Changes in airway reactivity to exogenous and endogenous acetylcholine and substance P after anaphylactic bronchoconstriction in anaesthetized guinea-pigs

Role of cyclooxygenase isoforms and nitric-oxide synthase in the modulation of tracheal motor responsiveness in normal and antigen-sensitized Guinea pigs

The effects of selective cyclooxygenase (COX) isoform (COX-1, COX-2) inhibition, alone or in combination with nitric-oxide synthase (NOS) blockade, on in vitro tracheal muscle responsiveness to histamine were investigated in healthy and ovalbumin (OVA)-sensitized guinea pigs. Immunohistochemistry showed that COX-1 and COX-2 are constitutively present in normal guinea pig trachea, particularly in the epithelial layer, and that COX-2 expression is enhanced in OVA-sensitized animals both in epithelial and subepithelial tissues. In normal guinea pigs, SC-560 [5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethylpyrazole] (COX-1 inhibitor) or DFU [5,5-dimethyl-3-(3-fluorophenyl)-4-(4-methylsulphonyl)phenyl-2(5H)-furanone] (COX-2 inhibitor) significantly increased the contractile response to histamine, these effects being not additive. NOS inhibition by l-N(G)-nitro-arginine methyl ester (l-NAME) did not affect histamine-induced contraction but reversed the increase caused by COX-1 blockade while not modifying the enhancement associated with COX-2 inhibition. In guinea pigs subjected to OVA sensitization and challenge, COX-2, but not COX-1, inhibition enhanced the motor responses to histamine without any influence by l-NAME. In normal, but not in sensitized animals, the removal of epithelial layer from tracheal preparations abolished the enhancing action of DFU on histamine-mediated contraction. A COX-2-dependent release of prostacyclin (PGI(2)), but not prostaglandin E(2), was observed in tracheal tissues from normal and OVA-sensitized guinea pigs. In conclusion, both COX-1 and COX-2 are constitutive in guinea pig trachea, and COX-2 expression is enhanced by OVA sensitization; in normal animals, epithelial COX-2 exerts a PGI(2)-dependent inhibitory control on tracheal contractility, and this isoform is subjected to upstream regulation by epithelial COX-1 and NOS through a complex interplay; and following antigen sensitization, the inhibitory control on tracheal contractility is maintained by COX-2 induced at subepithelial cell sites.

Maximal forced expiratory maneuver to measure airway obstruction in allergen challenged mice

Our purpose was to develop a method of using a maximal forced expiratory maneuver (MFEM) for the study of bronchoconstriction and bronchial hyperreactivity (BHR) induced in mice by ovalbumin (OA) inhalation challenge. Eight mice (group I) were sensitized and then provocated with OA. Pulmonary function testing (PFT) at baseline and after varying doses of acetylcholine challenge was performed. Eight weight-matched normal mice served as controls (group II). Pulmonary functions include MFEM, dynamic respiratory system compliance (Crs) and respiratory system resistance (Rrs). The results showed that mice treated with OA had worse PFTs than normal controls, characterized by lower MFEF 50%, FEV0.1 and Crs but higher Rrs. The OA-sensitized mice also had more severe bronchoconstriction in response to acetylcholine, characterized by greater decreases in MFEF 50%, FEV0.1 and Crs but a higher Rrs than the controls. There was a good correlation between PD20MFEF50%Ach and PD20FEV0.1Ach with PD20CrsAch and PD20RrsAch. In conclusion, the MFEM can be used to evaluate airway obstruction and BHR induced in mice by allergen challenge.

