Tracheal mucosal blood flow responses to autonomic agonists

Differential effects of inhaled methacholine on circumferential wall and vascular smooth muscle of third-generation airways in awake sheep

Evolution and natural selection ensure that specific mechanisms exist for selective airway absorption of inhaled atmospheric molecules. Indeed, nebulized cholinoceptor agonists used in asthma-challenge tests may or may not enter the systemic circulation. We examined the hypothesis that inhaled cholinoceptor agonists have selective access. Six sheep were instrumented under general anesthesia (propofol 5 mg/kg iv, 2-3% isoflurane-oxygen), each with pulsed-Doppler blood flow transducers mounted on the single bronchial artery and sonomicrometer probes mounted on the intrapulmonary third-generation lingula lobe bronchus. Continuous measurements were made of bronchial blood flow (Q(br)), Q(br) conductance (C(br)), bronchial hemicircumference (CIRC(br)), and bronchial wall thickness (WALL TH(br)) in recovered, standing, awake sheep. Methacholine (MCh; 0.125-2.0 μg/kg iv), at the highest dose, caused a 233% rise in Q(br) (P < 0.05) and a 286% rise in C(br) (P < 0.05). CIRC(br) fell to 90% (P < 0.05); WALL TH(br) did not change. In contrast, nebulized MCh (1-32 mg/ml), inhaled through a mask at the highest dose, caused a rise in ventilation and a rise in Q(br) proportional to aortic pressure without change in C(br). CIRC(br) fell to 91% (P < 0.01), and WALL TH(br) did not change. Thus inhaled MCh has access to cholinoceptors of bronchial circumferential smooth muscle to cause airway lumen narrowing but effectively not to those of the systemic bronchovascular circulation. It is speculated that the mechanism is selective neuroparacrine inhibition of muscarinic acetylcholine receptors (M3 bronchovascular cholinoceptors) by prostanoids released by intense MCh activation of epithelial and mucosal cells lining the airway.

The bronchial circulation--worth a closer look: a review of the relationship between the bronchial vasculature and airway inflammation

Until recently, the bronchial circulation has been relatively ignored in the research and clinical arenas, perhaps because of its small volume and seeming dispensability relative to the pulmonary circulation. Although the bronchial circulation only receives around 1% of the cardiac output in health, it serves functions that are critical to maintaining airway and lung function. The bronchial circulation also plays an important role in many lung and airway diseases; through its ability to increase in size, the bronchial circulation is able to provide lung parenchymal perfusion when the pulmonary circulation is compromised, and more recently the role of the bronchial circulation in the pathogenesis of inflammatory airway disease has been explored. Due to the anatomic variability and small volume of the bronchial circulation, much of the research to date has necessitated the use of animal models and invasive procedures. More recently, non-invasive techniques for measuring bronchial blood flow in the mucosal microvascular network have been developed and offer a new avenue for the study of this circulation in humans. In conjunction with molecular research, measurement of airway blood flow (Q(aw)) may help elucidate the role of the bronchial circulation in inflammatory airway disease and become a useful tool for monitoring therapy.

Effect of fudosteine, a cysteine derivative, on blood flow of tracheal microvasculature increased by airway inflammation

We examined the effect of fudosteine, a cysteine derivative, on blood flow of tracheal microvasculature increased by airway inflammation. Airway inflammation was elicited by sulfur dioxide (SO(2)) exposure for 2 weeks in rabbits. Each drug (500 mg/kg, p.o.) or 0.5% carboxymethylcellulose-Na (control group) was daily administered just before SO(2) exposure. After final SO(2) exposure was finished, blood flow of tracheal microvasculature was measured by blood perfusion monitor. Fudosteine or S-carboxymethylcysteine (S-CMC) significantly suppressed blood flow of tracheal microvasculature increased by SO(2) exposure. However, no effect of fudosteine was observed on the pharmacological microvascular response in trachea of SO(2)-exposed rabbits. On the other hand, fudosteine or S-CMC scavenged superoxide anion generated from rat neutrophils, and enzymatically generated from xanthine oxidase-acetaldehyde reaction. The results suggest that suppressive action in increased tracheal blood flow of fudosteine is due to anti-inflammatory activity, at least in part, via scavenging of superoxide anion.

Status asthmaticus in children: a review

About 10% of American children have asthma, and its prevalence, morbidity, and mortality have been increasing. Asthma is an inflammatory disease with edema, bronchial constriction, and mucous plugging. Status asthmaticus in children requires aggressive treatment with beta-agonists, anticholinergics, and corticosteroids. Intubation and mechanical ventilation should be avoided if at all possible, as the underlying dynamic hyperinflation will worsen with positive-pressure ventilation. If mechanical ventilation becomes necessary, controlled hypoventilation with low tidal volume and long expiratory time may lessen the risk of barotrauma and hypotension. Unusual and nonestablished therapies for severe asthma are discussed.

