A Comparison of the Effects of FMRFamide-Like Peptides on Locust Heart and Skeletal Muscle

1. The responses of the semi-isolated heart preparation of the locust to the peptide FMRFamide and a range of its analogues is described. 2. The nature of the response observed depended on the structure of the analogue and its concentration. In some cases the responses were cardioexcitatory, in some they were cardioinhibitory, and in others they were biphasic. The cardioexcitatory responses consisted of increases in both the amplitude and the frequency of spontaneous heart contractions. The nature of the latter effect depended on the form of the basal contractile activity pattern exhibited before the application of the peptide. 3. The pharmacological profile of the cardioexcitatory responses observed in the locust heart preparation was very similar to that observed for the potentiation of neurally evoked tension in the extensor tibiae muscle preparation of the locust. In addition, both the profiles show similarities with the responses of various molluscan hearts and non-cardiac muscle preparations to the same peptides. 4. The results are discussed in terms of possible physiological roles for FMRFamide-like peptides in the regulation of contractile activity of the locust heart.