Alpha bag cell peptide reduces stimulated cAMP levels and pro-ELH synthesis in bag cells.

alpha-Bag cell peptide (alpha-BCP), one of the multiple secretory products of the precursor to the egg-laying hormone (proELH) produced by the neurosecretory bag cells of Aplysia, has been variously claimed to have excitatory or inhibitory feedback effects on the cells which secrete it and to exert some of these effects via a reduction in cAMP levels. Since it was shown previously that proELH synthesis is modulated by cAMP, the present study was undertaken to determine if alpha-BCP causes parallel alterations in cAMP levels and peptide synthesis. The peptide had no effect on bag cell levels of cAMP in the absence of stimulation and no effect on the elevation of this nucleotide by the phosphodiesterase inhibitor, isobutylmethylxanthine. However, it reduced the elevations induced by forskolin, dopamine, and high external potassium. Consistent with the above and with the hypothesis that proELH synthesis is accelerated by cAMP, alpha-BCP reduced proELH synthesis and cAMP production in parallel in forskolin-stimulated preparations, but did not inhibit stimulation of peptide synthesis by isobutylmethylxanthine or zero external Ca2+. These results support an autoinhibitory role for alpha-BCP in modulating bag cell discharge and suggest a mechanism by which it can mediate negative feedback inhibition of proELH biosynthesis.