Neutral endopeptidase 24.11 inhibition reduces pulmonary vascular remodeling in rats exposed to chronic hypoxia

Additive protective effects of sacubitril/valsartan and bosentan on vascular remodelling in experimental pulmonary hypertension

AIMS

Although right ventricular (RV) function is an important determinant of morbidity and mortality in patients with pulmonary arterial hypertension (PAH), there is no treatment targeting directly the RV. We evaluate the efficacy of sacubitril/valsartan (LCZ 696) as add-on therapy to bosentan in rats with severe pulmonary hypertension (PH).

METHODS AND RESULTS

Combination therapy of LCZ 696 and bosentan has additive vascular protective effects against the pulmonary vascular remodelling and PH in two preclinical models of severe PH. Compared with monotherapy, co-treatment of LCZ 696 (30 or 68 mg/kg/day for 2 weeks, per os) and bosentan (100 mg/kg/day for 2 weeks, per os) started 7 days after monocrotaline (MCT) injection substantially reduces pulmonary pressures, vascular remodelling, and RV hypertrophy and fibrosis in rats. Consistent with these observations, co-treatment of rats with established PH induced by sugen/hypoxia (SuHx) with LCZ 696 (30 mg/kg/day for 3 weeks, per os) and bosentan (100 mg/kg/day for 3 weeks, per os) started 5 weeks after Sugen injection partially attenuate total pulmonary vascular resistance and cardiovascular structures. We also obtained evidence showing that LCZ 696 has anti-proliferative effect on cultured human pulmonary artery smooth muscle cells derived from patients with idiopathic PAH, an effect that is more pronounced in presence of bosentan. Finally, we found that the plasma levels of atrial natriuretic peptide (ANP) and cyclic guanosine monophosphate (cGMP) are higher in rats co-treated with LCZ 696 (30 mg/kg/day) and bosentan (100 mg/kg/day) than in MCT and SuHx rats treated with vehicle.

CONCLUSION

Dual therapy with LCZ 696 plus bosentan proved significantly superior beneficial effect to LCZ 696 or bosentan alone on vascular remodelling and severity of experimental PH.

Sacubitril/valsartan treatment relieved the progression of established pulmonary hypertension in rat model and its mechanism

AIMS

Pulmonary hypertension (PH) is a fatal disease identified by progressive elevated pulmonary arterial pressure, which neurohormonal activation is a notable contributor to its development. Sacubitril/valsartan is a complex of sacubitril [via enhancing the natriuretic peptide (NP) system] and valsartan [via blocking the renin-angiotensin-aldosterone system (RAAS)]. Regulation of the two neurohormonal system had been shown to attenuate PH. This study was to explore the role of sacubitril/valsartan in both monocrotaline (MCT)-induced and hypoxia-induced rat models and the underlying mechanism.

MAIN METHODS

The rats were treated with MCT or hypoxic environment for 14 days, after that sacubitril/valsartan were given for another 14 days. Hemodynamic measurements and histological assessments were performed. The expression of NPs was measured using RT-PCR and ELISA, while the protein level of natriuretic peptide receptors (NPRs) and AT1 receptor were detected by western blot, the concentrations of cGMP, IL-1β, IL-6, TNF-α and TGF-β1 were tested by ELISA.

KEY FINDINGS

We found that sacubitril/valsartan significantly improved the hemodynamic and histological data of two PH models. Sacubitril/valsartan suppressed the protein expression of AT1 receptor (P < 0.05). The intervention increased the expression of ANP and CNP (P< 0.05) and therefore upregulated the protein expression of NPRs (P < 0.05), raised the concentration of cGMP (P < 0.05). In addition, the treatment reduced the concentration of IL-1β, IL-6 and TNF-α (P < 0.05) but have no effects on TGF-β1.

SIGNIFICANCE

Sacubitril/valsartan alleviated PH in MCT-induced and hypoxia-induced rat models by inhibiting the activated RAAS, promoting ANP/NPR-A/cGMP and CNP/NPR-B/cGMP pathway, restoring the NPR-C signaling and the anti-inflammatory effects.

Neprilysin inhibition for pulmonary arterial hypertension: a randomized, double-blind, placebo-controlled, proof-of-concept trial

BACKGROUND AND PURPOSE

Pulmonary arterial hypertension (PAH) is an incurable, incapacitating disorder resulting from increased pulmonary vascular resistance, pulmonary arterial remodelling, and right ventricular failure. In preclinical models, the combination of a PDE5 inhibitor (PDE5i) with a neprilysin inhibitor augments natriuretic peptide bioactivity, promotes cGMP signalling, and reverses the structural and haemodynamic deficits that characterize PAH. Herein, we conducted a randomized, double-blind, placebo-controlled trial to assess the efficacy and safety of repurposing the neprilysin inhibitor, racecadotril, in PAH.

