Maturation of neuromodulatory effect of substance P in rabbit airways.

The maturation of the neuromodulatory action of substance P (SP) was investigated in tracheal smooth muscle (TSM) segments isolated from rabbits aged 2-24 wk. The tissues were placed in baths containing Krebs-Ringer solution and contracted with electrical field stimulation (ES) with ES frequencies ranging from 1 to 75 Hz. In tissues greater than 1 mo of age, the ES frequency-response relationships were progressively shifted in the presence of a maximally effective neuromodulatory SP dose (10(-7) M) such that by 24 wk of age the mean (+/- SEM) maximal tension (Tmax) significantly increased from 380.4 (+/- 41.9) to 502.3 (+/- 64.2) g/g TSM, and the corresponding mean (+/- SEM) log ES frequency producing 50% of Tmax (log ES50) significantly decreased from 1.209 (+/- 0.069) to 1.055 (+/- 0.046) Hz. By contrast, relative to methacholine, the direct contractile effects of SP did not significantly vary with age. In further analyzing the basis for the above age-related difference in the neuromodulatory action of SP, we found that the magnitude of SP-induced neuromodulation was highly correlated to the tissue's intrinsic sensitivity to ES. Indeed, after accounting for the tissue's sensitivity to ES, the effect of age alone on the magnitude of SP-induced neuromodulation was not statistically significant. These findings provide new evidence that: (a) SP-induced neuromodulation of acetylcholine release at the airway neuromuscular junction is significantly enhanced during postnatal development; and (b) that the latter age-dependent action of SP is based on a close coupling of the magnitude of SP-induced neuromodulation to the tissue's intrinsic sensitivity to neurally mediated contraction.

Regulation of TH1- and TH2-type cytokine expression and action in atopic asthmatic sensitized airway smooth muscle.

CD4(+) T helper (TH)1- and TH2-type cytokines reportedly play an important role in the pathobiology of asthma. Recent evidence suggests that proasthmatic changes in airway smooth muscle (ASM) responsiveness may be induced by the autocrine release of certain proinflammatory cytokines by the ASM itself. We examined whether TH1- and TH2-type cytokines are expressed by atopic asthmatic sensitized ASM and serve to autologously regulate the proasthmatic phenotype in the sensitized ASM. Expression of these cytokines and their receptors was examined in isolated rabbit and human ASM tissues and cultured cells passively sensitized with sera from atopic asthmatic patients or control subjects. Relative to controls, atopic sensitized ASM cells exhibited an early increased mRNA expression of the TH2-type cytokines, interleukin-5 (IL-5) and granulocyte-macrophage colony-stimulating factor (GM-CSF), and their receptors. This was later followed by enhanced mRNA expression of the TH1-type cytokines, IL-2, IL-12, and interferon-gamma (IFN-gamma), as well as their respective receptors. In experiments on isolated ASM tissue segments (a) exogenous administration of IL-2 and IFN-gamma to atopic asthmatic serum-sensitized ASM ablated both their enhanced constrictor responsiveness to acetylcholine (ACh) and their attenuated relaxation responsiveness to beta-adrenoceptor stimulation with isoproterenol, and (b) administration of IL-5 and GM-CSF to naive ASM induced significant increases in their contractility to ACh and impaired their relaxant responsiveness to isoproterenol. Collectively, these observations provide new evidence demonstrating that human ASM endogenously expresses both TH1- and TH2-type cytokines and their receptors, that these molecules are sequentially upregulated in the atopic asthmatic sensitized state, and that they act to downregulate and upregulate proasthmatic perturbations in ASM responsiveness, respectively.

Maturational regulation of inositol 1,4,5-trisphosphate metabolism in rabbit airway smooth muscle.