Protective effect of silymarin in antigen challenge- and histamine-induced bronchoconstriction in in vivo guinea-pigs

The effects of silymarin on bronchoconstriction induced by antigen challenge and on post-antigen challenge hyperresponsiveness to substance P were evaluated in sensitized guinea-pigs. Silymarin significantly decreased the bronchoconstriction due to antigen administration in the early phase of the response. In contrast, the dose-response curve for substance P recorded 1 h after antigen challenge was not modified by pretreatment with silymarin. The influence of the flavonoid on hyperresponsiveness to histamine in propranolol- and PAF (platelet-activating factor)-treated animals was also assessed. Silymarin did not affect hyperresponsiveness to histamine induced by either propranolol or PAF although it had inhibitory activity on the bronchial contractile response to the autacoid. These results suggest that silymarin has a protective effect in the early phase of allergic asthma, an effect, which may be related to a negative influence of the flavonoid on bronchial responsiveness to histamine.

Role of tachykinins in asthma

The sensory neuropeptides substance P (SP) and neurokinin A (NKA) are localized to sensory airway nerves, from which they can be released by a variety of stimuli, including allergen, ozone, or inflammatory mediators. Sensory nerves containing these peptides are relatively scarce in human airways, but it is becoming increasingly evident that inflammatory cells such as eosinophils, macrophages, lymphocytes, and dendritic cells can produce the tachykinins SP and NKA. Moreover, immune stimuli can boost the production and secretion of SP and NKA. SP and NKA have potent effects on bronchomotor tone, airway secretions, and bronchial circulation (vasodilation and microvascular leakage) and on inflammatory and immune cells. Following their release, tachykinins are degraded by neutral endopeptidase (NEP) and angiotensin-converting enzyme. The airway effects of the tachykinins are largely mediated by tachykinin NK1 and NK2 receptors. Tachykinins contract smooth muscle mainly by interaction with NK2 receptors, while the vascular and proinflammatory effects are mediated by the NK1 receptor. In view of their potent effects on the airways, tachykinins have been put forward as possible mediators of asthma, and tachykinin receptor antagonists are a potential new class of antiasthmatic medication.

Airway hyperresponsiveness in anaesthetised guinea-pigs 18-24 hours after antigen inhalation does not occur with all intravenously administered spasmogens

Actively sensitised guinea-pigs were exposed to inhalation challenges with ovalbumin aerosol (macro- and microshock) and airway responsiveness to six intravenously administered spasmogens was evaluated 18 to 24 hr later in the anaesthetised animal. An increase in airway sensitivity was defined as a significant leftward shift of the dose-response curve when compared with saline-challenged control sensitized animals. After ovalbumin-macroshock (1% ovalbumin for 2 min. with mepyramine cover against fatal anaphylaxis), airway hyperresponsiveness was seen to 5-HT, the thromboxane A2-mimetic, U-46619, and bradykinin but not to methacholine, histamine or substance P. A similar pattern was seen after ovalbumin-microshock (0.010% ovalbumin for 60 min.), with induction of airway hyperreactivity to 5-HT and U-46619 but not methacholine or histamine. When the U-46619 dose-response curve was constructed following treatment of the animals with atropine (1 mg/kg, intravenously), airway hyperresponsiveness was no longer significant. As an index of airway inflammation, lung weights were significantly heavier in ovalbumin-challenged animals, than in saline-challenged controls. The results of this study with intravenously administered spasmogens does not support claims that ovalbumin-induced airway hyperreactivity in the guinea-pig is a 'non-specific' phenomenon.

Different bronchial responsiveness to Ach between normal and OA-sensitized guinea pigs after acoustic stress: a role for adenosine

