DIFFERENTIAL-EFFECTS OF GUANINE-NUCLEOTIDES ON [I-125] HCGRP(8-37) BINDING TO PORCINE LUNG AND HUMAN NEUROBLASTOMA CELL-MEMBRANES

Calcitonin gene-related peptide (CGRP) mediates its effects by binding to specific receptors which are positively coupled to adenylyl cyclas e. CGRP(8-37), a CGRP fragment devoid of the N-terminal region, was sh own to be a competitive CGRP receptor antagonist. Only a limited amoun t of data exists on the usefulness of this ligand in studying CGRP rec eptors. In the present study, we used [I-125]-hCGRP(8-37) to character ize CGRP receptors in porcine lung and human neuroblastoma cell (SK-N- MC) membranes. [I-125]-hCGRP(8-37) displayed specific and high affinit y binding in both membrane preparations. Displacement studies using [I -125]-hCGRP(8-37) and the agonist CGRP revealed the presence of high a nd low affinity CGRP binding sites in SK-N-MC cell and porcine lung me mbranes. Addition of guanylimidodiphosphate [Gpp(NH)p] shifted the com petition curve to the right and changed the two affinity states of the receptor to a single affinity in SK-N-MC cell membranes. On the other hand, in porcine lung membranes, the whole competition curve was shif ted to the right while maintaining the two affinity states. Thus, our data indicate that the new radioligand [I-125]-hCGRP(8-37) is a useful tool for characterizing CGRP receptors and their coupling to guanine nucleotide binding proteins.