PREJUNCTIONAL M1 AND POSTJUNCTIONAL M3-MUSCARINIC-RECEPTORS IN THE CIRCULAR MUSCLE OF THE GUINEA-PIG ILEUM

The effects of subtype-selective muscarinic receptor antagonists on electrically evoked release of acetylcholine and muscle contraction were compared in circular muscle preparations of the guinea-pig ileum. Incubation of the preparation with [H-3]choline resulted in the formation of [H-3]acetylcholine. Electrical stimulation caused the release of [H-3]acetylcholine which was abolished by tetrodotoxin and omission of calcium from the medium. 5-Hydroxytryptamine (10 mu M) and the nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium (300 mu M) did not change acetylcholine release. The muscarinic antagonists pirenzepine (M1 selective), AF-DX 116 (M2 selective) and hexa-hydrosiladifenidol (M3 selective) caused concentration-dependent increases in the evoked release of acetylcholine, and inhibitions of the circular muscle contraction. The postjunctional affinity constants (pA(2) values) obtained for hexahydrosiladifenidol (8.06), pirenzepine (6.95) and AF-DX 116 (6.60) identified the muscular receptor as an M3 subtype. Pirenzepine was more potent in facilitating the evoked release than hexahydrosiladifenidol and AF-DX 116. These findings suggest that the release of acetylcholine in the circular muscle is inhibited by M1 muscarinic autoreceptors whereas muscle contraction is mediated by M3 receptors.