POSTNANTAL 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 by 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 μg/day, mean and SEM, to their maximum of 78.6 ± 18.1 ng/ml/hr, P < 0.01,6.8 ± 3.7 ng/ml, P < 0.05, and 26.4 + 2.9 fig/ day, P < 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 jug/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. © 1979 International Pediatric Research Foundation, Inc.