The role of muscarinic M1 and M2 receptors in airway constriction in the cat.

The role of prejunctional inhibitory and facilitatory muscarinic receptors was investigated in cats with tracheal hyperresponsiveness to vagal stimulation. Intrathoracic airway caliber (total lung resistance (RL) and dynamic compliance (Cdyn] and the diameter of tracheal ring 4 were measured during vagal stimulation and local acetylcholine (ACh) injection before and after administration of the M1 receptor antagonist pirenzepine or the M2 receptor antagonist gallamine. The responses of tracheal ring 4, RL, and Cdyn to ACh were unaltered by gallamine or pirenzepine. Changes in RL and Cdyn during vagal stimulation were enhanced by gallamine, but the magnitude of tracheal constriction was unchanged. Vagally induced tracheal constriction was decreased by pirenzepine in hyperresponsive but not in control cats. The M2 receptors limit intrathoracic airway constriction, but a functional role for M2 receptors in the cervical trachea could not be demonstrated. However, these data suggest that M1 excitatory receptors may play a role in vagally mediated tracheal hyperreactivity.