Angiotensin III Depressor Action in the Conscious Rabbit Is Blocked by Losartan but not PD 123319

Vasodilator and vasodepressor properties of angiotensins have been reported, and mediation by prostaglandins or nitric oxide has been proposed. Other studies indicate that angiotensin AT2 receptors might mediate a depressor action, and the present study was designed to delineate and explore this possibility in a conscious rabbit model. Large intravenous boluses of angiotensin III (15 nmol/kg) produced a predictable pressor peak (82±4 mm Hg) followed by a depressor phase (20±3 mm Hg), whereas equipressor doses of angiotensin II were less effective at producing depressor responses. Angiotensin-(1–7) did not exert a depressor action, and the reduced potency of angiotensin IV (relative to angiotensin III) was similar for both the pressor and depressor phases (≈100-fold). It is clear that specific angiotensin IV or angiotensin-(1–7) receptors do not mediate depressor effects in this model. The AT1 antagonist losartan (1 mg/kg) blocked both the pressor and depressor components of the angiotensin III response, whereas the AT2 antagonist PD 123319 (35 mg/kg) had no effect on either element of the response. The data obtained with the angiotensin receptor subtype–selective compounds, losartan and PD 123319, suggest that the depressor action is an AT1-mediated effect and give no indication that AT2 receptors could be involved. Paradoxically, the greater potency of angiotensin III as a vasodepressor belies the conclusion that the response is AT1-mediated, because AT1 receptors have a greater affinity for angiotensin II versus angiotensin III.