Airway reactivity has been shown to vary with age; however, the mechanism(s) underlying this process remain unidentified. To elucidate the role of ontogenetic changes in phosphoinositide-linked signal transduction, we examined whether age-related differences in tracheal smooth muscle (TSM) contractility to carbachol (CCh) are associated with developmental changes in the production and metabolism of the second messenger, inositol 1,4,5-trisphosphate (Ins (1,4,5)P3). In TSM segments isolated from 2-wk-old and adult rabbits, both the maximal isometric contractile force and sensitivity (i.e., -logED50) to CCh (10(-10)-10(-4) M) were significantly greater in the immature vs. adult tissues (P less than 0.001). Similarly, Ins(1,4,5)P3 accumulation elicited by either receptor-coupled stimulation with CCh (10(-10)-10(-4) M) or post-receptor-mediated guanine nucleotide binding protein activation of permeabilized TSM with GTP gamma S (100 microM) was also significantly enhanced in 2-wk-old vs. adult TSM. Measurement of the activities of the degradative enzymes for Ins(1,4,5)P3 demonstrated that: (a) mean +/- SE maximal Ins(1,4,5)P3 3'-kinase activity was significantly reduced in the immature vs. adult TSM (i.e., approximately 71.7 +/- 6.0 vs. 137.8 +/- 10.0 pmol/min per mg protein, respectively; P less than 0.005); (b) by contrast, maximal Ins(1,4,5)P3 5'-phosphatase activity was significantly increased in the immature vs. adult TSM (i.e., 27.9 +/- 1.2 vs. 15.6 +/- 1.5 nmol/min per mg protein, respectively; P less than 0.001); and (c) the Km values for Ins(1,4,5)P3 5'-phosphatase were 14- and 19-fold greater than those for Ins(1,4,5)P3 3'-kinase in the 2-wk-old and adult TSM, respectively. Collectively, the findings suggest that the age-related decrease in agonist-induced rabbit TSM contractility is associated with a diminution in Ins(1,4,5)P3 accumulation which is attributed, at least in part, to ontogenetic changes in the relative activities of the degradative enzymes for Ins(1,4,5)P3.

Autocrine role of interleukin 1beta in altered responsiveness of atopic asthmatic sensitized airway smooth muscle.

The role of IL-1beta in regulating altered airway responsiveness in the atopic/asthmatic sensitized state was examined in isolated rabbit tracheal smooth muscle (TSM) tissue and cultured cells passively sensitized with sera from atopic asthmatic patients or nonatopic/nonasthmatic (control) subjects. During half-maximal isometric contraction of the tissues with acetylcholine, relative to control TSM, the atopic sensitized TSM exhibited significant attenuation of both their maximal relaxation (P < 0.001) and sensitivity (i.e., -log dose producing 50% maximal relaxation) to isoproterenol and PGE2 (P < 0.05), whereas the relaxation responses to direct stimulation of adenylate cyclase with forskolin were similar in both tissue groups. The impaired relaxation responses to isoproterenol and PGE2 were ablated in sensitized TSM that were pretreated with either the IL-1 recombinant human receptor antagonist or an IL-1beta-neutralizing antibody. Moreover, extended studies demonstrated that, in contrast to their respective controls, both passively sensitized rabbit TSM tissue and cultured cells exhibited markedly induced expression of IL-1beta mRNA at 6 h after exposure to the sensitizing serum, a finding similar to that also obtained in passively sensitized human bronchial smooth muscle tissue. Finally, unlike their respective controls, passively sensitized TSM tissue and cultured cells also displayed progressively enhanced release of IL-1beta protein into the culture media for up to 24 h after exposure to atopic/asthmatic serum. Collectively, these observations provide new evidence demonstrating that the altered responsiveness of atopic/asthmatic sensitized airway smooth muscle is largely attributed to its autologously induced expression and autocrine action of IL-1beta.

Mechanism of cytokine-induced modulation of beta-adrenoceptor responsiveness in airway smooth muscle.

