Water and sodium
Martin Andrew Crook in Clinical Biochemistry & Metabolic Medicine, 2013
Antidiuretic hormone, by regulating aquaporin 2, enhances water reabsorption in excess of solute from the collecting ducts of the kidney and so dilutes the extracellular osmolality. Aquaporins are cell membrane proteins acting as water channels that regulate water flow. When ADH secretion is a response to a high extracellular osmolality with the danger of cell dehydration, this is an appropriate response. However, if its secretion is in response to a low circulating volume alone, it is inappropriate to the osmolality. The retained water is then distributed throughout the TBW space, entering cells along the osmotic gradient; the correction of extracellular depletion with water alone is thus relatively inefficient in correcting hypovolaemia. Plasma osmolality normally varies by less than 1–2 per cent, despite great variation in water intake, which is largely due to the action of ADH.
Respiratory System
Pritam S. Sahota, James A. Popp, Jerry F. Hardisty, Chirukandath Gopinath, Page R. Bouchard in Toxicologic Pathology, 2018
Gas exchange occurs within the alveoli across the thin epithelial barrier (<0.1 μm in places) of Type I pneumocytes (ATI) that cover approximately 98% of the alveolar surface and also function to regulate solute and water movement between blood and the airspace (Dobbs et al. 2010; Williams 2003). These cells are terminally differentiated and normally lack mitotic activity. Aquaporin-5 is an ATI specific marker in the lung parenchyma (Nielsen et al. 1997). Type II pneumocytes (ATII), which are located at the intersections of alveolar walls, act as progenitors for ATI cells (Evans et al. 1975), produce and secrete surfactant, and are involved in transepithelial fluid and electrolyte transport (Edelson et al. 1988). By transmission electron microscopy (TEM), ATII are characterized by the presence of cytoplasmic lamellar and multivesicular bodies, and numerous microvilli are present on their apical surface. These cells may also be highlighted by alkaline phosphatase histochemistry or IHC (Edelson et al. 1988). The other epithelial cell found lining the alveoli (and respiratory tract) in some species is the pulmonary brush cell or Type III pneumocyte. These cells have been seen in rat and hamster alveoli and may be identified by TEM, having typically up to 140 microvilli on their narrow apical surface. Their function is unknown (Reid et al. 2005).
Basics of CSF production
Jyotirmay S. Hegde, Hemanth Vamanshankar in CSF Rhinorrhea, 2020
Transport of water, solutes, and ions in a bi-directional manner, in response to both hydraulic pressure and passive osmotic gradients, is controlled by aquaporins. These are highly selective of the molecule being transported, and at the same time provide rapid transport.35 Of the many aquaporins found (about 14 are identified), six are reported to be present in the brain.36–38 Permeability is specific to each of them; AQP3 and 9- permeable to small solutes and water; AQP8 to ions; and AQP 1, 4, 5 are water permeable. Of these, AQP1 is important in CSF formation; AQP9 in energy metabolism; AQP4 in the clearance of K+ released in neuronal activity and formation/resolution of brain edema.36 AQP4 channels move water by simple diffusion and vesicular transport – this is currently the most accepted theory of the function of AQP4 (Figure 2.2 C).39 Further, AQP4-rich areas like BBB and glia limitans at the subpial zones may also act as water channels for ISF-CSF drainage in extreme conditions.40 The discovery of aquaporins by Peter Agre, which resulted in his receiving a Nobel Prize in Chemistry in 2003, has led us to a greater understanding of biological fluid homeostasis and turnover.41
Carbonic anhydrase 12 mutation modulates membrane stability and volume regulation of aquaporin 5
Published in Journal of Enzyme Inhibition and Medicinal Chemistry, 2019
Soyoung Hwang, Jung Yun Kang, Min Jae Kim, Dong Min Shin, Jeong Hee Hong
Aquaporin 5 (AQP5) is a member of the transmembrane protein channels involved in transcellular water permeability and a dominant water channel in the epithelia of the salivary and lacrimal glands and the lung alveoli1,2. A study using AQP5 knockout mice showed that AQP5 plays a central role in salivary secretion3. Indeed, AQP5 classically prevents excessive water reabsorption and modulates cell shape and volume4. Recently, several new roles of AQP5, in addition to water transport, have been discussed. AQP5 is involved in migration by activating Ras and Rac signalling in cancer cells5 and is associated with cancer cell invasion and tumour metastasis through the NF-κB signalling pathway6. Additionally, hyperosmotic stress-mediated AQP5 expression mediates the expression of inflammatory cytokines and cell death through the JNK1/2 MAPK signalling pathway7.
Anti-cataract therapies: is there a need for a new approach based on targeting of aquaporins?
Published in Expert Opinion on Therapeutic Targets, 2021
An important class of lens proteins, which are far lower in proportion that the crystallins, but which play a vital role in water transport, microcirculation and homeostasis in protein/water balance in the lens are the aquaporins [21]. Aquaporins are small membrane proteins found in a number of tissues in humans and animals. The three main aquaporins that are expressed in the human lens are aquaporin 0, 1, and 5 [21]. These aquaporins are water transporting proteins [22]. Aquaporin 0 (also known as Membrane Intrinsic Protein) is found in the fiber cells throughout the lens, constituting over half of the protein concentration of the cell membranes [22]. Aquaporin 1 is found in the epithelial cells [22] and aquaporin 5 has been detected in all parts of the lens [23] and is the second most prevalent of the aquaporins in mature lens fiber cells [23]. Aquaporin 7, an aquaglyceroporin which facilitates transport of small molecules as well as water and may be active in transport of nutrients, has also been detected in the eye lens epithelium [22]; it has a significantly lower water permeability than aquaporin 1 [24].
Optimization of an oral mucosa in vitro model based on cell line TR146
Published in Tissue Barriers, 2020
Grace C. Lin, Tamara Leitgeb, Alexandra Vladetic, Heinz-Peter Friedl, Nadine Rhodes, Angela Rossi, Eva Roblegg, Winfried Neuhaus
Aquaporins (AQP), integral membrane proteins forming water channels for rapid fluid transport, are very strongly expressed by, e.g., the salivary glands. Therefore, characterization of aquaporins is mostly referred to mouse, rat, or human salivary glands, but a thorough characterization for the human oral mucosa is still missing.70 In our oral mucosa model, we demonstrated the expression of AQP1, AQP3, AQP6, AQP7, AQP9, AQP10, and AQP11. The biopsy samples of the human oral mucosa showed a similar expression pattern as our model. A consistent expression for AQP1, AQP3, AQP7, AQP9, AQP10, and AQP11, a weak expression for AQP4 and no expression for AQP6, AQP8 and AQP12A was obtained. This confirms the similarity of our cell line-based model to the human model. Even though the expression of aquaporins was mostly described in the salivary glands, AQP3 and AQP9 were also found in rat buccal mucosa epithelium at the mRNA and protein level. Moreover, a correlation was suggested between AQP3 localization and the differentiation of keratinocytes.71 To understand the expression of aquaporins in our human buccal mucosa model, further investigations on their functionality in the oral mucosa are necessary.
Related Knowledge Centers
- Alpha Helix
- Asparagine
- Bacteria
- Lipid Bilayer
- Proline
- Cell Membrane
- Ion Channel
- Protein Family
- Major Intrinsic Proteins
- Cell