Depressor Role of Angiotensin AT2 Receptors in the (mRen-2)27 Transgenic Rat

The (mRen-2)27 transgenic rat (Tg+), a hypertensive model dependent on increased expression of the renin angiotensin system, was used to explore the role of angiotensin AT2 receptors in the control of cardiovascular and renal excretory function. Experiments tested the effect of blockade of AT2 receptors on basal blood pressure and the pressor, renal excretory, and vasopressin (VP) responses to intravenous hypertonic saline (HS). Chronically catheterized male Tg+ and normotensive Sprague-Dawley rats (Tg−) were housed in metabolic cages. PD123319 (AT2 antagonist) or 0.9% NaCl was given by intravenous bolus (3 mg/kg) followed by infusion (50 μg/kg/min). Blockade of AT2 receptors both in Tg+ and Tg− rats produced no change in basal mean arterial pressure (MAP). The pressor response to intravenous HS (10% NaCl; 325 μL/100 g body weight) was significantly greater in Tg+ than in Tg− rats. PD123319 did not affect the peak rise in MAP but extended the time course of the response only in Tg+ rats. MAP was increased 39 ± 4 and 36 ± 3 mm Hg in Tg+ rats with and without the antagonist as compared to 20 ± 2 and 24 ± 2 mm Hg in Tg− rats. In the antagonist-treated Tg+ rats, MAP remained elevated for 60 min as compared to 5 min for Tg+ control or Tg− control or antagonist-treated rats. Hypertonic saline caused similar increases in plasma Na, VP, and in the natriuretic and diuretic responses in both Tg+ and Tg− rats, with no effect of antagonist treatment. These results demonstrate that Tg+ rats are sensitive to the effects of peripheral osmotic stimulation showing an increased pressor response, not attributed to greater secretion of VP or diminished natriuresis. These data also suggest that angiotensin AT2 receptors play a depressor role in the sodium-induced pressor response in this model.