Role of the alpha2B-adrenergic receptor in the development of salt-induced hypertension.

Salt sensitivity is a common trait in patients with essential hypertension and seems to have both an inherited and an acquired component (eg, is influenced by aging and renal insufficiency). Experimental evidence suggests that salt loading induces hypertension via a neurogenic mechanism mediated by the alpha2-adrenergic receptors (alpha2-AR). To explore the alpha2-AR subtype involved in this mechanism, we studied 2 groups of mice genetically engineered to be deficient in one of the 3 alpha2-AR subtype genes (either alpha2B-AR +/- or alpha2C-AR -/- knockout mice) compared with their wild-type counterparts. The mice (n=10 to 14 in each group) were submitted to subtotal nephrectomy and given 1% saline as drinking water for up to 35 days. Blood pressure (BP) was monitored by tail-cuff readings and confirmed at the end point by direct intra-arterial BP recording. The alpha2B-AR-deficient mice had an attenuated BP response in this protocol (baseline 101.8+/-2.7 versus end point 109.9+/-2.8 mm Hg), whereas the BP of their wild-type counterparts went from a baseline 101.9+/-2.3 to an end point 141.4+/-7.1 mm Hg. The other 2 groups had BP increases of 44. 6+/-5.17 and 46.7+/-7.01 mm Hg, with no difference between the mice deficient in the alpha2C-AR gene subtype versus their wild-type counterparts. Body weight, renal remnant weight, and residual renal function were no different among groups. These data suggest that a full complement of alpha2B-AR genes is necessary to raise BP in response to dietary salt loading, whereas complete absence of the alpha2C-AR subtype does not preclude salt-induced BP elevation. It is unclear whether the mechanism(s) involved in this process are of central origin (inability to increase sympathetic outflow), vascular origin (inability to vasoconstrict), or renal origin (inability to retain excess salt and fluid).