Comparative effects of alpha-receptor stimulation and nitrergic inhibition on bronchovascular tone

Adrenergic agonists are known to influence bronchial blood flow and bronchovascular resistance. Recently, the nitrergic system has also been implicated in the control of bronchovascular tone. In this study, we compared the effects of the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) and the alpha(1)-receptor agonist phenylephrine on bronchovascular resistance in anesthetized sheep (n = 9). Bronchial blood flow, cardiac output, and systemic and pulmonary arterial pressures were continuously monitored. Phenylephrine (1.2-3.4 microg. kg(-1). min(-1)) was infused intravenously to increase mean systemic arterial pressure above 95 Torr for 10 min and then was discontinued. When hemodynamic parameters returned to baseline, nebulized phenylephrine (10 mg) was given over 10 min. When parameters again normalized, L-NAME (30 mg/kg) was infused intravenously over 1 min. Intravenous phenylephrine increased systemic vascular resistance by 40% at 10 min with no concurrent increase in bronchovascular resistance, but inhaled phenylephrine increased bronchovascular resistance by 66% at 10 min. By comparison, intravenous L-NAME produced a rapid and sustained fivefold increase in bronchovascular resistance at 10 min. We conclude that, although alpha-agonist stimulation has some influence on bronchovascular resistance in sheep, the nitrergic system has predominant control of bronchovascular tone.

Altered contractile sensitivity of isolated bronchial artery to phenylephrine in ovalbumin-sensitized rabbits

We tested the hypothesis that atopy and/or allergic lung inflammation enhances alpha1-adrenoceptor-mediated contractions of the bronchial artery. Bronchial arterial resistance vessels were isolated from rabbits that had undergone either systemic ovalbumin (OVA) sensitization followed by saline aerosol challenge (OVA/saline rabbits), or OVA sensitization followed by OVA aerosol challenge (OVA/OVA rabbits), or no sensitization followed by saline aerosol challenge (control rabbits). In OVA/OVA rabbits, bronchoalveolar lavage and lung histology revealed lymphocytic and eosinophilic inflammation. Arterial rings were contracted with phenylephrine (PE). In endothelium-intact arteries isolated from OVA/saline and OVA/OVA rabbits, PE responsiveness was enhanced compared with that of arteries isolated from controls. The nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester increased the contractile response to PE in all three experimental groups to a similar degree, suggesting that depressed NOS activity was not involved in the enhanced PE responsiveness in OVA/saline and OVA/OVA rabbits. After endothelium removal, arteries from OVA/saline and control rabbits showed similar PE responsiveness, indicating that the enhancement of PE responsiveness was endothelium dependent, possibly due to an endothelial constricting factor. In OVA/OVA rabbits, endothelium-denuded arteries showed decreased PE responsiveness compared with the other two groups; this difference was abolished by NG-nitro-L-arginine methyl ester. We conclude that systemic sensitization with OVA per se enhances PE-induced contractions of isolated bronchial arteries in rabbits by an endothelium-dependent mechanism and that allergic lung inflammation attenuates this effect by increased nonendothelial NOS activity.

Effect of aerosolized acetylcholine on bronchial blood flow

We studied the effects of aerosolized as well as intravenous infusion of acetylcholine on bronchial blood flow in six anesthetized sheep. Intravenous infusion of acetylcholine, at a dose of 2 microg/kg, increased bronchial blood flow from 45 +/- 15 (SE) to 74 +/- 30 ml/min, and vascular conductance increased by 76 +/- 22%. In contrast, aerosolized acetylcholine at doses of 2 and 20 microg/kg decreased bronchial vascular conductance by approximately 10%. At an aerosolized dose of 200 microg/kg, the bronchial vascular conductance increased by approximately 15%, and there was no further increase in conductance when the aerosolized dose was increased to 2,000 microg/kg. Pretreatment of animals with a nitric oxide synthase inhibitor, Nomega-nitro-L-arginine methyl ester hydrochloride, partially blocked the vasodilatory effects of intravenous acetylcholine and completely blocked the vasodilatory effects of high-dose aerosolized acetylcholine. These data suggest that aerosolized acetylcholine does not readily penetrate the vascular wall of bronchial circulatory system and, therefore, has minimal vasodilatory effects on the bronchial vasculature.

