Role of angiotensin AT1 and AT2 receptors in mediating the renal effects of angiotensin II in the anaesthetized dog

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Abstract
1 Experiments were performed using the selective AT1 receptor antagonist, GR117289, and the selective AT2 receptor antagonist, PD123177, to assess the relative importance of AT1 versus AT2 receptors in mediating the renal effects of angiotensin II (AII) in vivo, in salt-replete pentobarbitone-anaesthetized dogs. 2 The AT1 receptor antagonist, GR117289 (0.5 mg kg−1 + 1 μg kg−1 min−1, i.v.), caused renal vasodilatation, characterized by a mean increase of 21 ± 5% in renal blood flow, 45 min post-dose. GR117289 also caused a fall in mean blood pressure (12 ± 4%), but despite this, sodium and urine excretion were not reduced. Indeed, there was a tendency for urine output and sodium excretion to increase, although the changes were not statistically significant. GR117289 caused a reduction in plasma aldosterone levels (−35 ± 16%) 45 min post-dose, despite increasing plasma renin activity (+ 173 ± 42%). In contrast to GR117289, the AT2 receptor antagonist, PD123177 (20 μg kg−1 min−1 intra-renal artery; i.r.a.) caused no significant change in blood pressure, renal blood flow, or sodium and urine excretion, indicating that the renal effects of endogenous AII in these salt-replete animals are mediated predominantly by AT1 receptors. 3 Intra-renal artery infusion of AII (1–300 ng kg−1 min−1) caused dose-related renal vasoconstriction, and decreases in urine output, sodium excretion, fractional excretion of sodium, and glomerular filtration rate (GFR). The AT1 receptor antagonist, GR117289 (0.5 mg kg−1 + 1 μg kg−1 min−1, i.v.) antagonized these renal effects of AII, causing 15–38 fold rightward displacements of mean dose-response curves for these parameters. In contrast, PD123177 (20 μg kg−1 min−1, i.r.a.) failed to antagonize the renal haemodynamic and excretory effects of lower doses of AII (1–10 ng kg−1 min−1, i.r.a.). However, at higher doses of AII (30–300 ng kg−1 min−1, i.r.a.), while PD123177 still failed to antagonize the effects of the peptide on urine output, sodium excretion and GFR, it did cause a small, but significant, degree of inhibition of AII-induced renal vasoconstriction. In addition, at a higher dose (50 μg kg−1 min−1, i.r.a.), PD123177 caused a greater degree of antagonism of AII-induced renal vasoconstriction, while renal excretory responses to AII remained unaffected. 4 This study shows that the renal haemodynamic and excretory effects of AII in salt-replete anaesthetized dogs are mainly mediated by angiotensin AT1 receptors. However, the inhibitory effect of PD123177 on renal vasoconstrictor responses to high doses of AII, raises the possibility that functionally important AT2 receptors are present in the canine renal vasculature.