Losartan attenuates renal vasoconstriction in response to acute unilateral ureteral occlusion in pigs

Unilateral ureteral obstruction in pigs is associated with an enhanced, de novo generation of angiotensin II from the ipsilateral kidney. In order to further investigate the role of this system during unilateral ureter obstruction, the renal hemodynamic response to the non-peptide angiotensin II antagonist losartan was investigated. Danish land race pigs were operated on under general anesthesia. Catheters were placed in both renal veins by x-ray and ultrasonic flow probes were mounted on the renal arteries. Losartan (2 mg/kg/h) was administered intravenously to an experimental group ( n=9) continuously over 8 h of unilateral ureteral occlusion. This group was then compared to a matched control group which received only saline ( n=6). Ipsilateral pelvic pressure, renal blood flow using ultrasound transit time, glomerular filtration rate, mean arterial pressure and heart rate were measured. Renal handling of angiotensin II was examined by determining the renal extraction and secretion rates of immunoreactive angiotensin II. The anticipated reduction in ipsilateral renal blood flow after the onset of obstruction was attenuated in the losartan treated pigs, but the ipsilateral glomerular filtration rate was unaffected as compared with the controls. In the losartan group, the increase in renal vascular resistance was significantly reduced compared with un-treated controls (141+/-25% vs 217+/-24%, P<0.05). Plasma immunoreactive angiotensin II increased significantly from all three sample locations in both groups after the onset of obstruction, being more pronounced in the losartan treated group in which immunoreactive angiotensin II from the ipsilateral renal vein increased from 5.1+/-0.5 pmol/l to 41.6+/-19.6 pmol/l, P=0.027. In the controls immunoreactive angiotensin II increased from 2.7+/-0.3 pmol/l to 24.8+/-10.2 pmol/l. Furthermore, plasma aldosterone was significantly reduced after losartan administration (from 80.4 pmol/l to 36.0 pmol/l, P=0.005), indicating effective blockade of the angiotensin II type-1 receptor. The results from the present study suggest that continuous intravenous administration of losartan blocks the angiotensin II receptor mediated effects in the pig. Losartan is able to reduce ipsilateral vasoconstriction in the obstructed kidney during unilateral ureter obstruction supporting the view that angiotensin II is an important mediator of vasoconstriction during unilateral ureter obstruction in the pig model with acute unilateral occlusion of the ureter.