CHARACTERIZATION OF BINDING OF THE ANNELIDAN MYOACTIVE PEPTIDES, GGNGPEPTIDES, TO TISSUES OF THE EARTHWORM, EISENIA-FOETIDA

The GGNG peptides are myoactive peptides which we have isolated from s everal species of annelids. Two types of peptides have been isolated a s GGNG peptides: earthworm excitatory peptides (EEP) and leech excitat ory peptide (LEP). The EEP and LEP are highly homologous with each oth er but act specifically on earthworms and leeches, respectively. To ap proach the mechanism of the specific action, we attempted to character ize the receptors for EEP in the earthworm Eisenia foetida, from which EEP have been isolated, by a radioreceptor assay. To prepare a radiol abeled ligand, we first coupled EEP to Bolton-Hunter reagent and then labeled it with I-125 ([I-125] BH-EEP). Binding of the [I-125] BH-EEP to the membrane preparation of the earthworm tissue (crop-gizzard) was reversible, saturable, and specific with a Kd of 4.9 +/- 1.2 nM and a B-max of 15.9 +/- 2.0 fmol/mg wet tissue. For the earthworm membrane preparation, EEP showed a much greater potency than LEP in displacing [I-125] BH-EEP, suggesting that the earthworm tissues contain EEP-spec ific receptors. This result would account for the specific biological activity of EEP and LEP on the earthworm and leech tissues, respective ly. The binding capacity was high in the anterior part of digestive tr act including the esophagus, crop and gizzard, and the nephridia. From these results, together with the observed myotropic activity of EEP o n gut tissues, it is assumed that EEP is physiologically involved in r egulation of the gut motility in the earthworm E. foetida. (C) 1997 Wi ley-Liss, Inc.