The Role of the Medullary Collecting Ducts in Postobstructive Diuresis H. SONNENBERG and D. R. WISON From the Departments of Physiology and Medicine, University of Toronto, Ontario, Canada A B S T R A C T Medullary collecting duct function was mal nephron; (b) the net addition of sodium to the studied by direct microcatheterization techniques in rats medullary collecting duct observed during postobstruc- undergoing postobstructive diuresis. Significant net ad- tive diuresis is probably a direct effect of obstruction, dition of water and sodium to the duct was demonstrated since it was found during postobstructive diuresis after during postobstructive diuresis after relief of 24-h bi- relief of bilateral or unilateral ureteral ligation, but not lateral ureteral ligation. This striking abnormality in with urine reinfusion alone; and (c) blood-borne fac- function was associated with reduced delivery of sodium tors are important in the development of postobstructive and water to the collecting duct compared to sham- natriuresis and diuresis, and probably act by increasing operated controls. To examine the role of circulating the fraction of filtered sodium and water delivered from factors in this phenomenon, another group of rats was the proximal and distal tubule to the collecting duct. studied that underwent 24 h of total urine reinfusion into the femoral vein. Natriuresis and diuresis were INTRODUCTION similar to the postobstructive group, but absolute collect- Decreased salt and water reabsorption in the proximal, ing duct reabsorption of sodium and water was normal. and particularly in the distal nephron, has been demon- The natriuresis and diuresis in rats with urine reinfu- strated during postobstructive diuresis by micropuncture sion resulted from increased delivery of fluid and sodium methods (1-3). Impaired reabsorption in the collecting to the medullary collecting duct. A third group of rats ducts and disproportionate impairment of function in the was studied with 24-h unilateral ureteral ligation as deep as compared to the superficial nephrons of the post- well as urine reinfusion from the contralateral normal obstructive kidney have also been suggested indirectly kidney. Without urine reinfusion there was no diuresis- by these studies. Blood-borne factors, including. accumu- natriuresis but with urine reinfusion the diuresis and lation of urea, probably have a major role in producing natriuresis after relief of unilateral obstruction was sim- postobstructive diuresis, as indicated by recent urine re- ilar to that after relief of bilateral obstruction. More- infusion (4) and cross-circulation experiments (5). over, net addition of sodium and no significant water The importance of the collecting ducts in determining reabsorption were demonstrated in the medullary collect- urinary excretion of salt and water during saline-induced ing duct of such animals. natriuresis has recently been demonstrated by Sonnen- The results indicate that (a) the medullary collecting berg (6, 7), using a method of direct cannulation of the duct is the critical nephron segment affected by ureteral medullary collecting ducts (8) in the rat. This technique obstruction, since postobstructive diuresis occurred measures the contribution of both superficial and deep despite reduced delivery of fluid from the more proxi- nephrons to the final urine, thus eliminating any changes due to abnormalities in distribution of nephron function. This work was presented in part at the National Meeting Accordingly, it was decided to directly study collecting of the American Federation for Clinical Research in At- duct function in the postobstructive kidney after relief lantic City, May 1975 and published in abstract form of bilateral or unilateral ureteral ligation, and to further (Clin. Res. 23: 377A. 1975.) examine the role of circulating natriuretic factors in Dr. Wilson's address is: Division of Nephrology, Toronto General Hospital, Toronto, Ontario, Canada M5G 1L7. postobstructive diuresis. Received for publication 5 May 1975 and in revised fortn The results indicate that the marked diuresis-natriure- 5 February 1976. sis after relief of bilateral ureteral ligation is associated 1564 The Journal of Clinical Investigation Volume 57 June 1976-1564-1574 with net addition of sodium and water to the medullary ,lCi over the course of the experiment. A 40-min equilibra- collecting duct, presumably due to decreased reabsorp- tion was allowed before beginning the collection of urine and samples of collecting duct fluid. In both groups III and tion and increased back diffusion. An increase in the IV the patency of the ureterovenous shunt was confirmed fraction of filtered sodium remaining along the collecting before and after the experiment by injection of small vol- duct of the postobstructive kidney was found despite umes of dyed saline into the reinfusion line. Several rats reduced absolute delivery of fluid to the duct. Unilateral could not be used for experiments because urine flow in urine reinfusion from the contra- the reinfusion line had stopped during the previous 24 h. ureteral ligation