To elucidate the role of specific proinflammatory cytokines in regulating airway responsiveness, we examined the effects and mechanisms of action of IL-1beta, TNF-alpha, and IL-2 on the beta-adrenoceptor- and postreceptor-coupled transmembrane signaling mechanisms regulating relaxation in isolated rabbit tracheal smooth muscle (TSM) segments. During half-maximal isometric contraction of the tissues with acetylcholine, relaxation responses to isoproterenol, PGE2, and forskolin were separately compared in control (untreated) TSM and tissues incubated for 18 h with IL-1beta (10 ng/ml), TNF-(alpha (100 ng/ml), or IL-2 (200 ng/ml). Relative to controls, IL-1beta- and TNF-alpha-treated TSM, but not IL-2-treated tissues, depicted significant attenuation of their maximal relaxation and sensitivity (i.e., -log dose producing 50% maximal relaxation) to isoproterenol (P < 0.001) and PGE2 (P < 0.05); whereas the relaxation responses to direct stimulation of adenylate cyclase with forskolin were similar in the control and cytokine-treated tissues. Further, the attenuated relaxation to isoproterenol and PGE2 was ablated in the IL-1beta-treated TSM that were pretreated with either the muscarinic M2-receptor antagonist, methoctramine (10(-6) M), or pertussis toxin (100 ng/ml). Moreover, Western immunoblot analysis demonstrated that: (a) Gi protein expression was significantly enhanced in membrane fractions isolated from IL-1beta-treated TSM; and (b) the latter was largely attributed to induced enhanced expression of the Gi alpha2 and Gi alpha3 subunits. Collectively, these observations provide new evidence demonstrating that IL-lbeta and TNF-alpha induce impaired receptor-coupled airway relaxation in naive TSM, and that the latter effect is associated with increased muscarinic M2-receptor/Gi protein-coupled expression and function.

Autocrine interaction between IL-5 and IL-1beta mediates altered responsiveness of atopic asthmatic sensitized airway smooth muscle.

T-helper type 2 (Th2) cytokines have been implicated in the pathogenesis of the pulmonary inflammatory response and altered bronchial responsiveness in allergic asthma. To elucidate the mechanism of Th2-dependent mediation of altered airway responsiveness in the atopic asthmatic state, the expression and actions of specific cytokines were examined in isolated rabbit and human airway smooth muscle (ASM) tissues and cultured cells passively sensitized with sera from atopic asthmatic patients or nonatopic/nonasthmatic (control) subjects. Relative to control tissues, the atopic asthmatic sensitized ASM exhibited significantly enhanced maximal isometric contractility to acetylcholine and attenuated relaxation responses to isoproterenol. These proasthmatic changes in agonist responsiveness were ablated by pretreating the atopic sensitized tissues with either an IL-5 receptor blocking antibody (IL-5ra) or the human recombinant IL-1 receptor antagonist (IL-1ra), whereas an IL-4 neutralizing antibody had no effect. Moreover, relative to controls, atopic asthmatic sensitized ASM cells demonstrated an initial, early (after 3 hours of incubation) increased mRNA expression and protein release of IL-5. This was followed (after 6 hours of incubation) by an enhanced mRNA expression and release of IL-1beta protein, an effect that was inhibited in sensitized cells pretreated with IL-5ra. Extended studies demonstrated that naive ASM exposed to exogenously administered IL-5 exhibited an induced upregulated mRNA expression and protein release of IL-1beta associated with proasthmatic-like changes in ASM constrictor and relaxant responsiveness, and that these effects were ablated in tissues pretreated with IL-1ra. Taken together, these observations provide new evidence that (a) the Th2 cytokine IL-5 and the pleiotropic proinflammatory cytokine IL-1beta are endogenously released by atopic asthmatic sensitized ASM and mechanistically interact to mediate the proasthmatic perturbations in ASM responsiveness; and (b) the nature of this interaction is given by an initial endogenous release of IL-5, which then acts to induce the autologous release of IL-1beta by the sensitized ASM itself, resulting in its autocrine manifestation of the proasthmatic phenotype.

Mechanism of rhinovirus-induced changes in airway smooth muscle responsiveness.

