Prejunctional Inhibitory Muscarinic Receptors on Cholinergic Nerves in Human and Guinea Pig Airways

We have investigated whether prejunctional inhibitory muscarinic receptors ("autoreceptors") exist on cholinergic nerves in human airways in vitro and whether guinea pig trachea provides a good model for further pharmacological characterization of these receptors. Pilocarpine was used as a selective agonist and gallamine as a selective antagonist of these autoreceptors. Acetylcholine (ACh) release from postganglionic cholinergic nerves was elicited by electrical field stimulation (EFS) (40 V, 0.5 ms, 32 Hz). In human bronchi, pilocarpine inhibited the contractile response to EFS in a dose-related fashion; the dose inhibiting 50% of the control contraction was 2.2 +/- 0.4 x 10(-7) (SE) M (n = 22), and the inhibition was 96% at 3 x 10(-5) M. The inhibitory effects of pilocarpine were antagonized by gallamine in a dose-related fashion. The results were qualitatively the same in the guinea pig. Gallamine significantly enhanced the contractile response to EFS in the guinea pig, whereas pirenzepine failed to do so, which suggests that M2-receptors are involved. We conclude that prejunctional muscarinic receptors that inhibit ACh release are present on cholinergic nerves in human airways and that guinea pig trachea is a good model for further pharmacological characterization of these receptors, which appear to belong to the M2-subtype.