Dissociation between bronchial hyperreactivity in vivo and reduced beta-adrenoceptor sensitivity in vitro in allergen-challenged guinea pigs

Transient receptor potential vanilloid receptor-1 does not contribute to slowly adapting airway receptor activation by inhaled ammonia

Inhalation of ammonia influences the activity of slowly adapting airway receptors (SARs), but the mechanism(s) is uncertain. Release of inflammatory mediators by transient receptor potential vanilloid receptor-1 (TRPV1) containing nerve endings could affect SAR response to ammonia. We examined how sensitization and subsequent desensitization of the TRPV1 by resiniferatoxin (RTX), affected the responses of SARs to inhaled ammonia. In pentobarbital-anesthetized, paralyzed and artificially ventilated rats, the left cervical vagus nerve was exposed, sectioned rostrally, and desheathed. Single fibers of SARs were identified and recorded. Two milliliters of ammonia vapor (from a 30% NH(4)OH solution) was inhaled over 20 s and responses to ammonia were measured. RTX was injected intravenously at 2 microg/Kg. Twenty minutes later, ammonia inhalation was repeated. Isoproterenol (ISO, 100 microg/kg, i.v.) was used in another set of experiments to block possible ammonia-induced bronchoconstriction. Ammonia increased tonic activity of SARs (n=10, P<0.0001), with complex changes in ventilator-related activity. SAR firing rate began to increase 2.3+/-0.2 min after RTX and returned to control levels at 13.6+/-1.4 min (n=10). By 20 min after RTX cardiovascular responses to ammonia were abolished, but effects on SAR activity were essentially unchanged. ISO did not modify the response of SARs to ammonia (n=8). These data suggest that responses of SARs to ammonia in rats do not depend on release of mediators by nerve endings containing TRPV1 and are not secondary to bronchoconstriction. However, when TRPV1 containing nerve endings were initially activated by RTX, the release of mediators may have affected SAR discharges.

Cloning, pharmacological characterization, and polymorphism screening of the guinea pig beta(2)-adrenoceptor

In asthma, beta(2)-adrenoceptor agonist responsiveness has been associated with Arg16Gly and Gln27Glu polymorphisms of the beta(2)-adrenoceptor. Since the guinea pig is extensively used as an animal model for asthma, we investigated the occurrence of possible polymorphism of the guinea pig beta(2)-adrenoceptor. The guinea pig beta(2)-adrenoceptor coding region was amplified by sequence homology-based cloning. Homology of the translated protein with the human beta(2)-adrenoceptor was 88% with Ala at position 16 and Glu at position 27. Radioligand binding and cAMP- accumulation experiments of Chinese hamster ovary (CHO) cells transfected with the guinea pig beta(2)-adrenoceptor revealed a homogeneous population of functional receptors. Five degenerate single nucleotide polymorphisms were found within the beta(2)-adrenoceptor coding region of outbred Dunkin Hartley guinea pigs, at residues 354, 453, 483, 534 and 642. In conclusion, we have cloned the guinea pig beta(2)-adrenoceptor, which shows to be functional upon expression in a recombinant system and contains five single nucleotide polymorphisms dissimilar to human polymorphisms.

Contractile responses to allergen and cholinergic stimulation in bronchus and trachea of cockroach allergen-sensitized guinea pigs

Airway hyperreactivity is a hallmark of asthma. Yet, the role of allergic sensitization in the hyperreactivity remains controversial. This study examined the effects of airborne cockroach allergen (CRa) sensitization on the contractility of bronchial (BSM) and tracheal (TSM) smooth muscle rings to cholinergic stimulations, and to specific antigen, in vitro. Guinea pigs were actively immunized with CRa, 5 mg (high dose, HD), 0.5 mg (low dose, LD), or saline aerosols (2x/day, 5 days/week, for 4 weeks) and challenged by CRa inhalation 5 days after last exposure. Twenty-four hours postchallenge, the contractions of the TSM and BSM to electrical field stimulation (EFS), acetylcholine (ACh), and to CRa were measured in vitro. In addition, the contraction to these stimuli were also examined in the passively sensitized TSM with the sera of CRa-immunized guinea pigs. The contractile responses of actively immunized and passively sensitized tissues to EFS and ACh were very similar to those of control tissues. CRa induced dose-dependent contractions in both actively immunized and passively sensitized, but not in control, tissues. The CRa-induced contraction was stronger in the HD group than in the LD group (p<0.0001). Leukotriene C4/D4 receptor antagonist LY-171883 inhibited the CRa-induced contraction by 86% in actively immunized TSM, and by 9% in the passively sensitized TSM. Pyrilamine inhibited the contraction by 57% in actively immunized TSM and 70% in passively sensitized TSM. The results indicate that CRa sensitization does not cause increased airway smooth muscle contractility to cholinergic stimulations, but induces antigen-specific contractions in vitro. Leukotrienes appear to play a significant role in the CRa-specific contractions, more in the actively immunized than those in the passively sensitized tissues, while histamine exerts a moderate effect on the CRa-induced contractions.

