Reduced osmotic water permeability of the peritoneal barrier in aquaporin-1 knockout mice

Aquaporin-1 (AQP1) water channels are expressed widely in epithelia and capillary endothelia involved in fluid transport. To test whether AQP1 facilitates water movement from capillaries into the peritoneal cavity, osmotically induced water transport rates were compared in AQP1 knockout [(−/−)], heterozygous [(+/−)], and wild-type [(+/+)] mice. In (+/+) mice, RT-PCR showed detectable transcripts for AQP1, AQP3, AQP4, AQP7, and AQP8. Immunofluorescence showed AQP1 protein in capillary endothelia and mesangium near the peritoneal surface and AQP4 in adherent muscle plasmalemma. For measurement of water transport, 2 ml of saline containing 300 mM sucrose (600 mosM) were infused rapidly into the peritoneal cavity via a catheter. Serial fluid samples (50 μl) were withdrawn over 60 min, with albumin as a volume marker. The albumin dilution data showed significantly decreased initial volume influx in AQP1 (−/−) mice: 101 ± 8, 107 ± 5, and 42 ± 4 (SE) μl/min in (+/+), (+/−), and (−/−) mice, respectively [ n = 6–10, P < 0.001, (−/−) vs. others]. Volume influx for AQP4 knockout mice was 100 ± 8 μl/min. In the absence of an osmotic gradient,3H2O uptake [half time = 2.3 and 2.2 min in (+/+) and (−/−) mice, respectively], [14C]urea uptake [half time = 7.9 and 7.7 min in (+/+) and (−/−) mice, respectively], and spontaneous isosmolar fluid absorption from the peritoneal cavity [0.47 ± 0.05 and 0.46 ± 0.04 ml/h in (+/+) and (−/−) mice, respectively] were not affected by AQP1 deletion. Therefore, AQP1 provides a major route for osmotically driven water transport across the peritoneal barrier in peritoneal dialysis.

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In vivo inhibition of transcellular water channels (aquaporin-1) during acute peritoneal dialysis in rats

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.1996.271.6.h2254 ◽

1996 ◽

Vol 271 (6) ◽

pp. H2254-H2262 ◽

Cited By ~ 10

Author(s):

O. Carlsson ◽

S. Nielsen ◽

el-R. Zakaria ◽

B. Rippe

Keyword(s):

Peritoneal Dialysis ◽

Water Transport ◽

Water Flow ◽

Peritoneal Cavity ◽

Critical Role ◽

Water Channels ◽

Capillary Endothelium ◽

Tissue Fixation ◽

Solute Permeability ◽

Aquaporin 1

During peritoneal dialysis (PD), a major portion of the osmotically induced water transport to the peritoneum can be predicted to occur through endothelial water-selective channels. Aquaporin-1 (AQP-1) has recently been recognized as the molecular correlate to such channels. Aquaporins can be inhibited by mercurials. In the present study, HgCl2 was applied locally to the peritoneal cavity in rats after short-term tissue fixation, used to protect the tissues from HgCl2 damage. Dianeal (3.86%) was employed as dialysis fluid, 125I-albumin as an intraperitoneal volume marker, and 51Cr-EDTA (constantly infused intravenously) to assess peritoneal small-solute permeability characteristics. Immunocytochemistry and immunoelectron microscopy revealed abundant AQP-1 labeling in capillary endothelium in peritoneal tissues, representing sites for HgCl2 inhibition of water transport. HgCl2 treatment reduced water flow and inhibited the sieving of Na+ without causing any untoward changes in microvascular permeability, compared with that of fixed control rats, in which the peritoneal cavity was exposed to tissue fixation alone. In fixed control rats, the mean intraperitoneal volume (IPV) increased from 20.5 +/- 0.15 to 25.0 +/- 0.52 ml in 60 min, whereas in the HgCl2-treated rats, the increment was only from 20.7 +/- 0.23 to 23.5 +/- 0.4 ml. In fixed control rats, the dialysate Na+ fell from 135.3 +/- 0.97 to 131.3 +/- 1.72 mM, whereas in the HgCl2-treated rats the dialysate Na+ concentration remained unchanged between 0 and 40 min, further supporting that water channels had been blocked. Computer simulations of peritoneal transport were compatible with a 66% inhibition of water flow through aquaporins. The observed HgCl2 inhibition of transcellular water channels strongly indicates a critical role of aquaporins in PD and provides evidence that water channels are crucial in transendothelial water transport when driven by crystalloid osmosis.

