Abstract

The mechanism by which Z-338, a novel gastroprokinetic agent, stimulates gastric motility was studied in relation to muscarinic receptors in the guinea pig. Z-338 (3–30 μM) enhanced electrically stimulated contractions and the release of acetylcholine (ACh) that was tetrodotoxin sensitive and extracellular Ca²⁺ dependent, in gastric strips. Membrane-binding assay revealed that Z-338 possessed binding affinity for muscarinic M₁and M₂, but not M₃ receptors. InXenopus oocytes expressing M₁ and M₂ muscarinic receptors, Z-338 did not produce any response, but inhibited ACh-induced outward currents, thereby indicating that Z-338 acts on the M₁ and M₂muscarinic receptors as an antagonist. The M₁ receptor antagonist pirenzepine (0.5 μM) and M₂ receptor antagonist AF-DX 116 (1 μM) also enhanced electrically stimulated release of ACh. These results indicate that Z-338 facilitates ACh release from cholinergic nerve terminals by blocking muscarinic M₁ and M₂ autoreceptors, which regulate the release of ACh.