Aquaporin-1 water channels in short and long loop descending thin limbs and in descending vasa recta in rat kidney. Nielsen, Sren, Thomas Pallone, Barbara L. Smith, Erik Ils Christensen, Peter Agre, and Arvid B. Maunsbach. Department of Cell Biology, Institute of Anatomy, University of Aarhus, DK-8000 Aarhus C, Denmark and Division of Nephrology, The M.S. Hershey Medical Center, Pennsylvania State University, Hershey, Pennsylvania 19033, Departments of Medicine and Biological Chemistry, Johns Hopkins University Medical School, Baltimore, MD 21205, U.S.A.
APStracts 2:0008F, 1995.
The localization of Aquaporin-1 water channels (AQP1) in nephron and vascular structures in rat kidney were characterized, since vascular bundles are known to play a key role in urinary concentration. Immunohistochemistry and immunoelectron microscopy were applied on thin cryosections or ultrathin Lowicryl-sections using an optimized freeze-substitution method. Within the vascular bundles, AQP1 is localized in descending thin limbs (DTL) of short nephrons in apical and basolateral membranes. The expression in DTL of short nephrons is considerably lower compared to the expression in long nephrons, consistent with the known lower osmotic water permeability of this segment. Furthermore, DTL of short nephrons expressing AQP1, continue abruptly into a thin limb segment without AQP1. This suggests the existence of a novel thin limb epithelium in the outer medulla. Extensive expression of AQP1 is observed in apical and basolateral membranes of DTL of long nephrons, which are localized in the periphery of the vascular bundles. The expression decreases along the axis of long nephron DTLs in correlation with the known water permeability characteristics of thin limb segments. DTLs of both short and long nephrons continue abruptly into thin limb segments without AQP1 expression, revealing an abrupt cell to cell transistion. In vasa recta, AQP1 is selectively localized in the non -fenestrated endothelium of descending vasa recta, whereas the fenestrated endothelium of ascending vasa recta and peritubular capillaries do not express AQP1. AQP1 is localized in both apical and basolateral plasma membranes, which is logical for transendothelial water transport. Isolated perfused descending vasa recta, display high water permeability, and unlike sodium permeability, diffusional water permeability is partly inhibited by mercurials, thus, substantiating the presence of mercurial sensitive water channels in descending vasa recta. Thus, AQP1 is localized in DTL and descending vasa recta within vascular bundles, and AQP1 expression in DTL segments are in exact concordance with the known water permeability characteristics, strongly supporting that AQP1 is the major constitutive water channel of the nephron. 

Received 28 September 1994; accepted in final form 8 December
1994.
APS Manuscript Number F348-4.
Article publication pending Am. J. Physiol. (Renal Fluid Electrolyte
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 February 1995.