Effects of Aldosterone and Atrial Natriuretic Peptide on Water and Electrolyte Homeostasis of Sick Neonates

Abstract

ABSTRACT. To clarify fluid homeostasis in neonates, 27 sick neonates (51 had respiratory distress syndrome, 1 2 had hyperbilirubinemia, and nine were low birth wt infants) were studied on 13 to 1 5 days of age. The infants were stabilized and required neither ventilation nor intravenous fluids a t the time of study. All infants were breast fed, and their sodium intake ranged from 1.7 to 2.9 (2.2 f 0.3) mEq/kg/d. Gestational age ranged from 30 to 39 (34 f 3.4) wk and birth wt ranged from 1080 to 3280 (2,070 f 672) g. Creatinine clearance (Ccr), fractional excretion of sodium (FENa), fluid and electrolyte balance, plasma aldosterone and human atrial natriuretic peptide (ANP) concentrations were determined and analyzed according to conceptional age (CA) to clarify their maturational changes. None of the infants had a negative sodium balance or hyponatremia. Ccr remained almost constant until the 36th wk of CA, after which an abrupt increase occurred. FENa and plasma aldosterone level of infants less than the 32nd wk of CA were high (0.75 f 0.33%,2868 f 1153 pg/ mi, respectively) when compared with those in infants of the 3 3 to 34th wk of CA (0.47 f 0.12%, 1,663, f 488 pg/ ml, respectively). There was a negative correlation between FENa and plasma aldosterone level in all cases (p < 0.01).Plasma A N P level of all infants were higher than those of healthy adults (40 f 20 pg/ml). A good correlation was observed between Ccr and plasma A N P level in infants less than 36th wk of CA. These results show that the high plasma aldosterone concentrations possibly act to enhance the reabsorptive capacity of the distal nephron for sodium during development under conditions of low sodium intake and suggest that A N P may be released to control the extracellular space in premature infants with a limited Ccr. (Pediatr Res 25: 591-594,1989) Abbreviations ANP, a-human atrial natriuretic peptide CA, conceptional age Ccr, creatinine clearance ECV, extracellular fluid volume FENa, fractional excretion of sodium GFR, glomerular filtration rate LBW, low birth wt RAAS, renin-angiotensin-aldosterone system RDS, respiratory distress syndrome Plasma aldosterone concentration is relatively high in human newborn infants (1, 2). The cause of increased activity of the RAAS during the neonatal period is not clear. In the newborn the homeostatic function of the kidney may be limited (3). Although full-term infants are able to maintain the volume of body fluids within a narrow range, the physiologic limitations of the renal control of salt and water balance in low birth wt infants are not well established. In addition, recent reports demonstrated that ANP may play an important part in regulating water balance in the newborn period (4, 5). The purpose of this study was to clarify the roles of RAAS and ANP in neonatal fluid homeostasis. MATERIALS AND METHODSStudy protocol. A total of 27 hospitalized sick neonates (15 boys and 12 girls; six had RDS, 12 had hyperbilirubinemia, and nine were LBW infants) were the subjects of this study. Informed co...

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