EXPERIMENTAL APPROACH

Twenty-one PAH patients stable on PDE5i therapy were recruited. Acute haemodynamic and biochemical changes following a single dose of racecadotril or matching placebo were determined; this was followed by a 14-day safety and efficacy evaluation. The primary endpoint in both steps was the maximum change in circulating atrial natriuretic peptide (ANP) concentration (Δ_max ), with secondary outcomes including pulmonary and systemic haemodynamics plus mechanistic biomarkers.

KEY RESULTS

Acute administration of racecadotril (100 mg) resulted in a 79% increase in the plasma ANP concentration and a 106% increase in plasma cGMP levels, with a concomitant 14% fall in pulmonary vascular resistance. Racecadotril (100 mg; t.i.d.) treatment for 14 days resulted in a 19% rise in plasma ANP concentration. Neither acute nor chronic administration of racecadotril resulted in a significant drop in mean arterial BP or any serious adverse effects.

CONCLUSIONS AND IMPLICATIONS

This Phase IIa evaluation provides proof-of-principle evidence that neprilysin inhibitors may have therapeutic utility in PAH and warrants a larger scale prospective trial.

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.

Long-term neprilysin inhibition - implications for ARNIs

Neprilysin has a major role in both the generation and degradation of bioactive peptides. LCZ696 (valsartan/sacubitril, Entresto), the first of the new ARNI (dual-acting angiotensin-receptor-neprilysin inhibitor) drug class, contains equimolar amounts of valsartan, an angiotensin-receptor blocker, and sacubitril, a prodrug for the neprilysin inhibitor LBQ657. LCZ696 reduced blood pressure more than valsartan alone in patients with hypertension. In the PARADIGM-HF study, LCZ696 was superior to the angiotensin-converting enzyme inhibitor enalapril for the treatment of heart failure with reduced ejection fraction, and LCZ696 was approved by the FDA for this purpose in 2015. This approval was the first for chronic neprilysin inhibition. The many peptides metabolized by neprilysin suggest many potential consequences of chronic neprilysin inhibitor therapy, both beneficial and adverse. Moreover, LBQ657 might inhibit enzymes other than neprilysin. Chronic neprilysin inhibition might have an effect on angio-oedema, bronchial reactivity, inflammation, and cancer, and might predispose to polyneuropathy. Additionally, inhibition of neprilysin metabolism of amyloid-β peptides might have an effect on Alzheimer disease, age-related macular degeneration, and cerebral amyloid angiopathy. Much of the evidence for possible adverse consequences of chronic neprilysin inhibition comes from studies in animal models, and the relevance of this evidence to humans is unknown. This Review summarizes current knowledge of neprilysin function and possible consequences of chronic neprilysin inhibition that indicate a need for vigilance in the use of neprilysin inhibitor therapy.

Serine carboxypeptidase SCPEP1 and Cathepsin A play complementary roles in regulation of vasoconstriction via inactivation of endothelin-1

The potent vasoconstrictor peptides, endothelin 1 (ET-1) and angiotensin II control adaptation of blood vessels to fluctuations of blood pressure. Previously we have shown that the circulating level of ET-1 is regulated through its proteolytic cleavage by secreted serine carboxypeptidase, cathepsin A (CathA). However, genetically-modified mouse expressing catalytically inactive CathA S190A mutant retained about 10-15% of the carboxypeptidase activity against ET-1 in its tissues suggesting a presence of parallel/redundant catabolic pathway(s). In the current work we provide direct evidence that the enzyme, which complements CathA action towards ET-1 is a retinoid-inducible lysosomal serine carboxypeptidase 1 (Scpep1), a CathA homolog with previously unknown biological function. We generated a mouse strain devoid of both CathA and Scpep1 activities (DD mice) and found that in response to high-salt diet and systemic injections of ET-1 these animals showed significantly increased blood pressure as compared to wild type mice or those with single deficiencies of CathA or Scpep1. We also found that the reactivity of mesenteric arteries from DD mice towards ET-1 was significantly higher than that for all other groups of mice. The DD mice had a reduced degradation rate of ET-1 in the blood whereas their cultured arterial vascular smooth muscle cells showed increased ET-1-dependent phosphorylation of myosin light chain 2. Together, our results define the biological role of mammalian serine carboxypeptidase Scpep1 and suggest that Scpep1 and CathA together participate in the control of ET-1 regulation of vascular tone and hemodynamics.

Synergy between natriuretic peptides and phosphodiesterase 5 inhibitors ameliorates pulmonary arterial hypertension

RATIONALE

Phosphodiesterase 5 (PDE5) inhibitors (e.g., sildenafil) are selective pulmonary vasodilators in patients with pulmonary arterial hypertension. The mechanism(s) underlying this specificity remains unclear, but studies in genetically modified animals suggest it might be dependent on natriuretic peptide bioactivity.

OBJECTIVES

We explored the interaction between PDE5 inhibitors and the natriuretic peptide system to elucidate the (patho)physiological relationship between these two cyclic GMP (cGMP)-regulating systems and potential of a combination therapy exploiting these cooperative pathways.