An important interplay exists between specific viral respiratory infections and altered airway responsiveness in the development and exacerbations of asthma. However, the mechanistic basis of this interplay remains to be identified. This study addressed the hypothesis that rhinovirus (RV), the most common viral respiratory pathogen associated with acute asthma attacks, directly affects airway smooth muscle (ASM) to produce proasthmatic changes in receptor-coupled ASM responsiveness. Isolated rabbit and human ASM tissue and cultured ASM cells were inoculated with human RV (serotype 16) or adenovirus, each for 6 or 24 h. In contrast to adenovirus, which had no effect, inoculation of ASM tissue with RV induced heightened ASM tissue constrictor responsiveness to acetylcholine and attenuated the dose-dependent relaxation of ASM to beta-adrenoceptor stimulation with isoproterenol. These RV-induced changes in ASM responsiveness were largely prevented by pretreating the tissues with pertussis toxin or with a monoclonal blocking antibody to intercellular adhesion molecule-1 (ICAM-1), the principal endogenous receptor for most RVs. In extended studies, we found that the RV-induced changes in ASM responsiveness were associated with diminished cAMP accumulation in response to dose-dependent administration of isoproterenol, and this effect was accompanied by autologously upregulated expression of the Gi protein subtype, Gialpha3, in the ASM. Finally, in separate experiments, we found that the RV-induced effects on ASM responsiveness were also accompanied by autologously induced upregulated mRNA and cell surface protein expression of ICAM-1. Taken together, these findings provide new evidence that RV directly induces proasthmatic phenotypic changes in ASM responsiveness, that this effect is triggered by binding of RV to its ICAM-1 receptor in ASM, and that this binding is associated with the induced endogenously upregulated expression of ICAM-1 and enhanced expression and activation of Gi protein in the RV-infected ASM.

Corticosteroid modulation of Na(+)-K+ pump-mediated relaxation in maturing airway smooth muscle.

1. The ontogeny of the relaxant influence of the airway electrogenic Na(+)-K+ pump and its potential modulation by corticosteroids were examined in airway smooth muscle (ASM) segments isolated from newborn and adult rabbits. 2. Control and methylprednisolone-treated (MP) ASM segments were half-maximally contracted with methacholine in K(+)-free buffer and the ASM relaxant responses to Na(+)-K+ pump activation were subsequently evaluated. Relative to adult ASM, control newborn ASM showed significantly enhanced maximal relaxation (Rmax) to KCl (62.5 +/- 5.2% vs. 47.8 +/- 5.2%), but no difference in sensitivity (pC2 = -log concentration producing 50% Rmax: 2.18 +/- 0.12 vs. 2.29 +/- 0.09-log M). 3. Exposure of ASM segments to 500 microM methylprednisolone for 1 h potentiated the airway Na(+)-K+ pump activity. A more pronounced effect was obtained in newborn ASM, where both the Rmax and pC2 values were significantly enhanced. In mature ASM, only the Rmax response to KCl was increased in the presence of MP. 4. Collectively, these data demonstrate that: (i) the functional activity of the airway electrogenic Na(+)-K+ pump decreases with post-natal maturation in the rabbit: (ii) corticosteroid treatment potentiates Na(+)-K+ pump activity in rabbit ASM; and (iii) the latter effect of corticosteroids is enhanced in immature airways. 5. The above findings provide new evidence that the airway relaxant response to activation of the electrogenic Na(+)-K+ pump varies ontogenetically and that corticosteroids potentiate the Na(+)-K+ pump activity in an age-dependent manner.

The effects of acute hypoxia and hypercapnia on pulmonary mechanics in normal subjects and patients with chronic pulmonary disease.

The influence of progressive hypoxia and hypercapnia on respiratory mechanics was evaluated in 26 subjects (six normal subjects, seven asthmatic subjects, seven patients with IPD, and six patients with COPD). During separate rebreathing runs of progressive isocapnic hypoxia and normoxic hypercapnia, breath-to-breath changes in RL and Cdyn were determined. In five of the six normal subjects, seven of the seven asthmatic subjects, and six of the seven subjects with IPD, RL decreased with both progressive hypoxia and hypercapnia without a change in Cdyn. In the patients with COPD, the effects of hypoxia and hypercapnia on RL and Cdyn were variable. Compared to normal subjects, the changes in RL during hypoxia and hypercapnia were not significantly different in the asthmatic subjects and the patients with IPD. These data provide evidence that acute progressive hypoxia and hypercapnia are associated with significant changes in Raw in both normal subjects and patients with chronic pulmonary disease.

