Pre-junctional muscarinic autoreceptors in bovine airways.

Abstract 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.