METHODS

Pharmacological evaluation of vascular reactivity was conducted in rat isolated conduit and resistance vessels from the pulmonary and systemic circulation in vitro, and in anesthetized mice in vivo. Parallel studies were undertaken in an animal model of hypoxia-induced pulmonary hypertension (PH).

MEASUREMENTS AND MAIN RESULTS

Sildenafil augments vasodilatation to nitric oxide (NO) in pulmonary and systemic conduit and resistance arteries, whereas identical vasorelaxant responses to atrial natriuretic peptide (ANP) are enhanced only in pulmonary vessels. This differential activity is mirrored in vivo where sildenafil increases the hypotensive actions of ANP in the pulmonary, but not systemic, vasculature. In hypoxia-induced PH, combination of sildenafil plus the neutral endopeptidase (NEP) inhibitor ecadotril (which increases endogenous natriuretic peptide levels) acts synergistically, in a cGMP-dependent manner, to reduce many indices of disease severity without significantly affecting systemic blood pressure.

CONCLUSIONS

These data demonstrate that PDE5 is a key regulator of cGMP-mediated vasodilation by ANP in the pulmonary, but not systemic, vasculature, thereby explaining the pulmonary selectivity of PDE5 inhibitors. Exploitation of this mechanism (i.e., PDE5 and neutral endopeptidase inhibition) represents a novel, orally active combination therapy for pulmonary arterial hypertension.

Atrial natriuretic peptide in hypoxia

A growing number of mammalian genes whose expression is inducible by hypoxia have been identified. Among them, atrial natriuretic peptide (ANP) synthesis and secretion is increased during hypoxic exposure and plays an important role in the normal adaptation to hypoxia and in the pathogenesis of cardiopulmonary diseases, including chronic hypoxia-induced pulmonary hypertension and vascular remodeling, and right ventricular hypertrophy and right heart failure. This review discusses the roles of ANP and its receptors in hypoxia-induced pulmonary hypertension. We and other investigators have demonstrated that ANP gene expression is enhanced by exposure to hypoxia and that the ANP so generated protects against the development of hypoxic pulmonary hypertension. Results also show that hypoxia directly stimulates ANP gene expression and ANP release in cardiac myocytes in vitro. Several cis-responsive elements of the ANP promoter are involved in the response to changes in oxygen tension. Further, the ANP clearance receptor NPR-C, but not the biological active NPR-A and NPR-B receptors, is downregulated in hypoxia adapted lung. Hypoxia-sensitive tyrosine kinase receptor-associated growth factors, including fibroblast growth factor (FGF) and platelet derived growth factor (PDGF)-BB, but not hypoxia per se, inhibit NPR-C gene expression in pulmonary arterial smooth muscle cells in vitro. The reductions in NPR-C in the hypoxic lung retard the clearance of ANP and allow more ANP to bind to biological active NPR-A and NPR-B in the pulmonary circulation, relaxing preconstricted pulmonary vessels, reducing pulmonary arterial pressure, and attenuating the development of hypoxia-induced pulmonary hypertension and vascular remodeling.

Comparison of angiotensin-converting enzyme (ACE), neutral endopeptidase (NEP) and dual ACE/NEP inhibition on blood pressure and resistance arteries of deoxycorticosterone acetate-salt hypertensive rats

OBJECTIVES

Omapatrilat, an inhibitor of neutral endopeptidase (NEP) and angiotensin-converting enzyme (ACE), is an effective antihypertensive agent. Here, we studied the relative roles of NEP and ACE inhibition and their effect on resistance artery structure and function of deoxycorticosterone acetate (DOCA)-salt hypertensive rats.

METHODS

Omapatrilat (40 mg/kg per day), the NEP inhibitor CGS 25462 (CGS, 100 mg/kg per day) and the ACE inhibitor enalapril (10 mg/kg per day), were given for 3 weeks to DOCA-salt hypertensive rats. Effects on small mesenteric resistance arteries were studied on a pressurized myograph. Collagen deposition was evaluated by confocal microscopy.

RESULTS

Systolic blood pressure of DOCA-salt rats was significantly reduced (P < 0.05) by omapatrilat and CGS. Omapatrilat and CGS treatment increased lumen diameter and decreased media width and media/lumen ratio of small arteries of DOCA-salt rats (P < 0.05). Small artery relaxation responses to acetylcholine improved under omapatrilat or CGS treatment. The stress-strain curve shifted leftward in mesenteric arteries from DOCA-salt rats compared to control rats. Omapatrilat or CGS treatment resulted in a rightward shift, which was significantly different from that induced by enalapril. Omapatrilat and CGS decreased collagen deposition in the vessel wall of DOCA-salt rats. Enalapril had no effect on blood pressure, vascular structure, endothelial function or collagen deposition in the vessel wall of DOCA-salt rats.

CONCLUSIONS

Dual inhibition of ACE/NEP in DOCA-salt hypertensive rats resulted in potent anti-hypertensive effects, prevented vascular remodelling and improved endothelial function of resistance arteries. NEP inhibition is involved to a large extent in the effect of omapatrilat in DOCA-salt rats. These actions of omapatrilat may confer protection against end-organ damage characteristic of severe hypertension.