Noise-exposure makes non-sensitized guinea pigs hyporesponsive to Acetylcholine (Ach), while in Ovalbumin (OA)-sensitized guinea pigs the responsiveness to the cholinergic mediator is not modified by acoustic stress (Nieri et al., 1996). The occurrence of bronchial hyporesponsiveness after acoustic stress in non-sensitized guinea pigs was verified also with histamine, obtaining a result similar to that observed with Ach. Moreover, the role of adenosine as modulator of the bronchial responsiveness to Ach after noise-exposure was assessed both in normal and in sensitized guinea pigs. In non-sensitized noise-exposed guinea pigs, the hyporesponsiveness to Ach was abolished by pretreatment of the animals with the peripheral A1/A2 antagonist 8-p-(sulfophenyl)theophylline (8-pSPT, 3 mg/kg i.v.) or with the A2-selective blocker 3,7-dimethyl-1-propargylxanthine (DMPX, 80 microg/kg i.v.) but not with the A1-selective antagonist Xanthine Amine Congener (XAC, 0.1 mg/kg i.v.). In sensitized guinea pigs, pretreatment with theophylline (25 mg/kg i.v.) makes noise-exposed animals again hyporesponsive to Ach, while no effect was obtained with the selective A1 and A2 antagonists employed. Also enprofylline (10 mg/kg i.v.), a phosphodiesterase inhibitor more potent than theophylline, does not modify the responsiveness to Ach in sensitized noise-exposed guinea pigs. The overall data presented suggest the involvement of the peripheral purinergic system in the regulation of airway reactivity after the stressful condition and indicate an altered functionality of this system as a consequence of sensitization. Furthermore, noise-exposure makes it possible to reveal in guinea pigs an opposite influence by theophylline on airway responsiveness to Ach, in sensitized, with respect to normal, animals.

Effect of vagotomy on airway hyperreactivity to endogenously released neurotransmitters at 18-24 h after inhaled antigen

Airway reactivity was examined in anaesthetized guinea-pigs 18-24 h after inhalation challenge of ovalbumin-sensitized animals with ovalbumin. Bronchoconstrictor responses were measured from the increases in pulmonary inflation pressure. The study was undertaken to examine whether ovalbumin challenge induced airway hyperreactivity to neurotransmitters released endogenously by vagal nerve stimulation. Stimulation parameters were selected to cause release of either acetylcholine (0.3 ms pulse width for 3 s, 20 V, 2-40 Hz), both acetylcholine and neuropeptide (5 ms pulse width for 15 s, 20 V, 0.5-8 Hz) or neuropeptide only, using the latter parameters in the presence of atropine. The vagi were paired for stimulation and in some experiments were cut central to the stimulation point. Frequency-response curves for acetylcholine- and neuropeptide-mediated bronchoconstrictor responses to vagal stimulation when the nerves were intact revealed no airway hyperreactivity after ovalbumin challenge. The presence of atropine failed to reveal airway hyperreactivity. However, when the vagi were cut, the frequency-response curves were displaced to the left after ovalbumin challenge compared with saline challenged animals, indicating airway hyperreactivity. This airway hyperreactivity was significant after atropine and suggests an increase in sensitivity to endogenously released neuropeptides rather than acetylcholine. It also indicates that the airway hyperreactivity is dependent on removal of the afferent vagal pathways. Frequency-response curves for cholinergic stimulation (0.3 ms) with intact vagi revealed no airway hyperreactivity after antigen challenge. Comparisons of exogenously administered 5-hydroxytryptamine (5-HT, 300 ng/100 g i.v.) and a single vagal stimulation of 0.3 ms pulse width (cholinergic) revealed no airway hyperreactivity to either stimulus after ovalbumin challenge. However if the vagi were cut, airway hyperreactivity was observed, again suggesting that removal of afferent pathways is important for revealing airway hyperreactivity in the anaesthetized guinea-pig. Ovalbumin challenge caused significant increases in the bronchoconstrictor responses to a single dose of capsaicin (50 microg/100 g i.v.) or dose-response curves to bradykinin. Since these agents release neuropeptides from sensory C-fibres, this is further support for a raised sensitivity to endogenously released neuropeptides.

