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Aquaporin-1 Is not Expressed in Descending Thin Limbs of Short-Loop Nephrons

Xiao-Yue Zhai^*,, Robert A. Fenton, Arne Andreasen, Jesper Skovhus Thomsen^|| and Erik I. Christensen^*

Departments of ^* Cell Biology; Neurobiology; and ^|| Connective Tissue Biology; The Water and Salt Research Center, Institute of Anatomy, University of Aarhus, Aarhus C, Denmark; and Department of Histology and Embryology, China Medical University, Shen Yang, China

Correspondence: Dr. Erik Ilsø Christensen, Department of Cell Biology, Institute of Anatomy, University of Aarhus, DK-8000 Århus C, Denmark. Phone: +45-89-42-30-57; Fax: +45-86-19-86-64; E-mail: eic{at}ana.au.dk

Received for publication January 16, 2007. Accepted for publication June 19, 2007.

In mammalian kidneys, aquaporin-1 is responsible for water reabsorption along the proximal tubule and is also thought to be involved in the concentration of urine that occurs in the medulla. It has been suggested, however, that aquaporin-1 is not expressed in the last part of the descending thin limbs of short loop nephrons in rats and mice, and its expression in this region in humans has not been studied. We examined the expression of aquaporin-1 and the urea transporter UT-A2 in serial sections of mouse nephrons in the inner stripe of the outer medulla using immunohistochemistry. In contrast to previous observations, we demonstrate a complete absence of aquaporin-1 along the entire length of descending thin limbs of 90% of short loop nephrons. Conversely, as expected, we identified aquaporin-1 in proximal tubules, descending thin limbs of long loop nephrons, and medullary descending vasa recta. We also observed this abrupt transition from aquaporin-1–positive proximal tubules to aquaporin-1–negative descending thin limbs of short loop nephrons in sections of human and rat kidneys. UT-A2 was restricted to the last 28% to 44% of the descending thin limbs of all short loop nephrons. Because the majority of nephrons are of the short loop variety, our findings suggest that the mechanisms of water transport in the descending thin limbs of short loop nephrons should be reevaluated. Likewise, the roles of aquaporin-1 and UT-A2 in the countercurrent multiplier and water conversation may need to be readdressed.

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Copyright © 2008 by the American Society of Nephrology. Online ISSN: 1533-3450 Print ISSN: 1046-6673