Objective: Chronic feeding of a purified synthetic diet induces renin-angiotensin system-dependent moderate high blood pressure in normal Sprague-Dawley rats. The present study was designed to characterize the angiotensin II (Ang II) receptor type 2 (AT2)-specific mechanism of blood pressure regulation in these rats.
Methods: The effect of the AT2 receptor antagonist PD123319 (PD) on blood pressure was examined in vivo in synthetic diet-fed rats. Ang II-dependent contraction of aortic rings prepared from the synthetic diet-fed rats was also investigated.
Results: After 8 weeks of feeding the synthetic diet, the mean arterial pressure (MAP) was significantly elevated above levels measured in control rats (117 +/- 2 versus 102 +/- 3 mmHg, P < 0.05). Intravenous administration of PD to conscious hypertensive rats elicited an immediate dose-dependent increase in MAP that was sustained for approximately 7.4 min with 3 mg/kg PD. The angiotensin converting enzyme inhibitor captopril, but not the Ang II type 1 receptor blocker losartan, significantly attenuated the effect of PD on blood pressure. PD did not increase the plasma level of catecholamines. The PD-dependent blood pressure increase was not observed in normotensive control rats. Aortic ring assays revealed that functional activation of the AT2 receptor occurs only in the hypertensive rats, and this AT2 response is abolished by indomethacin (5 micromol/l) but not by Nomega-nitro-L-arginine methyl ester (100 Fmol/l).
Conclusion: These results clearly demonstrate that AT2 receptor-mediated blood pressure regulation is functional in this experimental model of hypertension. Furthermore, cyclooxygenase metabolites might be the key mediators for the AT2 receptor-mediated blood pressure-lowering action.