1. Changes in the urinary concentrations or urine/plasma (U/P) ratios of creatinine, urea, sodium and potassium were determined during the transition from water diuresis to antidiuresis when normal subjects and patients with diabetes insipidus were given injections of vasopressin.

2. In confirmation of previous work, it was found that after vasopressin administration the urinary concentration or U/P ratio of urea rose to a lesser degree than that of creatinine, indicating an increase in the tubular reabsorption of urea.

3. The urinary concentration, or U/P ratio, of sodium also rose to a lesser degree than that of creatinine. Thus vasopressin also increases the tubular reabsorption of sodium.

4. In contrast, the urinary concentration or U/P ratio of potassium tended to rise more than that of creatinine, indicating a decrease in the net tubular reabsorption of potassium.

5. Quantitative changes in the tubular handling of these urinary solutes were assessed by calculating the ratio—solute concentration during antidiuresis/solute concentration during diuresis—and then expressing this as a percentage of the corresponding ratio for creatinine. In subjects on a normal or high sodium intake the values thus obtained were: urea 72% (99% confidence limits 65–81%); sodium 79% (99% confidence limits 68–92%) and potassium 119% (99% confidence limits 102–139%).

6. In salt depleted subjects the values for these ratios were: urea 63% (99% confidence limits 60–65%) and sodium 36% (99% confidence limits 33–40%). Vasopressin had no consistent effect on potassium reabsorption in salt depleted subjects.

7. The effects of vasopressin on normal subjects and on patients with diabetes insipidus were similar.

8. It is suggested that these effects of vasopressin on sodium and potassium handling by the kidney occur in the collecting duct.

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