The perinatal expression of aquaporin-2 and aquaporin-3 in developing kidney

The kidney provides an important contribution to permit the fetus to successfully transition to an independent existence by production of urine with significantly different osmolality compared with plasma. Although recent work has uncovered many aspects of the maturation and regulation of the renal concentrating and diluting mechanism, understanding of how alterations in the expression of aquaporin (AQP) water channels contribute to the formation of urine in the perinatal period is incomplete. Here, we report that both AQP-2 and -3 are expressed during fetal life as early as embryonic d 18 in ureteric buds of rat kidneys, where each is localized to the apical and basolateral membranes of epithelial cells, respectively. Northern analyses demonstrate that the 1.9-kb AQP-2 transcript is present in fetal and postnatal rat kidneys similar to that observed in adults. AQP-2 mRNA expression increases after d 3 of postnatal life. Immunoblotting reveals an increase in total kidney AQP-2 protein particularly with respect to its glycosylated form after postnatal d 3. AQP-3 protein also exhibits a similar alteration likely due to a similar increase in its glycosylation state. Both AQP-2 and AQP-3 display a distribution in the collecting ducts of human postnatal infants and adults identical to that exhibited in rat kidneys. These data show that both AQP-2 and -3 are present in collecting duct epithelia of fetal and postnatal kidneys. Thus, the reduced AVP-responsiveness and decreased urinary concentrating ability of the kidney during the fetal and immediate postnatal period does not appear to be caused by lack of AQP-2 or AQP-3 proteins.