Neuronal muscarinic receptors attenuate vagally‐induced contraction of feline bronchial smooth muscle

Abstract

1 In anaesthetized cats, stimulation of the vagus nerves produced bradycardia and a bronchoconstriction which was measured as an increase in lung resistance (R_L) and a fall in dynamic lung compliance (C_dyn); these effects were abolished by atropine. 2 Gallamine potentiated vagally-mediated changes in R_L and C_dyn at doses that blocked muscarinic receptors in the heart and inhibited neuromuscular transmission. (+)-Tubocurarine and suxamethonium did not affect the response of the lung or the heart to vagal stimulation. 3 Bronchoconstriction induced by intravenous acetylcholine was not potentiated by gallamine, indicating that postsynaptic muscarinic receptors in the lung and changes in muscle tone were not involved. 4 Potentiation of vagally-induced bronchoconstriction appears to be due to blockade of inhibitory muscarinic receptors located in the pulmonary parasympathetic nerves innervating both central and peripheral airways. 5 Pilocarpine was an agonist for these neuronal receptors as it inhibited vagally-induced bronchoconstriction at low doses (10 ng to 1 μg kg⁻¹). 6 The results demonstrate that gallamine is an antagonist and pilocarpine an agonist at neuronal muscarinic receptors which attenuate parasympathetic nerve activity in feline lung.