Cardioregulatory Peptides in Limulus: Effects of Fl, F2 and an Endogenous FMRFamide-like Peptide on the Neurogenic Heart.

Abstract

The neurogenic heart of the horseshoe crab, Limulus polyphemus, has been used as a model system with which to study the cellular and molecular mechanisms underlying specific actions of peptide and amine neuromodulators (1). The peptide Phe-Met-Arg-Phe-NH, (FMRFamide), or an analog, has been proposed as a candidate cardioregulator in Limulus. FMRFamide-like peptides have been detected in insects and crustaceans (2) and FMRFamide-like immunoreactivity has been detected in the Limulus CNS and cardiac ganglion (3). Although FMRFamide itself is cardioactive in the horseshoe crab only at high (> 1O-5 M) doses, a potent endogenous cardioactive FMRFamide-like peptide has been reported to occur in Limulus (4). We have characterized the action of this native FMRFamidelike peptide on the neurogenic heart of the horseshoe crab, and have compared these effects with those of synthetic identified analogs. A factor partially purified from the Limulus prosomal CNS produces characteristic excitatory chronotropic and inotropic effects when applied to the intact Limulus heart preparation (Fig. 1). This factor also produces FMRFamide-like contracture of the Busycon radula protractor muscle, a characterized bioassay for the peptide (5). The peptide character of this factor was determined by enzymatic degradation (loss of biological activity) with trypsin or pronase. A single peak of cardioactivity is detected by high performance liquid chromatography of the prosomal extract, eluting at 38% acetonitrile on a linear gradient of 66% to 48% in 70 min. The chronotropic effect of this peptide factor on the intact heart is probably a result of actions on the cardiac ganglion, because it also increases the frequency of bursts recorded from the isolated ganglion. Moreover, the factor increases levels of cyclic AMP in the cardiac ganglion, and previous studies have shown that cyclic AMP mediates chronotropic excitation by amines (6). While the cellular site of inotropic action by this