The Characteristics of Renal Hypoperfusion in Dogs with Acute and Chronic Reductions in Glomerular Filtration Rate as Disclosed by the Pattern of Water and Solute Excretion after Hypotonic Saline Infusions *

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Abstract
The effects of acute and chronic reductions in glomerular filtration rate on the urine excretion of solutes and water were studied in dogs during water diuresis and hypotonic saline diuresis. In animals with renal hypoperfusion, the different effects of water diuresis and hypotonic saline diuresis are attributable to differences in the relative rates of free water formation and free water back diffusion. During water diuresis, slowed tubular flow secondary to reduced GFR augments free water back-diffusion disproportionately to free water formation (since sodium is viritually absent from the urine), and urine osmolality rises although urine sodium concentration may not fall. During hypotonic saline diuresis, free water formation is relatively unrestricted (since sodium is now readily available for reabsorption in the diluting segment) and reduced tubular flow augments free water formation disproportionately to free water back-diffusion; consequently urine osmolality and urine sodium concentration fall.