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Developmental changes in rabbit juxtamedullary proximal convoluted tubule water permeability

AJP Renal Physiology ◽

10.1152/ajprenal.1996.271.4.f871 ◽

1996 ◽

Vol 271 (4) ◽

pp. F871-F876 ◽

Cited By ~ 4

Author(s):

R. Quigley ◽

M. Baum

Keyword(s):

Proximal Tubule ◽

Water Transport ◽

Water Permeability ◽

Water Movement ◽

Adult Rabbit ◽

Osmotic Water ◽

Glomerular Filtrate ◽

Convoluted Tubules ◽

Insight Into

The mammalian proximal tubule reabsorbs the bulk of the glomerular filtrate in a nearly isosmotic fashion due to the high osmotic water permeability (Pf) of this segment. Although the characteristics of proximal tubule water transport have been studied in the adult proximal tubule, little is known about the neonatal segment. The present study directly measured the Pf and diffusional water permeability (PDW) of neonatal (10 +/- 2 day old) and adult rabbit juxtamedullary proximal convoluted tubules (PCT) using in vitro microperfusion. The Pf of neonatal juxtamedullary PCT was greater than the Pf of adult juxtamedullary PCT. In contrast, the PDW was not different between the two groups. The Pf and PDW values of both neonatal and adult tubules were inhibited to the same degree by p-chloromercuribenzene sulfonate and had identical activation energies. The transepithelial reflection coefficients of NaCl and NaHCO3 were also found to be similar in both the neonatal and adult proximal tubules. Thus neonatal and adult juxtamedullary PCT have many characteristics of water transport that are identical; however, neonatal Pf is three to five times that of the adult value. This difference in Pf with identical PDW values may give an insight into the transepithelial pathway for water movement in the neonatal tubule.

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New Insights into the Physiology of Peritoneal Fluid Transport

Peritoneal Dialysis International ◽

10.1177/089686080802803s27 ◽

2008 ◽

Vol 28 (3_suppl) ◽

pp. 144-149

Author(s):

Raymond T. Krediet ◽

Annemieke M. Coester ◽

Alena Parikova ◽

Watske Smit ◽

Dirk G. Struijk

Keyword(s):

Water Transport ◽

Fluid Transport ◽

Free Water ◽

Osmotic Gradient ◽

Ultrafiltration Failure ◽

Ultra Filtration ◽

Relationship Of ◽

The Relationship

A review is given on the mechanisms of free water transport, the various methodologies for its measurement, its dependency on the osmotic gradient, and the assessment of osmotic conductance in individual patients. The importance of impaired free water transport in long-term ultra-filtration failure is discussed, relative to peritoneal solute transport status. Furthermore, the relationship of free water transport with locally released potassium is considered, together with a potential role of impaired K+ channel function with peritoneal alterations. Finally, the role of impaired osmotic conductance to glucose and its effects on free water transport in long-term patients with ultrafiltration failure is reviewed.

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Evidence for transcellular osmotic water flow in rat proximal tubules

AJP Renal Physiology ◽

10.1152/ajprenal.1985.249.1.f124 ◽

1985 ◽

Vol 249 (1) ◽

pp. F124-F131 ◽

Cited By ~ 10

Author(s):

P. A. Preisig ◽

C. A. Berry

Keyword(s):

Surface Area ◽

Water Flow ◽

Water Permeability ◽

Channel Length ◽

Osmotic Gradient ◽

Osmotic Water Permeability ◽

Osmotic Water ◽

Major Route ◽

Osmotic Water Flow

To determine the predominant pathway for transepithelial osmotic water flow, the transepithelial osmotic water permeability [Pf(TE)] and the apparent dimensions of paracellular pores and slits were determined in rat proximal convoluted tubules microperfused in vivo. To measure Pf(TE), tubules were perfused with a hyposmotic, cyanide-containing solution. Pf(TE), calculated from the observed volume flux in response to the measured log mean osmotic gradient, was 0.12-0.15 cm/s, assuming sigmaNaCl equal to 1.0-0.7, respectively. The dimensions of the paracellular pathways were determined using measured sucrose and mannitol permeabilities (nonelectrolytes confined to the extracellular space). These were 0.43 and 0.87 X 10(-5) cm/s, respectively. By using the ratio of these permeabilities, their respective free solution diffusion coefficients and molecular radii, and the Renkin equation, the radius of the nonelectrolyte-permeable pores and the total pore area/cm2 surface area/channel length were calculated to be 1.4 nm and 3.56 cm-1, respectively. Similar calculations for slits yielded a slit half-width of 0.8 nm and a total slit area/cm2 surface area/channel length of 3.16 cm-1. The osmotic water permeability of these nonelectrolyte-permeable pathways was calculated by Poiseuille's law to be 0.0018 cm/s (pores) or 0.0014 cm/s (slits), at most 2% of Pf(TE). We conclude that the nonelectrolyte-permeable pathway in the tight junctions is not the major route of transepithelial osmotic water flow in the rat proximal tubule.