Bronchoconstriction and eosinophil recruitment in guinea pig lungs after platelet activating factor administration

Using a forced expiratory maneuver to measure flow volume loops, we evaluated the ability of platelet activating factor (PAF) to induce acute bronchospasm and in histological changes associated with bronchial asthma in guinea pigs. We determined both the dose-response curve and the time course of PAF-induced bronchoconstriction. Eight guinea pigs with weights ranging from 350 to 450 g were anesthetized, tracheotomized, and then paralyzed with gallamine. Baseline pulmonary function tests (PFT) were done. Different doses (25, 50, 100, 200, and 500 ng/kg) of PAF were injected through the jugular vein, and serial PFTs were done at 30 sec, 2, 5, and 20 min after each dose of PAF. Acetylcholine provocation testing was done following the 200 ng/kg dose of PAF. The PFTs included a forced expiratory maneuver, airway opening pressure (PaO), and total lung compliance (TLC). After all PFTs were completed, cell counts were done on fluid obtained from bronchoalveolar lavage (BAL) and the lungs were removed for histological study. Eight other guinea pigs were used as controls. The results showed that with increasing doses of PAF from 25 ng/kg to 200 ng/kg, all lung function parameters, including vital capacity, peak flow, MFEF 75%, MFEF 50%, MFEF 25%, and total lung compliance, gradually decreased. However, a further increase of the dose of PAF up to 500 ng/kg did not result in continued worsening of PFTs. The most severe bronchoconstriction occurred 30 sec after PAF was injected, and it gradually resolved thereafter. PAF injection also induced a severe inflammatory reaction of the airway and lung tissue, characterized by congestion, edema, inflammatory cell (especially lymphocytes and eosinophils) infiltration, and desquamation of bronchial epithelial cells. In conclusion, in the guinea pig model, PAF can induce acute reversible bronchospasm and bronchial hyperreactivity, as well as the typical histological changes of bronchial asthma.

Acetylcholine-induced bronchoconstriction modified by noise exposure in normal but not in sensitized guinea-pigs

1. The acute (6h) exposure of guinea-pigs to white noise (110 dB) as a stress stimulus, reduced bronchial reactivity to acetylcholine (Ach) (3-1000 micrograms kg-1 i.v.) in anaesthetized animals. 2. The hyporesponsiveness to Ach in stressed animals was not confirmed in vitro on tracheal preparations (Ach 1 x 10(-9)-1 x 10(-4) g ml-1) and disappeared in vivo when the animals were sensitized with ovalbumin (OA, 100 mg kg-1 i.p. + 100 mg kg-1 s.c.). The hyporesponsiveness was also absent in ovalbumin sensitized guinea-pigs exposed to an aerosol of ovalbumin 60 min before testing with Ach. 3. In non-sensitized guinea-pigs, pretreatment with butoxamine (1 mg kg-1 i.v.) or with theophylline (25 mg kg-1 i.v.), completely abolished the effect of noise-exposure. In contrast, pretreatment with L-NG-nitro-arginine methyl ester (L-NAME, 10 mg kg-1 i.v.), alpha-chymotrypsin (2 U kg-1 i.v.) or with enprofylline (10 mg kg-1 i.v.), did not affect it. 4. In conclusion, our experiments reveal inhibitory mechanisms upon Ach-induced bronchoconstriction activated by a stress stimulus and this is absent in sensitized animals. These mechanisms seem to be linked to the adrenergic beta 2-receptors and a role for the purinergic system (via A-receptors) may also be present.

Differences in the distribution and characteristics of tachykinin NK1 binding sites between human and guinea pig lung

