Urea Reabsorption by the Proximal Tubule of the Dog. ∗

Abstract

Both micropuncture and tissue analytical techniques have been used extensively to study urea reabsorption by the renal tubule of the rat (1–3). These procedures have made it possible to quantitate urea reabsorption in various portions of the rat renal tubule. However, in the dog, the percentage contribution of various portions of the nephron to urea reabsorption has not yet been documented. For this reason, simultaneous renal micro-puncture and renal clearance techniques were used in the following experiments to study urea reabsorption by the proximal tubule of the dog.

Methods. Nine micropuncture experiments were performed on normal anti-diuretic dogs anesthetized with pentobarbital (30' mg/kg). The surgical procedure and the technique of renal micropuncture in this species have been described(4). After the urine flow has stabilized, serial urine samples of 15 minutes duration were obtained during the collection of samples of proximal tubular fluid. Blood was drawn at the mid-point of each collection period and concentrations of inulin and urea were determined. Five to seven samples of tubular fluid (0.1 to 0.3 μl) were obtained from surface convolutions of the proximal tubule during each study. Plasma and urine concentrations of inulin were measured by the anthrone method of Fuhr and co-work-ers(5). The urea concentration in proximal tubular fluid was determined in duplicate by an ultra-micro modification of the colori-metric method by Chaney and Marbach(6). This method involves the urease-induced liberation of ammonia from urea and subsequent formation of a blue color by the reaction of ammonia with phenol reagent in the presence of hypochorite and nitroprusside. The urea concentration of 24 samples of plasma ultrafiltrate determined by this method averaged 99.2 (S.D. ± 6.1%) of simultaneous values obtained with the Conway microdiffusion method.