Hormones and Homeostatic Mechanisms 167 



of the kidneys to eliminate an administered water load 

 changes and its ability to concentrate increases. According 

 to Falk (1955), however, infant rats older than three days 

 already react to vasopressin by cessation of diuresis and an 

 increased excretion of chloride. As both authors use different 

 methods it seemed useful to study this problem first, using 

 several methods, and also to study the effect of vasopressin 

 on the elimination of sodium and potassium. Opinions on the 

 natriuretic effect of vasopressin also differ and we believe 

 that this is due to different methodological approaches. 

 Schaumann (1949) and Heller and Stephenson (1950) observed 

 that vasopressin decreases the excretion of sodium in adult 

 rats, while Sawyer (1952) observed an increased elimination 

 of this electrolyte. The former authors administered the 

 hormone at the same time as the water load. Sawyer first 

 slightly prehydrated his animals and then gave them the 

 hormone and the water load. According to Heller (1952) the 

 ability of the rat kidney to eliminate a water load changes at 

 the time of weaning. We therefore always used rats with a 

 water load. 



Infant rats were weaned on the 15th- 16th day after birth 

 and the whole litter left in one cage. They received a standard 

 synthetic diet without sodium chloride. They were allowed 

 to choose between water and a 3 per cent sodium chloride 

 solution. As we expected changes in the mechanisms studied 

 to occur at the end of the fourth week, infant animals aged 

 23 and 33 days were used. Loads of warm distilled water 

 were administered via a stomach tube in amounts of 4-5 

 ml./lOO g. body weight. Subcutaneously the animals received 

 saline (0-5 ml./lOO g. body weight) in which the substances 

 studied were dissolved. The elimination of a water load was 

 studied for three hours after its administration or, in the 

 case of vasopressin, for three hours from the first micturition. 

 Urine was collected at hourly intervals. The amount of urine, 

 together with the concentration of sodium and potassium, 

 was determined by use of a flame photometer. 



Adult rats rapidly excrete urine with a low content of 