Methacholine-induced temporal changes in airway geometry and lung density by CT.

PURPOSE: Electron-beam CT (EBCT) was utilized to assess the time course of changes in airways cross-sectional area (CSA) and lung density during methacholine-induced bronchoconstriction. MATERIALS AND METHODS: EBCT scans (200 ms, 3-mm thickness, 2 mm increments) were obtained before (baseline) and 30 s, 2 min, and 4 min after bolus IV injection of methacholine to pigs receiving mechanical ventilation. A total of seven experiments were analyzed using custom-made image analysis software. With each challenge, five different airways and 50 lung regions of interest were studied. RESULTS: The time course of lung density changes paralleled the time course for CSA changes. The maximal response to methacholine, measured in terms of both CSA and lung density changes, occurred 30 s after injection. Lung density changes were unaffected by reconstruction algorithm, normal (standard) or sharp (high resolution). Overall, there was increased air content in the lung during bronchoconstriction. This effect was significantly greater at the dependent lung regions. CONCLUSIONS: EBCT is an effective tool to assess temporal and regional changes in the lung during bronchoconstriction. Measurements of lung density during bronchoconstriction allow for assessment of peripheral changes that are beyond the CT spatial resolution of airways anatomy.

Use of TAO without methylprednisolone in the treatment of severe asthma.

The antimicrobial agent troleandomycin (TAO) has been shown to be effective in reducing corticosteroid requirements in patients with corticosteroid-dependent asthma. To our knowledge, the efficacy of TAO without concomitant use of corticosteroids has never been documented. We report the case of a 12-year-old patient with corticosteroid-dependent asthma who has remained asymptomatic and without any evidence of pulmonary deterioration during treatment with TAO without concomitant use of corticosteroids.

Respiratory influence of peripheral chemoreceptor stimulation in maturing rabbits.

To evaluate whether the influence of peripheral chemoreceptor (PCR) stimulation is centrally modulated by hypoxia, the respiratory effects of sodium cyanide (NaCN) infusion were compared during room air and 10% O2 inhalation in 18 lightly anesthetized, tracheotomized rabbits of varying postnatal age (1-33 days). During normoxia, noncumulative infusions of NaCN (5-400 micrograms/kg body wt) produced dose-dependent ventilatory (VE) stimulation. Maximal VE stimulation (VEmax) and ventilatory sensitivity to NaCN [i.e., log dose producing 50% of VEmax (log ED50)] did not significantly vary with age, with the average VEmax and log ED50 values amounting to 238% above base line and 1.564 micrograms/kg, respectively. During hypoxia, after initial stimulation (average: 152%), VE progressively decreased and stabilized to 67% above the normoxic base-line level. In contrast to normoxia, subsequent NaCN administration during steady-state hypoxia produced dose-dependent VE depression, occasionally manifested by abrupt apnea. The NaCN effect during hypoxia was significantly related to age (P less than 0.05), as well as to the estimated change in CO2 production during hypoxia (P less than 0.01). Both the respiratory depressant effects of hypoxia alone and in combination with NaCN were abolished after denervation of the peripheral chemoreceptors. These findings demonstrate that while PCR stimulation during normoxia produces ventilatory stimulation, the influence of enhanced PCR input during hypoxia is centrally modulated to produce ventilatory depression.

Effect of substance P on neurally mediated contraction of rabbit airway smooth muscle.