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Aquaporin-1 Facilitates Transmesothelial Water Permeability: In Vitro and Ex Vivo Evidence and Possible Implications in Peritoneal Dialysis

International Journal of Molecular Sciences ◽

10.3390/ijms222212535 ◽

2021 ◽

Vol 22 (22) ◽

pp. 12535

Author(s):

Francesca Piccapane ◽

Andrea Gerbino ◽

Monica Carmosino ◽

Serena Milano ◽

Arduino Arduini ◽

...

Keyword(s):

Plasma Membrane ◽

Peritoneal Dialysis ◽

Tight Junctions ◽

Water Transport ◽

Water Flux ◽

Water Channel ◽

Bathing Solution ◽

Ussing Chambers ◽

Aquaporin 1 ◽

Osmotic Water

We previously showed that mesothelial cells in human peritoneum express the water channel aquaporin 1 (AQP1) at the plasma membrane, suggesting that, although in a non-physiological context, it may facilitate osmotic water exchange during peritoneal dialysis (PD). According to the three-pore model that predicts the transport of water during PD, the endothelium of peritoneal capillaries is the major limiting barrier to water transport across peritoneum, assuming the functional role of the mesothelium, as a semipermeable barrier, to be negligible. We hypothesized that an intact mesothelial layer is poorly permeable to water unless AQP1 is expressed at the plasma membrane. To demonstrate that, we characterized an immortalized cell line of human mesothelium (HMC) and measured the osmotically-driven transmesothelial water flux in the absence or in the presence of AQP1. The presence of tight junctions between HMC was investigated by immunofluorescence. Bioelectrical parameters of HMC monolayers were studied by Ussing Chambers and transepithelial water transport was investigated by an electrophysiological approach based on measurements of TEA+ dilution in the apical bathing solution, through TEA+-sensitive microelectrodes. HMCs express Zo-1 and occludin at the tight junctions and a transepithelial vectorial Na+ transport. Real-time transmesothelial water flux, in response to an increase of osmolarity in the apical solution, indicated that, in the presence of AQP1, the rate of TEA+ dilution was up to four-fold higher than in its absence. Of note, we confirmed our data in isolated mouse mesentery patches, where we measured an AQP1-dependent transmesothelial osmotic water transport. These results suggest that the mesothelium may represent an additional selective barrier regulating water transport in PD through functional expression of the water channel AQP1.

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Knockdown of hepatocyte aquaporin-8 by RNA interference induces defective bile canalicular water transport

AJP Gastrointestinal and Liver Physiology ◽

10.1152/ajpgi.90410.2008 ◽

2009 ◽

Vol 296 (1) ◽

pp. G93-G100 ◽

Cited By ~ 32

Author(s):

M. Cecilia Larocca ◽

Leandro R. Soria ◽

M. Victoria Espelt ◽

Guillermo L. Lehmann ◽

Raúl A. Marinelli

Keyword(s):

Rna Interference ◽

Water Transport ◽

Hepg2 Cells ◽

Water Movement ◽

Rat Hepatocytes ◽

Conclusive Evidence ◽

Osmotic Gradient ◽

Bile Formation ◽

Canalicular Membrane ◽

Water Secretion

Aquaporin-8 (AQP8) water channels, which are expressed in rat hepatocyte bile canalicular membranes, are involved in water transport during bile formation. Nevertheless, there is no conclusive evidence that AQP8 mediates water secretion into the bile canaliculus. In this study, we directly evaluated whether AQP8 gene silencing by RNA interference inhibits canalicular water secretion in the human hepatocyte-derived cell line, HepG2. By RT-PCR and immunoblotting we found that HepG2 cells express AQP8 and by confocal immunofluorescence microscopy that it is localized intracellularly and on the canalicular membrane, as described in rat hepatocytes. We also verified the expression of AQP8 in normal human liver. Forty-eight hours after transfection of HepG2 cells with RNA duplexes targeting two different regions of human AQP8 molecule, the levels of AQP8 protein specifically decreased by 60–70%. We found that AQP8 knockdown cells showed a significant decline in the canalicular volume of ∼70% ( P < 0.01), suggesting an impairment in the basal (nonstimulated) canalicular water movement. We also found that the decreased AQP8 expression inhibited the canalicular water transport in response either to an inward osmotic gradient (−65%, P < 0.05) or to the bile secretory agonist dibutyryl cAMP (−80%, P < 0.05). Our data suggest that AQP8 plays a major role in water transport across canalicular membrane of HepG2 cells and support the notion that defective expression of AQP8 causes bile secretory dysfunction in human hepatocytes.

