Methacholine-induced bronchoconstriction in rats: effects of intravenous vs. aerosol delivery

Peták, Ferenc, Zoltán Hantos, ÁgnesAdamicza, Tibor Asztalos, and Peter D. Sly. Methacholine-inducedbronchoconstriction in rats: effects of intravenous vs. aerosoldelivery. J. Appl. Physiol. 82(5):1479-1487, 1997.To determine the predominant site of action ofmethacholine (MCh) on lung mechanics, two groups of open-chestSprague-Dawley rats were studied. Five rats were measured duringintravenous infusion of MCh (iv group), with doubling of concentrationsfrom 1 to 16 µg · kg¹ · min¹.Seven rats were measured after aerosol administration of MCh with dosesdoubled from 1 to 16 mg/ml (ae group). Pulmonary input impedance(ZL) between 0.5 and 21 Hz wasdetermined by using a wave-tube technique. A model containing airwayresistance (Raw) and inertance (Iaw) and parenchymal damping (G) andelastance (H) was fitted to theZL spectra. In the iv group, MChinduced dose-dependent increases in Raw [peak response 270 ± 9 (SE) % of the control level; P < 0.05] and in G (340 ± 150%;P < 0.05), with no increase inIaw (30 ± 59%) orH (111 ± 9%). In the ae group, thedose-dependent increases in Raw (191 ± 14%;P < 0.05) andG (385 ± 35%; P < 0.05) were associated with a significant increase in H (202 ± 8%; P < 0.05).Measurements with different resident gases [air vs. neon-oxygenmixture, as suggested (K. R. Lutchen, Z. Hantos, F. Peták,Á. Adamicza, and B. Suki. J. Appl.Physiol. 80: 1841-1849, 1996)] in thecontrol and constricted states in another group of rats suggested thatthe entire increase seen in G during the ivchallenge was due to ventilation inhomogeneity, whereas the aechallenge might also have involved real tissue contractions viaselective stimulation of the muscarinic receptors.

     

Partitioning airway and lung tissue resistances in humans: effects of bronchoconstriction

Kaczka, David W., Edward P. Ingenito, Bela Suki, and KennethR. Lutchen. Partitioning airway and lung tissue resistances inhumans: effects of bronchoconstriction. J. Appl.Physiol. 82(5): 1531-1541, 1997.The contributionof airway resistance(Raw) and tissue resistance(Rti) to totallung resistance(RL)during breathing in humans is poorly understood. We have recentlydeveloped a method for separating Rawand Rti from measurements ofRLand lung elastance (EL)alone. In nine healthy, awake subjects, we applied a broad-band optimalventilator waveform (OVW) with energy between 0.156 and 8.1 Hz thatsimultaneously provides tidal ventilation. In four of the subjects,data were acquired before and during a methacholine (MCh)-bronchoconstricted challenge. TheRLandELdata were first analyzed by using a model with a homogeneous airwaycompartment leading to a viscoelastic tissue compartment consisting oftissue damping and elastance parameters. Our OVW-based estimates ofRaw correlated well with estimatesobtained by using standard plethysmography and were responsive toMCh-induced bronchoconstriction. Our data suggest thatRti comprises ~40% of totalRLat typical breathing frequencies, which corresponds to ~60% ofintrathoracic RL. During mildMCh-induced bronchoconstriction, Rawaccounts for most of the increase inRL. At high doses of MCh, therewas a substantial increase in RLat all frequencies and inEL athigher frequencies. Our analysis showed that bothRaw andRti increase, but most of the increaseis due to Raw. The data also suggestthat widespread peripheral constriction causes airway wall shunting toproduce additional frequency dependence inEL.