The neuromodulatory action of substance P (SP) was investigated in isolated rabbit tracheal smooth muscle (TSM) segments contracted with electrical field stimulation (ES). The tissues were placed in organ baths containing modified Krebs-Ringer solution and stimulated at a constant voltage (8 V; 24.5 mA) and pulse duration (2 ms) with ES frequencies ranging from 1 to 100 Hz. In the presence of SP, there occurred a dose-dependent augmentation of the TSM contractile response to any given ES, with the maximal effect of SP obtained at a dose of 10(-7) M. Accordingly, with the administration of 10(-7) M SP, the ES frequency-response relationship was altered so that 1) the mean (+/- SE) maximal tension (Tmax) induced by ES significantly increased (P less than 0.02) from a base-line value of 273 +/- 53 to 402 +/- 45 g/g TSM; and 2) the mean (+/- SE) log ES frequency producing 50% of Tmax (ES50) significantly decreased from a base-line value of 1.278 +/- 0.069 to 1.102 +/- 0.070 Hz (P less than 0.01). In contrast to these effects on ES-induced contraction, SP administration did not affect the TSM contractile response to administered methacholine chloride (10(-8) to 10(-3) M). On the other hand, the effects of SP on ES-induced contraction were independently blocked by the cholinergic antagonist, atropine (10(-6) M); the neurotoxin, tetrodotoxin (10(-6) g/ml); and the SP antagonist, D-Arg1,D-Pro2,D-Trp7,9,Leu11-SP (10(-5) M).(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny of respiratory control and pulmonary mechanics in newborn rabbits.

Maturation of the respiratory pattern and the active and passive mechanical properties of the respiratory system were assessed in 19 tracheotomized rabbits (postnatal age range: 1-26 days) placed in a body plethysmograph. With maturation both minute ventilation and tidal volume significantly increased, whereas respiratory frequency decreased. When normalized for body weight (kg) both the passive (Rrs X kg) and active (R'rs X kg) resistances of the respiratory system significantly increased with age, whereas the corresponding passive (Crs X kg-1) and active (C'rs X kg-1) compliances significantly decreased. At any given age R'rs X kg only slightly exceeded Rrs X kg, whereas C'rs X kg-1 was significantly lower than Crs X kg-1. Moreover, the maturational increases in Rrs X kg and R'rs X kg exceeded the corresponding decreases in Crs X kg-1 and C'rs X kg-1, resulting in significant age-related increases in both the passive (tau rs) and active (tau'rs) time constants of the respiratory system. Due to the age-related increases in tau'rs, producing a delayed volume response to any given inspiratory driving pressure, the relative volume loss obtained at any time during inspiration was greater in the maturing rabbit. On the other hand, because of concomitant compensatory changes in respiratory pattern, evidenced by increases in inspiratory duration with age, the end-inspiratory tidal volume loss in the maturing animal was maintained generally less than 10% at all postnatal ages. Thus maturational changes in respiratory pattern appear coupled to changes in the active mechanical properties of the respiratory system. The latter coupling serves to optimize the transduction of inspiratory pressure into volume change in a manner consistent with establishing the minimum inspiratory work of breathing during postnatal development.

Mechanisms of protein kinase C regulation of airway contractility.

To elucidate the role of protein kinase C (PK-C) in regulating airway contractility, the effects of PK-C activation with phorbol esters, 12-deoxyphorbol 13-isobutyrate (DPB), and phorbol 12-myristate 13-acetate (PMA), and with the diacylglycerol analogue 1-oleoyl-2-acetate-rac-glycerol (OAG) were separately evaluated in isolated rabbit tracheal smooth muscle (TSM) segments. The latter agents produced dual and opposing contractile effects, with DPB being the most potent. Lower doses of DPB (less than or equal to 10(-6) M) elicited significant increases in isometric tension in both untreated TSM, as well as in TSM half-maximally precontracted with methacholine. These potentiated TSM contractions were inhibited by the Ca2+ channel blockers, nifedipine (10(-4) M) and diltiazem (10(-5) M). In contrast, higher doses of DPB (greater than or equal to 10(-6) M) induced airway relaxation, which was ablated by preinhibition of the electrogenic Na+-K+ pump with ouabain (5 x 10(-6) M) or K+-free buffer. Indeed, in separate experiments DPB (10(-7) M) was found to significantly potentiate the functional activity of the Na+-K+ pump, an effect occurring independent of inhibition of Na+-H+ exchange with amiloride (10(-4) M) or extracellular Ca2+ influx with nifedipine (10(-4) M).(ABSTRACT TRUNCATED AT 250 WORDS)

Vasoactive effects of substance P on isolated rabbit pulmonary artery.

