A Method for Assaying Small Amounts of Antidiuretic Substances With Notes on Some Properties of Vasopressin

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Abstract
An intravenous method of high precision for bioassay of antidiuretic substances at low concentrations is described. Sensitivity is high and occurrence of nonspecific antidiuresis is largely avoided. It uses the rat, maintained under constant water load and ethanol anesthesia. Antidiuretic activity of commercial vasopressin in solution, studied under varying conditions of temperature and pH, withstood short boiling at any pH but was reduced after longer boiling, especially in alkaline solutions. Loss of pressor activity did not parallel that of antidiuretic activity at all pH levels. Vasopressin solutions lyophilized or frozen for long periods lost no antidiuretic activity. There was appreciable loss of activity on standing at room temperature for 72 hours. Plasma samples containing vasopressin may be acidified, boiled and centrifuged while the antidiuretic activity remains intact in the supernatant fluid.