Dysfunctional muscarinic M2 autoreceptors in vagally induced bronchoconstriction of conscious guinea pigs after the early allergic reaction

We studied the function of autoinhibitory muscarinic M2 receptors on vagal nerve endings in the airways of conscious, unrestrained, ovalbumin-sensitized guinea pigs after the early and late allergic reaction. For this purpose, the effects of the selective muscarinic M2 receptor antagonist gallamine were examined on unilateral vagus nerve stimulation-induced bronchoconstriction, which was determined as an increase in basal respiration amplitude, measured as changes in pleural pressure. Under control conditions, i.e., before antigen challenge, a significant increase in the pleural pressure was found after inhalation of 0.1 mM and, even more pronounced, 1.0 mM gallamine, at medium stimulation frequencies (2-16 Hz), leading to a leftward shift of the frequency-response curve. After inhalation of 10 mM of gallamine, a complete reversal of the left-shift was observed and the frequency-response curve was depressed. However, 6 h after challenge with ovalbumin (i.e., after the early allergic reaction) no increase in nerve stimulation-induced bronchoconstriction by gallamine was found; a decrease in this bronchoconstriction was again observed with the highest concentration. At this moment, bronchial responsiveness to histamine was enhanced 4.5-fold compared to control, i.e., prior to antigen provocation. Both after the late allergic response (24 h after challenge; 1.6-fold histamine hyperresponsiveness) and 4 days after allergen challenge (normal histamine responsiveness) the gallamine-induced potentiation of the bronchoconstriction was restored, similar to the responses under control conditions. The results clearly demonstrate that prejunctional muscarinic M2 receptors control bronchoconstriction in conscious, unrestrained guinea pigs in vivo. Furthermore, these autoinhibitory receptors appear to be completely dysfunctional after the early allergic phase, but their function is largely restored after the late phase. The results indicate that dysfunction of autoinhibitory muscarinic M2 receptors might contribute to the strongly enhanced responsiveness to histamine after the early allergic response.

Deficiency of nitric oxide in allergen-induced airway hyperreactivity to contractile agonists after the early asthmatic reaction: an ex vivo study

1. Using a guinea-pig model of allergic asthma, we investigated the role of nitric oxide (NO) in allergen-induced airway hyperreactivity after the early asthmatic reaction, by examining the effects of the NO-synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) on the responsiveness to methacholine and histamine of isolated perfused tracheae from unchallenged (control) animals and from animals 6 h after ovalbumin challenge. 2. All animals developed airway hyperreactivity to inhaled histamine at 6 h after ovalbumin challenge, with a mean 3.11 +/- 0.45 fold increase in sensitivity to the agonist (P < 0.001). 3. In perfused tracheal preparations from the ovalbumin-challenged guinea-pigs, the maximal responses (Emax) to methacholine and histamine were significantly enhanced compared to controls, both after intraluminal (IL) and extraluminal (EL) administration of the contractile agonists. In addition, a small but significant increase in the pD2 (-log10 EC50) for IL and EL methacholine and for IL histamine was observed. As a consequence, the delta pD2 (EL-IL) for histamine was slightly decreased from 1.67 +/- 0.13 to 1.23 +/- 0.14 (P < 0.05). However, the delta pD2 for methacholine was unchanged (1.85 +/- 0.11 and 1.77 +/- 0.12, respectively; NS). 4. Incubation of control tracheae with 100 microM L-NAME (IL) significantly enhanced the Emax for both IL and EL methacholine and histamine to approximately the same degree as observed after ovalbumin challenge, with no effect on the pD2 and delta pD2 for both agonists. On the contrary, L-NAME had no effect on Emax and pD2 values of tracheal preparations from ovalbumin-challenged guinea-pigs. 5. L-NAME (10 microM-1 mM) had no effect on methacholine-induced contraction of isolated tracheal strip preparations obtained from control animals, indicating that L-NAME has no antimuscarinic effect on tracheal smooth muscle. 6. Histological examination of the intact tracheal preparations indicated epithelial and subepithelial infiltration of eosinophils after ovalbumin challenge. However, no apparent damage of the airway epithelium was observed in these preparations. 7. The results indicate that a deficiency of NO contributes to allergen-induced airway hyperreactivity after the early asthmatic reaction and that this deficiency appears not to be due to epithelial shedding.

The importance of eosinophil activation for the development of allergen-induced bronchial hyperreactivity in conscious, unrestrained guinea-pigs

Using a newly developed guinea-pig model of asthma, characterized by allergen-induced early and late phase asthmatic reactions, bronchial hyperreactivity (BHR) and airway inflammation, the importance of eosinophil activation for the development of BHR to inhaled histamine was investigated at 6 h (after the early reaction) and 24 h (after the late reaction) after allergen provocation. Eosinophil activation was assessed by a sensitive kinetic assay for eosinophil peroxidase (EPO) activity, suitable for bronchoalveolar lavage (BAL) analysis. A significant 2.9-fold (P < 0.01) increase in bronchial reactivity to histamine was observed at 6 h after allergen exposure, which was associated with a 2.9-fold increase in the number of eosinophils (P < 0.05) and a 6.7-fold increase in EPO activity (P < 0.01) in the BAL fluid. At 24 h after allergen exposure the bronchial reactivity to histamine was lower (1.7-fold), but still significantly enhanced (P < 0.01). By contrast, the number of eosinophils was further increased compared with 6 h after provocation (3.8-fold, P < 0.05), while the EPO activity remained stable at 6 h levels. The number of eosinophils was significantly correlated with EPO activity at 6 h (r = 0.62; P < 0.05), but not at 24 h after provocation. No significant correlation was observed between the number of eosinophils in the BAL fluid and BHR to histamine at either time point.(ABSTRACT TRUNCATED AT 250 WORDS)