Low-renin hypertension accounts for approximately 25% of essential hypertensive patients. It is modeled in animals by chronic delivery of deoxycorticosterone acetate and excess dietary sodium (the DOCA-salt model). Previous studies have demonstrated that DOCA-salt hypertension is mediated through activation of the brain renin-angiotensin system. Here, we demonstrate robust metabolic phenotypes of DOCA-salt treatment. Male C57BL/6J mice (6–8 weeks old) received a subcutaneous pellet of deoxycorticosterone acetate (50 mg, 21-day), and were offered a 0.15 mol/L NaCl drink solution in addition to regular chow and tap water. Treatment resulted in mild hypertension, a blunting of weight gain, gross polydipsia, polyuria, and sodium intake, alterations in urinary sodium and potassium turnover, and serum sodium retention. Most strikingly, DOCA-salt mice exhibited no difference in food intake, but a large elevation in basal metabolic rate. Normalization of blood pressure by hydralazine (500 mg/L in drink solutions) attenuated the hydromineral phenotypes and renal renin suppression effects of DOCA-salt, but had no effect on the elevated metabolic rate. In contrast, intracerebroventricular infusion of the angiotensin II type 1 receptor antagonist, losartan (5 μg/hr), attenuated the elevation in metabolic rate with DOCA-salt treatment. Together, these data illustrate the necessity of angiotensinergic signaling within the brain, independent of blood pressure alterations, in the metabolic consequences of DOCA-salt treatment.

An indispensable role for the brain renin-angiotensin system (RAS) has been documented in most experimental animal models of hypertension. To identify the specific efferent pathway activated by the brain RAS that mediates hypertension, we examined the hypothesis that elevated arginine vasopressin (AVP) release is necessary for hypertension in a double-transgenic model of brain-specific RAS hyperactivity (the "sRA" mouse model). sRA mice experience elevated brain RAS activity due to human angiotensinogen expression plus neuron-specific human renin expression. Total daily loss of the 4-kDa AVP prosegment (copeptin) into urine was grossly elevated (≥8-fold). Immunohistochemical staining for AVP was increased in the supraoptic nucleus of sRA mice (~2-fold), but no quantitative difference in the paraventricular nucleus was observed. Chronic subcutaneous infusion of a nonselective AVP receptor antagonist conivaptan (YM-087, Vaprisol, 22 ng/h) or the V(2)-selective antagonist tolvaptan (OPC-41061, 22 ng/h) resulted in normalization of the baseline (~15 mmHg) hypertension in sRA mice. Abdominal aortas and second-order mesenteric arteries displayed AVP-specific desensitization, with minor or no changes in responses to phenylephrine and endothelin-1. Mesenteric arteries exhibited substantial reductions in V(1A) receptor mRNA, but no significant changes in V(2) receptor expression in kidney were observed. Chronic tolvaptan infusion also normalized the (5 mmol/l) hyponatremia of sRA mice. Together, these data support a major role for vasopressin in the hypertension of mice with brain-specific hyperactivity of the RAS and suggest a primary role of V(2) receptors.

Hypertension in many animal models is sensitive to inhibition of the brain renin-angiotensin system (RAS). We examined mice with transgenic hyperactivity of the brain RAS (sRA mice) to determine whether this manipulation is sufficient to cause hypertension, and to identify the causative mechanism. sRA mice exhibit brain-specific increases in angiotensin (ANG) peptide production through neuron-specific expression of human renin (synapsin promoter) and expression of human angiotensinogen through its own promoter. We determined both through tail-cuff and radiotelemetric methods that sRA mice are hypertensive (SBP; control 112±2 vs sRA 127±5 mmHg, P=0.02). Despite normal plasma osmolality and moderate hyponatremia, sRA mice exhibit double the number of vasopressin-expressing neurons in the supraoptic nucleus (P=0.002), as detected by immunohistochemistry. Plasma levels of the vasopressin pro-segment, copeptin, were reduced in sRA mice (140±19 vs 68±21 pg/mL, P=0.02), which correlated with severe polyuria (1.8±0.3 vs 12.3±1.6 mL/day, P<0.001). Indeed, total daily copeptin loss in the urine was significantly increased almost twenty-fold in sRA mice (7.9±4.3 vs 154.4±62.4 pg/day, P=0.03), highlighting an increase in vasopressin secretion per unit time. The baseline hypertension of sRA mice was completely reversed by chronic infusion of the dual V 1A / V 2 receptor antagonist, conivaptan (22 ng/hr, 10 days, s.c.; 113±5 mmHg, P=0.02). Preliminary experiments demonstrate that infusion of the selective V 2 receptor antagonist, tolvaptan (22 ng/hr, 10 days, s.c.), has similar effects. Further, while abdominal aorta and mesenteric arteries demonstrate selective desensitization to vasopressin and down-regulation of the V 1A receptor (38% of control, P<0.05), renal V 2 receptor expression remained normal in sRA mice. Together, these data demonstrate major roles for vasopressin and its V 2 receptor in the hypertension caused by elevated brain RAS activity.

Selective induction of the brain renin-angiotensin system (RAS) results in hypertension, polydipsia, and elevated resting metabolic rate. Transgenic sRA mice exhibit brain-specific RAS hyperactivity through expression of human renin via the synapsin promoter, and human angiotensinogen via its own promoter. The circulating RAS in these animals is suppressed due to chronic hypertension, and the elevated resting metabolic rate of sRA mice is sensitive to peripheral propranolol or angiotensin replacement. Cultured 3T3L1 adipocytes exhibited a dose-dependent induction of uncoupling protein 1 (UCP1) by the AT 2 receptor (AT 2 R) antagonist, PD-123,319, and suppression by the AT 2 R agonist, CGP-42112a (CGP). UCP1 mRNA was specifically induced within inguinal adipose of sRA mice (25-fold, P=0.02). Treatment of sRA mice with CGP (50 ng/kg/min, 8 weeks, s.c.) normalized metabolic rate (control 2.8±0.2, sRA 3.6±0.2, sRA+CGP 3.1±0.2 mL O 2 /100g/min) and inguinal UCP1 mRNA (sRA 24.6 vs sRA+CGP 4.6-fold of control, P=0.02), blood pH (control 7.26±0.02, sRA 7.37±0.03, sRA+CGP 7.28±0.06), bicarbonate (control 22±1, sRA 31±1, sRA+CGP 21±6 mM) and perigenital adipose mass (control 317±100, sRA 127±16, sRA+CGP 590±127 mg). Fluid and sodium intake, urine volume and electrolytes, and heart and kidney masses were different in sRA mice but unchanged by CGP treatment, and food intake was unchanged in all groups, highlighting the specificity of effects elicited by CGP. We conclude that AT 2 R activation blunts the thermogenic response to increased brain RAS activity. Interestingly, sRA gain weight at the same rate as control mice when fed a high fat diet, prompting the hypothesis that an elevated circulating and/or adipose RAS may result in attenuated thermogenic responses during obesity. This may suggest a positive-feedback loop between the circulating RAS and adiposity, and identify adipose AT 2 R as a novel therapeutic target for obesity.