Characterization of muscarinic receptors in rat bronchial smooth muscle in vitro.

This study was undertaken to assess the important muscarinic receptor subtype in acetylcholine (ACh)-induced rat bronchial smooth muscle contraction. Ring smooth muscle strips of the left main bronchus were used. Isometrical contraction was measured in response to ACh in cumulative concentrations (10(-7)-10(-3) M) with and without preincubations with the muscarinic receptor antagonists, pirenzepine (an M1 antagonist), methoctramine (an M2 antagonist), and 4-diphenylacetoxy N-methylpiperidine (4-DAMP; an M1/M3 antagonist). Preincubation with these antagonists resulted in concentration-dependent rightward shifts of the concentration-response curves to ACh. pA2 values (means+/-sem) were 8.80+/-0.10 for 4-DAMP, 7.03+/-0.06 for pirenzepine and 5.91+/-0.36 for methoctramine, indicating that the most important muscarinic receptor mediating ACh-induced contraction of rat bronchial smooth muscle is of the M3 type.