Denielli da Silva Gonçalves Bós, Cathelijne E. E. Van Der Bruggen, Kondababu Kurakula, Xiao-Qing Sun, Karina R. Casali, Adenauer G. Casali, Nina Rol, Robert Szulcek, Cris dos Remedios, Christophe Guignabert, Ly Tu, Peter Dorfmüller, Marc Humbert, Paul J.M. Wijnker, Diederik W.D. Kuster, Jolanda van der Velden, Marie-José Goumans, Harm-Jan Bogaard, Anton Vonk-Noordegraaf, Frances S. de Man, and M. Louis Handoko. Circulation. 2018; 137:910-924.

Article summary by Kara Goss, MD, Assistant Professor of Medicine and Pediatrics, University of Wisconsin.
Expert commentary by Samar Farha, MD, Assistant Professor of Medicine, Cleveland Clinic Lerner College of Medicine, Cleveland Clinic Abu Dhabi

Article Summary:

Background: Autonomic imbalance, characterized by increased sympathetic activity and parasympathetic withdrawal, is associated with disease severity and worsened prognosis in pulmonary arterial hypertension (PAH). Although restoration of parasympathetic activity is a cornerstone of left heart failure (for example through treatment with beta blockers such a carvedilol), the therapeutic effect of parasympathetic modulation in right heart failure remains unknown. Importantly, the use of beta-blockade in PAH and right ventricular (RV) failure is controversial, and an evaluation of alternate strategies to modulate parasympathetic function is needed. Here, the authors identify autonomic dysfunction mediated through reduced acetylcholinesterase activity in patients with PAH and RV failure, and utilize pyridostigmine, an oral agent that stimulates the parasympathetic nervous system through acetylcholinesterase inhibition, to assess the potential therapeutic effects of parasympathetic activation in an animal model of PAH.

Methods and Results: First, the authors retrospectively reviewed 112 patients with idiopathic and hereditary PAH who completed a maximal cardiopulmonary exercise test at the VU University Medical Center in Amsterdam. Patients with PAH had impaired heart rate recovery from maximal exercise at 30 and 60 seconds, a marker of impaired parasympathetic reactivation. Notably, the severity of impairment correlated with the severity of RV dysfunction and ejection fraction. As acetylcholine is the primary parasympathetic neurotransmitter, the authors assessed acetylcholine receptor expression and acetylcholinesterase activity in explanted RV tissue samples from patients with PAH undergoing heart/lung transplantation. Compared to control tissue, nicotinic acetylcholine receptor expression was significantly higher and RV acetylcholine activity significantly lower among RV failure tissues.

Acetylcholinesterase activity was also reduced in the pulmonary artery and pulmonary microvascular endothelial cells isolated from PAH patients undergoing transplant.

Next, the authors evaluated the effect of parasympathetic activation with pyridostigmine in 3D engineered heart tissue, generated from iCell cardiomyocytes, and in the Sugen-hypoxia rat model of PAH. In engineered heart tissue, pyridostigmine reduced the heart rate and relaxation velocity of spontaneously beating tissue, with no significant effects on maximum force and contraction velocity. In Sugen-hypoxia rats, pyridostigmine was dosed to achieve a heart rate reduction by 10% with minimal effect on systemic blood pressure, and treatment initiated after rats developed PAH. Cardiovascular autonomic function was assessed by heart rate and blood pressure variability analysis in pyridostigmine-treated PAH rats, demonstrating a significant reduction in heart rate and increased parasympathetic activity as measured by the high-frequency component of heart rate variability obtained by spectral analysis. Remarkably, pyridostigmine treated rats demonstrated improved survival and slower progression of RV failure, marked by relatively preserved RV systolic function, reduced pulmonary vascular resistance, reduced RV hypertrophy, and reduced RV end-diastolic diameter. Pressure-volume loop analysis demonstrated improved RV contractility, reduced diastolic stiffness, and reduced afterload, resulting in partial restoration of RV-pulmonary vascular coupling.

To understand the mechanisms of improved RV function in pyridostigmine-treated PAH rats, the authors assessed RV and pulmonary vascular histology. Structural changes included reduced RV cardiomyocyte cross-sectional area, decreased RV fibrosis, and reduced RV leukocyte infiltration/inflammation. With respect to cholinergic receptors, pyridostigmine reduced nicotinic (α-7nAchR) and muscarinic (m2AchR) expression in the RV. Assessment of pulmonary vascular remodeling demonstrated decreased wall thickness and decreased formation of occlusive vascular lesions, consistent with reduced RV afterload. Intriguingly, follow-up studies of human pulmonary microvascular endothelial cells also demonstrated reduced proliferation when treated with pyridostigmine, confirming the anti-proliferative and anti-inflammatory effects seen in the animal model.

