MINIMUM URINE FLOW-RATE DURING WATER-DEPRIVATION - IMPORTANCE OF THE NONUREA VERSUS TOTAL OSMOLALITY IN THE INNER MEDULLA

Antidiuretic hormone leads to an increase in the permeability for wate r and urea in the inner medullary collecting duct. Hence, urea may not be an ''effective'' osmole in the inner medulla during maximal renal water conservation. Accordingly, the purpose of this study was to eval uate whether differences in the rate of urea excretion would influence maximum renal water conservation in humans. In water-deprived rats, t he concentration of urea and total osmolality were somewhat higher in the urine exiting the inner medullary collecting duct than in intersti tial fluid obtained from the entire papillary tip. Nevertheless, the ' 'nonurea'' (total osmolality minus urea in millimolar terms) osmolalit y was virtually identical in both locations, Chronically fasted human subjects that were water-deprived for 16 h had a lower rate of urea ex cretion (71 +/- 7 versus 225 +/- 14 mu mol/min) and a somewhat lower u rine osmolality (745 +/- 53 versus 918 +/- 20 mosmol/kg H2O). Neverthe less, they had identical urine flow rates (0.5 +/- 0.01 and 0.5 +/- 0. 02 ml/min, respectively), and their nonurea osmolality also was simila r (587 +/- 25 and 475 +/- 14 mosmol/kg H2O, respectively) to the water -deprived normal subjects. The composition of their urine differed in that the principal nonurea osmoles became NH4+ and beta-hydroxybutyrat e rather than Na and Cl. During water deprivation in normal subjects, the ingestion of urea caused a twofold rise in urine flow rate, a fall in the nonurea osmolality, and a rise in the rate of excretion of non urea osmoles. The nonurea osmolality of the urine, and presumably the medullary interstitial fluid as well, was inversely related to the ure a excretion rate. In chronic fasting, the nature, but not the quantity , of nonurea osmoles changed. The similar minimum urine volume was pre dictable from an analysis based on nonurea osmole considerations.