MECHANISM OF ACTION OF MUSCARINE ON THE LONGITUDINAL MUSCLE OF THE GUINEA‐PIG ISOLATED ILEUM

Abstract

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 (2.0 × 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 β-Bungarotoxin (10 μg/ml) significantly (P < 0.025) reduced the contraction elicited by dl-muscarine (2.5 × 10⁻⁸ m) suggesting presynaptic release of acetylcholine as an indirect mechanism of action of dl-muscarine. 5 Morphine (1.0 × 10⁻⁸ m) significantly (P < 0.05) reduced the contractions elicited by dl-muscarine (2.5 × 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 × 10⁻¹⁰ m) potentiated the effect of acetylcholine (2.5 × 10⁻⁸ m) and the potentiation was blocked by β-bungarotoxin.