Muscarinic blockade of methacholine induced airway and parenchymal lung responses in anaesthetised rats

Abstract

Background-It has previously been shown that M 1 cholinergic receptors are involved in the parenchymal response to inhaled methacholine in puppies using the M 1 selective antagonist pirenzepine. Although M 3 receptors are responsible for acetylcholine induced bronchoconstriction in isolated rat lung, the role of M 1 receptors has not been determined in the rat in vivo. Methods-Anaesthetised, paralysed, open chested Brown Norway rats were mechanically ventilated and the femoral vein cannulated for intravenous injection of drugs. Low frequency forced oscillations were applied to measure lung input impedance (ZL) and computerised modelling enabled separation of ZL into airway and parenchymal components. Atropine (500 µg/kg iv) and pirenzepine (50, 100 or 200 µg/kg iv) were administered during steady state constriction generated by continuous inhalation (1 mg/ml) or intravenous (10 or 15 µg/kg/min) administration of methacholine. Results-Continuous inhalation of methacholine produced a 185% increase in frequency dependent tissue resistance (G) which was eVectively inhibited by atropine 500 µg/kg iv (p<0.01, n = 6). Pirenzepine (50, 100 or 200 µg/kg) had a minimal eVect on the parenchymal response to inhaled methacholine. A 258% increase in airway resistance (Raw) was induced by continuous intravenous infusion of methacholine and this response was eVectively abolished by pirenzepine (p<0.001, n = 5). Cutting the vagi in the cervical region did not alter baseline airway mechanics. Vagotomy did not aVect lung responses to intravenous methacholine nor the ability of pirenzepine to reduce these responses. Conclusions-In the rat, M 1 -subtype receptors are functional in airways but not in the tissue. (Thorax 1999;54:531-537) Keywords: forced oscillation technique; muscarinic blockade; lung parenchyma Methacholine is commonly used as a challenge agent for the assessment of hyperresponsiveness in asthmatic subjects. It is a chemical analogue of acetylcholine and induces smooth muscle contraction by stimulating muscarinic cholinergic receptors located on the smooth muscle. In a number of animal studies the way in which methacholine alters lung function has been shown to depend on the route of delivery. Intravenous methacholine acts mainly on the airway producing an increase in airway resistance whereas inhaled methacholine alters the mechanical properties of both the airways and the lung tissues.1-3 One possible explanation for these findings is that diVerent receptors are involved.Muscarinic cholinergic receptors exist in at least four major subtypes that can be demonstrated pharmacologically. 4 Multiple subtypes have, however, been identified biochemically in the lung and binding studies have shown the presence of M 1 , M 2 and M 3 subtypes.5 6 A species diVerence in distribution of muscarinic receptors of the lung has been established. In isolated rat lung, stimulation of the M 3 receptor subtype is responsible for acetylcholine induced bronchoconstriction.7 However, previous experiments in our laboratory using...

Acetylcholine-induced Calcium Signaling and Contraction of Airway Smooth Muscle Cells in Lung Slices

“…Many studies on airway contraction have been performed using methacholine (MCH), a chemical analogue of ACH acting via muscarinic receptors that may vary in subtype selectivity depending on species and tissue ( Tulic et al, 1999 ). The choice to use MCH may have been based on the slower metabolism of MCH and the widespread use of MCH in clinical lung function tests.…”

Inertance measurements by jet pulses in ventilated small lungs after perfluorochemical liquid (PFC) applications

“…Recent adaptation of low-frequency FOT enabled a separate assessment of airway and tissue mechanical parameters by model fitting (Sly et al 1996, Hantos et al 1990. However, these measurements are difficult to perform and still limited to special research purposes (Tulic et al 1999, Hantos et al 2003, Brewer et al 2003.…”

New insights into the pathophysiology of the small airways in asthma

“…Methacholine (MCh)-induced increase in parenchymal resistance has also been reported in New Zealand white rabbits,[ 41 42 ] guinea-pigs[ 43 ] and rats. [ 38 44 45 ]…”