Non-cAMP-mediated bronchial arterial vasodilation in response to inhaled beta-agonists

We studied the dose-dependent effects of inhaled isoetharine HCl, a beta-adrenergic bronchodilator (2.5, 5.0, 10.0, and 20.0 mg), on bronchial blood flow (Qbr) in anesthetized sheep. Isoetharine resulted in a dose-dependent increase in Qbr. With a total dose of 17.5 mg, Qbr increased from baseline values of 22 +/- 3.4 (SE) to 60 +/- 16 ml/min (P < 0.001), an effect independent of changes in cardiac output and systemic arterial pressure. To further study whether synthesis of endogenous nitric oxide (NO) affects beta-agonist-induced increases in Qbr, we administered isoetharine (20 mg) by inhalation before and after the NO-synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME). Intravenous L-NAME (30 mg/kg) rapidly decreased Qbr by approximately 80% of baseline, whereas L-NAME via inhalation (10 mg/kg) resulted in a delayed and smaller (approximately 22%) decrease. Pretreatment with L-NAME via both routes of administration attenuated bronchial arterial vasodilation after subsequent challenge with isoetharine. We conclude that isoetharine via inhalation increases Qbr in a dose-dependent manner and that beta-agonist-induced relaxation of vascular smooth muscle in the bronchial vasculature is partially mediated via synthesis of NO.

Effect of beta 2-agonists on histamine-induced airway microvascular leakage in ozone-exposed guinea pigs

beta 2-adrenergic agonists exhibit antipermeability effects in the airways. However, it is not known whether beta 2-agonists have this beneficial effect in airway mucosa that is already inflamed. We evaluated the effects of two inhaled beta 2-agonists, salbutamol and formoterol, on the histamine-induced bronchoconstriction and plasma extravasation in the airways of guinea pigs with or without ozone exposure. Total pulmonary resistance (RL) was measured before and after histamine inhalation in anesthetized animals that were pretreated with inhaled salbutamol, formoterol, or saline. Plasma extravasation in the airways was measured using Evans blue dye. In the control animals not exposed to ozone, salbutamol and formoterol each significantly reduced both the histamine-induced bronchoconstriction and the plasma extravasation in the trachea and main bronchi. In the ozone-exposed animals, the increase in RL after histamine was greater than that in control animals. Salbutamol and formoterol each significantly reduced histamine-induced bronchoconstriction, even in the ozone-exposed animals. Salbutamol did not affect the histamine-induced plasma extravasation, whereas formoterol reduced the plasma extravasation in the main bronchi, but not in the trachea, of the animals exposed to ozone. These results suggest that the anti-inflammatory properties of formoterol in inflamed airways may contribute to the beneficial effects in the treatment of airway inflammation.

Norepinephrine-induced contraction of isolated rabbit bronchial artery: role of alpha 1- and alpha 2-adrenoceptor activation

The contractile effect of norepinephrine (NE) on isolated rabbit bronchial artery rings (150-300 microns in diameter) and the role of alpha 1- and alpha 2-adrenoceptors (AR) on smooth muscle and endothelium were studied. In intact arteries, NE increased tension in a dose-dependent manner, and the sensitivity for NE was further increased in the absence of endothelium. In intact but not in endothelium-denuded arteries, the response to NE was increased in the presence of both indomethacin (Indo; cyclooxygenase inhibitor) and NG-nitro-L-arginine methyl ester [L-NAME; nitric oxide (NO) synthase inhibitor], indicating that two endothelium-derived factors, NO and a prostanoid, modulate the NE-induced contraction. The alpha 1-AR antagonist prazosin shifted the NE dose-response curve to the right, and phenylephrine (alpha 1-AR agonist) induced a dose-dependent contraction that was potentiated by L-NAME or removal of the endothelium. The sensitivity to NE was increased slightly by the alpha 2-AR antagonists yohimbine and idazoxan, and this effect was abolished by Indo or removal of the endothelium. Similarly, contractions induced by UK-14304 (alpha 2-AR agonist) were potentiated by Indo or removal of the endothelium. These results suggest that NE-induced contraction is mediated through activation of alpha 1- and alpha 2-ARs on both smooth muscle and endothelium. Activation of the alpha 1- and alpha 2-ARs on the smooth muscle causes contraction, whereas activation of the endothelial alpha 1- and alpha 2-ARs induces relaxation through release of NO (alpha 1-ARs) and a prostanoid (alpha 2-ARs).