     

Inhibitory effects of CO2 on airway defensive reflexes in enflurane-anesthetized humans

We investigated responses of respiration, blood pressure, and heart rate to tracheal mucosa irritation induced by injection of distilled water at three different levels of CO2 ventilatory drive in 11 spontaneously breathing female patients under a constant depth of enflurane anesthesia [1.1 minimum alveolar concentration (MAC)]. The airway irritation at the resting level of spontaneous breathing caused a variety of respiratory responses such as coughing, expiration reflex, apnea, and spasmodic panting, with considerable increases in blood pressure and heart rate. Although the latency of respiratory responses after water injection was much shorter than those of blood pressure and heart rate responses, blood pressure and heart rate responses, once elicited, were prolonged much longer than was the respiratory response. An increase in CO2 ventilatory drive decreased the degree and duration of respiratory, blood pressure, and heart rate responses to the airway irritation, whereas a decrease in CO2 ventilatory drive had the opposite effect on these responses. Our results indicate that changes in CO2 ventilatory drive can modify reflex responses of respiration, blood pressure, and heart rate to airway irritation.     

Nonlinearity of respiratory mechanics during bronchoconstriction in mice with airway inflammation.

Respiratory system resistance (R) and elastance (E) are commonly estimated by fitting the linear equation of motion P = EV + RV + P0 (Eq. 1) to measurements of respiratory pressure (P), lung volume (V), and flow (V). However, the respiratory system is unlikely to behave linearly under many circumstances. We determined the importance of respiratory system nonlinearities in two groups of mechanically ventilated Balb/c mice [controls and mice with allergically inflamed airways (ova/ova)], by assessing the impact of the addition of nonlinear terms (E2V2 and R2V(V)) on the goodness of model fit seen with Eq. 1. Significant improvement in fit (51.85 +/- 4.19%) was only seen in the ova/ova mice during bronchoconstriction when the E2V2 alone was added. An improvement was also observed with addition of the E2V2 term in mice with both low and high lung volumes ventilated at baseline, suggesting a volume-dependent nonlinearity of E. We speculate that airway closure in the constricted ova/ova mice accentuated the volume-dependent nonlinearity by decreasing lung volume and overdistending the remaining lung.     

Computational model of airway narrowing: mature vs. immature rabbit.

Immature rabbits have greater maximal airway narrowing and greater maximal fold increases in airway resistance during bronchoconstriction than mature animals. We have previously demonstrated that excised immature rabbit lungs have more distensible airways, a lower shear modulus, and structural differences in the relative composition and thickness of anatomically similar airways. In the present study, we incorporated anatomic and physiological data for mature and immature rabbits into a computational model of airway narrowing. We then investigated the relative importance of maturational differences in these factors as determinants of the greater airway narrowing that occurs in the immature animal. The immature model demonstrated greater sensitivity to agonist, as well as a greater maximal fold increase in airway resistance. Exchanging values for airway compliance between the mature and immature models resulted in the mature model exhibiting a greater maximal airway response than the immature model. In contrast, exchanging the shear moduli or the composition of the airway wall relative to the airway size produced relatively small changes in airway reactivity. Our results strongly suggest that the mechanical properties of the airway, i.e., greater compliance of the immature airway, can be an important factor contributing to the greater airway narrowing of the immature animal.     

Larynx vs. esophagus as reflexogenic sites for acid-induced bronchoconstriction in dogs.

Bronchoconstriction in asthmatic patients is frequently associated with gastroesophageal reflux. However, it is still unclear whether bronchoconstriction originates from the esophagus or from aspiration of the refluxate into the larynx and larger airway. We compared the effect of repeated esophageal and laryngeal instillations of HCl-pepsin (pH 1.0) on tracheal smooth muscle activity in eight anesthetized and artificially ventilated dogs. Saline was used as control. We used pressure in the cuff of an endotracheal tube (Pcuff) as a direct index of smooth muscle activity at the level of the larger airways controlled by vagal efferents. The Pcuff values of the first 60 s after instillations were averaged, and the difference from the baseline values was evaluated. Changes in Pcuff were significantly greater with laryngeal than with esophageal instillations (P = 0.0166). HCl-pepsin instillation into the larynx evoked greater responses than did saline (P = 0.00543), whereas no differences were detected with esophageal instillations. Repeated laryngeal exposure enhanced the responsiveness significantly (P < 0. 001). Our data indicate that the larynx is more important than the esophagus as a reflexogenic site for the elicitation of reflex bronchoconstriction in response to acidic solutions.     