Atrial natriuretic peptide in severe primary and nonprimary pulmonary hypertension: response to iloprost inhalation

OBJECTIVES

The goal of this study was to assess atrial natriuretic peptide (ANP) levels during inhalation of iloprost in severe primary (PPH) and nonprimary pulmonary hypertension (NPPH).

BACKGROUND

The ANP system is activated in pulmonary hypertension and may help protect from right ventricular (RV) decompensation. It is unknown if ANP regulation is the same in severe PPH and NPPH and if the dynamic regulation is intact in a highly activated ANP system.

METHODS

In 11 patients with PPH and seven patients with NPPH, right heart catheter investigations were performed. Pulmonary and systemic artery ANP and cyclic guanosine monophosphate (cGMP) levels as well as hemodynamics were measured before and after iloprost inhalation.

RESULTS

The baseline hemodynamics of patients with PPH and patients with NPPH were comparable (mean pulmonary artery pressure [mPAP]: 61 +/- 5 mm Hg vs. 52 +/- 5 mm Hg, pulmonary vascular resistance [PVR]: 1,504 +/- 153 dyne.s.cm(-5) vs. 1,219 +/- 270 dyne.s.cm(-5). Atrial natriuretic peptide and cGMP levels were increased about tenfold and fivefold compared with controls in both PPH and NPPH. Iloprost inhalation significantly decreased mPAP (-9.1 +/- 2.5 mm Hg vs. -7.9 +/- 1.5 mm Hg), PVR (-453 +/- 103 dyne.s.cm(-5) vs. -381 +/- 114 dyne.s.cm(-5)), ANP (-99 +/- 63 pg/ml vs. -108 +/- 47 pg/ml) and cGMP (-4.6 +/- 0.9 nM vs. -4.2 +/- 1.6 nM). Baseline ANP including all patients significantly correlated with PVR, right atrial pressure, cardiac index, RV ejection fraction, mixed venous oxygen saturation and cGMP.

CONCLUSIONS

The ANP system is highly activated in patients with severe PPH and NPPH. Atrial natriuretic peptide levels are significantly correlated with parameters of RV function and pre- and afterload. Iloprost inhalation causes a rapid decrease in ANP and cGMP in parallel with pulmonary vasodilation and hemodynamic improvement.

Pulmonary vascular remodeling: a target for therapeutic intervention in pulmonary hypertension

Pulmonary vascular remodelling is an important pathological feature of pulmonary hypertension, leading to increased pulmonary vascular resistance and reduced compliance. It involves thickening of all three layers of the blood vessel wall (due to hypertrophy and/or hyperplasia of the predominant cell type within each layer), as well as extracellular matrix deposition. Neomuscularisation of non-muscular arteries and formation of plexiform and neointimal lesions also occur. Stimuli responsible for remodelling involve transmural pressure, stretch, shear stress, hypoxia, various mediators [angiotensin II, endothelin (ET)-1, 5-hydroxytryptamine, growth factors, and inflammatory cytokines], increased serine elastase activity, and tenascin-C. In addition, there are reductions in the endothelium-derived antimitogenic substances, nitric oxide, and prostacyclin. Intracellular signalling mechanisms involved in pulmonary vascular remodelling include elevations in intracellular Ca2+ and activation of the phosphatidylinositol pathway, protein kinase C, and mitogen-activated protein kinase. In animal models of pulmonary hypertension, various drugs have been shown to attenuate pulmonary vascular remodelling. These include angiotensin-converting enzyme inhibitors, angiotensin receptor antagonists, ET receptor antagonists, ET-converting enzyme inhibitors, nitric oxide, phosphodiesterase 5 inhibitors, prostacyclin, Ca2+ -channel antagonists, heparin, and serine elastase inhibitors. Inhibition of remodelling is generally accompanied by reductions in pulmonary artery pressure. The efficacy of some of the drugs varies, depending on the animal model of the disease. In view of the complexity of the remodelling process and the diverse aetiology of pulmonary hypertension in humans, it is to be anticipated that successful anti-remodelling therapy in the clinic will require a range of different drug options.

Effect of ECE and NEP inhibition on cigarette smoke-induced cell proliferation in the rat lung

To investigate the role of endothelins in cigarette smoke-induced cell proliferation, we assessed the effect of two dual nonselective neutral endopeptidase (NEP)/endothelin-converting enzyme (ECE) inhibitors, phosphoramidon and CGS 26303, and of a specific NEP inhibitor, CGS 24592, on cell proliferation in the airways and arterial vasculature of the rat lung. Eight groups of rats were exposed to either room air (group 1, control), the smoke of 10 cigarettes (group 2, smoke only) or groups 1 and 2 in addition to a continuous iv infusion of CGS 24592, CGS 26303, or phosphoramidon (10 mg/kg/24 h). Cigarette smoke produced significant cell proliferation in the airways (epithelium and wall) and in the perialveolar ductular vessels (endothelium and wall). CGS 26303 reduced the smoke-induced proliferation in the endothelium and walls of the vessels adjacent to the alveolar ducts, and in the airway walls, but did not affect proliferation in the airway epithelium. CGS 24592 reduced cell proliferation in the airway wall. Phosphoramidon had no effect. These findings indicate that acute cigarette smoke-induced cell proliferation of the rat airways and pulmonary arterial vessels is mediated, at least in part, through release and actions of endothelins. The effectiveness of the more potent inhibitor, CGS 26303, appears to conform to its site of predominant expression.

