METHODS

Experiments were performed in 25 adult mutant Wistar rats weighing 172-362 g and allowed free access to food and water prior to study. Rats were anesthetized with Inactin (100 mg/kg) and prepared for micropuncture as described previously (4, 5, 7). Eleven rats underwent isoncotic plasma expansion prior to study. These animals received the intravenous infusion of a volume of homologous rat plasma (obtained by arterial exsanguination of a litter mate on the morning of study) equal to 2.5% body weight administered in a 90-min period beginning 120 min prior to micropuncture. The remaining 14 rats were studied without prior volume expansion. Pressures, jlows, and resistances prior to reductions in renal perfusion pressure. Beginning 60 min prior to micropuncture, all rats received an intravenous infusion of isotonic NaCl at the rate of 0.02 ml/min. Inulin was present in a concentration of 10%, thereby resulting in final plasma concentrations of about 100 mg/lOO ml. Mean femoral arterial pressure ((AP)) was monitored by means of an electronic transducer(model P23AA, Statham Instruments, Los Angeles, Calif.) connected to a direct-writing recorder (model 7702B, Hewlett-Packard Co., Palo Alto, Calif.). Late surface convolutions of proximal tubules were located by observing the passage of lissamine green dye which was injected rapidly (0.05 ml of a 5% solution) into a right jugular vein catheter. Following this 60-min equilibration period, exactly timed (l-2 min) samples of fluid were collected from each experimental tubule for determination of flow rate and inulin concentration and calculation of single nephron(SN) GFR. The rate of fluid collection was adjusted to maintain a column of polymer oil (Kel F polymer oil, Minnesota Mining and Manufacturing Co., St. Paul, Minn.), three to four tubule diameters in length, in a relatively constant position just distal to the site of puncture. Using the collection technique of controlled suction recently