Suppressed baroreflex peripheral arc overwhelms augmented neural arc and incapacitates baroreflex function in rats with pulmonary arterial hypertension.

NEW FINDINGS: What is the central question of this study? The impact of pulmonary arterial hypertension on open-loop baroreflex function, which determines how powerfully and rapidly the baroreflex operates to regulate arterial pressure, remains poorly understood. What is the main finding and its importance? The gain of the baroreflex total arc, indicating the baroreflex pressure-stabilizing function, is markedly attenuated in rats with monocrotaline-induced pulmonary arterial hypertension. This is caused by a rightward shift of the baroreflex neural arc and a downward shift of the peripheral arc. These findings contribute greatly to our understanding of arterial pressure regulation by the sympathetic nervous system in pulmonary arterial hypertension. ABSTRACT: Sympathoexcitation has been documented in patients with established pulmonary arterial hypertension (PAH). Although the arterial baroreflex is the main negative feedback regulator of sympathetic nerve activity (SNA), the way in which PAH impacts baroreflex function remains poorly understood. In this study, we conducted baroreflex open-loop analysis in a rat model of PAH. Sprague-Dawley rats were injected with monocrotaline (MCT) s.c. to induce PAH (60 mg kg-1 ; n = 11) or saline as a control group (CTL; n = 8). At 3.5 weeks after MCT injection, bilateral carotid sinuses were isolated, and intrasinus pressure (CSP) was controlled while SNA at the coeliac ganglia and arterial pressure (AP) were recorded. To examine the static baroreflex function, CSP was increased stepwise while steady-state AP (total arc) and SNA (neural arc) responses to CSP and the AP response to SNA (peripheral arc) were measured. Monocrotaline significantly decreased the static gain of the baroreflex total arc at the operating AP compared with CTL (-0.80 ± 0.31 versus -0.22 ± 0.22, P < 0.05). Given that MCT markedly increased plasma noradrenaline, an index of SNA, by approximately 3.6-fold compared with CTL, calibrating SNA by plasma noradrenaline revealed that MCT shifted the neural arc to a higher SNA level and shifted the peripheral arc downwards. Monocrotaline also decreased the dynamic gain of the baroreflex total arc (-0.79 ± 0.16 versus -0.35 ± 0.17, P < 0.05), while the corner frequencies that reflect the speed of the baroreflex remained unchanged (0.06 ± 0.02 versus 0.08 ± 0.02 Hz, n.s.). In rats with MCT-induced PAH, the suppressed baroreflex peripheral arc overwhelms the augmented neural arc and, in turn, attenuates the gain of the total arc, which determines the pressure-stabilizing capacity of the baroreflex.

Late reversible complete atrioventricular block and PR interval normalization after antegrade slow pathway ablation for atrioventricular nodal re-entrant tachycardia with pre-existing PR prolongation.

Late complete atrioventricular block (CAVB) is a rare periprocedural complication in the treatment of atrioventricular (AV) nodal re-entrant tachycardia. However, it can necessitate permanent pacemaker implantation. We present a case of late CAVB that developed during the periprocedural period. Its pathogenesis was attributed to the indirect or functional effects on the fast pathway of the AV node due to the presence of paroxysmal supraventricular tachycardia with PR prolongation. Additionally, PR prolongation regressed to within the normal range after curing the late CAVB, and the advanced AV block with treadmill exercise stress test also improved 1:1 AV conduction with time. Periprocedural complications such as bradyarrhythmia may be reversible if late CAVB occurs within a few weeks after ablation. Thus, urgent permanent pacemaker implantation should be carefully considered. Learning objective: Late high-grade atrioventricular (AV) blocks can develop during the periprocedural period even if antegrade slow pathway ablation does not result in a complete AV block. Late high-grade AV block is a relatively rare periprocedural complication. However, it can necessitate permanent pacemaker implantation. Additionally, if a late high-grade AV block develops within a few weeks after ablation, bradyarrhythmia-such as periprocedural complications-may be reversible and indicate that permanent pacemaker implantation should be carefully considered.

Chronic Inhibition of Toll-Like Receptor 9 Ameliorates Pulmonary Hypertension in Rats.

Background Recent accumulating evidence suggests that toll-like receptor 9 (TLR9) is involved in the pathogenesis of cardiovascular diseases. However, its role in pulmonary hypertension remains uncertain. We hypothesized that TLR9 is involved in the development of pulmonary hypertension. Methods and Results A rat model of monocrotaline-induced pulmonary hypertension was used to investigate the effects of TLR9 on hemodynamic parameters, vascular remodeling, and survival. Monocrotaline-exposed rats significantly showed increases in plasma levels of mitochondrial DNA markers, which are recognized by TLR9, TLR9 activation in the lung, and interleukin-6 mRNA level in the lung on day 14 after monocrotaline injection. Meanwhile, monocrotaline-exposed rats showed elevated right ventricular systolic pressure, total pulmonary vascular resistance index and vascular remodeling, together with macrophage accumulation on day 21. In the preventive protocol, administration (days -3 to 21 after monocrotaline injection) of selective (E6446) or nonselective TLR9 inhibitor (chloroquine) significantly ameliorated the elevations of right ventricular systolic pressure and total pulmonary vascular resistance index as well as vascular remodeling and macrophage accumulation on day 21. These inhibitors also significantly reduced NF-κB activation and interleukin-6 mRNA levels to a similar extent. In the short-term reversal protocol, E646 treatment (days 14-17 after monocrotaline injection) almost normalized NF-κB activation and interleukin-6 mRNA level, and reduced macrophage accumulation. In the prolonged reversal protocol, E6446 treatment (days 14-24 after monocrotaline injection) reversed total pulmonary vascular resistance index and vascular remodeling, and improved survival in monocrotaline-exposed rats. Conclusions TLR9 is involved in the development of pulmonary hypertension concomitant via activation of the NF-κB‒IL-6 pathway. Inhibition of TLR9 may be a novel therapeutic strategy for pulmonary hypertension.

