STUDIES ON THE RENAL CONCENTRATING MECHANISM. IV. OSMOTIC DIURESIS*

Abstract
Renal concentrating ability during osmotic diuresis has been examined over a wide range of urine flow in hydropenic normal men. As urine flow (V) increased up to 20 ml/min./1.73 m2 there was a curvilinear increase in net water reabsorption (Tc H2O) and a curvilinear decrease in osmotic urine/ plasma (U/P) ratio, which fit the equation [image] = a V + b, where a and b are constants related to maximum net water reabsorption and maximum U/P ratio respectively. Data obtained with urea and mannitol loading at intermediate flows fit this equation and showed no difference between the responses to the 2 solutes. When urine flow was increased up to 40 ml/min./1.73 m2, this relationship was disrupted. Tc H2O usually decreased progressively with isotonic mannitol loading. Concentrating ability was greater with urea plus mannitol loading at these urine flows than with isotonic mannitol alone, while the response to hypertonic mannitol loading was intermediate.