Urinary aquaporin-2 excretion in preterm and full-term neonates.

The study was undertaken to define the role of aquaporin-2 (AQP2) in renal concentrating performance by measuring urinary AQP2 excretion and urine osmolality in healthy preterm and full-term neonates during early postnatal life. Random urine samples were obtained from 9 full-term newborn infants (mean birth weight 3,218 g, mean gestational age 39.2 weeks) at postnatal ages of 1, 3 and 5 days. Five premature infants with a mean birth weight of 1,570 g and mean gestational age of 30.6 weeks were studied at the end of the 1st week and then weekly up to the 6th week. Urine osmolality (Knauer osmometer), creatinine (modified Jaffé's method) and AQP2 concentrations (radioimmunoassay) were measured. In full-term neonates, urinary AQP2 excretion showed no consistent changes over the age period studied, while urine osmolality decreased significantly with advancing age. In premature infants, urinary AQP2 excretion remained practically unchanged during the first 4 weeks followed by an abrupt increase thereafter. Urine osmolality did not follow the developmental pattern of AQP2 excretion; its mean values varied only from 78 +/- 39 to 174 +/- 146 mosm/l during the experimental period. It is concluded that during the early postnatal period, urinary AQP2 excretion does not serve as a direct marker of the renal action of AVP and the renal capacity to concentrate urine.