THE MECHANISM OF POLYURIA OF DIABETES INSIPIDUS IN MAN. THE EFFECT OF OSMOTIC LOADING

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Abstract
Osmotic diuresis was produced by mannitol loading in 3 patients with diabetes insipidus. During hydropenia, urine flow was high, 7-10 cc./min., and osmolarity low, 50-100 mOsm/l. After loading, urine flow increased more than 2-fold, while osmolarity rose 4-fold, but remained hypotonic to plasma. For any given solute load, the rate of urine flow was 2-3 times that of normal subjects. After pitressin, the pattern of urine flow and solute excretion reverted to normal. The % composition of urinary solutes during loading was similar to that of normal subjects, suggesting that the solutes destined for excretion are assembled in a portion of the tubule proximal to the site of concn. or dilution of the urine. A tentative analysis of the data was made on the assumption that in diabetes insipidus there exists a basal dilute urine flow, possibly largely a secretion of the distal tubule. With loading, and increased amt. of proximal isotonic fluid is added to the urine without affecting the basal flow or being acted on by the distal tubule. The increment of urinary load divided by the plasma osmolarity, gives a calculated value for the added volume of proximal fluid. Calculated and observed flows were in reasonable agreement.