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A B S T R A C T Recently we demonstrated potassium secretion by the pars recta or by the descending limb of the juxtamedullary nephron. The purpose of this present investigation is to study the effect of a chronic high-potassium intake on this phenomenon. Fractional reabsorption of water and sodium by the juxtamedullary proximal nephron was decreased when compared to that in normal hydropenic rats. There was a striking increase in the fraction of filtered potassium at the end of the juxtamedullary descending limb from 94+11% to 180+18%, which was principally a result of enhanced potassium secretion. When the concentration of potassium in the collecting tubule fluid of potassium-loaded rats was reduced after the administration of amiloride, a sharp fall was observed in the amount of potassium which reached the end of the descending limb (64+8%). A direct correlation was observed between the fraction of filtered potassium at the end of the descending limb and the potassium concentration in the final urine (P < 0.001). The findings suggest that potassium, like urea, normally undergoes medullary recycling, which is enhanced by chronic potassium loading.Portions of this work were presented to

A B S T R A C T The reabsorption of water and solute by the papillary collecting duct was studied during water diuresis and vasopressin-induced antidiuresis in young rats with hereditary hypothalamic diabetes insipidus. The tip of the left renal papilla was exposed and fluid was obtained by micropuncture from loops of Henle and from collecting ducts at the papillary tip, and at an average of 1 mm proximal to the tip. In water diuresis the ratio of tubule fluid to plasma (TF/P) osmolality (osm) of rats. Thus, fractional reabsorption (and very likely the absolute volume) of water reabsorbed by the terminal collecting duct was less in antidiuresis than in water diuresis (mean difference, 1.01% +0.29, P < 0.005).INTRODUCTION During water diuresis, fluid in the lumina of collecting tubules is hyposmotic to systemic plasma, but the renal medullary interstitium remains hyperosmotic (1-4). In antidiuresis, the permeability to water of the collecting tubule epithelium is increased by antidiuretic hormone (5, 6); as a result, more water is absorbed and the osmolality of fluid in the collecting tubules approaches equilibrium with that of the medullary interstitium. At the same time, the osmolality of the medullary interstitium also increases (1, 2, 7). These observations seem inconsistent: how can the medullary interstitium become progressively more hypertonic if, simultaneously, more water is reabsorbed from the collecting duct? Among several hypotheses (8-12) is the suggestion (11,12) that the quantity of water reabsorbed from the medullary collecting duct in antidiuresis is actually less than that reabsorbed in water diuresis. The recent discovery of a strain of rats with diabetes insipidus (13) coupled with the observation that the papilla in young rats is accessible to micropuncture (14), have made it possible to test that hypothesis. The results are reported in this paper.

A B S T R A C T Recent models of the urinary concentrating mechanism have postulated that urea in the medullary interstitium creates a transtubular concentration gradient for sodium between fluid at the end of the descending limb of Henle's loop and the medullary interstitium, favoring the passive outward movement of sodium from Henle's thin ascending limb. These experiments were designed to determine whether such a gradient normally exists. Young nondiuretic Munich-Wistar rats were prepared for micropuncture of the exposed left renal papilla. Samples of loop of Henle fluid and vasa recta plasma (assumed to reflect the composition of interstitial fluid) were obtained from adjacent sites. Loop fluid values in 21 comparisons from 18 rats (mean+SE)were: sodium, 344+ 12 meq/liter; potassium, 26+2 meq/liter; osmolality, 938+37 mosmol/kg H20. Vasa recta plasma values (in corresponding units ofmeasurement) were: sodium, 284±11; potassium, 34±2; osmolality, 935±34. Mean values of paired differences (loop fluid minus vasa recta plasma) were: A sodium, 60±11.1 (P < 0.001); A potassium, -8.0±2.1 (P < 0.001); A osmolality, 4±16 (NS). Corrected for plasma water, the loop fluid minus vasa recta differences (in milliequivalents per kilogram H20) were:A sodium, 40±11.4 (P < 0.005); A potassium, -9.7 ±1.9 (P < 0.001). We interpret these findings to indicate that in the papilla of nondiuretic rats, a significant difference in sodium concentration exists across the thin loop of Henle favoring outward movement of sodium, which confirms a key requirement of the pasPortions of this work were presented to the American Society for Clinical Investigation.

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