Airway hyper‐ or hyporeactivity to inhaled spasmogens 24 h after ovalbumin challenge of sensitized guinea‐pigs

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Abstract
1 The aim of this study was to determine whether an inhalation of ovalbumin (OA, 10 or 20 mg ml−1) by conscious OA-sensitized guinea-pigs leads to airway hyperreactivity to spasmogens 24 h later. In contrast to most previous studies, the spasmogens (5-HT, methacholine (MCh), U-46619 and adenosine) were administered by inhalation and airway function was measured in conscious guinea-pigs. 2 Guinea-pigs were sensitized by i.p. injection of 10 μg OA and 100 mg aluminium hydroxide in 1 ml normal saline; 14–21 days later they were exposed to an inhalation of 5-HT, MCh, U-46619 or adenosine. Specific airway conductance (sGaw) was measured in conscious animals by whole body plethysmography. The spasmogens caused bronchoconstriction, measured as a reduction in sGaw from the pre-inhalation basal values. Dose-related bronchoconstrictions were observed with 5-HT, MCh and U-46619. 3 The effect of an ovalbumin macroshock challenge upon the responses to each spasmogen were examined by giving an inhalation of aerosolized OA at 24 h (or 7 days in the case of adenosine) after an initial spasmogen challenge. Eighteen to twenty-four hours after the OA macroshock, the same guinea-pigs were exposed to a repeated inhalation of 5-HT, MCh, U-46619 or adenosine. 4 U-46619 was the only spasmogen to demonstrate hyperresponsiveness, the peak change in sGaw being increased from −12.3±9.9 to −38.8±5.0% by 10 mg ml−1 OA challenge. In contrast, the ovalbumin challenge (20 mg ml−1) inhibited the bronchoconstrictions to 5-HT (50 μg ml−1) and MCh (100 μg ml−1). Adenosine demonstrated bronchoconstriction in sensitized guinea-pigs but no significant change in the response was observed after an OA challenge. 5 All results were compared with a control group of sensitized guinea-pigs receiving a NaC1 challenge. The bronchoconstrictor responses to 5-HT, MCh, U-46619 or adenosine did not differ significantly before and after the saline challenge, indicating reproducibility of the responses. 6 In further experiments, guinea-pigs were exposed to inhalation of 5-HT (50 μg ml−1) or MCh (300 μg ml−1) 24 h before atropine (10 μg, 100 μg or 1 mg ml−1) and again at 0.5 to 1.5 h afterwards. Atropine antagonized the 5-HT- and MCh-induced bronchoconstrictions over the same antagonist dose-range. This suggests that the bronchoconstriction induced in the conscious guinea-pig by 5-HT is mediated primarily via muscarinic receptors, possibly by a vagal reflex. The inhibition of the responses to 5-HT and MCh by OA challenge would therefore appear to be related to interference with a common cholinergic pathway for these spasmogens. 7 In summary, airway hyperresponsiveness was evident at 24 h after OA challenge as measured by an enhanced bronchoconstrictor response to inhaled U-46619. When 5-HT or MCh were used as the spasmogens, an opposing decrease in responsiveness was observed. This was presumed to be due to an inhibition of cholinergic pathways by the OA challenge. Adenosine caused a bronchoconstriction in the sensitized animals but this was not enhanced by the OA challenge.