Distinct or shared actions of peptide family isoforms: I. Peptide-specific actions of pyrokinins in the lobster cardiac neuromuscular system

Abstract

Although the crustacean heart is modulated by a large number of peptides and amines, few of these have been localized to the cardiac ganglion (CG) itself; most appear to reach the CG only by hormonal routes. Immunohistochemistry in the American lobster, Homarus americanus, indicates that pyrokinins are present not only in neuroendocrine organs (pericardial organ and sinus gland), but also in the CG itself, where pyrokinin-positive terminals were found in the pacemaker cell region, as well as surrounding the motor neurons. Surprisingly, the one pyrokinin peptide identified from H. americanus, FSPRLamide, which consists solely of the conserved FXPRLamide residues that characterize pyrokinins, did not alter the activity of the cardiac neuromuscular system. However, a pyrokinin from the shrimp, Litopenaeus vannamei, (ADFAFNPRLamide; i.e., Penaeus vannamei pyrokinin 2 (PevPK2)) increased both the frequency and amplitude of heart contractions when perfused through the isolated whole heart. None of the other crustacean pyrokinins tested (another from L. vannamei and two from the crab, Cancer borealis) had any effect on the lobster heart. Similarly, altering the PevPK2 sequence either by truncation or by the substitution of single amino acids resulted in much lower or no activity in all cases; only the conservative substitution of serine for alanine at position 1 resulted in any activity on the heart. Thus, in contrast to other systems (cockroach and crab) in which pyrokinins have been examined and shown to elicit similar bioactivities, activation of the pyrokinin receptor in the lobster heart appears to be highly isoform-specific.