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Pirenzepine blunts the pulmonary parenchymal response to inhaled methacholine
1Division of Clinical Sciences, Institute for Child Health Research, Perth, Australia.
Abstract:
To determine the role of M1 muscarinic receptors in the response of the pulmonary parenchyma to inhaled methacholine (MCh), 20 mongrel, out-bred puppies, 8-10 weeks of age were challenged following pretreatment with either saline (control), UH-AH37 (a combined M1 & M3 receptor blocker), or pirenzepine (a relatively selective M1 receptor blocker). In addition, eight fox hound-beagle puppies, born and raised in a clean animal house, were studied. Relatively selective doses of pirenzepine produced a dose-dependent shift to the right of the parenchymal dose-response curves (P = 0.031), with no effect on the airway dose-response curve (P = 0.102). The fox hound-beagle puppies showed less parenchymal response (P <0.0005), but equivalent airway response (P = 0.468), to MCh compared with the mongrel puppies. High doses of pirenzepine (10 000 mu g/kg) and UH-AH37 (3 mg/kg) markedly inhibited both the parenchymal and airway responses to MCh. Data from the present study demonstrate that: (1) while both the airway and pulmonary parenchyma respond to inhaled MCh, the mechanisms by which they respond differ; (2) stimulation of M1 subtype muscarinic receptors are responsible, at least partly, for the parenchymal response; and (3) experimental conditions, such as the breed and housing conditions of animals, may have major influences on the parenchymal response to inhalational challenge tests.
Insights
M1 muscarinic receptors partly mediate pulmonary parenchyma responses to methacholine (MCh). Different animal breeds and housing conditions significantly influence these parenchymal responses to MCh challenges.
Area of Science:
- Pulmonary pharmacology
- Respiratory system physiology
- Muscarinic receptor signaling
Background:
- Methacholine (MCh) inhalation challenges are used to assess airway hyperresponsiveness.
- The specific roles of muscarinic receptor subtypes in pulmonary parenchymal responses to MCh are not fully elucidated.
- Previous studies have primarily focused on MCh's effects on airways, with less attention to parenchymal tissue.
Purpose of the Study:
- To investigate the involvement of M1 muscarinic receptors in the pulmonary parenchymal response to inhaled methacholine (MCh).
- To compare MCh responses between different animal models and assess the impact of housing conditions.
- To differentiate the mechanisms underlying airway and parenchymal responses to MCh.
Main Methods:
- Inhaled methacholine (MCh) challenges were performed on puppies pretreated with saline, UH-AH37 (M1/M3 blocker), or pirenzepine (M1 blocker).
- Dose-response curves for both airway and parenchymal responses were analyzed.
- Responses were compared between mongrel puppies and fox hound-beagle puppies raised in a clean environment.
Main Results:
- Pirenzepine administration caused a dose-dependent rightward shift in parenchymal dose-response curves, indicating M1 receptor involvement (P = 0.031).
- Fox hound-beagle puppies exhibited significantly reduced parenchymal responses to MCh compared to mongrel puppies (P < 0.0005).
- High doses of pirenzepine and UH-AH37 significantly inhibited both parenchymal and airway responses to MCh.
Conclusions:
- Pulmonary parenchyma and airways exhibit distinct mechanisms in response to inhaled MCh.
- M1 muscarinic receptor stimulation plays a partial role in the parenchymal response to MCh.
- Animal breed and housing conditions can significantly influence pulmonary parenchymal responses in challenge tests.