Adrenomedullin-2/intermedin induces cAMP accumulation in dissociated rat spinal cord cells: evidence for the existence of a distinct class of sites of action.

Adrenomedullin-2/intermedin is structurally related to the calcitonin family of peptides, which includes calcitonin gene-related peptide (CGRP), adrenomedullin, and amylin. We recently reported that CGRP and adrenomedullin act through distinct receptors to induce cyclic adenosine monophosphate (cAMP) accumulation in dispersed cells from embryonic rat spinal cord. Here, we investigated the apparent affinity and efficacy of adrenomedullin-2/intermedin for these receptors. Adrenomedullin-2/intermedin competed with [(125)I]-CGRP for binding to specific embryonic spinal cord cells with a pIC(50) of 9.73 +/- 0.06. Interestingly, adrenomedullin-2/intermedin competed for specific [(125)I]-adrenomedullin binding in a biphasic manner with pIC(50) of 9.03 +/- 0.22 and 6.45 +/- 0.24, respectively. Cellular levels of cAMP were increased by adrenomedullin-2/intermedin (pEC(50) 7.84 +/- 0.08) when cells were exposed to this peptide for 10 min at 37 degrees C. This effect was partially inhibited by the non-peptide antagonist BIBN4096BS (pA(2) 6.56 +/- 0.12), the adrenomedullin antagonist hAM(22-52) (pA(2) 6.36 +/- 0.30), and the adrenomedullin/CGRP antagonist CGRP(8-37) (pA(2) 7.24 +/- 0.60). More interestingly, a highly significant effect of adrenomedullin-2/intermedin on cAMP accumulation (pEC(50) 7.3 +/- 0.14) was still observed even in the presence of a mixture of saturating concentrations of BIBN4096BS, hAM(22-52), and the amylin antagonist AC187. Taken together, these data provide evidence for the possible existence of a distinct class of receptor sites for adrenomedullin-2/intermedin in embryonic rat spinal cord cells.