure, pulmonary vascular resistance, RVH and muscularization of the pulmonary arteries, although pulmonary blood flow was increased in this group. In addition, significantly lower levels of plasma ET-1 and AngII and cardiac NPR-C mRNA expression were observed in VNP-treated compared with saline-treated HPH rats, whereas higher plasma concentrations of ANP were found in the former group. Acute intravenous administration of 50 microg/kg VNP significantly ameliorated pulmonary haemodynamics in HPH rats. 4. Taken together, the date indicate that VNP has certain preventative and therapeutic effects against HPH.