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 (U_NaV) [from 0.6 ± 0.01 to 0.05 ± 0.002 mM/day (P < 0.0001)] whereas neither SBP nor U_NaV 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 ≈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.

• blood pressure
• sodium
• bradykinin