Pre-junctional muscarinic autoreceptors in bovine airways.

ABSTRACT

UNLABELLED We searched for pre-junctional inhibitory muscarinic receptors in isolated bovine trachealis strips and bronchial rings. Electric stimulation (ES)-induced tritiated acetylcholine ([(3)H]-ACh)-release and isometric contractions were determined in muscles incubated with the non-selective muscarinic agonist pilocarpine, the non-selective muscarinic antagonist atropine, the selective M(2)-receptor antagonists methoctramine and gallamine, or the selective M(4)-receptor antagonist PD102807. Electric field stimulation (EFS)-induced isometric contractile responses were assessed in trachealis strips and bronchial rings treated with 10(-9)-10(-5)M methoctramine, gallamine or PD102807. Pilocarpine (10(-6) and 10(-5)M) and atropine (10(-7)M) significantly decreased and increased ES-evoked [(3)H]-ACh-release, respectively. The enhancing effect of atropine on [(3)H]-ACh-release prevailed over the inhibitory effect of pilocarpine. M(2)- and M(4)-receptor antagonists did not increase EFS-induced contraction or ES-induced [(3)H]-ACh-release. However, 10(-7)M methoctramine, gallamine or PD102807 significantly attenuated the inhibitory effects of pilocarpine 10(-5)M on ES-induced [(3)H]-ACh-release. CONCLUSIONS: Muscarinic autoregulation is present in bovine airways but is not fully accounted for by M(2)- and M(4)-receptor subtypes.