with However, cessation of urine flow in the bladder-femoral vein lateral kidney produced a similar diuresis-natriuresis and (reinfusion) line did not occur during any of the experi- net addition of sodium to the medullary collecting duct. ments. Urine was collected from the left kidney for 7-10 Total urine reinfusion for 24 h also resulted in a marked consecutive 20-min periods and arterial blood samples (0.05 diuresis-natriuresis but was associated with increased ml) were taken at the midpoint of each period. The technique of sampling of collecting duct fluid was delivery of fluid to the collecting duct and no intrinsic as described previously (6, 7). Briefly, fine polyethylene abnormality in duct function. catheters (outside diameter 16-40 ,um) were inserted to We conclude (a) that the medullary collecting duct varying distances into different collecting ducts via the is the critical nephron segment affected by ureteral ob- previously exposed papilla tip. It was occasionally possible to obtain paired samples in the same collecting duct system struction; (b) that the net addition of sodium to the from near the papillary tip and deep in the medulla. In collecting duct during postobstructive diuresis is prob- each animal an average of 12 samples of fluid (range 5-16) ably a direct effect of obstruction, and (c) that circu- were obtained with controlled suction just sufficient to lating factors are important in the development of post- overcome the tip resistance of the catheter (6). Previous diuresis and probably act by decreasing frac- experiments have shown that deliberate alterations in flow obstructive dynamics in the catheterized duct, by increasing or de- tional reabsorption in the nephron proximal to the col- creasing suction, could produce detectable changes in re- lecting duct. absorption, but such changes were not statistically sig- nificant and did not obscure net sodium and water secretion METHODS in the medullary duct during intravenous fluid loading (7). Male Sprague-Dawley rats (weight range 202-278 g) were The depth of insertion of the catheter was measured during maintained on Purina lab chow (Ralston Purina Co., St. withdrawal with a micrometer and was related to medullary Louis, Mo.) and water ad lib. 24 h before an experiment, length obtained from a post-mortem saggital section of animals were lightly anesthetized with pentobarbital and the kidney. subjected to one of four surgical procedures as follows: Sodium and potassium concentrations in plasma and urine sham operation (group I), both ureters were exposed but were determined by flame photometry and [3H] inulin de- not ligated through a midline suprapubic incision; bilateral termination was by liquid scintillation counting in a toluene- ureteral ligation (group II), both ureters were ligated based scintillant. Urinary excretion of sodium (UNaV) and through a midline suprapubic incision; urine reinfusion potassium (UKV) were calculated, as was glomerular fil- (group III), the bladder was cannulated through a midline tration rate. Sodium and potassium concentration in por- incision and the catheter was passed through the body wall tions (10 nl) of tubular fluid were determined in an Aminco in the groin area and inserted into a femoral vein; uni- helium glow photometer (American Instrument Co., Tra- lateral ureteral ligation with urine reinfusion (group IV), venol Laboratories Inc., Silver Spring, Md., and [3H] inulin the left ureter was ligated, and a bladder catheter connected (30 nl) by liquid scintillation counting. A Clifton nanoliter to a femoral vein as in group II to allow reinfusion of osmometer (Clifton Technical Physics, Hartford, N. Y.) urine from the right, nonobstructed kidney. Rats with uni- was used to measure total solute concentration in tubular lateral ureteral ligation were also prepared but insufficient fluid and urine samples. For each tubular fluid sample, the urine flow was obtained from left kidney to conduct satis- fluid to plasma concentration ratio of inulin (TF/Pi.) was factory collecting duct studies. Subsequently, animals of calculated, as were the fractions of filtered sodium ([TF/ the four groups were not allowed food or water and lost PNJ]/[TF/PIn]) and potassium ([TF/PK/[TF/Pin]) re- 6-10% of body weight before the experiment. maining at the collection site. Linear regression and t test 24 h after surgical preparations, rats were anesthetized analyses were used for statistical evaluation of data. with Inactin (Promonta, Hamburg, W. Germany) (10 mg/ 100 g body wt, i.p.), a tracheostomy was performed, and RESULTS a jugular vein and femoral artery were cannulated for in- Clearance The clearance results and the ex- fusion and for blood pressure measurement and sampling, data. respectively. After a priming dose of 1.5 ml of Ringer's cretion of solute, sodium, potassium, and water in the solution, a constant intravenous infusion (1.5 ml/h) was four experimental groups are shown in Table I. Glo- maintained. The left kidney was exposed and mobilized merular filtration rate was markedly decreased in the through a flank incision, and placed in a Lucite cup.