1. The distribution and characteristics of tachykinin NK1 binding sites have been compared in human and guinea pig lung using quantitative in vitro receptor autoradiography with [125I]-Bolton Hunter-labelled substance P ([125I]-BH-SP). In addition, the effects on these sites of ovalbumin sensitization and challenge have been determined in guinea pig lung. 2. [125I]-BH-SP bound specifically and with high affinity to microvascular endothelium in both human and guinea pig lung, but to bronchial smooth muscle and pulmonary artery media in only guinea pig lung. 3. Specific binding of [125I]-BH-SP to guinea pig bronchial smooth muscle was positively correlated with airway diameter in the range 150-800 microns and was less dense in trachea than in main bronchi. 4. [125I]-BH-SP binding was inhibited by tachykinins with rank orders of affinity of SP > NKA > NKB (human microvessels) and SP > NKA = NKB (guinea pig bronchi and pulmonary arteries). NKA displayed a higher affinity for [125I]-BH-SP binding sites in human microvessels than in guinea pig tissues (P < 0.0001), indicating differences in selectivity for tachykinins between human and guinea pig NK1 receptors. 5. In both human and guinea pig lung, [125I]-BH-SP binding was inhibited by the specific tachykinin receptor antagonists FK888 (NK1 selective antagonist) and FK224 (mixed NK1/NK2 antagonist), with FK888 displaying equal affinity to SP and > 500 times higher affinity than FK224. SP, NKA, NKB and FK888 exhibited similar affinities for [125I]-BH-SP binding sites in both guinea pig arteries and bronchi. 6. Similar distributions, densities and characteristics of [I251]-BH-SP binding sites were demonstrated in oval bumin-sensitized and -challenged guinea-pig lung and in naive animals.7. Differences in the distribution and characteristics of NKI binding sites labelled with [125I]-BH-SP between guinea pig and human lung suggest limitations in the use of guinea pig models for studying roles of tachykinins in pulmonary disease. However, the similar microvascular distributions of NK,binding sites in human and guinea pig lung suggest that the selective tachykinin receptor antagonistsFK888 and FK224 may be useful in the management of airway inflammation in man.

Histamine and serotonin released from the rat perfused heart by compound 48/80 or by allergen challenge influence noradrenaline or acetylcholine exocytotic release

Terminal nerve fibres of the autonomic nervous system closely approach mast cells in peripheral organs, and mutual influences between release of neurotransmitters or mast cell mediators may cause neuro-immunological interactions. We have studied the influence of mast cell degranulation on the release of endogenous noradrenaline and newly incorporated acetylcholine (such as 14C-choline/acetylcholine overflow) evoked by stimulation of extrinsic postganglionic sympathetic or preganglionic vagal nerves in the rat Langendorff heart perfused with Tyrode solution. Compound 48/80 perfused in normal hearts, or ovalbumin infused into hearts from rats sensitized to ovalbumin, enhanced the overflow of endogenous histamine and serotonin. Both stimuli increased the release of mediators to a similar extent and with fast kinetics. Maximum average concentrations in the perfusate of histamine were about 800 nmol/l, and of serotonin 40 nmol/l, in a sample collected within 4 min after mast cell degranulation. Stimulation of autonomic nerves did not affect basal histamine or serotonin overflow. Whereas basal overflows were unaffected, the stimulation-evoked releases of both noradrenaline and acetylcholine, were facilitated when compound 48/80 was perfused before and during nerve stimulation. The facilitation of noradrenaline overflow was more pronounced (by 60%) when compound 48/80-induced mediator overflow started 4 min before nerve stimulation as compared to 30 s (15%), and was reduced by cocaine (by 50%), and, in the presence of cocaine, abolished by cimetidine (but was unaffected by mepyramine and thioperamide) and NG-nitro-(L)-(-)-arginine. In the presence of cimetidine and cocaine, when the facilitatory components were abolished, the evoked noradrenaline overflow observed 30 s after the start of infusion of compound 48/80 was inhibited, and the inhibition was partly reduced by methiotepin and ketanserin. Ovalbumin infusion in hearts from sensitized animals caused an inhibition of evoked noradrenaline overflow sensitive to methiotepin and also partly to ketanserin, and no facilitation was observed. The facilitation (> 100%) of evoked overflow of acetylcholine observed at 4 min after the start of perfusion with compound 48/80 was partly reduced by thioperamide (but not mepyramine or cimetidine) and to a comparable extent either by tropisetron (3 mumol/l) alone or by tropisetron plus methiotepin. In conclusion, degranulation of immunological cells is followed by histamine and serotonin release in the rat heart and may affect the release of autonomic neurotransmitters in rather unusual ways, by i) an uptake1-dependent and ii) an H2-mediated facilitation which probably involves nitric oxide as a permissive mediator, and iii) a serotonergic inhibition, of noradrenaline release, and iv) an H3- and serotonergic facilitation of acetylcholine release.