Postnatal changes in plasma and renal renin of the rat.

Plasma renin activity (PRA) of infant rats is high until some time between the 3rd and 4th week after birth. Mothers, however, return PRA to normal by 2 weeks post partum. The rate of disappearance endogenous PRA of nephrectomized rats is slower in those animals having gigh PRA than in mature rats. Disappearance curves of endogenous PRA of mothers post partum is also the same as that of normal mature females. Part of the high PRA of the neonate can thereby be the related to the lower rate of destruction. Total kidney renin content increases with age. Renal renin activity (RRA) is low only during the 1st postnatal week when expressed in terms of whole kidney weight and compared to later postnatal times. Although there are some slight differences in the means in older animals, none are statistically significant.