A muscarinic receptor subtype modulates vagally stimulated bronchial contraction

An in vitro preparation was developed to study vagus nerve-stimulated (preganglionic) and field-stimulated (postganglionic) contraction of the rabbit main stem bronchus and to compare the inhibitory effects of muscarinic antagonists on that contraction. The maximal contractile responses (20 V, 0.5 ms, 64 Hz) for either field or vagal stimulation were completely abolished by atropine (60 nM). Hexamethonium (0.1 mM) abolished the response to vagal stimulation but did not affect the field-stimulated response. To compare the effectiveness of atropine and pirenzepine as antagonists at the nerve-smooth muscle junction, inhibition studies of field-stimulated contractions were performed. Pirenzepine was 102- to 178-fold less potent than atropine when compared at the inhibitory concentration of antagonist that produced 25, 50, and 75% inhibition (IC₂₅, IC₅₀, and IC₇₅, respectively), indicating that the muscarinic receptor at the nerve-smooth muscle junction is a muscarinic receptor with low affinity for pirenzepine (M₂ subtype). Atropine had similar inhibitory effects on vagal- and field-stimulated contractions. In contrast, pirenzepine was more potent in inhibiting vagally stimulated contraction than field-stimulated contraction, especially at the IC₂₅ where pirenzepine was only 8- to 22-fold less potent than atropine in inhibiting vagally stimulated contraction. These data suggest that an M₁ subtype of muscarinic receptor modulates excitatory neurotransmission through bronchial parasympathetic ganglia.