Analysis of pulmonary vascular responses in cats to sympathetic nerve stimulation under elevated tone conditions. Evidence that neuronally released norepinephrine acts on alpha 1-, alpha 2-, and beta 2-adrenoceptors

Atrial arrhythmias in chronic lung disease-associated pulmonary hypertension

Atrial arrhythmias are common during episodes of acute respiratory failure in patients with chronic lung disease-associated pulmonary hypertension. Expert opinion suggests that management of atrial arrhythmias in patients with pulmonary hypertension should aim to restore sinus rhythm. This is clinically challenging in pulmonary hypertension patients with coexisting chronic lung disease, as there is controversy on the use of rhythm control agents; generally, in regard to either their pulmonary toxicity profile or the lack of evidence supporting their use. Rate control methods are largely focused on the use of beta blockers and calcium channel blockers. Concerns regarding their use involve their negative inotropic properties in cor pulmonale, the risk of bronchospasm associated with beta blockers, and the potential for ventilation/perfusion mismatching associated with calcium channel blockers. While digoxin has been associated with promising outcomes during acute right ventricular failure, there is limited evidence to suggest its routine use. Electrical cardioversion is associated with a high failure rate and it frequently requires multiple attempts. Radiofrequency catheter ablation is a more definitive approach, but concerns surrounding mechanical ventilation and sedation limit its applicability in decompensated pulmonary hypertension. Individual approaches are needed to address atrial arrhythmia management during acute episodes of respiratory failure.

Lung Circulation

The circulation of the lung is unique both in volume and function. For example, it is the only organ with two circulations: the pulmonary circulation, the main function of which is gas exchange, and the bronchial circulation, a systemic vascular supply that provides oxygenated blood to the walls of the conducting airways, pulmonary arteries and veins. The pulmonary circulation accommodates the entire cardiac output, maintaining high blood flow at low intravascular arterial pressure. As compared with the systemic circulation, pulmonary arteries have thinner walls with much less vascular smooth muscle and a relative lack of basal tone. Factors controlling pulmonary blood flow include vascular structure, gravity, mechanical effects of breathing, and the influence of neural and humoral factors. Pulmonary vascular tone is also altered by hypoxia, which causes pulmonary vasoconstriction. If the hypoxic stimulus persists for a prolonged period, contraction is accompanied by remodeling of the vasculature, resulting in pulmonary hypertension. In addition, genetic and environmental factors can also confer susceptibility to development of pulmonary hypertension. Under normal conditions, the endothelium forms a tight barrier, actively regulating interstitial fluid homeostasis. Infection and inflammation compromise normal barrier homeostasis, resulting in increased permeability and edema formation. This article focuses on reviewing the basics of the lung circulation (pulmonary and bronchial), normal development and transition at birth and vasoregulation. Mechanisms contributing to pathological conditions in the pulmonary circulation, in particular when barrier function is disrupted and during development of pulmonary hypertension, will also be discussed.

Prognostic significance of sympathetic nervous system activation in pulmonary arterial hypertension

RATIONALE

The sympathetic nervous system has been reported to be activated in pulmonary arterial hypertension (PAH).

OBJECTIVES

We investigated the prognostic significance of muscle sympathetic nervous system activity (MSNA) in PAH.

METHODS

Thirty-two patients with PAH were included in the study and underwent a measurement of MSNA over a 6-year period of time. They had undergone a concomitant evaluation of New York Heart Association (NYHA) functional class, a 6-minute walk distance (6MWD), an echocardiographic examination, and a right heart catheterization for diagnostic or reevaluation purposes. The median follow-up time was 20.6 months (interquartile range, 45.8 mo). Clinical deterioration was defined by listing for transplantation or death.

MEASUREMENTS AND MAIN RESULTS

Seventeen patients presented with clinical deterioration. As compared with the 15 others, they had an increased MSNA (80 +/- 12 vs. 52 +/- 18 bursts/min; P < 0.001) and heart rate (88 +/- 17 vs. 74 +/- 12 bpm; P = 0.01), a lower 6MWD (324 +/- 119 vs. 434 +/- 88 m; P < 0.01) and a deteriorated NYHA functional class (3.6 +/- 0.5 vs. 2.9 +/- 0.8; P < 0.001). The hemodynamic variables were not different. MSNA was directly related to heart rate and inversely to 6MWD. A univariate analysis revealed that increased MSNA and heart rate, NYHA class IV, lower 6MWD, and pericardial effusion were associated with subsequent clinical deterioration. A multivariate analysis showed that MSNA was an independent predictor of clinical deterioration. For every increase of 1 burst/minute, the risk of clinical deterioration during follow-up increased by 6%.

