The action of 5-hydroxytryptamine on longitudinal gastric smooth muscle of the plaice, Pleuronectes platessa

Abstract

Abstract 1. The sensitivity of plaice gastric longitudinal muscle to agonist drugs decreases with time spent in aquaria. 2. The stimulant action of acetylcholine (Ach) and 5-hydroxytryptamine (5-HT) persists after treatment with tetrodotoxin (1·6 × 10−6 M) which blocks nervous conduction. 3. Dose-dependent responses to 5-HT varied between 2·2 × 10−7 and 2·3 × 10−6 M at the first test, and between 1·8 × 10−7 and 5·6 × 10−6 M at the second test. Phenoxybenzamine (up to 3 × 10−6 M) and methysergide (up to 2 × 10−6 M) blocked part of the 5-HT contraction but morphine (up to 3 × 10−6 M), ergotamine (up to 1·5 × 10−6 M) and 2-bromo- d -LSD (up to 2 × 10−6 M) were not blockers. Hemicholinium (1 × 10−6M) gave partial, non-competitive blockade. 4. On the basis of this and previous work, it is concluded that 5-HT primarily stimulates gastric longitudinal muscle by displacing Ach from cholinergic nerve endings in the enteric plexus, but a direct action on muscle cells is also present.