EFFECTS OF REDUCED GLOMERULAR FILTRATION ON URINE CONCENTRATION IN THE PRESENCE OF ANTIDIURETIC HORMONE*

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Abstract
The effects of reductions in glomerular filtration rate (GFR) on the elaboration of hypertonic urine have been studied in dogs in which the GFR was decreased reversibly in one kidney and urine collected separately from each kidney. When the GFR of one kidney was lowered 10-30% in dehydrated dogs infused with vasopressin, the concentration of the urine from that kidney often became up to 40% greater than that from the control kidney. Regularly when the reduction of GFR was 30% or more at low solute flows, and sometimes with smaller reductions in GFR, urine osmolality fell markedly. Under these conditions, analysis of renal tissue showed large decreases in urea concentration and moderate decreases in NaCl concentration in the medulla of the kidney whose GFR has been lowered. During urea or mannitol diuresis urine osmolality regularly increased, even when the GFR was reduced more than 60%. These data are consistent with the hypothesis that urine hypertonicity is developed by equilibration of fluid in the collecting ducts with hypertonic medullary interstitial fluid and that the medullary interstitial fluid is made hypertonic to plasma by deposition of Na salts from the loops of Henle. Accumulation of urea in the medulla, considered to be the result of passive diffusion from urea-permeable collecting ducts, makes possible the achievement of considerably higher osmolalities than could be developed in response to the interstitial Na salts alone.