Postural natriuresis and urine osmotic concentration in hydropenic subjects

Abstract

When previously ambulatory, hydropenic subjects (with or without vasopressin) assume the supine position, there ensues a transient osmotic diuresis attributable primarily to increased excretion of sodium. The excretion of potassium and urea are also usually increased, but only secondarily, it appears, to the increased natriuresis. This osmotic diuresis is characterized by a constant urine osmotic concentration (with or without subtraction of urea) over a small but physiologically significant range of urine flow. It is postulated that as long as the rate of influx of solute-free water into the medullary interstitium (calculated as T H 2O c) is less than a critical value ( ca. 2 ml. per minute), the transport of sodium by the loop of Henle into the medullary interstitium is limited either by the concentration of sodium in the interstitium or by the concentration gradient between the urine in Henle's loop and the interstitium, with the consequence that the interstitial osmotic pressure, and hence the urine osmotic pressure, remains constant in the face of increasing urine flow and increasing influx of solute-free water (T H 2O c) into the medulla. It is suggested that when the rate of influx of solute-free water (T H 2O c) is critically increased, as during marked osmotic diuresis, the ‘concentration’ limitation in sodium transport by the loop gives way to a maximal rate limitation, and the rate of removal of solute-free water from the urine attains the approximal constant and maximal value (Tm H 2O c) previously described during mannitol diuresis.