CONCLUSIONS

Sympathetic nervous system activation is an independent predictor of clinical deterioration in pulmonary arterial hypertension.

Sympathetic overactivity in patients with pulmonary stenosis and improvement after percutaneous balloon valvuloplasty

OBJECTIVE

Percutaneous balloon valvulotomy (PBV) is the procedure of choice for the treatment of valvular pulmonary stenosis (PS) with similar results comparable to surgical valvotomy but less invasive.

METHODS AND RESULTS

Twenty-seven consecutive patients with PS being evaluated for PBV were enrolled in the study. Peak instantaneous transvalvular gradient, right ventricle (RV) diameter, mean atrial pressures, RV systolic pressure (RVSP), pro-brain natriuretic peptide (proBNP) levels significantly decreased immediately after PBV. Regarding heart rate variability (HRV) parameters, mean HR (heart rate), LF (low frequency) day and night, LF/HF day and night significantly decreased and standard deviation of all NN intervals (SDNN), root mean square of successive differences (RMSSD), P number of NN intervals that differed by more than 50 ms from adjacent interval divided by the total number of all NN intervals (PNN50), HF (High frequency) day and night significantly increased 1 day after PBV and these changes were shown to be preserved at the first month. The increase in SDNN was correlated with the decrease in right atrial pressure (RAP) (r =-0.5, P = 0.04); the increase in standard deviation of the 5-minute mean RR intervals (SDANN) was correlated with the decrease in proBNP (r =-0.4, P = 0.03).

CONCLUSIONS

Sympathetic overactivity and increased proBNP levels were associated with the symptomatic status of patients with PS. Associated with a decrease in atrial pressures and proBNP levels, PBV yielded a decrease in adrenergic overactivity in the patients with PS.

Reflex regulation of airway sympathetic nerves in guinea-pigs

Sympathetic nerves innervate the airways of most species but their reflex regulation has been essentially unstudied. Here we demonstrate sympathetic nerve-mediated reflex relaxation of airway smooth muscle measured in situ in the guinea-pig trachea. Retrograde tracing, immunohistochemistry and electrophysiological analysis identified a population of substance P-containing capsaicin-sensitive spinal afferent neurones in the upper thoracic (T1-T4) dorsal root ganglia (DRG) that innervate the airways and lung. After bilateral vagotomy, atropine pretreatment and pre-contraction of the trachealis with histamine, nebulized capsaicin (10-60 microm) evoked a 63+/-7% reversal of the histamine-induced contraction of the trachealis. Either the beta-adrenoceptor antagonist propranolol (2 microm, administered directly to the trachea) or bilateral sympathetic nerve denervation of the trachea essentially abolished these reflexes (10+/-9% and 6+/-4% relaxations, respectively), suggesting that they were mediated primarily, if not exclusively, by sympathetic adrenergic nerve activation. Cutting the upper thoracic dorsal roots carrying the central processes of airway spinal afferents also markedly blocked the relaxations (9+/-5% relaxation). Comparable inhibitory effects were observed following intravenous pretreatment with neurokinin receptor antagonists (3+/-7% relaxations). These reflexes were not accompanied by consistent changes in heart rate or blood pressure. By contrast, stimulating the rostral cut ends of the cervical vagus nerves also evoked a sympathetic adrenergic nerve-mediated relaxation that were accompanied by marked alterations in blood pressure. The results indicate that the capsaicin-induced reflex-mediated relaxation of airway smooth muscle following vagotomy is mediated by sequential activation of tachykinin-containing spinal afferent and sympathetic efferent nerves innervating airways. This sympathetic nerve-mediated response may serve to oppose airway contraction induced by parasympathetic nerve activation in the airways.

