Objective: The present study investigated the consequences of partial AT(1) receptor blockade on the participation of catecholamines, thromboxane A(2) (TXA(2)), and endothelin-1 (ET-1) in the renal vasoconstriction induced by angiotensin II (Ang II).
Methods: For this purpose, the increase in renal perfusion pressure (RPP) produced by Ang II was studied in isolated kidneys from untreated or irbesartan-treated Wistar Kyoto and spontaneously hypertensive rats (SHR), in absence or presence of the alfa-1 receptor antagonist, prazosin, the TXA(2) receptor antagonist, ifetroban, or the ET(A)/ET(B) receptor antagonist, PD145065.
Results: Systolic arterial pressure (SAP) was higher (P < 0.05) in SHR than in WKY. Increases in RPP produced by Ang II were comparable in kidneys from both untreated groups. Treatment with irbesartan reduced SAP and RPP in both strains in a comparable extent. Presence of prazosin, ifetroban, or PD145065 in perfusion media reduced (P < 0.05) Ang II maximal response in all groups. Maximal inhibition of Ang II-induced vasoconstriction produced by the 3 antagonists was comparable in untreated WKY, but that of ifetroban and PD145065 was lower (P < 0.05) than that of prazosin in untreated SHR. Maximal inhibition of Ang II-induced vasoconstriction produced by the 3 antagonists was comparable in WKY treated with irbesartan, and not different to that observed in untreated WKY. Maximal inhibitory effect of the 3 antagonists was higher (P < 0.05) in treated than in untreated SHR.
Conclusion: The study further supports the importance of catecholamines, TXA(2), and ET-1 as mediators of the renal vasoconstriction induced by Ang II in both normotensive and hypertensive rats. Hypertensive conditions appeared to reduce the participation of TXA(2) and ET-1 in Ang II-induced vasoconstriction when compared with normotensive conditions. Chronic partial blockade of AT(1) receptors did not affect the participation of catecholamines, TXA(2), and ET-1 in normotensive rats, but increased the participation of the 3 mediators in SHR. This suggests that when AT(1) receptors are partially blocked, other vasoconstrictor factors could exert part of the renal vasoconstrictor effects of Ang II.