SPECIFIC RECEPTORS FOR ADRENOMEDULLIN IN CULTURED RAT VASCULAR SMOOTH-MUSCLE CELLS

The effects of synthetic rat adrenomedullin (rAM), a novel vasorelaxan t peptide originally isolated from human pheochromocytoma, on receptor binding and cAMP generation were studied in cultured rat vascular smo oth muscle cells (VSMC). A binding study using [I-125]rAM revealed the presence of a single class of high-affinity (K-d 1.3 x 10(-8) M) bind ing sites for rAM in VSMC. The apparent K-i of rat calcitonin gene-rel ated peptide (rCGRP) was 3 x 10(-7) M. Affinity labeling of VSMC membr anes with [I-125]rAM revealed two distinct labeled bands with apparent molecular weights of 120 and 70 kDa, both of which were abolished by excess unlabeled rAM or rCGRP. rAM stimulated cAMP formation with an a pproximate EC(50) of 10(-8) M, the effect of which was additive with i soproterenol, but not with rCGRP. The rAM-induced cAMP response was un affected by propranalol, indomethacin, or quinacrine, but inhibited by a CGRP receptor antagonist, human CGRP[8-37]. These data suggest that VSMC possesses specific AM receptors functionally coupled to adenylat e cyclase with which CGRP interacts.