The vasoactive properties of substance P (SP) were studied in isolated rabbit pulmonary artery (PA) segments in vitro. In the absence of active base-line tone, noncumulative administration of SP (10(-11) to 10(-4) M) produced dose-dependent increases in PA tension. The peak isometric tension (Tmax) with SP was similar to the Tmax response to epinephrine; however, the doses of the agonist producing a threshold contraction and 25% of Tmax (ED25) were significantly lower for SP. In the presence of active base-line tone, induced by epinephrine or 5-hydroxytryptamine, SP produced transient PA relaxation which was directly related to the magnitude of the precontracted PA tension. Blockade of neurotransmission with tetrodotoxin (1 microgram/ml) and antagonists to alpha 1-adrenergic and histamine receptor binding had no effect on the contractile response to SP. On the other hand, PA contraction to an ED50 dose of SP was 1) inhibited by a mean of 33 +/- 10% (SE) following pretreatment with the cholinesterase inhibitor, neostigmine (10(-6) M) and 2) augmented by 52 +/- 21% with the cholinergic antagonist, atropine (10(-4) M). The latter also completely blocked the relaxation response to SP in precontracted PA. Similarly, removal of the PA endothelium also abolished the relaxation response to SP. In contrast, SP-induced contraction was markedly inhibited by the cyclooxygenase inhibitor, meclofenamate (1 microgram/ml), as well as the SP antagonist, D-Pro2, D-Trp7,9-SP.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of positive end-expiratory pressure on lung tissue mechanics in rabbits.

The effects of positive end-expiratory pressure (PEEP) on lung tissue resistance (Rti) and dynamic elastance (Edyn,L) were examined separately during histamine-induced lung constriction and after saline lung lavage in anesthetized paralyzed New Zealand White rabbits. During mechanical ventilation in the open-chest state, Rti and Edyn,L were estimated by fitting the appropriate signals to the equation of motion of the single-compartment linear model of the lung. Data were analyzed in relation to the structural damping hypothesis, which assumes that energy dissipation (Rti) and energy storage (Edyn,L) within the lung tissues are coupled at a fundamental level; the coupling parameter, termed hysteresivity (eta), = Rti.omega/Edyn,L, where omega is angular frequency. Under baseline conditions, elevation in PEEP resulted in significant increases in both Rti and Edyn,L, with eta remaining unchanged. During induced constriction and after lung lavage, Rti and Edyn,L significantly increased relative to their baseline values. During histamine-induced constriction, increasing PEEP was associated with increases in Edyn,L, whereas Rti and eta were reduced. After lung lavage, elevation in PEEP from 5 to 7 cmH2O was associated with proportional increases in Rti and Edyn,L, resulting in a relative constancy of eta. By contrast, when PEEP was decreased from 5 to 3 cmH2O, the values of Rti increased, whereas Edyn,L remained unchanged, resulting in significant increases in eta. Collectively, these findings suggest that the effects of PEEP on Rti during agonist-induced constriction and after perturbations of the gas-liquid interface are dependent on the state of alveolar/airway stability.

Assessment of methacholine-induced airway constriction by ultrafast high-resolution computed tomography.

