Methods

Animal model. The present studies were performed on male rats of the Munich-Wistar strain (original breeding stock obtained from Dr. Klaus Thurau, Physiologic Institute of Munich), ranging in weight from 165 to 230 g. Rats were maintained on standard chow (Purina) diets and on ad lib H20 intake until the time of study. This strain has several glomeruli on the surface of the kidney, accessible to micropuncture.

Animals were anesthetized with mactin (100 mg/kg ip) and a tracheostomy tube (PE 240) was inserted. Polyethylene catheters were placed in the left jugular vein (PE 50) for the infusion of solutions and into the left femoral artery for the monitoring of blood pressure with a pressure transducer (Statham), registered on a recorder (Statham). Urine was collected from the right kidney via a bladder catheter (PE 50). The animal was then turned onto the right side on a heated micropuncture table, and a subcostal flank incision was made. The adrenal gland, perirenal fat and connective tissue were carefully dissected from the left kidney so that the pedicle was not stretched; the kidney was then placed in a plastic cup (Lucite). Urine was collected from the left kidney by three methods in group 2: 1) placement of a PE 10 catheter into the ureter (rats 6 and 7); 2) placement of a PE 50 catheter into the ureter (rats 8, 9, 12 and 13); and 3) allowing normal drainage down the left ureter into the bladder (rats 2 and 4). In all group 1 animals, a PE 50 catheter was placed into the left ureter. The plastic cup was then lined and the base packed loosely with cotton. Clear agar at 37 to 39 C was placed around the kidney, leav-ing the dorsal surface exposed. The kidney was then covered with heated (37 C), normal NaCl—NaHCO3 solution.