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Evidence for the absence of a functional role for muscarinic M2 inhibitory receptors in cat trachea in vivo: contrast with in vitro results

1 . The effect of the selective muscarinic M2 receptor antagonist, gallamine and the selective M2 receptor agonist, pilocarpine, on airway constriction induced by vagal stimulation was studied in anaesthetized cats. In addition, the effect of gallamine on contraction of cat isolated tracheal and bronchi preparations induced by electrical field stimulation was also investigated. 2 . In in vivo experiments, extrathoracic airway constriction was measured with an electromechanical caliper that was attached to the outer surface of tracheal ring 4. Intrathoracic airway constriction was determined by measuring the changes in total lung resistance and dynamic compliance during vagal stimulation. 3 . Intravenous gallamine (0.1, 1, and 10 mg kg−1) augmented the rise in total lung resistance induced by vagal stimulation in a dose‐ and frequency‐dependent manner. At stimulation frequencies of 8 and 12 Hz the fall in dynamic compliance provoked by vagal stimulation was also significantly increased by gallamine (10 mg kg−1). Gallamine was without effect on airway constriction induced by acetylcholine. 4 . Vagal stimulation at 4 Hz produced significant tracheal constriction, but the amount of constriction did not change following injection of increasing doses of gallamine. Similarly, there was no difference in tracheal constriction at any frequency of stimulation (0.5–16 Hz) when frequency‐response curves before and after gallamine injection (10 mg kg−1) were compared. 5 . Pilocarpine (0.01–10 μg kg−1, i.v.) diminished changes in total lung resistance and dynamic compliance induced by vagal stimulation, an effect that was reversed by gallamine (10 mg kg−1, i.v.). Vagally‐induced tracheal constriction was not significantly affected by any dose of pilocarpine, nor was it modified by gallamine (10 mg kg−1) given subsequently. 6 . Atropine (0.5 mg kg−1) completely blocked tracheal constriction induced by vagal stimulation, indicating that the changes in tracheal ring diameter provoked by stimulation were mediated by muscarinic receptors and that intravenous drugs could reach the cervical trachealis muscle. 7 . In vitro tissue bath studies demonstrated a significant leftward shift of the frequency‐response curve to electrical field stimulation in both tracheal strips and bronchial rings following gallamine (10−1m) administration. 8 . Although the functional presence of muscarinic M2 autoreceptors was demonstrated in feline isolated tracheal and bronchial preparations, a corresponding functional role was not detected in cat trachea in vivo. This was despite repeated demonstration of muscarinic M2 receptor‐mediated limitation of airway constriction of intrathoracic airways in vivo.