The identification and structure-activity relations of a cardioactive FMRFamide-related peptide from the blue crab Callinectes sapidus.

The pericardial organs and thoracic ganglia of the blue crab Callinectes sapidus were resected and extracted. The extracts were fractionated by HPLC and the fractions analyzed by a radioimmunoassay (RIA) to FMRFamide. Multiple peaks of immunoreactivity were present and one of these, upon fast atom bombardment mass spectrometry (FAB-ms) and microsequencing, yielded the sequence GYNRSFLRFamide. The amount of this peptide in each crab is between 7 and 13 pmol. Several incomplete sequences were also characterized, suggesting a precursor with multiple copies of peptides related to GYNRSFLRFamide might occur. The peptide caused a dose-dependent increase in heart rate; threshold was 10 to 30 nM, and the EC50 was 323 +/- 62 nM. A structure-activity study of GYNRSFLRFamide on the crab heart suggests that, for full potency, a peptide should be at least a heptapeptide with the sequence XXZFLRFamide, where X is any amino acid and Z is either asparagine or serine.