Lower atrial natriuretic activity in the newborn versus adult rat

Experiments were designed to compare the natriuretic activity contained within extracts of the atrial myocardium of the newborn and adult rat. Extracts prepared from adult rats (greater than 8 weeks of age) contained significantly greater natriuretic activity than extracts from newborn preweanling pups (19 days of age). Since it has been reported that the primary site of inappropriate sodium reabsorption by the newborn kidney during saline expansion is in the distal nephron and since the atrial natriuretic factor (ANF has been reported to inhibit sodium reabsorption in this region of the nephron, results of the current study are consistent with the hypothesis that the compromised ability of the newborn to excrete a sodium load is related to a deficiency of ANF.