Analysis of influence of extra- and intrarenally formed angiotensin II on renal blood flow.

Previous studies have indicated that intrarenal converting enzyme (CE) inhibition had no influence on renal blood flow (RBF). The present study examined the effect of intrarenal CE inhibition with captopril on the distribution of left (L) RBF in the anesthetized rabbit. Systemic arterial blood pressure and LRBF were measured in pentobarbital anesthetized rabbits, the latter by electromagnetic flowmetry. Radiolabeled 15-mu microspheres (85Sr and 141Ce) were used to determine RBF distribution. A dose of captopril that blocked the LRBF response to angiotensin I (ANG I), 0.3 microgram/kg/min i.a., but had no effect on the response to ANG I i.v. was established. Captopril administered i.a. at 0.2 microgram/kg/min for 20 min into the left renal artery decreased the response to ANG I i.a. by 71% without any effect on the response to ANG I i.v. The deep to superficial cortical blood flow ratio was unaffected by infusion of captopril i.a. in normal rabbits. In animals treated with furosemide, the ratio was increased by captopril infusion. However, the ratio change occurred in the infused and noninfused kidneys, negating a selective intrarenal effect. In the second part of this investigation the influence of a maximally effective i.a. dose of captopril was determined on intrarenal CE and RBF. The dose of 6.4 micrograms/kg/min of captopril increased LRBF slightly, but decreased the RBF response to ANG I i.a. only by 30%. Induction of a secondary pathway of intrarenal conversion of ANG I to ANG II is suggested by the latter results.