Sprague-Dawley rats, each weighing 210 to 300g. In experimental animals, renal ischemia was induced as follows: the animals were anesthetized with so-dium pentobarbital (50 mg/kg, ip) and placed on a heated animal board. A catheter (PE-50) was secured in the right external jugular vein, a midline abdominal incision was made, and the abdominal aorta was exposed by a careful blunt dissection. Three minutes after the administration of heparin (500 mg/kg) via the jugular catheter, a vascular clamp was placed across the aorta proximal to the origin of the left renal artery. Because of the posi-tion of the celiac artery, a clamp could not be placed proximal to the origin of the right renal artery. Therefore, a silastic sling was looped around the right renal artery just distal to its origin from the aorta and placed on traction to occlude the right renal artery. In all experiments, great care was taken not to disturb the inferior vena cava or renal veins. In five animals, strontium-85 microspheres (15 2 s, 3M Corp) were injected into the root of the aorta, via the carotid artery, 30 mm after both renal vessels had been occluded. No radioactivity, above background count, was found in any of the ten kidneys. Thus, this technique resulted in bilater-al renal artery occlusion and ischemia without ma-nipulation of the renal veins. Inulin clearance (C1) and renal blood flow (RBF) were measured with 3H-methoxy-inulin. The animals were anesthetized (mactin, 80 to 100 mg/kg, ip), placed on a heated animal board to maintain temperature between 36.5 and 3750 C, a tracheostomy was performed, and polyethylene catheters (PE-50) were secured into the external jugular vein, left carotid artery, and bladder. After replacement of surgical losses with isotonic saline (2 to 3% body wt), a priming dose of 10 pCi of 3H-methoxy-inulin (New Eng. Nuclear Co.) was given and followed by a sustaining infusion of 10 pCi/hr in a volume of 1.2 ml. After a 45-mm equilibration period, inulin clearance was determined by the average of three 10-mm urine collections. Blood samples were obtained from the tail at the midpoint of each urine collection, and at the end of the third clearance period a sample was obtained from the left renal vein. The concentration of 3H-methoxy-inulin was deter-mined with a liquid scintillation counter, and the inulin clearance and renal blood flow were calculated by standard formulae [26]. Urinary osmolality (U0) and fractional excretion of sodium (FENa) were determined from specimens obtained during the clearance periods.

After completion of the clearance periods, total renal blood flow (RBF) was determined with radio-active microspheres. A small volume (0.1 cc) of strontium-85 microspheres (15 2 js, 3M Corp), which had been mixed for 3 mm by an ultrasonic dismembranator (Artek Systems) and which contained approximately 60,000 spheres, was injected into the root of the aorta via the left carotid artery, and simultaneously blood was withdrawn for 1 mm from the left femoral artery at a constant and fixed rate of 1.03 cc/mm. The animals were then sacri-ficed, the kidneys removed, and the activity (counts per minute) of the isotope in each kidney was deter-mined in a gamma counter (Packard). Total renal blood flow was determined as: counts per minute of whole kidney÷ counts per minute of femoral artery