Specific adrenomedullin binding sites and hypotension in the rat systemic vascular bed

The potent vasodilator peptide, adrenomedullin, has been shown to be present in plasma, suggesting a physiological role in cardiovascular control. Here we investigated the hypotensive action of adrenomedullin in vivo, using the anaesthetised rat as the bioassay model, and adrenomedullin binding sites using ligand binding assays on rat blood vessel membranes. Rat alpha CGRP and both human and rat adrenomedullins induced dose-dependent, powerful and long-lasting hypotensive effects. At peptide doses used in this study (0.02-2 nmol/kg), the efficacy of both human and rat adrenomedullins was lower than that of rat alpha CGRP. The CGRP(1)-receptor antagonist, human CGRP(8-37) (200 nmol/kg) was able to completely inhibit the hypotensive effect of rat alpha CGRP (0.2 nmol/kg) but not that of rat adrenomedullin (2 nmol/kg), implying that the adrenomedullin action is independent of CGRP(1)-receptors. Ligand binding assays confirmed the presence of both CGRP and adrenomedullin binding sites in rat blood vessels. The I-125-rat adrenomedullin binding site has a K-d = 0.32 +/- 0.12 nM (n = 4) for rat adrenomedullin but has a K-i > 10(-6) M for rat alpha CGRP. Chemical cross-linking and SDS-PAGE analysis revealed theadrenomedullin binding protein to have a M(r) of 83 000 with a minor band of M(r) = 99 000. The results suggest that the hypotensive effect of adrenomedullin may be mediated via specific adrenomedullin binding sites, in vivo.