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Inhibition of the haemodynamic effects of angiotensin II in conscious rats by AT 2 ‐receptor antagonists given after the AT 1 ‐receptor antagonist, EXP 3174
Author(s) -
Widdop R.E.,
Gardiner S.M.,
Kemp P.A.,
Bennett T.
Publication year - 1992
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1992.tb14540.x
Subject(s) - medicine , endocrinology , hemodynamics , angiotensin ii , antagonist , chemistry , receptor antagonist , receptor
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) (125 pmol kg −1 , i.v.) and noradrenaline (1 nmol kg −1 , i.v.) in the absence and presence of the AT 2 ‐receptor antagonist, PD 123319 (10 mg kg −1 , i.v.) (day 1), the AT 1 ‐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 AT 2 ‐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 μg 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 AT 2 ‐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 AII 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 AII by the AT 2 ‐receptor antagonists, PD 123319 and PD 123177, when these were administered 24 h after the AT 1 ‐receptor antagonist, EXP 3174, may have been due to the functional activation of AT 2 ‐receptors and/or loss of AT 2 ‐receptor antagonist selectivity, and/or the displacement of nonspecifically bound EXP 3174 by AT 2 ‐receptor antagonists. While the latter explanation seems the most likely, these results raise the possibility that nonpeptide, AII‐receptor antagonists that act at both AT 1 ‐ and AT 2 ‐receptors may have therapeutic advantages over selective AT 1 ‐receptor antagonists.