Trophic effects induced by α 1D-adrenoceptors on endothelial cells are potentiated by hypoxia

Catecholamines have been shown to be involved in vascular remodeling through the stimulation of α₁-adrenoceptors (α₁-ARs). Recently, it has been demonstrated that catecholamines can stimulate angiogenesis in pathological conditions, even if the mechanisms and the AR subtypes involved still remain unclear. We investigated the influence of hypoxia (3% O₂) on the ability of picomolar concentrations of phenylephrine (PHE), which are unable to induce any vascular contraction, to induce a trophic effect in human endothelial cells through stimulation of the α_1D-subtype ARs. PHE, at picomolar concentrations, significantly promoted pseudocapillary formation from fragments of human mature vessels in vitro. Exposure to hypoxia significantly potentiated this effect, which was inhibited by the selective α_1D-AR antagonist BMY-7378 and by the nitric oxide synthase inhibitor L-NAME, suggesting that α_1D-ARs were involved in this effect through activation of the nitric oxide pathway. Proliferation and migration of HUVEC were also affected by picomolar PHE concentrations. Again, these effects were significantly potentiated in cells exposed to hypoxia and were inhibited by BMY-7378 and by N^G-nitro-L-arginine methyl ester. Conversely, the α_1A-AR-selective antagonist (S)-(+)-niguldipine hydrochloride and the α_1B-AR antagonist chloroethylclonidine dihydrochloride did not modify endothelial cell migration and proliferation in response to PHE. These results demonstrate that the stimulation of α_1D-ARs, triggered by picomolar PHE concentrations devoid of any contractile vascular effects, induces a proangiogenic phenotype in human endothelial cells that is enhanced in a hypoxic environment. The role of α_1D-ARs may become more prominent in the adaptive responses to hypoxic vasculature injury.