Parenchymal tethering, airway wall stiffness, and the dynamics of bronchoconstriction.

We do not yet have a good quantitative understanding of how the force-velocity properties of airway smooth muscle interact with the opposing loads of parenchymal tethering and airway wall stiffness to produce the dynamics of bronchoconstriction. We therefore developed a two-dimensional computational model of a dynamically narrowing airway embedded in uniformly elastic lung parenchyma and compared the predictions of the model to published measurements of airway resistance made in rats and rabbits during the development of bronchoconstriction following a bolus injection of methacholine. The model accurately reproduced the experimental time-courses of airway resistance as a function of both lung inflation pressure and tidal volume. The model also showed that the stiffness of the airway wall is similar in rats and rabbits, and significantly greater than that of the lung parenchyma. Our results indicate that the main features of the dynamical nature of bronchoconstriction in vivo can be understood in terms of the classic Hill force-velocity relationship operating against elastic loads provided by the surrounding lung parenchyma and an airway wall that is stiffer than the parenchyma.     

Airflow-induced bronchoconstriction: role of epithelium and eicosanoid mediators

We examined the role of cyclooxygenase-derived metabolites and epithelial cells in airflow-induced bronchospasm. Male dogs were anesthetized and collateral system resistance (Rcs) was measured with the wedged-bronchoscope technique. A 2-min high flow challenge with dry air in nine animals produced a mean increase in Rcs of 69 +/- 13% (SE). After treatment with indomethacin (5 mg/kg), the response was significantly attenuated; Rcs increased only 40 +/- 8%. Bronchoalveolar lavage performed 5 min after a dry air challenge yielded fluid with greater concentrations of prostaglandin D2 (PGD2) and thromboxane B2 than samples from unchallenged segments. Challenge with humidified air produced a smaller physiological response than did challenge with dry air. Lavage samples obtained after dry challenge had greater concentrations of PGD2 than samples taken after challenge with humidified air. After dry air challenge, epithelial cells in lavage fluid were increased by 454 and 515% when compared with control and humidified air challenge, respectively. Significant correlations were found between epithelial cell number and PGD2 recovered in lavage fluid after dry air challenges. We conclude that both epithelial cells and prostaglandins play an important role in peripheral lung responses to dry air.     

Cold-induced apoptosis in isolated rat hepatocytes: protective role of glutathione

Liver conservation for transplantation is usually made at 2-4 degrees C. We studied the effect of rewarming to 37 degrees C for up to 3 h of rat hepatocytes kept at 4 degrees C for 20 h, modulating intracellular glutathione (GSH) concentration either with a GSH precursor (N-acetyl-L-cysteine, NAC), or with GSH depleting agents (diethylmaleate and buthionine sulfoximine, DEM/BSO). Untreated hepatocytes showed time-dependent production of reactive oxygen species (ROS), lipid peroxidation, chromatin condensation and membrane blebbing, decrease in GSH concentration, and protein sulfhydryl groups. Fluorochromatization with Propidium Iodide (PI) and Annexin V (AnxV) of cells rewarmed for 1 h caused an increase of AnxV-positive cells without PI staining and any observed lactate dehydrogenase leakage. TUNEL and DNA-laddering tests were negative for all times and treatments, indicating that apoptosis may occur without DNA fragmentation. Cold preservation and rewarming in the presence of NAC induced a significant improvement in the morphology, less oxidative stress and apoptosis. Conversely, DEM/BSO caused a marked deterioration of morphology, increase of oxidative stress and apoptosis. These results suggested that marked changes in GSH status might play a critical role in triggering apoptosis during cold preservation of isolated rat hepatocytes. NAC, added before rewarming, might represent a therapeutic approach for preventing the early events of apoptosis during cold storage.     