Assessment of changes in airway dimensions during bronchoconstriction is conventionally based on measurements of respiratory mechanics. We evaluated the efficacy of ultrafast high-resolution computed tomography (UHRCT) to directly determine the dynamic changes in cross-sectional area (CSA) of airways in response to methacholine (MCh). UHRCT scans were obtained at functional residual capacity before (baseline) and after intravenous bolus injections of MCh (10(-8.5)-10(-7.0) mol/kg) to seven mechanically ventilated pigs. Changes in CSA of bronchi of varying baseline size (1-10 mm diam) were determined by using a customized image processing software package (VIDA) based on a user-directed computer-adjusted edge-finding algorithm. MCh induced dose-dependent decreases in CSA, which were paralleled by increases in airway opening pressure at higher doses of MCh; at lower doses of MCh, decreases in CSA of smaller airways were detected without concomitant changes in airway opening pressure. Changes in CSA were heterogeneous and variable, especially in the smaller airway ranges. The results of the present study support the concept that UHRCT can be used in conjunction with bolus challenges to effectively determine dose-response changes in airway caliber in both large and small airways. This technique provides data that may not be reflected by conventional lung function measurements and, hence, is a useful tool to study airway reactivity.

Effects of temperature on cholinergic contractility of rabbit airway muscle.

To elucidate the mechanism underlying temperature-induced changes in airway cholinergic contractility, the effects of organ bath cooling were evaluated in isolated rabbit airway smooth muscle (ASM) segments isometrically contracted with methacholine (METH) (10(-8)-10(-3) M) and electrical field stimulation (ES), wherein the ES stimulus frequency was varied between 1 and 100 Hz. Cooling from 37 to 25 degrees C produced systematic increases (P less than 0.01) in isometric tension at various administered doses of METH and at different levels of ES. Since the potentiated contractions to ES significantly exceeded (P less than 0.001) the corresponding increases in METH-induced contractility, we evaluated whether the latter was attributed to temperature-mediated changes in intrinsic airway neuronal acetylcholine (ACh) release. Accordingly, the effects of ASM cooling were independently determined before and after inhibition of the Na+-K+ electrogenic pump with ouabain (10(-5) M), and depletion of intrinsic neuronal ACh stores with hemicholinium-3 (HC-3) (10(-3) M). In the presence of either ouabain or HC-3 the above responses to temperature reduction were reversed, and airway cooling was associated with abrupt relaxation of ASM segments precontracted with METH. In contrast, neither inhibition of cyclooxygenase products with indomethacin (10(-6) M) nor cholinesterase inhibition with neostigmine (10(-3) M) notably influenced the ASM responses to organ bath cooling. Thus these findings demonstrate that 1) both METH-induced and neurally mediated cholinergic contractility are augmented during airway cooling; 2) the potentiated cholinergic responses are attributed to enhanced presynaptic release of ACh at the airway neuromuscular junction.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous opioids modulate fetal rabbit lung maturation.

To test the hypothesis that endogenous opioids modulate fetal lung development, separate groups of pregnant rabbits received daily injections of saline, morphine (1 mg/kg body wt), or the opioid antagonist naloxone (0.4 and 5.0 mg) for 10 days during their last trimester of pregnancy. The corresponding groups of fetuses were then delivered prematurely on day 28 of gestation (term approximately 31 days) and evaluated with respect to differences in body weight, lung weight, and the ratios of wet to dry lung weight and lung dry weight to body weight, the static inflation and deflation air and saline pressure-volume (P-V) characteristics of the lungs, and lung morphology. Mean values for body weight, lung weight, and the ratios of lung wet to dry weight and lung dry weight to body weight were not significantly different among the saline control (C), morphine (M)-, and naloxone (NLX)-treated fetuses. On the other hand, the fetal air P-V curves varied significantly (P less than 0.001), wherein the M-treated group depicted increased lung distensibility and alveolar stability on lung deflation, whereas the opposite was obtained in the NLX-treated fetuses. Moreover, morphometric analyses demonstrated that the mean alveolar air space-to-tissue ratio in lungs from M-treated fetuses were significantly greater than that observed either in C or in NLX-treated fetuses (P less than 0.05); however, the air space-to-tissue ratio did not significantly vary between the C and NLX-treated animals. These observations provide new evidence that endogenous opioids enhance fetal lung maturation.