Adrenomedullin induces endothelium-dependent vasorelaxation via the phosphatidylinositol 3-kinase/Akt-dependent pathway in rat aorta

To study the mechanisms by which adrenomedullin (AM) induces endothelium-de pendent vasorelaxation, we examined whether AM-induced endothelium-dependen t vasodilation was mediated by the phosphatidylinositol 3-kinase (PI3K)/Akt -dependent pathway in rat aorta, because it was recently reported that PI3K /Akt was implicated in the activation of endothelial NO synthase, AM-induce d vasorelaxation in thoracic aorta with intact endothelium was inhibited by pretreatment with PI3K inhibitors to the same level as that in endothelium -denuded aorta. AM elicited Akt phosphorylation in a time- and dose-depende nt manner. AM-induced Akt phosphorylation was inhibited by pretreatment wit h a calmodulin-dependent protein kinase inhibitor as well as with PI3K inhi bitors. When an adenovirus construct expressing a dominant-negative Akt mut ant (Ad/dnAkt) was injected into abdominal aortas so that the mutant was ex pressed predominantly in the endothelium layer, AM-induced vasodilation was diminished to the same level as that in endothelium-denuded aortas. Finall y, AM-induced cGMP production, which was used as an indicator for NO produc tion, was suppressed by PI3K inhibition or by Ad/dnAkt infection into the e ndothelium. These results suggested that AM induced Act activation in the e ndothelium via the Ca2+/calmodulin-dependent pathway and that this was impl icated in the production of NO, which in turn induced endothelium-dependent vasodilation in rat aorta.