ProELH-related peptides: influence on bag cell cAMP levels

ProELH is the prohormone to the bag cell egg-laying peptide of Aplysia. In addition to containing the structure of the hormone (ELH) itself, proELH also contains several other secreted peptides: AP (acidic peptide) and alpha-, beta-, and gamma-bag cell peptides (BCPs). The BCPs, ranging in length from 5 to 9 amino acids, are structurally similar in that they all contain the sequence Arg-Leu-Arg-Phe. An additional peptide from the atrial gland, Atrial A, also contains this sequence. The BCPs previously have been reported to have direct feedback (autocrine) effects on the bag cells, including electrophysiological excitation and inhibition. Moreover, some of these effects are temperature-dependent. The autocrine functions of these peptides were explored here by investigating their effects on bag cell cAMP levels. In addition, we monitored the effects of Atrial A, as well as ELH and AP, which are proELH products that do not have sequence homology with the BCPs. While ELH and AP have no effect on bag cell cAMP levels, the other peptides fall into two functional classes. alpha- and gamma-BCP produce an elevation of cAMP levels at 20 degrees and a depression at 15 degrees C. The elevation in cAMP is sensitive to low Ca2+/high Mg2+. beta-BCP and Atrial A elevate cAMP levels independently of temperature, and are insensitive to low Ca2+/high Mg2+. Our results suggest that there may be multiple bag cell receptors for these peptides with the Arg-Leu-Arg-Phe sequence representing a receptor-recognition motif.