Urine Flow and Solute Excretion During Osmotic Diuresis

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Abstract
Expts. were performed on hydropenic dogs to study the urinary flow-load relationship when filtered load was varied by 2 methods: (a) stepwise increases in plasma mannitol concn. and (b) maintenance of a constant plasma mannitol concn. and reduction of glomerular filtration rate (GFR). These 2 methods gave results in essential agreement over a wide range of urine flows. At extreme reductions of GFR, some nephrons cease to function and the concentrating capacity of the kidneys is thereby reduced. Studies on the excretion of Na and water at normal and reduced levels of GFR were carried out on one group of animals during osmotic diuresis and on another group during the infusion of isotonic saline. The decrease in water excretion with reduction of GFR was less extensive in the osmotic diuresis expts. because of the barrier to water reabsorption imposed by the high concn. of unreabsorbable solutes. On the other hand, reduction in Na excretion was nearly as great in the osmotic diuresis expts. as in the saline expts. The Na concn. gradient between urine and plasma is variable during osmotic diuresis and increases when GFR decreases. There is no overall limiting concn. gradient to the reabsorption of Na during osmotic diuresis. The concept that a limiting concn. gradient can be assigned to the proximal tubule was questioned, and it was suggested that the increased excretion of Na during osmotic diuresis may possibly be related to the diminished time that a fluid of low Na concn. is in contact with the renal tubular cells.

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