Role of the alpha(2B)-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 influ enced by aging and renal insufficiency). Experimental evidence suggests tha t salt loading induces hypertension via a neurogenic mechanism mediated by the alpha(2)-adrenergic receptors (alpha(2)-AR). To explore the (alpha(2)-A R subtype involved in this mechanism, we studied 2 groups of mice genetical ly engineered to be deficient in one of the 3 alpha(2)-AR subtype genes (ei ther alpha(2B)-AR +/- or alpha(2C)-AR -/- knockout mice) compared with thei r wild-type counterparts. The mice (n=10 to 14 in each group) were submitte d to subtotal nephrectomy and given 1% saline as drinking water for up to 3 5 days. Blood pressure (BP) was monitored by tail-cuff readings and confirm ed at the end point by direct intra-arterial BP recording. The alpha(2B)-AR -deficient mice had an attenuated BP response in this protocol (baseline 10 1.8+/-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 alpha(2C)-A R gene subtype versus their wild-type counterparts. Body weight, rend remna nt weight, and residual renal function were no different among groups. Thes e data suggest that a full complement of alpha(2B)-AR genes is necessary to raise BP in response to dietary salt loading, whereas complete absence of the alpha(2C)-AR subtype does not preclude salt-induced BP elevation. It is unclear whether the mechanism(s) involved in this process are of central o rigin (inability to increase sympathetic outflow), vascular origin (inabili ty to vasoconstrict), or renal origin (inability to retain excess salt and fluid).