Two separate groups (n = 8) of Brattleboro rats, chronically instrumented for the measurement of regional haemodynamics, underwent a 2-stage experimental protocol. Initially, animals had their drinking water removed and cardiovascular recordings were made every 30 min for the following 6 h. Then animals received either the AT1-receptor antagonist, losartan (3 mg kg-1, i.v., n = 8), or an initially equi-hypotensive dose of its active metabolite, EXP 3174 (1 mg kg-1, i.v., n = 8), and the resultant cardiovascular changes were monitored for a further 2 h. A third group of Brattleboro rats (n = 8) was water-deprived for 8 h to serve as a time control. 2. In all 3 groups of animals, mesenteric and hindquarters vasoconstriction (beginning approximately 30 and 120 min, respectively, after water was removed) occurred much earlier than changes in blood pressure, since increases in blood pressure were significant only after 5-6 h of water deprivation; renal perfusion was largely unchanged. The observation of a differential onset of haemodynamic changes (i.e. mesenteric > hindquarters >> renal) extends our previous findings, in which measurements were made only at the end of a 14 h period of water-deprivation. 3. When given after 6 h of water deprivation, losartan and EXP 3174 produced directionally similar, but temporally disparate, haemodynamic effects. EXP 3174 caused a depressor response associated with marked renal, mesenteric and hindquarters vasodilatations which were well maintained over 2 h. Losartan evoked similar changes to EXP 3174 in the first 5 min, but in contrast to EXP 3174, blood pressure showed some recovery and all 3 vascular conductance values returned to baseline (i.e. pre-drug)levels over the following 10-20 min. Thereafter, hypotension and renal, mesenteric and hindquarters vasodilatation again occurred, and these changes were maintained for the rest of the 2 h. However,compared with losartan, EXP 3174 caused significantly greater mesenteric and hindquarters vasodilatation,even at times when both compounds lowered blood pressure to the same extent.4. The biphasic cardiovascular response caused by losartan is consistent with the conversion of the parent compound to EXP 3174. Whether or not the enhanced vasodilator effect of EXP 3174 over losartan is related to pharmacodynamic differences (i.e., noncompetitive versus competitive antagonism,respectively), and/or to differences in the amount of EXP 3174 generated from losartan is not known at present.