Diaphragmatic pressures: transvenous vs. direct phrenic nerve stimulation

Diaphragmatic force, determined by stimulating the phrenic nerve while simultaneously measuring the pressures in a closed respiratory system, was assessed in five anesthetized dogs over a 5-h period to evaluate the inherent variability of this technique. Transdiaphragmatic pressure (Pdi) was measured at functional residual capacity during stimulation (120 Hz, 0.2-ms duration) of one phrenic nerve by either direct phrenic nerve stimulation (DPNS) or transvenous phrenic nerve stimulation (TPNS). An analysis of variance showed no significant (P greater than 0.50) change during the 5-h period. There was a significant correlation (r = 0.94, P less than 0.001) between Pdi obtained by TPNS and that obtained by DPNS. It is concluded that either DPNS or TPNS can be used to evaluate diaphragmatic strength over a 5-h period and that TPNS can be used in lieu of DPNS.     

Handgrip-induced airway dilation in asthmatic patients with bronchoconstriction induced by MCh inhalation.

We investigated the effects of static and rhythmic handgrip on the time course of recovery of airway resistance measured with the interrupter technique (Rint) following bronchoconstriction induced by methacholine (MCh) inhalation in 17 asthmatic patients. On three separate occasions, a 100 +/- 5% increase in baseline Rint was induced by MCh inhalation. Subsequently, patients either rested [control trials (CTs)] or performed 3-min bouts of static or rhythmic handgrip. Respiratory and cardiovascular variables were continuously monitored. Rint changes were assessed at 1-min intervals for 30 min after rest and both types of handgrip. Plasma catecholamine concentrations were also determined at scheduled intervals. Bronchoconstriction increased ventilation (P < 0.01) but did not affect cardiovascular variables and plasma catecholamine concentrations. Handgrip provoked an increase in cardiovascular variables (P < 0.01) and plasma norepinephrine concentrations (P < 0.05) but caused no additional changes in ventilation. Rint only partially recovered within 30 min after CTs, whereas it consistently decreased 1 min after both handgrip paradigms and remained lower than after CTs (P always <0.01) for the whole 30-min observation period. Sympathetic activation and withdrawal of cholinergic input to the airway smooth muscle reflexly induced by activation of skeletal muscle and carotid sinus receptors may be the primary events accounting for the bronchodilator response induced by handgrip. Mediators co-released in response to sympathetic activation may also have contributed.     

Attenuation of induced bronchoconstriction in healthy subjects: effects of breathing depth.

The effects of breathing depth in attenuating induced bronchoconstriction were studied in 12 healthy subjects. On four separate, randomized occasions, the depth of a series of five breaths taken soon (approximately 1 min) after methacholine (MCh) inhalation was varied from spontaneous tidal volume to lung volumes terminating at approximately 80, approximately 90, and 100% of total lung capacity (TLC). Partial forced expiratory flow at 40% of control forced vital capacity (V(part)) and residual volume (RV) were measured at control and again at 2, 7, and 11 min after MCh. The decrease in V(part) and the increase in RV were significantly less when the depth of the five-breath series was progressively increased (P < 0.001), with a linear relationship. The attenuating effects of deep breaths of any amplitude were significantly greater on RV than V(part) (P < 0.01) and lasted as long as 11 min, despite a slight decrease with time when the end-inspiratory lung volume was 100% of TLC. In conclusion, in healthy subjects exposed to MCh, a series of breaths of different depth up to TLC caused a progressive and sustained attenuation of bronchoconstriction. The effects of the depth of the five-breath series were more evident on the RV than on V(part), likely due to the different mechanisms that regulate airway closure and expiratory flow limitation.     

