Abstract
Receptors for calcitonin gene-related peptide (CGRP) in the lung membranes of guinea pig were characterized by using a millititer plate precoated with polyethylenimine. Specific binding of 125I-CGRP was time-dependent, rapid, and reversible, and the binding increased linearly with increasing concentrations of membrane protein. Scatchard analysis revealed two classes of CGRP binding sites: high affinity sites with a KD value of 7.17×10-11 M and a Bmax of 364 fmol/mg protein and low affinity sites with a KD value of 1.7×10-8 M and a Bmax of 39594 fmol/mg protein. Furthermore, the specific binding of 125I-CGRP was dissociated in the presence of GTP or Gpp(NH)p, suggesting that CGRP receptors in guinea pig lung membranes were coupled to the guanine nucleotide regulatory protein. Scatchard analysis of CGRP binding in the presence of GTP revealed selective inhibition of the binding to high affinity binding sites. Unlabeled CGRP displaced the binding of 125I-CGRP to guinea pig lung membranes with an IC50 value of 3.1×10-10 M. In contrast, salmon calcitonin and human calcitonin displaced the binding at 600-fold higher concentrations. We suggest that both low and high affinity binding sites for CGRP exist in the lung membranes of the guinea pig, and the high affinity binding sites for CGRP may be coupled to GTP binding regulatory protein.
