α_1D-Adrenoceptors Cause Endothelium-Dependent Vasodilatation in the Rat Mesenteric Vascular Bed

Abstract

The vasodilator activity of α₁-adrenoceptor agonists was tested in the rat mesenteric vascular bed (MVB), and the mechanism involved was investigated in cultured endothelial cells isolated from the bovine coronary vascular bed. In preparations preconstricted byU46619, noradrenaline and phenylephrine induced a slight relaxant effect at nanomolar concentrations. This effect was abolished in endothelium-denuded preparations and in preparations pretreated with 100 μM N^ω-nitro-l-arginine methyl ester plus 3 μM indomethacin. Both the phospholipase C inhibitor U73122 and the endoplasmic reticulum Ca²⁺-ATPase inhibitor thapsigargin inhibited the vasorelaxant effect of phenylephrine. The cellular level of inositol monophosphate (IP₁) in bovine endothelial cells doubled after a 15-min exposure to 0.03 to 0.1 nM phenylephrine. The activity of cNOS was significantly increased following exposure to the same concentrations of phenylephrine. Both chloroethylclonidine and the selective α_1D-adrenoceptor antagonist BMY 7378 reduced, in a concentration-dependent manner, the relaxant effect induced by phenylephrine, whereas the selective α_1A-adrenoceptor antagonist (+)-niguldipine was ineffective. BMY 7378 also blocked the cNOS activation induced by phenylephrine. Conversely, the increase in perfusion pressure induced by micromolar concentrations of phenylephrine was blocked by 1 nM (+)-niguldipine, but was unaffected by BMY 7378. These findings demonstrate that nanomolar concentrations of phenylephrine, which are devoid of any contractile effect, induced a slight endothelium-dependent vasorelaxation in the rat MVB through the stimulation of α_1D-adrenoceptors, located on endothelial cells, which act through phospholipase C stimulation, followed by IP₁ generation, and nitric-oxide synthase activation. Conversely, the increase in perfusion pressure induced by micromolar concentrations of phenylephrine is attributable to the stimulation of α_1A-adrenoceptors.