Adenovirus-mediated atrial natriuretic protein expression in the lung protects rats from hypoxia-induced pulmonary hypertension

Endogenous as well as exogenous atrial natriuretic peptide (ANP) attenuates the development of chronic hypoxic pulmonary hypertension (CHPH) in rats. We built a recombinant adenovirus type 5 containing ANP cDNA under the control of the Rous sarcoma virus long terminal repeat (Ad.ANP). The efficiency of this vector in delivering the ANP gene was first examined in rat primary cultures of pulmonary vessel smooth muscle cells (SMCs) in comparison with Ad.beta GAL. Conditioned medium collected from Ad.ANP-infected cells (1000 TCID(50)/cell) contained 5 x 10(9) M immunoreactive ANP and elicited relaxation of isolated rat pulmonary arteries preconstricted with phenylepinephrine. To examine the effects of adenovirus-mediated ANP expression in the CHPH rat lung, Ad.ANP or Ad.beta GAL was administered via the tracheal route. Immunoreactive ANP was detected in bronchoalveolar fluid as early as 4 days and until 10-17 days after Ad.ANP administration (5 x 10(8) TCID(50)). Lung ANP immunostaining was mainly localized in bronchial and alveolar epithelial cells. As compared with Ad.beta GAL-treated controls, rats given Ad.ANP (5 x 10(8) TCID(50)) on the day before a 2-week exposure to hypoxia (10% O(2)) had lower values for pulmonary artery pressure (32.1 +/- 1.93 vs. 35.5 +/- 2 mmHg, p < 0.01) and Fulton's index (0.52 +/- 0.089 vs. 0.67 +/- 0.12, p < 0.001) and less severe right ventricular hypertrophy and distal vessel muscularization. These results suggest that induction of ANP expression in the lung may hold promise in the treatment of pulmonary hypertension.

NPR-A-Deficient mice show increased susceptibility to hypoxia-induced pulmonary hypertension

BACKGROUND

Mice in which the gene encoding NPR-A, a guanylyl cyclase-linked natriuretic peptide receptor, has been disrupted were used to examine the contribution of natriuretic peptides to maintaining pulmonary vascular homeostasis in normal- and low-oxygen environments.

METHODS AND RESULTS

Wild-type (+/+), heterozygous (+/-), and homozygous null mutants (-/-) were studied. The response of the pulmonary vasculature to atrial, B-type, and C-type natriuretic peptides (ANP, BNP, and CNP) during acute hypoxia was studied in isolated perfused lungs. Right ventricular systolic pressure (RVSP), RV weight, and pulmonary vascular remodeling were measured in each genotype exposed to normal air and after 7 and 21 days in a hypoxic atmosphere (10% O2). ANP and BNP (300 ng) reduced pulmonary artery pressure during acute hypoxia-induced pulmonary vasoconstriction in +/+ mice, but this effect was attenuated in +/- and absent in -/- mice. CNP (600 ng) had little effect in all 3 genotypes. RVSP and RV weight were similar in the 3 genotypes housed in a normal-O2 environment. Seven and 21 days of hypoxia produced a pronounced and significantly greater increase in RVSP and RV weight in -/- mice compared with +/+ or +/- mice and more rapid muscularization of distal pulmonary arterioles.

CONCLUSIONS

ANP and BNP do not contribute to maintaining normal pulmonary artery pressure but play an important role in attenuating the pulmonary vascular response to hypoxia. NPR-A mediates the vasorelaxant effect of ANP in pulmonary vasculature.

Enhanced natriuretic response to neutral endopeptidase inhibition in patients with moderate chronic renal failure

