FMRF-NH2 peptides which contain a conserved, identical C-terminal tetrapeptide but unique N terminus modulate cardiac contractility; yet, little is known about the mechanisms involved in signaling. Here, the structure-activity relationships (SARs) of the Drosophila melanogaster FMRF-NH2 peptides, PDNFMRF-NH2, SDNFMRF-NH2, DPKQDFMRF-NH2, SPKQDFMRF-NH2, and TPAEDFMRF-NH2, which bind FMRFa-R, were...