Distribution of airway narrowing during hyperpnea-induced bronchoconstriction in guinea pigs

Increasing minute ventilation of dry gas shifts the principal burden of respiratory heat and water losses from more proximal airway to airways farther into the lung. If these local thermal transfers determine the local stimulus for bronchoconstriction, then increasing minute ventilation of dry gas might also extend the zone of airway narrowing farther into the lung during hyperpnea-induced bronchoconstriction (HIB). We tested this hypothesis by comparing tantalum bronchograms in tracheostomized guinea pigs before and during bronchoconstriction induced by dry gas hyperpnea, intravenous methacholine, and intravenous capsaicin. In eight animals subjected to 5 min of dry gas isocapnic hyperpnea [tidal volume (VT) = 2-5 ml, 150 breaths/min], there was little change in the diameter of the trachea or the main stem bronchi up to 0.75 cm past the main carina (zone 1). In contrast, bronchi from 0.75 to 1.50 cm past the main carina (zone 2) narrowed progressively at all minute ventilations greater than or equal to 300 ml/min (VT = 2 ml). More distal bronchi (1.50-3.10 cm past the main carina; zone 3) did not narrow significantly until minute ventilation was raised to 450 ml/min (VT = 3 ml). The estimated VT during hyperpnea needed to elicit a 50% reduction in airway diameter was significantly higher in zone 3 bronchi [4.3 +/- 0.8 (SD) ml] than in zone 2 bronchi (3.5 +/- 1.1 ml, P less than 0.012).(ABSTRACT TRUNCATED AT 250 WORDS)     

Peptidase modulation of noncholinergic vagal bronchoconstriction and airway microvascular leakage

We investigated whether inhibition of neutral endopeptidase 24.11 (NEP) and/or angiotensin-converting enzyme (ACE) modifies vagally induced nonadrenergic noncholinergic (NANC) airflow obstruction and airway microvascular leakage as measured by extravasation of Evans blue dye (intravenous) in anesthetized guinea pigs. We gave phosphoramidon to inhibit NEP and enalapril maleate or captopril to inhibit ACE. Animals pretreated with inhaled phosphoramidon (7.5 or 75 nmol), enalapril maleate (87 or 870 nmol), or captopril (350 nmol) reached higher peak lung resistance (RL) values (14.3 +/- 2.7, 15.7 +/- 3.8, 16.7 +/- 3.8, 11.4 +/- 1.6, and 24.6 +/- 3.5 cmH2O.ml-1.s, respectively) than saline-treated animals (5.9 +/- 1.1; P less than 0.05) after bilateral vagus nerve stimulation (5 Hz, 10 V, 10 ms, 150 s). Intravenous phosphoramidon (1 mg/kg), but not intravenous captopril (6 mg/kg), potentiated peak RL (22.9 +/- 6.9 and 7.1 +/- 1.5 cmH2O.ml-1.s, respectively). Vagal nerve stimulation (1 and 5 Hz) increased the extravasation of Evans blue dye in tracheobronchial tissues compared with sham-stimulated animals, but this was not potentiated by inhaled enzyme inhibitors or intravenous captopril. However, intravenous phosphoramidon significantly augmented the extravasation of Evans blue dye in main bronchi and intrapulmonary airways. We conclude that degradative enzymes regulate both NANC-induced airflow obstruction and airway microvascular leakage.     

In vivo loads on airway smooth muscle: the role of noncontractile airway structures.

The degree of airway smooth muscle contraction and shortening that occurs in vivo is modified by many factors, including those that influence the degree of muscle activation, the resting muscle length, and the loads against which the muscle contracts. Canine trachealis muscle will shorten up to 70% of starting length from optimal length in vitro but will only shorten by around 30% in vivo. This limitation of shortening may be a result of the muscle shortening against an elastic load such as could be applied by tracheal cartilage. Limitation of airway smooth muscle shortening in smaller airways may be the result of contraction against an elastic load, such as could be applied by lung parenchymal recoil. Measurement of the elastic loads applied by the tracheal cartilage to the trachealis muscle and by lung parenchymal recoil to smooth muscle of smaller airways were performed in canine preparations. In both experiments the calculated elastic loads applied by the cartilage and the parenchymal recoil explained in part the limitation of maximal active shortening and airway narrowing observed. We conclude that the elastic loads provided by surrounding structures are important in determining the degree of airway smooth muscle shortening and the resultant airway narrowing.