INHIBITION OF THE HEMODYNAMIC-EFFECTS OF ANGIOTENSIN-II IN CONSCIOUS RATS BY AT2-RECEPTOR ANTAGONISTS GIVEN AFTER THE AT1-RECEPTOR ANTAGONIST, EXP 3174

1 Conscious, Long Evans rats (n = 10), chronically instrumented for the measurement of regional haemodynamics, were studied on 3 consecutive experimental days to assess responses to angiotensin II (AII) (I 25 pmol kg-1, i.v.) and noradrenaline (1 nmol kg-1, i.v.) in the absence and presence of the AT2-receptor antagonist, PD 123319 (10 mg kg-1, i.v.) (day 1), the AT1-receptor antagonist, EXP 3174 (1 mg kg-1, i.v.) (day 2), and PD 123319 (10 mg kg-1, i.v.) given 24 h after EXP 3174 (day 3). 2 In naive rats (day 1), PD 123319 did not antagonize the haemodynamic effects of AII or noradrenaline. EXP 3174 (day 2) caused a marked, prolonged blockade of the haemodynamic effects of AII but not those of noradrenaline. Twenty four h after administration of EXP 3174 (day 3) there was still significant attenuation of the haemodynamic effects of AII. However, administration of PD 123319 at this time caused a further inhibition (lasting 1 h) of the effects of AII but not those of noradrenaline. 3 An identical 3 day protocol was used in a separate group of rats (n = 6) in which the AT2-receptor antagonist, PD 123177, was given instead of PD 123319, and the results were essentially the same, i.e., PD 123177 significantly attenuated the haemodynamic effects of AII but only when given 24 h after EXP 3174. 4 In a separate group of rats (n = 4), a low dose of EXP 3174 (60 mug kg-1 i.v.) was given to naive rats in order to simulate the degree of inhibition of the effects of AII seen after administration of AT2-receptor antagonists in animals pretreated with EXP 3174. This low dose of EXP 3174 did not produce a sustained inhibition of the effects of AII and the time course of recovery of All responses was similar to that seen with PD 123319 or PD 123177 given after the high dose of EXP 3174. 5 The apparent inhibition of the effects of All by the AT2-receptor antagonists, PD 123319 and PD 123177, when these were administered 24 h after the AT1-receptor antagonist, EXP 3174, may have been due to the functional activation of AT2-receptors and/or loss of AT2-receptor antagonist selectivity, and/or the displacement of nonspecifically bound EXP 3174 by AT2-receptor antagonists. While the latter explanation seems the most likely, these results raise the possibility that nonpeptide, AII-receptor antagonists that act at both AT1- and AT2-receptors may have therapeutic advantages over selective AT1-receptor antagonists.