Osmotic Diuresis in the Unanesthetized Hydropenic Rat

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Abstract
The osmotic pressure of the plasma and urine of hydropenic female rats was determined by the freezing point technique during solute loading. The urinary osmolarity decreased asymptotically toward the plasma level when the rate of total solute excretion was increased by infusion of mannitol with or without vasopressin. The data can be quantitatively accounted for by assuming that urinary hypertonicity is achieved by reabsorption of solute-free water, up to a maximal rate of about 37 µl/min/100 cm2, from isotonic fluid present in the tubules distal to the sites of tubular solute transfer. Thus the rat resembles man and the dog in this respect. Similar results were obtained when the loading solute was NaCl, Na2SO4, PAH, creatinine or sucrose. Results during urea loading, however, constitute an exception to the general rule.

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