Maturation of Renal Function in Infant Rats

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Abstract
Maturation of the renal responses of infant rats to water excesses, to water deprivation and to hyper-tonic solutions was studied. A diuretic response to water, absent at birth, developed gradually. Deprived of food and water, infant rats had comparable water losses, but urine was not concentrated as in adults. Urine, markedly hypertonic to plasma, was excreted by 3-week-old rats. Ability to excrete urine of high osmolarity was correlated with the ability to concentrate urine with respect to a given solute. Possible factors in the maturation of response to water were investigated. Water absorption from the alimentary tract in infants was significantly slower than in adults, but at all ages absorption was faster than excretion. Lower urine flows of infants could not be attributed to the lesser amount of functional renal tissue, since further reduction by unilateral nephrectomy did not alter normal response to hydration. Inulin clearances were low at birth, less than 1/5 on a body weight basis, or 1/20 on a body surface area basis, and rose most rapidly during first 2 weeks of life. By 3 days of age diuresis could be inhibited by Pitressin and by nicotine, the latter probably acting via the neurohypophysis. Adrenal cortical extract increased rate of elimination of water, but the character of diuresis remained infantile. The adrenal meduallary hormones imparted to the water diuresis of 2-day old rats the major adult characteristics. Experimental evidence was obtained to indicate that the sympathetic amines affected glomerular filtration and tubular reabsorption directly.