Activity of the renin-angiotensin-aldosterone system in full-term newborn infants during the first week of life.

A study was performed in 7 healthy full-term newborn infants with mean birth weight of 3130 g (range, 2650-2850 g) and a mean gestational age of 38.8 weeks (range, 37-41 weeks) to investigate the postnatal changes of plasma renin activity (PRA), plasma aldosterone concentration (PA) and urinary aldosterone excretion (UAE) in relation to electrolyte balance. PRA, PA and UAE fell significantly from their initially high values of 28.36 +/- 3.6 ng/ml/hr 1.5 +/- 3 ng/ml and 7.1 +/- 1.9 microgram/day on the first day to 6.84 +/- 2.9 ng/ml/hr (p less than 0.01), 0.71 +/- 0.1 ng/ml (p less than 0.01) and 5.5 +/- 1.6 microgram/day (p less than 0.05), respectively, on the 5th day. No significant correlation was found between either PRA and PA or PA and UAE. PRA and UAE showed a significant negative correlation with dietary sodium intake, the other parameters of sodium and potassium homeostasis, however, had no apparent influence on the activity of RAAS. The initial high level and the rapid postnatal decline in RAAS activity has been attributed to perinatal stress imposed by labour and delivery and not to the changes in sodium or potassium balance in the early days of life.