SALMFamide neuropeptides cause relaxation and eversion of the cardiac stomach in starfish

Feeding in starfish of the species Asterias rubens involves eversion of the cardiac stomach over prey such as mussels and oysters. For eversion to be accomplished the cardiac stomach must be relaxed. Here we show that two neu ropeptides (S1 and S2) belonging to a family of echinoderm neuropeptides ca lled SALMFamides cause concentration-dependent relaxation of the cardiac st omach in vitro, with S2 being 10-20 times more potent than S1. Previously, we have obtained evidence that nitric oxide mediates neural control of card iac stomach relaxation in Asterias. However, S2-induced relaxation of the c ardiac stomach is not affected by an inhibitor of the nitric oxide 'recepto r' soluble guanylyl cyclase. Therefore, cardiac stomach relaxation in starf ish appears to be controlled by at least two neural signalling pathways act ing in parallel. To assess the involvement of the SALMFamides in mediating cardiac stomach eversion in Asterias, experiments were performed in which w ater (control) or S1 or S2 was injected into the perivisceral coelom. Cardi ac stomach eversion was observed after 5 min in 3% of tests with water, in 11% of tests with S1 and in 57% of tests with S2. Importantly, the effectiv eness of S1 and S2 in promoting eversion corresponds with their relative po tency as cardiac stomach relaxants in vitro. Collectively, these data indic ate that SALMFamide neuropeptides may be involved in regulating the process of cardiac stomach eversion in starfish.