Structure of a molluscan cardioexcitatory neuropeptide.

A cardioexcitatory substance from ganglia of the clam Macrocallista nimbosa, formerly designated peak C, is the tetrapeptide amide Phe-Met-Arg-Phe-NH2. Its structure was determined by the combined use of Edman dansyl degradation and tryptic digestion. The structure was confirmed by synthesis. This neuropeptide is active at about 10(-8)M when assayed on molluscan muscle.