THE EFFECT OF HORMONES OF THE 



PITUITARY AND ADRENAL GLANDS ON THE 



ELIMINATION OF SODIUM, POTASSIUM AND 



A WATER LOAD IN INFANT RATS DURING 



THE WEANING PERIOD 



JiRi Krecek, Helena Dlouha, JiM Jelinek, 

 Jarmila KreCkova and Zdenek Vacek 



Department of Ontogenetic Physiology, Institute of Physiology, Czechoslovak 



Academy of Sciences, Prague, and Institute of Embryology of the 



Medical Faculty of Charles^ University, Prague 



HoMEOSTATic mcchanisms in infant animals differ from 

 those in adults of the same species. Mechanisms regulating the 

 metabolism of water and electrolytes change immediately 

 after birth, during the period the eyes open, at the time of 

 weaning, in connexion with sexual maturation and perhaps 

 also at other stages of postnatal development. In the present 

 paper we should like to draw attention to the time of weaning, 

 which seems to us to be one of the important stages in the 

 development of the regulation of water and electrolyte 

 metabolism. 



The preweaning period in rats is relatively long. Up to the 

 14th day of life infant rats cannot survive without the mother 

 rat. They are usually weaned at the end of the third week but 

 according to breeders natural weaning occurs only at the end 

 of the fourth week. This agrees with the development of 

 thermoregulation, for infant rats can survive very low 

 environmental temperatures without the mother only at the 

 end of the fourth week (Capek et ah, 1956). 



Up to the 14th or 18th day infant rats live on breast milk 

 only. This is the only source of water and electrolytes, if we 

 disregard the urine of litter-mates that is sometimes sucked 

 by the infant animals. From that time onward infant rats in 

 addition to breast milk also actively feed on solid food and 



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