Washout of renal countercurrent Na gradient by osmotic diuresis

Abstract

The antidiuretic hormone regulates the ability of the kidney to concentrate urine. There are two theories as to the nature of the renal concentrating mechanism: 1) the existence of an active water pump in a distal area which removes free water from the urine and 2) the existence of a countercurrent multiplier system in the renal pyramid which allows for passive movement of water out of the collecting ducts in response to an osmotic gradient. During stop flow, the kidneys are unable to alter appreciably the total osmotic pressure of urine. Measurement of Na concentrations along the length of the renal pyramid may be used as an index of the magnitude of the countercurrent gradient. Such determinations were made before and during mannitol diuresis in dogs. An osmotic gradient along the pyramids was evident in control kidneys and disappeared upon establishment of osmotic diuresis. These data suggest that high rates of urine flow through the loop of Henle may wash out the pre-existing countercurrent gradient. This accounts for the inability of the kidneys to concentrate stop flow urine. It also demonstrates that osmotic diuresis, which is generally thought of as only a proximal event, must also result from diminished distal water reabsorption due to the washout of the countercurrent gradient.