Atrial natriuretic factor (ANF) has natriuretic, renin-suppressing and chronic hypotensive actions that may be utilized by inhibition of ANF degradation by neutral endopeptidase, E.C.24.11 (NEP). Three groups of 8 male patients [GFR 103 +/- 8 (Normal), 64 +/- 6 (Moderate CRF), and 16 +/- 2 ml/min (Severe CRF)] received 100 mg i.v. bolus of the NEP inhibitor candoxatrilat or placebo in random order in a double-blind crossover study. GFR (51CR-EDTA), ERPF (125I-hippuran). ANF (IRMA), urinary cGMP (RIA) and albumin (RIA) and sodium excretion and flow rate were measured hourly for two hours before and for seven hours after candoxatrilat administration. After candoxatrilat plasma ANF rose two- to threefold from baseline, and remained elevated for 5(N) and 7(M,S) hours (P < 0.01(N,S), P < 0.03(M)) associated with an immediate rise in urine cGMP excretion from 23.5(N), 25.4(M) and 10.4(S) nmol/hr (base) to 51.7(N), 73.8(M) and 27.5(S)(peak) lasting 7(N,M,S) hours (P < 0.01(N,M,S)). There was a marked natriuresis in all three groups, the cumulative sodium excretion at seven hours post-candoxatrilat being 104(N), 140(M), 102(S) mmol (P < 0.05(N,M,S)). This was greatest in those with moderate CRF (moderate CRF vs. normal, P = 0.036, moderate vs. severe CRF, P = 0.01, normal vs. severe CRF, P = 0.74). Following candoxatrilat there was a near doubling of the urine flow rate (P < 0.01(N,S), P < 0.02(M)). Urine albumin excretion increased in patients with renal failure (P < 0.01), but there was no change in GFR, ERPF or systemic blood pressure. We conclude that the marked natriuretic effects of acute NEP inhibition seen in normal subjects are enhanced in the presence of moderate CRF and sustained even in severe renal impairment.

Atrial natriuretic peptide and brain natriuretic peptide in cor pulmonale. Hemodynamic and endocrine effects

We have studied the hemodynamic and hormonal effects of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) in eight patients with cor pulmonale. Subjects were studied twice and were given a 20-min placebo infusion followed by either ANP or BNP (3 pmol/kg/min then 10 pmol/kg/ min for 20 min each). Responses were measured after placebo infusion and following low-dose then high-dose ANP or BNP. Placebo infusion had no significant effects on either study day. Low-dose ANP and BNP significantly reduced mean pulmonary artery pressure (MPAP) from baseline by 3.7 mm Hg (95% confidence interval [CI], 1.4 to 6.1) and 3.0 mm Hg (95% CI, 0.6 to 5.4), respectively. High-dose ANP and BNP further reduced MPAP from baseline by 7.1 mm Hg (95% CI, 4.8 to 9.4) and 7.1 mm Hg (95% CI, 4.7 to 9.6), respectively. Effects on total pulmonary vascular resistance were similar. ANP and BNP had no confounding systemic hemodynamic effects. Plasma aldosterone was significantly suppressed from baseline by ANP: 156 pmol/L (95% CI, 93 to 220) after low dose, 275 pmol/L (95% CI, 207 to 343) after high dose; and by BNP: 92 pmol/L (95% CI, 30 to 153) after low dose, 159 pmol/L (95% CI, 98 to 220) after high dose. ANP and BNP produced dose-related pulmonary vasodilatation in patients with cor pulmonale, without worsening oxygen saturation or affecting systemic hemodynamics. ANP and BNP also exerted favorable neurohormonal effects by suppressing aldosterone.

Angiotensin II receptor expression and inhibition in the chronically hypoxic rat lung

1. Angiotensin II (AII) binding density and the effect of chronic AII receptor blockade were examined in the rat model of hypoxia-induced pulmonary hypertension. 2. [125I]-[Sar1,Ile2]AII binding capacity was increased in lung membranes from rats exposed to hypoxia (10% fractional inspired O2) for 7 days compared to normal rats (Bmax 108 +/- 12 vs 77 +/- 3 fmol mg-1 protein; P < 0.05), with no significant change in dissociation constant. Competition with specific AII receptor subtype antagonists demonstrated that AT1 is the predominant subtype in both normal and hypoxic lung. 3. Rats treated intravenously with the AT1 antagonist, GR138950C, 1 mg kg-1 day-1 rather than saline alone during 7 days of exposure to hypoxia developed less pulmonary hypertension (pulmonary arterial pressure: 21.3 +/- 1.7 vs 28.3 +/- 1.1 mmHg; P < 0.05), right ventricular hypertrophy (right/left ventricle weight ratio: 0.35 +/- 0.01 vs 0.45 +/- 0.01; P < 0.05) and pulmonary artery remodelling (abundance of thick-walled pulmonary vessels: 9.6 +/- 1.4% vs 20.1 +/- 0.9%; P < 0.05). 4. The reduction in cardiac hypertrophy and pulmonary remodelling with the AT1 antagonist was greater than that achieved by a dose of sodium nitroprusside (SNP) that produced a comparable attenuation of the rise in pulmonary arterial pressure during hypoxia. 5. The data suggest that AII, via the AT1 receptor, has a role in the early pathogenesis of hypoxia-induced pulmonary hypertension in the rat.

