Requirement of aquaporin-1 for NaCl-driven water transport across descending vasa recta
Thomas L. Pallone, Aurélie Edwards, Tonghui Ma, Erik P. Silldorff, A.S. Verkman
Thomas L. Pallone, Aurélie Edwards, Tonghui Ma, Erik P. Silldorff, A.S. Verkman
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Requirement of aquaporin-1 for NaCl-driven water transport across descending vasa recta

Abstract

Deletion of AQP1 in mice results in diminished urinary concentrating ability, possibly related to reduced NaCl- and urea gradient–driven water transport across the outer medullary descending vasa recta (OMDVR). To quantify the role of AQP1 in OMDVR water transport, we measured osmotically driven water permeability in vitro in microperfused OMDVR from wild-type, AQP1 heterozygous, and AQP1 knockout mice. OMDVR diameters in AQP1–/– mice were 1.9-fold greater than in AQP1+/+ mice. Osmotic water permeability (Pf) in response to a 200 mM NaCl gradient (bath > lumen) was reduced about 2-fold in AQP1+/– mice and by more than 50-fold in AQP1–/– mice. Pf increased from 1015 to 2527 μm/s in AQP1+/+ mice and from 22 to 1104 μm/s in AQP1–/– mice when a raffinose rather than an NaCl gradient was used. This information, together with p-chloromercuribenzenesulfonate inhibition measurements, suggests that nearly all NaCl-driven water transport occurs by a transcellular route through AQP1, whereas raffinose-driven water transport also involves a parallel, AQP1-independent, mercurial-insensitive pathway. Interestingly, urea was also able to drive water movement across the AQP1-independent pathway. Diffusional permeabilities to small hydrophilic solutes were comparable in AQP1+/+ and AQP1–/– mice but higher than those previously measured in rats. In a mathematical model of the medullary microcirculation, deletion of AQP1 resulted in diminished concentrating ability due to enhancement of medullary blood flow, partially accounting for the observed urine-concentrating defect.

Authors

Thomas L. Pallone, Aurélie Edwards, Tonghui Ma, Erik P. Silldorff, A.S. Verkman

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Osmotic water transport across AQP1+/+ and AQP1–/– OMDVR wall. (a) Left:...
Osmotic water transport across AQP1+/+ and AQP1–/– OMDVR wall. (a) Left: Collectate fluorescence reversibly rose as raffinose or NaCl was added to and then removed from the bath of a microperfused AQP1+/+ OMDVR. Despite the much larger transmural NaCl gradient (400 mOsm/L), raffinose (200 mM) concentrated the FITCDx volume marker with equal effectiveness. After 30 minutes of incubation in 2 mM pCMBS, raffinose continued to drive water efflux, but NaCl was ineffective. Right: Collectate fluorescence reversibly rose as raffinose was added to and then removed from the bath, but NaCl was ineffective at driving water movement across the wall of a microperfused AQP1–/– OMDVR. Raffinose-driven water efflux was not reduced by pCMBS. Raf, raffinose. (b) The effect of pCMBS on Pf of AQP1+/+ OMDVR was tested. Paired measurements of Pf were obtained using transmural raffinose and NaCl gradients in random order. In all vessels, raffinose was more effective than was NaCl at inducing osmotic water movement (P < 0.05). After 30 minutes of incubation with pCMBS (2 mM), Pf measured with NaCl was reduced to nearly zero, but Pf measured with raffinose was only partly inhibited (P < 0.05 for both comparisons). A 5-minute treatment with DTT (5 mM) reversed the pCMBS effects. (c) The effect of pCMBS of inhibiting water flux across the AQP1–/– OMDVR wall was tested. Pf was measured in OMDVR from AQP1–/– mice as water efflux was driven by the addition of raffinose (200 mM) to the bath. Vessels were incubated for 30 minutes in pCMBS (2 mM, n = 7) or vehicle (n = 4). The pathway across which raffinose drives water efflux in AQP1–/– OMDVR wall is insensitive to mercurials.