Renal Function in Man during Transition from Hydropenia to Water Diuresis with Superimposed Mannitol Load

Renal function studies have been carried out in seven healthy volunteers during the transition from hydropenia to water diuresis with superimposed mannitol load. The glomerular filtration rate increased significantly during the transition from hydropenia to water diuresis, hut remained unchanged when hypotonic mannitol was infused during water diuresis. During osmotic diuresis in hydrated man the free water formation at first increased as a function of increased osmotic load demonstrating a reserve capacity of the distal tubular Na+ reabsorption. At the osmolality clearance exceeding 8 ml/ 100 ml GFR/min however, the free water formation appeared to level off. It is suggested that this level was reached only because of a general Na+ depletion. By comparing the data on urine osmolality, urine flow and osmolar clearance in the same individual during the transition from hydropenia to water diuresis further information could he gained on distal tubular function. It was thus found that the actual dilution of urine exceeded the expected dilution of urine in all individuals even when discrepancies in osmolar clearance were corrected for. Those results were interpreted to mean an increased water permeability in distal tubule of the human nephron during hydropenia.