The mode of action of acetylcholine and 5-hydroxytryptamine at the neuromuscular junctions in a molluscan muscle (radular protractor)

Abstract

1. In the radular protractor of Rapana thomasiana, the nerve endings among muscle fibers contained 2 types of synaptic vesicles; granular and agranular. In most nerve endings both types of vesicles were found together. 2. Electrical stimulations of the muscle showed elevation of excitability of the muscle by 5-hydroxytryptamine (5-HT). 3. A mixture of 5-HT and acetylcholine (ACh) induced repetitive activities of the muscle. These rhythmic responses were abolished in Ca 2+-free solution or by Mn 2+, indicating that the rhythmic responses depended on Ca 2+. 4. Peak potential of ACh-induced depolarization was not affected by [Ca 2+] 0, though the resting potential was lowered at decreased [Ca 2+] 0. At decreased [Na +] 0, membrane hyperpolarized and ACh-induced depolarization was depressed remarkably. These results suggest that ACh-induced depolarization depends mainly on Na + inflow.