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Lung fluid transport in aquaporin-1 and aquaporin-4 knockout mice

Journal of Clinical Investigation ◽

10.1172/jci4138 ◽

1999 ◽

Vol 103 (4) ◽

pp. 555-561 ◽

Cited By ~ 173

Author(s):

Chunxue Bai ◽

Norimasa Fukuda ◽

Yualin Song ◽

Tonghui Ma ◽

Michael A. Matthay ◽

...

Keyword(s):

Knockout Mice ◽

Fluid Transport ◽

Aquaporin 4 ◽

Aquaporin 1 ◽

Lung Fluid

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Water transport in neonatal and adult rabbit proximal tubules

AJP Renal Physiology ◽

10.1152/ajprenal.00341.2001 ◽

2002 ◽

Vol 283 (2) ◽

pp. F280-F285 ◽

Cited By ~ 8

Author(s):

Raymond Quigley ◽

Michel Baum

Keyword(s):

Water Transport ◽

Water Permeability ◽

Water Movement ◽

Chloride Transport ◽

Basolateral Membrane ◽

Membrane Vesicles ◽

Proximal Tubules ◽

Adult Rabbit ◽

Osmotic Gradient ◽

Intracellular Compartment

We have recently demonstrated that although the osmotic water permeability ( P f) of neonatal proximal tubules is higher than that of adult tubules, the P f of brush-border and basolateral membrane vesicles from neonatal rabbits is lower than that of adults. The present study examined developmental changes in the water transport characteristics of proximal convoluted tubules (PCTs) in neonatal (9–16 days old) and adult rabbits to determine whether the intracellular compartment or paracellular pathway is responsible for the maturational difference in transepithelial water transport. The permeability of n-butanol was higher in the neonatal PCT than the adult PCT at all temperatures examined, whereas the diffusional water permeability was identical. Increasing the osmotic gradient increased volume absorption in both the neonatal and the adult PCT to the same degree. The P f was not different between the neonatal and the adult PCT at any osmotic gradient studied. To assess solvent drag as a measure of the paracellular transport of water, the effect of the osmotic gradient on mannitol and chloride transport were measured. There was no change in chloride or mannitol transport with the increased osmotic gradient in either group, indicating that there was no detectable paracellular water movement. In addition, the mannitol permeability of the neonatal PCT was found to be lower than that of the adult PCT with the isotonic bath (8.97 ± 4.01 vs. 40.49 ± 13.89 μm/s, P < 0.05). Thus the intracellular compartment of the neonatal PCT has a lower resistance for water transport than the adult PCT and is responsible for the higher than expected P f in the neonatal PCT.

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Very high aquaporin-1 facilitated water permeability in mouse gallbladder

AJP Gastrointestinal and Liver Physiology ◽

10.1152/ajpgi.90680.2008 ◽

2009 ◽

Vol 296 (4) ◽

pp. G816-G822 ◽

Cited By ~ 18

Author(s):

Lihua Li ◽

Hua Zhang ◽

Tonghui Ma ◽

A. S. Verkman

Keyword(s):

Bile Salt ◽

Water Transport ◽

Water Permeability ◽

Salt Concentration ◽

Knockout Mice ◽

Osmotic Gradient ◽

Gallbladder Epithelium ◽

Wild Type ◽

Bile Salt Concentration ◽

Very High

Water transport across gallbladder epithelium is driven by osmotic gradients generated from active salt absorption and secretion. Aquaporin (AQP) water channels have been proposed to facilitate transepithelial water transport in gallbladder and to modulate bile composition. We found strong AQP1 immunofluorescence at the apical membrane of mouse gallbladder epithelium. Transepithelial osmotic water permeability (Pf) was measured in freshly isolated gallbladder sacs from the kinetics of luminal calcein self-quenching in response to an osmotic gradient. Pf was very high (0.12 cm/s) in gallbladders from wild-type mice, cAMP independent, and independent of osmotic gradient size and direction. Although gallbladders from AQP1 knockout mice had similar size and morphology to those from wild-type mice, their Pf was reduced by ∼10-fold. Apical plasma membrane water permeability was greatly reduced in AQP1-deficient gallbladders, as measured by cytoplasmic calcein quenching in perfluorocarbon-filled, inverted gallbladder sacs. However, neither bile osmolality nor bile salt concentration differed in gallbladders from wild-type vs. AQP1 knockout mice. Our data indicate constitutively high water permeability in mouse gallbladder epithelium involving transcellular water transport through AQP1. The similar bile salt concentration in gallbladders from AQP1 knockout mice argues against a physiologically important role for AQP1 in mouse gallbladder.

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