Effect of phorbol myristate acetate-induced lung injury on airway blood flow

The effects of phorbol myristate acetate (PMA) induced lung injury on the pulmonary and systemic blood flow contributions to the trachea and main bronchi (upper airways) were assessed in anesthetized dogs by injecting 15 microns radiolabeled microspheres into the right and left heart, respectively. Upper airway blood flow was studied in lungs given the following treatments: (1) PMA; (2) PMA in lungs pretreated with the thromboxane synthetase inhibitor OKY-046, and (3) PMA in lungs pretreated with the antioxidant catalase. After microsphere injections, the tracheal cartilage, tracheal muscle-mucosa, and main bronchi were excised. The results of this study indicate that under normal conditions, tracheal mucosa [33-52 ml.min-1.(100 g)-1] and tracheal cartilage [18-27 ml.min-1.(100 g)-1] blood flow is primarily systemic while both the systemic [12-18 ml.min-1.(100 g)-1] and pulmonary [6-12 ml.min-1.(100 g)-1] circulations contribute substantial amounts of blood flow to the main bronchi. PMA significantly decreased the systemic blood flow contribution to the tracheal cartilage and muscle-mucosa, and both the systemic and pulmonary blood flow contributions to the main bronchi to less than 50% of control values, an effect that was inhibited by catalase, but not by OKY-046. These results suggest that the effect of PMA-induced lung injury on the pulmonary and systemic blood flow contributions to the upper airways is at least partially mediated by oxygen radical production, probably hydrogen peroxide (H2O2), but not by the production of the arachidonic acid metabolite thromboxane.

Microvascular function. Transvascular exchange of fluid in the airways

Fluid exchange in the airway microcirculation is presented relative to whether the airway epithelium is in an absorbing or a secreting state. The effects of increasing microvascular pressures and damaging the endothelial barrier are also examined relative to fluid dynamics and lymph flow, especially with regard to "filtration secretion." The microvessel pressure profile of the tracheal circulation is discussed relative to our recent data and to the published literature. These micropuncture data indicate that the major resistance to tracheal blood flow resides in arterioles with diameters less than 50 microns. It is hoped that this review will stimulate research on the airway microcirculation since the available data on the physiology and biophysics of circulation in the large and small airways are insufficient to define the microcirculatory exchange characteristics in this capillary bed.

Airway blood flow modifies allergic airway smooth muscle contraction

We tested the hypothesis that airway perfusion modifies the contractile response of airway smooth muscle to allergen challenge by influencing the clearance of locally released spasmogens. In six intact, lightly sedated, sheep allergic to Ascaris suum, we measured tracheal mucosal blood flow (Qtr) with a soluble gas uptake method and tracheal dead space (Vtr), an index of airway smooth muscle tone, by helium dilution before and serially after local aerosol challenge with A. suum extract or ragweed extract (control). The former challenge was repeated during continuous intravenous infusion of either vasopressin or nitroglycerin, which by themselves had no effect on Vtr and decreased and increased Qtr, respectively. Ragweed had no effect on Qtr and Vtr, whereas A. suum increased mean (+/- SE) Qtr by 111 +/- 31% (p less than 0.05) and decreased mean Vtr by 15 +/- 2% (p less than 0.05) immediately after challenge, with Qtr returning to baseline by 40 min and Vtr by 80 min. Vasopressin infusion prevented the A. suum-induced increase in Qtr and prolonged the decrease in mean Vtr (p less than 0.05). During nitroglycerin infusion, A. suum failed to alter Qtr or Vtr. Vasopressin and nitroglycerin had no effect on the contractile responses of tracheal smooth muscle to A. suum in vitro. These results indicate that the effects of vasopressin and nitroglycerin on antigen-induced airway smooth muscle contraction in vivo were due to alterations in airway blood flow rather than to alterations in the release of or airway smooth muscle responsiveness to chemical mediators.

Vasomotion influences airflow in peripheral airways

The purpose of this study was to test the hypothesis that blood flow, by its effect on blood volume, influences airflow resistance in peripheral airways. In conscious ewes, forced sinusoidal flow oscillations (5 Hz) were applied through a balloon-tipped, dual-channel fiberoptic bronchoscope placed in a segmental bronchus, and peripheral airflow resistance (Rp) was determined from flow and bronchial pressure. Drugs with predominant vascular or airway smooth muscle effects were administered locally through the bronchoscope. Nitroglycerin (NTG) produced a dose-dependent increase in mean Rp (+288% at 1,000 micrograms), which was blocked by methylene blue (p less than 0.05) and not reversed by atropine. Carbachol (CARB) also increased mean Rp in a dose-dependent manner (+605% at 400 micrograms); this effect was not blocked by methylene blue, but it was reversed by atropine (p less than 0.05). The increase in mean Rp after a single dose of NTG (250 micrograms) was sustained for at least 20 min and transiently reversed by vasopressin (0.2 units, p less than 0.05) but not by isoproterenol (100 micrograms). Conversely, the sustained increase in Rp after a single dose of CARB (50 micrograms) was transiently reversed by isoproterenol (p less than 0.05) but not by vasopressin. We conclude that NTG increased Rp by vasodilation and CARB by bronchoconstriction. This supports the hypothesis that vasodilation limits airflow in the lung periphery, presumably because of vascular congestion.