Regulation of renal water excretion in newborn full-term infants

The renal response to low (45 ml/kg) and high (73 ml/kg) fluid intake was studied during an 8-hour period in healthy 3-4-day-old full-term infants. 20 infants received low fluid (LF) intake and 15 infants received high fluid (HF) intake. HF significantly increased urine flow and significantly decreased urine osmolality but did not influence glomerular filtration rate measured as the clearance of creatinine. Serum arginine vasopressin (s-AVP) was not different in the LF and HF groups and did not correlate to urine osmolality. Urinary sodium excretion was significantly correlated to the diuresis.

Following high fluid intake full-term infants are capable to adaptively excrete larger urine volumes and more dilute urine by mechanisms independent of AVP. S-AVP appears to relate differently to the state of hydration and to urine osmolality in infants than in adults.