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

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Abstract
To clarify fluid homeostasis in neonates, 27 sick neonates (51 had respiratory distress syndrome, 12 had hyperbilirubinemia, and nine were low birth wt infants) were studied on 13 to 15 days of age. The infants were stabilized and required neither ventilation nor intravenous fluids at the time of study. All infants were breast fed, and their sodium intake ranged from 1.7 to 2.9 (2.2 ± 0.3) mEq/kg/d. Gestational age ranged from 30 to 39 (34 ± 3.4) wk and birth wt ranged from 1080 to 3280 (2,070 ± 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 ± 0.33%, 2868 ± 1153 pg/ml, respectively) when compared with those in infants of the 33 to 34th wk of CA (0.47 ± 0.12%, 1,663, ± 488 pg/ml, respectively). There was a negative correlation between FENa and plasma aldosterone level in all cases (p<0.01). Plasma ANP level of all infants were higher than those of healthy adults (40 ± 20 pg/ml). A good correlation was observed between Ccr and plasma ANP 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 ANP may be released to control the extracellular space in premature infants with a limited Ccr.