Loss of α2B-adrenoceptors increases magnitude of hypertension following nitric oxide synthase inhibition

Abstract

Vascular α2B-adrenoceptors (α2B-AR) may mediate vasoconstriction and contribute to the development of hypertension. Therefore, we hypothesized that blood pressure would not increase as much in mice with mutated α2B-AR as in wild-type (WT) mice following nitric oxide (NO) synthase (NOS) inhibition with Nω-nitro-l-arginine (l-NNA, 250 mg/l in drinking water). Mean arterial pressure (MAP) was recorded in heterozygous (HET) α2B-AR knockout mice and WT littermates using telemetry devices for 7 control and 14 l-NNA treatment days. MAP in HET mice was increased significantly on treatment days 1 and 4 to 14, whereas MAP did not change in WT mice ( days 0 and 14 = 113 ± 3 and 114 ± 4 mmHg in WT, 108 ± 0.3 and 135 ± 13 mmHg in HET, P < 0.05). MAP was significantly higher in HET than in WT mice days 10 through 14 ( P < 0.05). Thus blood pressure increased more rather than less in mice with decreased α2B-AR expression. We therefore examined constrictor responses to phenylephrine (PE, 10−9 to 10−4 M) with and without NOS inhibition to determine basal NO contributions to arterial tone. In small pressurized mesenteric arteries (inner diameter = 177 ± 5 μm), PE constriction was decreased in untreated HET arteries compared with WT ( P < 0.05). l-NNA (100 μM) augmented PE constriction more in HET arteries than in WT arteries, and responses were not different between groups in the presence of l-NNA. Acetylcholine dilated preconstricted arteries from HET mice more than arteries from WT mice. Endothelial NOS expression was increased in HET compared with WT mesenteric arteries by Western analysis. Griess assay showed increased NOx concentrations in HET plasma compared with those in WT plasma. These data demonstrate that diminished α2B-AR expression increases the dependence of arterial pressure and vascular tone on NO production and that vascular α2B-AR either directly or indirectly regulates vascular endothelial NOS function.