Differences between Saline and Mannitol Diuresis in

In the hydropenic intact animal, knowledge of solute and water transfers in the renal concen-trating operation may be obtained by utilizing osmotic diuresis (2-5). Changes in the urine to plasma osmotic ratio (Uosm/Poum) and tubular reabsorption of solute-free water (TcH2o) with in-creasing rates of total. solute clearance (Co.m) and urine flow (V) may give information pertain-ing to the limits of the mechanisms involved in the concentration of urine. Much of the data ob-tained from normal man with this technique has been derived from studies employing hypertonic mannitol as the loading solute (3, 5). These studies have characteristically shown that, during mannitol diuresis in hydropenia, TCH2O achieves an upper limit or maximum when Co.m exceeds 15 ml per minute and that urine osmolality ap-proaches the osmolality of plasma as a limit. If TCH2O is an index of water movement from the collecting duct into the hypertonic medullary interstitium, then it is probably a reflection of the rate of transport and accumulation of solute (chiefly sodium chloride) in the renal medulla. The observed upper limit to TCH2O with mannitol diuresis might indicate an intrinsic maximal ca-pacity for the active transport of sodium in the ascending limb of the loop of Henle (5, 6). This interpretation, however, is at variance with the observations made in micropuncture studies in rats during saline loading that fail to show any *Submitted for publication August 25, 1964; accepte