Mechanism of action of muscarine on the longitudinal muscle of the guinea-pig isolated ileum.

1 DL-Muscarine elicited a contraction of the ileal longitudinal muscle of the guinea-pig and the contraction was characterized by an after-response. 2 Physostigmine (20 x 10(8) M) potentiated the contraction of the longitudinal muscle elicited by DL-muscarine. 3 Hemicholinium-3 (HC-3) caused a rightward shift of the dose-response curve to DL-muscarine on the ileal longitudinal muscle of the guinea-pig ileum. 4 beta-Bungarotoxin (10 microgram/ml) significantly (P < 0.025) reduced the contraction elicited by DL-muscarine (2.5 X 10(-8) M) suggesting presynaptic release of acetylcholine as an indirect mechanism of action of DL-muscarine. 5 Morphine (1.0 x 10(-8) M) significantly (P < 0.05) reduced the contractions elicited by DL-muscarine (2.5 x 10(-8) M) further suggesting presynaptic release of acetylcholine as an indirect mechanism of action of DL-muscarine. 6 A subthreshold dose of DL-muscarine (2.0 x 10(-10) M) potentiated the effect of acetylcholine (2.5 x 10(-8) M) and the potentiation was blocked by beta-bungarotoxin.