Background: We have found that low-dose infusion of angiotensin II (Ang II) selectively potentiates the mean arterial pressure (MAP) response to phenylephrine in pentobarbital-anesthetized rabbits.
Objective: To examine whether endogenous Ang II levels in normotensive rabbits maintained on a normal-salt diet exert a potentiating effect on the MAP response to phenylephrine.
Methods: We compared the effects of enalaprilat (0.3 mg/kg per min for 5 min bolus, 1 mg/kg per h infusion), losartan (DuP753; 4 mg/kg bolus, 2 mg/kg per h infusion) and vehicle administration on the MAP response to infusions of phenylephrine that were increased incrementally (2.5, 5 and 10 micrograms/kg per min).
Results: Enalaprilat decreased MAP significantly, whereas no maintained change was observed with losartan or vehicle. Phenylephrine infusions elevated MAP significantly and dose-dependently in all of the rabbits studied, but this effect was attenuated significantly in the rabbits given losartan compared with in those given vehicle or enalaprilat. The heart rate responses were not significantly different among the three groups.
Conclusion: We conclude that inhibition of the renin-angiotensin system at two distinct sites results in different MAP responses to phenylephrine infusion.