Hypertension in mice with transgenic activation of the brain renin-angiotensin system is vasopressin dependent

Abstract

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.

Keywords: Vaprisol; antidiuretic hormone.

Fig. 1.

Elevated vasopressin in sRA mice. A: arginine vasopressin (AVP) immunoreactivity in the supraoptic (SON, top and middle rows, from four separate animals) and paraventricular (PVN, bottom row, from two separate animals) nuclei in female sRA and control animals. Note the increased numbers of strongly immunoreactive AVP neurons in the retrochiasmatic part of the SON in sRA animals. ON, optic tract; 3V, third ventricle. Bars = 200 μm. B: total immunoreactive cell fragments per side, greater than 10 μm in diameter, in four serial sections (spaced 200 μm apart) through the PVN and SON of littermate control and sRA mice (n = 3 females each group). C: plasma copeptin levels (n = 4 male + 4 female control, 4 male + 4 female sRA). D: urine copeptin concentration, total daily urine volume, and total daily copeptin loss into urine (n = 12 male + 5 female control, 10 male + 7 female sRA). All data are means ± SE. *P < 0.05 vs. control.

Fig. 2.

Blood pressure responses to vasopressin receptor antagonists. A: systolic blood pressure (BP), monitored by tail-cuff, at baseline and with 10 days of chronic subcutaneous infusion (22 ng/h) of the V_1A/V₂ nonpeptide antagonist conivaptan (n = 2 male + 4 female control, 2 male + 4 female sRA). Hourly telemetric blood pressure (B, MAP) and heart rate (C, HR) recordings for 3 days preceding and 18 days during subcutaneous infusion of the nonselective V_1A/V₂ receptor antagonist conivaptan (22 ng/h) in a female sRA mouse are shown. D: spontaneous ambulatory physical activity counts during conivaptan infusion experiment (in B and C). E: systolic BP, monitored by tail-cuff, at baseline and with 10 days of chronic subcutaneous infusion (22 ng/h) of the V₂-selective antagonist tolvaptan (n = 4 male + 5 female control, 4 male + 6 female sRA). F: hourly average radiotelemetric MAP recordings from (n = 4 female) sRA mice at baseline and after 10 days of subcutaneous tolvaptan infusion (Drug × Time, P = 0.029). All data are means ± SE. *P < 0.05 vs. control, †P < 0.05 vs. baseline sRA.

Fig. 3.

Vascular reactivity of abdominal aorta. A: maximum contractile response to 100 mmol/l KCl. B and C: relaxation responses to graded doses of acetylcholine and sodium nitroprusside after half-maximal contraction to PGF_2α. D–H: contractile responses to graded doses of arginine vasopressin, phenylephrine, endothelin-1, angiotensin II, and prostaglandin-F_2α (PGF_2α) (n = 6 male control, 5 male sRA). All data are means ± SE. *P < 0.05 vs. control.

Fig. 4.

Mesenteric artery vascular reactivity. A: maximum contractile response to 100 mmol/l KCl. B: contractile responses to graded doses of arginine vasopressin, phenylephrine, and endothelin-1 (n = 6 male control, 6 male sRA). C: external and lumen diameters, wall thickness, media-to-lumen ratio, and cross-sectional area of mesenteric arteries maintained at 75 mmHg lumen pressure, in calcium-free conditions. D: mesenteric artery mRNA expression of the AVP V_1A receptor, the endothelin-1 ET_A receptor, RGS2, and RGS5 (V_1A, RGS2, and RGS5; n = 4 male + 5 female control, 4 male + 3 female sRA. ET_A, n = 4 male control, 4 male sRA). All data are means ± SE. *P < 0.05 vs. control.

Fig. 5.

Serum electrolytes. A: serum sodium concentration. B: serum-ionized calcium concentration (baseline: n = 8 male and 12 female control, 5 male and 8 female sRA; tolvaptan: n = 4 male and 5 female control, 4 male and 6 female sRA). All data are means ± SE. *P < 0.05 vs. control, †P < 0.05 vs. baseline sRA.