Sustained hypersensitivity to angiotensin II and its mechanism in mice lacking the subtype-2 (AT2) angiotensin receptor

Abstract

The vast majority of the known biological effects of the renin-angiotensin system are mediated by the type-1 (AT1) receptor, and the functions of the type-2 (AT2) receptor are largely unknown. We investigated the role of the AT2 receptor in the vascular and renal responses to physiological increases in angiotensin II (ANG II) in mice with targeted deletion of the AT2 receptor gene. Mice lacking the AT2 receptor (AT2-null mice) had slightly elevated systolic blood pressure (SBP) compared with that of wild-type (WT) control mice (P < 0.0001). In AT2-null mice, infusion of ANG II (4 pmol/kg/min) for 7 days produced a marked and sustained increase in SBP [from 116 +/- 0.5 to 208 +/- 1 mmHg (P < 0.0001) (1 mmHg = 133 Pa)] and reduction in urinary sodium excretion (UNaV) [from 0.6 +/- 0.01 to 0.05 +/- 0.002 mM/day (P < 0.0001)] whereas neither SBP nor UNaV changed in WT mice. AT2-null mice had low basal levels of renal interstitial fluid bradykinin (BK), and cyclic guanosine 3', 5'-monophosphate, an index of nitric oxide production, compared with WT mice. In WT mice, dietary sodium restriction or ANG II infusion increased renal interstitial fluid BK, and cyclic guanosine 3', 5'-monophosphate by approximately 4-fold (P < 0.0001) whereas no changes were observed in AT2-null mice. These results demonstrate that the AT2 receptor is necessary for normal physiological responses of BK and nitric oxide to ANG II. Absence of the AT2 receptor leads to vascular and renal hypersensitivity to ANG II, including sustained antinatriuresis and hypertension. These results strongly suggest that the AT2 receptor plays a counterregulatory protective role mediated via BK and nitric oxide against the antinatriuretic and pressor actions of ANG II.

Figure 1

SBP (A), 24-h urine sodium excretion (U_NaV) (B), and 24-h urine volume (V) (C) of mice (n = 10) lacking the subtype-2 (AT₂) angiotensin receptor (open bars) and WT mice (n = 10; closed bars) on normal or low sodium intake. ∗, P < 0.01 from WT mice; +, P < 0.0001 from normal sodium intake.

Figure 2

RIF, BK (A), and cGMP (B) in mice (n = 10) lacking the subtype-2 (AT₂) angiotensin receptor (open bars) and WT mice (n = 10; closed bars) on normal or low sodium intake. ∗∗, P < 0.0001 from WT mice; +, P < 0.0001 from normal sodium intake.

Figure 3

SBP (A), 24-h urine sodium excretion (U_NaV) (B), and 24-h urine volume (UV) (C) of mice (n = 10) lacking the subtype-2 (AT₂) angiotensin receptor (squares) and WT mice (n = 10; triangles) during vehicle infusion (day 0), during sustained s.c. infusion of ANG II (days 1–7, solid line) and during vehicle (V) infusion in the postexperimental control period (days 8–14). Baseline data for AT₂-null mice infused with vehicle (V) throughout the 14-day period were 104 ± 3 mmHg (SBP), 0.23 ± 0.04 milliequivalent/24 h (U_NaV), and 2.4 ± 0.6 ml/24 h (UV). There was no change from baseline for any of these parameters on any day of the 14-day vehicle infusion (P was not significant). ∗, P < 0.0001 from vehicle control (day 0); +, P < 0.0001 from WT mice.

Figure 4

RIF, BK (A), and cGMP (B) in mice (n = 10) lacking the subtype-2 (AT₂) angiotensin receptor (open bars) and WT mice (n = 10; closed bars) during the vehicle control period (Control) and after 7 days of continuous infusion of ANG II. ∗, P < 0.0001 from WT mice; +, P < 0.0001 from vehicle control (day 0).