RENAL WATER CHANNEL EXPRESSION IN NEWBORNS - MEASUREMENT OF URINARY-EXCRETION OF AQUAPORIN-2

Aquaporin-2 (AQP-2) encodes the vasopressin-regulated ''water channels '' of the renal collecting duct and is excreted in human urine. We mea sured urinary excretion of AQP-2 by radioimmunoassay in 15 term and 10 preterm infants on day 1 and day 4 of life to determine the molecular basis of water balance during the newborn period. AQP-2 was detectabl e in the urine of term and preterm newborns, but AQP-2 excretion was s everalfold less than the reported level in normal adults. Urinary excr etion of AQP-2 significantly decreased postnatally, in parallel with a reduction in urine osmolality and arginine vasopressin (AVP) excretio n. Urinary AQP-2 correlated positively and significantly with urine os molality on days 1 and 4 and with AVP on day 1 in both groups. No sign ificant differences were detected in AQP-2 levels between term and pre term newborns. Our findings suggest that vasopressin-regulated water c hannels are expressed in the renal collecting duct of both term and pr eterm newborns, although to a lesser extent as compared with adults, a nd these channels encoded by AQP-2 contribute to the urine concentrati ng power of the newborn kidney. Copyright (C) 1998 by W.B. Saunders Co mpany.