Increased Lung Uric Acid Deteriorates Pulmonary Arterial Hypertension.

Background Recent studies have demonstrated that uric acid (UA) enhances arginase activity, resulting in decreased NO in endothelial cells. However, the role of lung UA in pulmonary arterial hypertension (PAH) remains uncertain. We hypothesized that increased lung UA level contributes to the progression of PAH. Methods and Results In cultured human pulmonary arterial endothelial cells, voltage-driven urate transporter 1 (URATv1) gene expression was detected, and treatment with UA increased arginase activity. In perfused lung preparations of VEGF receptor blocker (SU5416)/hypoxia/normoxia-induced PAH model rats, addition of UA induced a greater pressure response than that seen in the control and decreased lung cGMP level. UA-induced pressor responses were abolished by benzbromarone, a UA transporter inhibitor, or L-norvaline, an arginase inhibitor. In PAH model rats, induction of hyperuricemia by administering 2% oxonic acid significantly increased lung UA level and induced greater elevation of right ventricular systolic pressure with exacerbation of occlusive neointimal lesions in small pulmonary arteries, compared with nonhyperuricemic PAH rats. Administration of benzbromarone to hyperuricemic PAH rats significantly reduced lung UA levels without changing XOR (xanthine oxidoreductase) activity, and attenuated right ventricular systolic pressure increase and occlusive lesion development. Topiroxostat, a XOR inhibitor, significantly reduced lung XOR activity in PAH rats, with no effects on increase in right ventricular systolic pressure, arterial elastance, and occlusive lesions. XOR-knockout had no effects on right ventricular systolic pressure increase and arteriolar muscularization in hypoxia-exposed mice. Conclusions Increased lung UA per se deteriorated PAH, whereas XOR had little impact. The mechanism of increased lung UA may be a novel therapeutic target for PAH complicated with hyperuricemia.

Inhibition of TLR9-NF-κB-mediated sterile inflammation improves pressure overload-induced right ventricular dysfunction in rats.

AIMS: Recent accumulating evidence suggests that sterile inflammation plays a crucial role in the progression of various cardiovascular diseases. However, its contribution to right ventricular (RV) dysfunction remains unknown. The aim of this study was to elucidate whether toll-like receptor 9 (TLR9)-NF-κB-mediated sterile inflammation plays a critical role in the pathogenesis of RV dysfunction. METHODS AND RESULTS: We performed main pulmonary artery banding (PAB) in rats to induce RV pressure overload and dysfunction. On Day 14 after PAB, the pressure overload impaired RV function as indicated by increased RV end-diastolic pressure concomitant with macrophage infiltration and fibrosis, as well as maximal activation of NF-κB and TLR9. Short-term administration (days 14-16 after PAB) of a specific TLR9 inhibitor, E6446, or an NF-κB inhibitor, pyrrolidine dithiocarbamate (PDTC) significantly attenuated NF-κB activation. Furthermore, long-term administration of E6446 (treatment: days 14-28) or PDTC (prevention: days -1 to 28; treatment: days 14 to 28) improved RV dysfunction associated with mitigated macrophage infiltration and fibrosis in right ventricle and decreased serum brain natriuretic peptide levels. CONCLUSION: Inhibition of TLR9-NF-κB pathway-mediated sterile inflammation improved PAB-induced RV dysfunction in rats. This pathway plays a major role in the progression of pressure overload-induced RV dysfunction and is potentially a novel therapeutic target for the disorder.

Inhibition of nitric oxide synthase unmasks vigorous vasoconstriction in established pulmonary arterial hypertension.

It is widely accepted that impaired bioavailability of endothelial nitric oxide (NO) plays a critical role in the pathophysiology of pulmonary arterial hypertension (PAH). However, there are published data that show that relatively many PAH patients respond favorably to acetylcholine-induced pulmonary vasodilation during their follow-up period, when diverse stages of the disorder are included. We hypothesized that NO bioavailability varies depending on the progression of PAH Adult rats were exposed to the VEGF receptor blocker Sugen5416 and 3 weeks of hypoxia followed by return to normoxia for various additional weeks. All rats developed increased right ventricular systolic pressure (RVSP) and occlusive lesion formation at 1, 3, 5, and 8 weeks after the Sugen5416 injection. Acute NO synthase blockade did not change the elevated RVSP at the 1-week time point, while it further increased RVSP markedly at the 3-, 5-, and 8-week time points, leading to death in all rats tested at 8 weeks. Acetylcholine caused significant reduction in RVSP at the 8-week but not the 1-week time point, whereas sodium nitroprusside decreased the pressure similarly at both time points. Increased NO-mediated cGMP production was found in lungs from the 8-week but not the 1-week time point. In conclusion, despite its initial impairment, NO bioavailability is restored and endogenous NO plays a critical protective role by counteracting severe pulmonary vasoconstriction in established stages of PAH in the Sugen5416/hypoxia/normoxia-exposed rats. Our results provide solid pharmacological evidence for a major contribution of a NO-suppressed vasoconstrictor component in the pathophysiology of established PAH.