The effects of selective phosphodiesterase III and V inhibitors on adrenergic and non-adrenergic, non-cholinergic relaxation responses of guinea-pig pulmonary arteries

1. The aim of the present study was to investigate the role of several possible neurotransmitters in mediating non-adrenergic, non-cholinergic (NANC) relaxation, and the effects of phosphodiesterase (PDE) III and V inhibitors on adrenergic and NANC relaxation in branch pulmonary artery (PA) of guinea-pig. 2. Under the NANC conditions, electrical field stimulation (EFS, 60 V, 0.2 ms, 20 Hz) induced a tetrodotoxin-sensitive relaxation of the histamine-precontracted PA rings. The nitric oxide (NO) synthase inhibitor NG-nitro-l-arginine methyl ester (l-NAME, 10(-4) m) and the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10(-5) m) partially inhibited the EFS-induced relaxation. The inhibitory effect of l-NAME was reversed completely by l-arginine (10(-3) m), but not d-arginine (10(-3) m). 3. This NANC relaxation was attenuated by 8-phenyltheophylline (10(-5) m), a P1-purinoceptor antagonist. 4. The NANC response was potentiated by 10-6 m zaprinast, a type V PDE inhibitor, but was unaffected by 3 x 10-6 m milrinone, a type III PDE inhibitor. 5. Sodium nitroprusside (SNP) caused a concentration-dependent vasodilator effect which was potentiated by zaprinast, but unaffected by milrinone. Moreover, the effect of combination of zaprinast with milrinone was not significantly different from that observed with zaprinast alone. 6. Isoprenaline produced a concentration-dependent vasodilatation in branch PA of guinea-pig which was potentiated by both zaprinast and milrinone, the efficacy of milrinone being greater than zaprinast. 7. These results suggest that both nitrergic and purinergic pathways are involved in mediating the NANC relaxation in branch PA of guinea-pig. The combination of PDE III or V inhibitors with vasorelaxant drugs may be a hopeful approach for the treatment of pulmonary hypertension.

Remodeling of lung interstitium but not resistance vessels in canine pacing-induced heart failure

We previously showed that pacing-induced heart failure in dogs results in an enhancement of pulmonary vascular reactivity. In the present study we hypothesized that enhanced matrix deposition and structural remodeling of lung resistance microvessels would underlie these functional changes. Using biochemical measures, we found no difference in the normalized lung content of hyaluronan, uronic acid, and collagen between control dogs and dogs paced for 1 mo, although lung dry weight and noncollagen protein content increased significantly in the paced group (P < 0.05). From separate Formalin-fixed lung lobes, 5-microm frozen sections were prepared and stained with Masson's trichrome, and vascular structure was evaluated using standard morphometric techniques. When perivascular fluid cuffs were excluded from the measure of wall thickness, collagen and media volume fractions in any size range did not differ between paced and control groups. Similarly, in the paced group, medial thickness in <400-microm arterial or venular microvessels did not vary significantly from that in the controls. In contrast, the relationship of interstitial fluid pressure to lung water was significantly shifted to the right in the paced group, such that normal tissue pressures were observed, despite the increased water content. We conclude that although 1 mo of pacing-induced heart failure results in altered interstitial function, the attendant pulmonary hypertension and/or hormonal responses are insufficient to induce medial hypertrophy or other remodeling of the extra-alveolar microvasculature.

Potentiation of cyclic AMP-mediated vasorelaxation by phenylephrine in pulmonary arteries of the rat

Alpha1-adrenoceptor agonists may potentiate relaxation to beta-adrenoceptor agonists, although the mechanisms are unclear. We compared relaxations induced by beta-adrenoceptor agonists and cyclic AMP-dependent vasodilators in rat pulmonary arteries constricted with prostaglandin F2alpha (PGF2alpha) or the alpha1-adrenoceptor agonist phenylephrine (PE). In addition, we examined whether differences were related to cyclic AMP- or nitric oxide (NO) and cyclic GMP-dependent pathways. Isoprenaline-induced relaxation was substantially potentiated in arteries constricted with PE compared with PGF2alpha. Methoxamine was similar to PE, whereas there was no difference between PGF2alpha and 30 mM KCl. The potentiation was primarily due to a marked increase in the NO-independent component of relaxation, from 9.1+/-1.7% for PGF2alpha to 55.1+/-4.4% for PE. NO-dependent relaxation was also enhanced, but to a lesser extent (50%). Relaxation to salbutamol was almost entirely NO-dependent in both groups, and was potentiated approximately 50% by PE. Relaxation to forskolin (activator of adenylate cyclase) was also enhanced in PE constricted arteries. Part of this relaxation was NO-dependent, but the major effect of PE was to increase the NO-independent component. Propranolol diminished but did not abolish the potentiation. There was no difference in response to CPT cyclic AMP (membrane permeant analogue) between PE and PGF2alpha, suggesting that mechanisms distal to the production of cyclic AMP were unchanged. Relaxation to sodium nitroprusside (SNP) was the same for PE and PGF2alpha, although relaxation to acetylcholine (ACh) was slightly depressed. This implies that potentiation by PE does not involve the cyclic GMP pathway directly. Mesenteric arteries constricted with PE did not show potentiation of isoprenaline-induced relaxation compared to those constricted with PGF2alpha, suggesting that this effect may be specific to the pulmonary circulation. These results clearly show that PE potentiates both the NO-independent and -dependent components of cyclic AMP-mediated relaxation in pulmonary arteries of the rat, although the effect on the former is more profound. We suggest that potentiation of both components is largely due to direct activation of adenylate cyclase via alpha1-adrenoceptors, within the smooth muscle and endothelial cells respectively.