Neutral endopeptidase inhibitors and the pulmonary circulation

1. Neutral endopeptidase (NEP) EC 3.4.24.11 is a zinc-metallopeptidase which is partly responsible for the degradation of atrial natriuretic peptide (ANP) in vivo. 2. ANP inhibits vascular smooth muscle cell proliferation, and elicits vasorelaxation of the systemic and, more potently, the pulmonary vasculature. Plasma ANP levels are elevated in human disease states characterized by pulmonary hypertension, and in animal models of these diseases. 3. However, the short in vivo half-life of ANP suggests that it has limited therapeutic potential. Therefore, it has been hypothesized that inhibition of the metabolism of ANP may prove successful in the treatment of pulmonary hypertension. 4. Several inhibitors of NEP have been shown to reduce the development of pulmonary hypertension secondary to chronic hypoxia in rats. In addition, the inhibitor SCH 42495, partially reversed the established cardio-pulmonary remodelling associated with this disease model, without elevating plasma ANP levels. 5. The physiological actions of ANP are many of the properties desirable in a treatment for pulmonary hypertension. Thus, attenuating the metabolism of this peptide using NEP inhibitors, should potentially enhance the effects of ANP, either by maintaining plasma levels or at a local, tissue level.

The role of the renin-angiotensin and natriuretic peptide systems in the pulmonary vasculature

1. The role of vasoactive peptide systems in the pulmonary vasculature has been studied much less extensively than systemic vascular and endocrine effects. The current understanding of the role of the renin-angiotensin (RAS) and natriuretic peptide systems (NPS) in the pulmonary circulation is therefore reviewed. 2. Plasma concentrations of angiotensin II, the main vasoactive component of the RAS, are elevated in pulmonary hypertension and may interact with hypoxaemia to cause further pulmonary vasoconstriction. Pharmacological manipulation of angiotensin II can attenuate hypoxic pulmonary vasoconstriction but larger studies are needed to establish the efficacy of this therapeutic strategy in established pulmonary hypertension. 3. Although all the known natriuretic peptides, ANP, BNP and CNP are elevated in cor pulmonale, only ANP and BNP appear to have pulmonary vasorelaxant activity in humans. ANP and BNP can also attenuate hypoxic pulmonary vasoconstriction, suggesting a possible counter-regulatory role for these peptides. Inhibition of ANP/BNP metabolism by neutral endopeptidase has been shown to attenuate development of hypoxic pulmonary hypertension but this property has not been tested in humans. 4. It is also well established that there are potentially important endocrine and systemic circulatory interactions between the RAS and NPS. This also occurs in the pulmonary circulation and in humans, where at least BNP acts to attenuate angiotensin II induced pulmonary vasoconstriction. This interaction may be particularly relevant as a mechanism to counter-regulate overactivity of the RAS.(ABSTRACT TRUNCATED AT 250 WORDS)

Neutral endopeptidase (NEP) inhibition in rats with established pulmonary hypertension secondary to chronic hypoxia

1. Atrial natriuretic peptide (ANP) causes vasorelaxation in the pulmonary vasculature. ANP levels are elevated in conditions characterized by pulmonary hypertension and it has been hypothesized that ANP may be autoregulatory in the pulmonary circulation. 2. One route of ANP metabolism in vivo is by the action of the enzyme neutral endopeptidase (NEP). We have studied the effects of the NEP inhibitor, SCH 42495, in rats with established pulmonary hypertension secondary to chronic hypoxia. 3. Rats (n = 32) were divided into 4 groups. Normoxic controls were kept in air for 10 days (NC10) and all other animals were placed in a normobaric hypoxic chamber (F1 O2 10%). Chronic hypoxic controls were studied at 10 days (CHC10). After 10 days hypoxia the two remaining groups received oral treatment for a further 10 days, consisting of either SCH 42495 (30 mg kg-1, twice daily CHT20) or methyl cellulose vehicle (0.4%, twice daily, CHV20). 4. Animals were anaesthetized and blood collected for measurement of plasma ANP. Hearts were dissected and ventricles weighed and the histology of the pulmonary vasculature examined. 5. CHC10 rats had significant right ventricular hypertrophy (0.53 +/- 0.08) and pulmonary vascular remodelling (29.0 +/- 0.01%) and had gained significantly less body weight (33.2 +/- 5.5 g) than NC10 rats (0.31 +/- 0.04, 10.9 +/- 0.01%, and 59.2 +/- 11.9 g respectively). CHC10 rats had significantly elevated plasma ANP levels (58.4 +/- 9.9 pM) compared with NC10 rats (23.9 +/- 32 pM). Treatment with SCH 42495 caused a significant reduction in pulmonary vascular remodelling (25.0 +/- 0.01%) and right ventricular hypertrophy (0.52 +/- 0.09) in CHT20 rats compared with CHV20 controls (33.0 +/- 0.02% and 0.61 +/- 0.09 respectively). Pulmonary vascular remodelling was also significantly lower in CHT20 rats than CHC1O animals.6. Thus, short term inhibition of NEP causes regression of established pulmonary vascular remodelling and may be a useful therapeutic strategy in pulmonary hypertension.

Haemodynamic effects of atrial natriuretic peptide in hypoxic chronic obstructive pulmonary disease

BACKGROUND

Pulmonary artery pressure is elevated in patients with advanced chronic obstructive pulmonary disease (COPD). Release of atrial natriuretic peptide (ANP) is increased in pulmonary hypertension and this hormone may both selectively vasodilate pulmonary vessels and inhibit pulmonary vascular remodelling. The hypothesis that ANP has a physiological role in protection of the pulmonary circulation from pressure overload, and that it may be beneficial in patients with COPD, has been examined.

