Postnatal development of renin-angiotensin-aldosterone system, RAAS, in relation to electrolyte balance in premature infants

In an attempt to provide information about the role of RAAS in development of late hyponatremia in low-birthweight neonates, simultaneous measurement of plasma renin activity, (PRA), plasma aldosterone concentration (PA), and urinary aldosterone excretion (UAE) was made using RIA methods along with determination of Na and K balance weekly up to the 6th week of life. Seven healthy male infants with mean birthweight of 1580 g, range: 1160-1850 g, and mean gestational age of 31 weeks, range: 30-32 weeks, were selected for the study. Due to the increased urinary Na loss, negative Na balance developed in the first 2 weeks followed positive balance thereafter. PRA, PA, and UAE increased tremendously from the initially high values of 18.2 +/- 4.1 ng/ml/hr, 1.7 +/- 0.5 ng/ml, and 2.6 +/- 0.4 microgram/day, mean and SEM, to their maximum of 78.6 +/- 18.1 ng/ml/hr, p less than 0.01, 6.8 +/- 3.7 ng/ml, p less than 0.05, and 26.4 +/- 2.9 microgram/dayp less than 0.01, in the 3rd week, respectively. Later on, gradual declines occurred, however, PRA, PA, and UAE remained highly elevated even at the 6th week with values of 45.5 +/- 15 ng/ml/hr, 1.6 +/- 0.5 ng/ml, and 14.5 +/- 1.4 microgram/day, respectively. It is suggested that late hyponatremia of premature infants is due to tubular unresponsiveness to aldosterone and not to inadequate response of RAAS to stimulation.