Possible mechanism of the dual action of the new polypeptide (anthopleurin-B) from sea anemone in the isolated ileum and taenia caeci of the guinea-pig.

1 Anthopleurin-B (AP-B), a newly isolated polypeptide from a sea anemone (Anthopleura xanthogrammica) caused relaxation of the guinea-pig isolated ileum following a transient contraction at concentrations greater than 3 x 10(-9) M. 2 AP-B caused a tonic contraction followed by rhythmic relaxation of the guinea-pig isolated taenia caeci at a concentration of 3 x 10(-9) M or more. 3 The other polypeptides, anthopleurin-A (AP-A) from the same species of anthopleurin-C (AP-C) from Anthopleura elegantissima elicited similar effects but higher concentrations were required in both tissues. 4 These responses induced by AP-B in both tissues were abolished by treatment of each tissue with tetrodotoxin or incubation in a low-Na+ medium. 5 In the ileum, the AP-B-induced contraction was markedly inhibited by atropine but not by mecamylamine, whereas the AP-B-induced relaxation was not affected by phentolamine of guanethidine. 6 The AP-B-induced contraction of the taenia caeci was also inhibited by atropine, but not mecamylamine. However, in contrast to the ileum, the spontaneous relaxation of this tissue was completely abolished in the presence of phentolamine or guanethidine. 7 These results suggest that the AP-B-induced contractions of both tissues are mainly caused by acetylcholine (ACh) release from the cholinergic nerve terminals and that the AP-B-induced relaxation of the taenia caeci is due to the excitation of adrenergic nerves, while the relaxation of the ileum is mediated through non-adrenergic inhibitory mechanisms.