Actions of noradrenaline and some other biogenic amines on the body-wall muscles of an echiuroid, Urechis unicinctus.

1. Noradrenaline inhibited contractions of the body-wall strips of U. unicinctus in response to electrical pulse of stimulation and L-proline, while serotonin enhanced them. In addition, serotonin increased the rate of relaxation of twitch contraction. Octopamine enhanced the contractions but was less potent than serotonin. 2. Acetylcholine-contraction of the isolated inner circular body-wall muscle was also inhibited by noradrenaline and enhanced by serotonin. 3. Phentolamine, an alpha-adrenoreceptor blocker, potentiated tetanic contraction of the strip, though it did not alter twitch contraction. The inhibitory action of noradrenaline was blocked by phentolamine. 4. Noradrenaline hyperpolarized the fibre membrane of the inner circular muscle, while serotonin did not alter the membrane potential. The action of noradrenaline was blocked by phentolamine. 5. Bioassay of the body-wall extract and the ventral nerve-cord extract suggests that these extracts contain serotonin-like and noradrenaline-like substances, respectively. 6. These results suggest that noradrenaline and serotonin may be neurotransmitters or neurohormones modulating muscle contraction in the body wall of U. unicinctus.