Noradrenaline, beta-adrenoceptor mediated vasorelaxation and nitric oxide in large and small pulmonary arteries of the rat

1. Noradrenaline induces a meagre vasoconstriction in small muscular pulmonary arteries compared to large conduit pulmonary arteries. We have examined whether this may be partially related to differences in the beta-adrenoceptor-mediated vasorelaxation component and, in particular, beta-adrenoceptor-mediated NO release. 2. Noradrenaline induced a bell-shaped concentration-response in large (1202+/-27 microm) and small (334+/-12 microm) pulmonary arteries of the rat. In large arteries tension increased to 95.6+/-1.8% of 75 mM KCl (KPSS; n=8) at 2 microM, above which tension declined. The response in small arteries was meagre (12+/-1.5% KPSS, n=9), peaking at 0.2 microM. N(G)-monomethyl-L-arginine (L-NMMA; 100 microM) abolished the decline in tension induced by higher concentrations of noradrenaline in large arteries, and increased maximum tension (117+/-3.5% KPSS, n=5, P<0.05). In small arteries peak tension doubled (22.0+/-3.4% KPSS, n=6, P<0.01), but still declined above 0.2 microM. 3. Propranolol (1 microM) abolished the decline in tension at higher concentrations of noradrenaline in both groups, but increased tension substantially more in small (37.4+/-3.7% KPSS, n=5, P<0.001) than in large arteries (112.2+/-3.7% KPSS, n=9, P<0.05). In the presence of L-NMMA, propranolol had no additional effect on large arteries, whereas in small arteries there was greater potentiation than for either agent alone (67.8+/-5.9% KPSS, n=4). 4. Beta-adrenoceptor-mediated relaxation was examined in arteries constricted with prostaglandin F2alpha (50 microM). In the presence of propranolol isoprenaline caused an unexpected vasoconstriction, which was abolished by phentolamine (10 microM). In the presence of phentolamine, isoprenaline caused a maximum relaxation of 43.3+/-2.1% (n=6) in large, and 49.0+/-4.5% (n=6) in small arteries. L-NMMA substantially reduced relaxation in large arteries (7.4+/-1.5%, n=6, P<0.01), but was less effective in small arteries (26.8+/-5.8, n=5, P<0.05). 5. Atenolol (beta1-antagonist, 5 microM) reduced relaxation to isoprenaline (large: 34.8+/-4.5%, n=5; small: 35.0+/-1.9%, n=6), but in combination with L-NMMA had no additional effect over L-NMMA alone. ICI 118551 (beta2-antagonist, 0.1 microM) reduced isoprenaline-induced relaxation more than atenolol (large: 18.0+/-4.6%, n=6, P<0.05; small: 25.6+/-10.7%, n=6, P<0.05). ICI 118551 in combination with L-NMMA substantially reduced relaxation (large: 4.8+/-2.6%, n=9; small: 6.5+/-3.6%, n=5). 6. Salbutamol-induced relaxation was reduced substantially by L-NMMA in large arteries (control: 34.7+/-6.4%, n=6; +L-NMMA: 8.3+/-1.3%, n=5, P<0.01), but to a lesser extent in small arteries (control: 50.9+/-7.5%, n=6; +L-NMMA: 23.0+/-0.7%, n=5, P<0.05). Relaxation to forskolin was also partially antagonized by L-NMMA. 7. These results suggest that the meagre vasoconstriction to noradrenaline in small pulmonary arteries is partially due to a greater beta-adrenoceptor-mediated component than in large arteries. Beta-mediated vasorelaxation in large arteries was largely NO-dependent, whereas in small arteries a significant proportion was NO-independent. Noradrenaline stimulation was also associated with NO release that was independent of beta-adrenoceptors.