METHODS

Ten patients with hypoxic COPD were infused for 30 minute periods with saline followed by ANP at 0.4, 2, and 10 pmol/kg/min respectively via a pulmonary artery catheter whilst monitoring haemodynamics and oxygenation.

RESULTS

Levels of immunoreactive ANP (irANP) increased from a mean (SD) of 23 (15) pmol/l to a maximum of 94 (41) pmol/l. Neither systemic blood pressure, cardiac output nor total systemic vascular resistance showed any correlation with irANP levels. There were negative correlations between levels of ANP and mean pulmonary artery pressure which fell from 28.7 to 25.9 mm Hg, pulmonary artery wedge pressure which fell from 6.5 to 4.6 mmHg, and total pulmonary vascular resistance which fell from 489 to 428 dynes s cm-5. There was a small fall in PaCO2 from 6.2 to 5.9 kPa, whilst venous admixture and oxygen delivery both increased non-significantly.

CONCLUSIONS

At these pathophysiological concentrations there was evidence that ANP selectively reduced right ventricular afterload. These data support the hypotheses that increased plasma levels of ANP may be beneficial in hypoxic COPD, and that endogenous ANP may ameliorate pulmonary hypertension in humans.

Effects of atrial natriuretic peptide and nitroprusside on isolated pulmonary resistance and conduit arteries from rats with pulmonary hypertension

1. The relaxant effects of atrial natriuretic peptide (ANP) and nitroprusside were studied on prostaglandin F2 alpha (PGF2 alpha)-contracted preparations of pulmonary resistance vessels (internal diameter 200-500 microns) and main pulmonary arteries taken from rats with pulmonary hypertension induced by monocrotaline (105 mg kg-1, s.c., 4 weeks previously). Control rats received saline. 2. In preparations from monocrotaline-treated rats, the potencies (negative log EC50) of ANP on resistance vessels (8.45) and main pulmonary arteries (8.25) were similar to those obtained in vessels from control rats (8.78 and 8.53 respectively), whereas those of nitroprusside were significantly less than in controls in both resistance vessels (7.13 compared with 7.63 in controls; three fold decrease in potency) and main pulmonary arteries (6.92 compared with 8.17 in controls; 18 fold decrease in potency). 3. On pulmonary resistance vessels from monocrotaline-treated rats, the maximum relaxant responses to ANP and nitroprusside, and also to pinacidil, were increased compared with controls, and reversal of the PGF2 alpha-induced contraction by these drugs was greater than 100%. In contrast, on main pulmonary arteries from monocrotaline-treated rats, the maximum relaxations to ANP and nitroprusside were not increased relative to controls, and reversal of PGF2 alpha was not greater than 100%. 4. Since the high potency of ANP on pulmonary resistance and conduit arteries is retained in vessels from rats with pulmonary hypertension, whether induced by monocrotaline (this study) or by chronic hypoxia (previous findings), it is postulated that elevation of plasma levels of ANP by use of drugs that inhibit the breakdown of this endogenous peptide, may be one approach to the pharmacological treatment of pulmonary hypertension.

Early changes in the calcitonin gene-related peptide (CGRP) content of pulmonary endocrine cells concomitant with vascular remodeling in the hypoxic rat

Morphologic changes are reported to occur in rat lung vasculature after 3 days of hypoxia. We have previously shown that immunoreactivity for the vasodilator calcitonin gene-related peptide (CGRP) is increased in pulmonary endocrine cells by 7 days of hypoxia. Because these cells may be among the earliest mediators of the hypoxic response, we examined endocrine cell CGRP content in rat lung after 0, 2, 4, and 8 h and 1, 5, 10, 15, 20, 28, and 35 days of normobaric hypoxia, using optimal and supraoptimal dilutions of CGRP antibodies to demonstrate changes in CGRP immunoreactivity. This was compared with temporal changes in pulmonary vascular smooth muscle after 1, 5, and 20 days of hypoxia exposure by evaluating vascular immunoreactivity for alpha-smooth muscle actin (alpha-SM actin), platelet-derived growth factor (PDGF) beta-receptor, and proliferating cell nuclear antigen (PCNA). Significant increases in endocrine cell CGRP immunoreactivity were found after 4 h of hypoxia, and levels increased up to 1 day, followed by a decrease (at 5 days) and then a progressive increase up to 35 days. After 1 day of hypoxia, the number of vessels displaying immunoreactivity for alpha-SM actin, PDGF beta-receptor, and PCNA were also significantly increased. Whereas PDGF beta-receptor and PCNA returned to control values by day 20, alpha-SM actin reached a plateau that persisted until 20 days. The results indicate that modulation of endocrine cell CGRP content in response to hypoxia is rapid and characterized by a significant and persistent increase, paralleled by a proliferation of vascular cells leading to vascular muscularization.(ABSTRACT TRUNCATED AT 250 WORDS)