Opposite modulation by muscarinic M1 and M3 receptors of acetylcholine release from guinea pig ileum as measured directly.

Muscarinic receptor agonist and antagonist effects on acetylcholine release evoked by electrical or dimethylphenylpiperazinium stimulation from guinea pig ileum were evaluated by measuring acetylcholine with a high performance liquid chromatography-electrochemical detector system. AF102B (cis-2-methylspiro-(1,3-oxathiolane-5,3')-quinuclidine), a muscarinic M1 receptor agonist, increased markedly the evoked release of acetylcholine. In contrast, pirenzepine decreased the evoked acetylcholine release. 4-DAMP (4-diphenyl-acetoxy-N-methylpiperidine methiodide) and p-F-HHSiD (p-fluoro-hexahydrosiladifenidol), muscarinic M3 antagonists, increased the release of acetylcholine. Atropine enhanced acetylcholine release to a similar extent while bethanechol reduced the electrically evoked acetylcholine release. This reduction was virtually unaffected by methoctramine, but was antagonized by 4-DAMP or p-F-HHSiD. The results from direct determination of acetylcholine suggest that, in contrast to autoinhibition by stimulation of muscarinic M3 receptors, stimulation of presynaptic muscarinic M1 receptors is predominantly involved in enhancement of the acetylcholine release from guinea pig ileal nerves, and that AF102B functions as a muscarinic M1 agonist in this peripheral neuron.