Conclusions: Patients with RV failure from PAH demonstrate impaired global parasympathetic activity and reduced RV acetylcholinesterase activity. In PAH rats, treatment with pyridostigmine increased parasympathetic activity and delayed progression toward RV failure, associated with decreased RV hypertrophy, inflammation, and fibrosis. In addition, pyridostigmine reduced RV afterload through anti-proliferative and anti-inflammatory remodeling effects on the pulmonary vasculature. Overall, this translational study demonstrates a possible therapeutic role for parasympathetic activation with pyridostigmine in RV failure, and future studies of the effect of parasympathetic modulation on RV and pulmonary vascular function in humans with PAH are warranted.

Expert Commentary: Cardiac autonomic dysfunction has been well studied in heart failure both left sided and biventricular. Therapies aiming at regulating the autonomic system and renin-angiotensin-aldosterone system (RAAS) are the mainstay of therapy in left heart failure. In pulmonary arterial hypertension (PAH), pulmonary vascular remodeling, obliteration and thrombosis lead to increased right ventricle (RV) afterload resulting in isolated RV dysfunction and ultimately failure. RV dysfunction has been shown to progress independent of its afterload and is the main determinant of survival in patients with PAH. Improving our understanding of RV pathophysiology in PAH and developing RV targeted therapies are essential. Over the last thirty years, there has been growing interest in the role of the autonomic nervous system in PAH and isolated RV failure.

Studies have shown profound alteration of cardiac autonomic control in PAH with increased sympathetic nervous system activity and parasympathetic nervous system withdrawal (1-7). The observed changes parallel those seen in chronic left heart failure and relate to disease severity. However, therapies targeting the autonomic system have lagged behind in PAH despite the substantial evidence of their beneficial effects in left heart failure. The use of β-blockers, an approved therapy in left heart failure currently is not recommended for patients with PAH. This is based on the presumption that PAH patients are unable to increase stroke volume during exercise and as such are heart rate dependent to increase their cardiac output. Nonetheless, therapies targeting adrenergic receptors have been shown to reverse cardiac and vascular remodeling and improve outcomes in experimental pulmonary hypertension models (8, 9). The effectiveness of β-blockers in clinical settings remains uncertain. In a small pilot study, carvedilol in patients with stable PAH and RV dysfunction was safe and improved RV function (10). In another study, bisoprolol was well tolerated in patients with PAH, but was associated with a decrease in cardiac index and 6-minute walk distance (11). In a recent 6-month double-blinded, randomized, controlled trial, carvedilol was safe and well tolerated (12). Patients treated with carvedilol had an accelerated heart rate recovery without a reduction in exercise capacity during 6-minute walk testing. Carvedilol therapy was associated with improved RV function and preserved cardiac output. The clinical benefits of carvedilol were coupled with reduced RV glycolysis and increased β-adrenergic receptor levels (12). Based on current data, a larger multicenter trial to establish the use of beta-blockers in PAH is warranted.

In this study, da Silva Goncalves Bos et al. investigate the role of the parasympathetic system in PAH and its effects on the RV (13). They show that reduced heart rate recovery, a surrogate of impaired parasympathetic activity is more delayed in PAH patients with worse RV function as measured by RVEF by cardiac MRI. In vivo, looking at RV tissue from PAH patients undergoing transplantation, they found increased presynaptic nicotinic receptor expression and reduced acetylcholinesterase activity. There were no differences in nicotinic or muscarinic receptors in the lung tissue and the pulmonary artery of PAH patients. However, acetylcholinesterase activity was reduced in microvascular endothelial cells and pulmonary arteries in PAH. The authors then used pyridostigmine, an acetylcholinesterase inhibitor to stimulate the parasympathetic activity in the sugen-hypoxia rat model. They found that pyridostigmine in the sugen-hypoxia model increased parasympathetic activity and delayed progression to RV failure. RV function improved and was related to a reduction in RV fibrosis, hypertrophy and inflammation. In parallel, RV afterload decreased and was associated with decreased pulmonary vascular remodeling.

The authors are commended for their work. The findings are significant in that they shed light on the role of the parasympathetic system in PAH and on RV function and propose an acetylcholinesterase inhibitor, pyridostigmine, as a potential therapy to be further investigated in a proof of concept pilot study. Although PAH is associated with cardiac autonomic dysfunction, the precise role of autonomic nervous system involvement in the pathogenesis of PAH is still not fully understood and the crosstalk between the sympathetic and parasympathetic systems remains unclear. It is an adaptive compensatory mechanism but is insufficient. In addition, the chronic sympathetic overactivity has, in the long run, detrimental effects on cardiac function as shown in left heart failure. Adrenergic blockade and activation of the parasympathetic system may be beneficial in PAH. Studies are needed to further determine the role of the autonomic nervous system in PAH and to assess the effectiveness and safety of targeting the autonomic nervous system for the treatment of PAH patients.

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