Aquaporin‐1 is expressed in the thick ascending limb.

Abstract

The thick ascending limb (TAL) is the water‐impermeable segment of the nephron. Isolated TALs exhibit low luminal water permeability even in the presence of AVP, a characteristic that allows this segment to dilute the forming urine. Thus it is assumed that TALs do not express water channels. However, TALs rapidly shrink in response to increased basolateral osmolality suggesting the presence of water channels. We hypothesized that the TAL expresses AQP1. We measured AQP1 by: 1) Western blot, 2) RTPCR, and 3) immunofluorescence. By Western blot, homogenates of rat TAL suspensions exhibited two bands characteristic of glycosylated (0.8 ± 0.5 OD units) and unglycosylated (3.4 ± 0.5 OD units) AQP1 (n=5). To ensure that the signal was not due to other cell types in our preparation, we used microdissected TALs. AQP1 was detected in microdisected rat and mice TALs. No bands were detected in TALs from AQP1 ‐/‐ mice. By RTPCR, microdissected rat TALs, showed the predicted 430 bp and 603 bp products for AQP1 and the NaK2Cl co‐transporter (specific for TALs). Double immunofluorescence labeling showed both, basolateral localization of AQP1 and luminal localization for Tham Horsfall protein (specific for TALs). We conclude that AQP1 is expressed in the basolateral side of the TAL. Since the TAL is water impermeable, these data suggest AQP1 could be involved in transport of molecules other than water in this nephron segment.