ADRENOMEDULLIN STIMULATES 2 SIGNAL-TRANSDUCTION PATHWAYS, CAMP ACCUMULATION AND CA2-CELLS( MOBILIZATION, IN BOVINE AORTIC ENDOTHELIAL)

The biological action of adrenomedullin, a novel hypotensive peptide, on bovine aortic endothelial cells, was examined. The specific binding of adrenomedullin to these cells was observed, and adrenomedullin was found to induce intracellular cAMP accumulation in a dose-dependent m anner. EC(50) for the cAMP accumulation was about 100 times lower than the apparent IC50 for the binding assay. Adrenomedullin also induced increase of intracellular free Ca2+ in endothelial cells in a dose-dep endent manner. The Ca2+ response to adrenomedullin was biphasic with a n initial transient increase due to the release from thapsigargin-sens itive intracellular Ca2+ storage and a prolonged increase by influx th rough the ion channel on the plasma membrane. This intracellular free Ca2+ increase resulted from phospholipase C activation and inositol 1, 4,5-trisphosphate formation, and seemed to cause nitric oxide synthase activation by monitoring intracellular cGMP accumulation. Both cAMP a ccumulation and Ca2+ increased responses to adrenomedullin were mediat ed by cholera toxin-sensitive G protein, but the two signal transducti on pathways were independent. Thus, the results suggest that adrenomed ullin elicits the hypotensive effect through at least two mechanisms, a direct action on vascular smooth muscle cells to increase intracellu lar cAMP and an action on endothelial cells to stimulate nitric oxide release, with both leading to vascular relaxation.