Evidence for a functional cardiac interaction between losartan and angiotensin‐(1–7) receptors revealed by orthostatic tilting test in rats

Studies have shown that the angiotensin II (Ang II) AT1 receptor antagonist, losartan, accentuates the orthostatic hypotensive response in anesthetized rats, and there is evidence indicating that this effect is not exclusively mediated by AT1 receptors. We investigated whether the pronounced orthostatic cardiovascular response observed in losartan-treated rats involves an interference with angiotensin-(1–7) (Ang-(1–7)) receptors. Urethane-anesthetized rats were submitted to orthostatic stress (90° head-up tilt for 2 min). Intravenous injection of losartan (1 mg kg−1, n=9) significantly accentuated the decrease in mean arterial pressure (MAP) induced by head-up tilt (−33±6% after losartan vs −15±8% control tilt). This effect was accompanied by a significant bradycardia (−8±3% after losartan vs −3±3% control tilt). Another AT1 antagonist, candesartan, did not potentiate the decrease of MAP and did not change the cardiac response induced by head-up tilt. Strikingly, administration of the Ang-(1–7) antagonist, A-779 (10 nmol kg−1, n=5), totally reversed the bradicardiac effect caused by losartan and this effect was accompanied by a tendency towards attenuation of the hypotensive response caused by losartan. These findings indicate that the marked orthostatic cardiovascular response is specific for losartan, and that it may be due, in part, to an interaction of this antagonist with Ang-(1–7) receptors, probably at the cardiac level. Keywords: Ang antagonists, ang receptors, tilt test, cardiovascular system Introduction Previous studies have shown that, in the rat model of orthostatic challenge, or tilting test, the angiotensin II (Ang II) AT1 receptor antagonist, losartan, produces a pronounced orthostatic hypotensive response (Ohlstein et al., 1992; Hashimoto et al., 1999). The same effect is not observed with other Ang II AT1 receptor antagonists (Ohlstein et al., 1992; Hashimoto et al., 1999), or even with converting enzyme inhibitors (Ohlstein et al., 1992). This suggests that, at least peripherally, the orthostatic hypotensive effect caused by losartan is beyond antagonism of Ang II AT1 receptor blockade. However, the mechanisms involved in mediating the orthostatic hypotensive effect of losartan in rats were not investigated in these previous studies (Ohlstein et al., 1992; Hashimoto et al., 1999). During the last decade, studies using the selective angiotensin-(1–7) (Ang-(1–7)) antagonist D-Ala7-Ang-(1–7) (A-779) (Santos et al., 1994) provided pharmacological evidence for an Ang-(1–7) receptor distinct from the classical Ang II receptors AT1 and AT2 (Fontes et al., 1994; Ferrario et al., 1998; Santos et al., 2000; Heringer-Walther et al., 2001). These findings were recently corroborated by the identification of an Ang-(1–7) receptor, the G protein-coupled receptor Mas (Santos et al., 2003). Von Bohlen Und Halbach et al. (2000) reported that an interaction between this Ang-(1–7) receptor and Ang II AT1 receptors exists. In addition, several previous studies have shown that, under certain conditions, Ang-(1–7) actions can be blocked by relatively low doses of losartan (Santos et al., 2000). Furthermore, reports suggesting a previously unsuspected nonspecificity for losartan provided additional arguments to the hypothesis that Ang-(1–7) receptors, in the kidney (Chansel et al., 1994; Gironacci et al., 1999) and perhaps in the heart (Gironacci et al., 1994), are sensitive to losartan. Therefore, the aim of the present study was to investigate whether the previously reported orthostatic hypotension caused by losartan involves AT1 receptor-independent mechanisms and specifically interaction with Ang-(1–7) receptors. For this purpose, the effects of losartan alone on the cardiovascular response to orthostatic tilt were compared with another AT1 antagonist, candesartan, and with that obtained in the presence of blockade of Ang-(1–7) receptors.