Water transportation, cellular

Epilepsy is an example of excessive neural signaling in the central nervous system. Relative cellular and extracellular space (ECS) volume has been demonstrated to play an important role in the propensity for epileptic seizures. For example, reducing ECS volume by exposure to hypotonic medium produces hyperexcitability and enhanced epileptiform activity, whereas hyperosmolar medium reduces excitability. The hypothesis that AQP4-dependent water transport in astrocytes might modulate intrinsic brain excitability was tested by seizure susceptibility in response to the GABAa antagonist convulsant pentylenetetrazol... 

The q-space imaging method, which deals with signals only after long diffusion times, discards all information relevant to dynamic aspects of water diffusion and transport, especially the restriction of water transport by membrane and cell wall permeability barriers in cellular tissues. This information is contained in the functional dependence of the pulsed gradient spin echo amplitude S(q,A,x) on the three independent variables q, A, and x (x is the 90-180 degree pulse spacing) [53]. As the tool to explore the q and A dependence of S(q,A,x), generalized diffusion times and their associated fractional populations are introduced and a multiple exponential time series expansion is used to analyze the dependence [53]. 

Conjugation of lipophilic xenobiotics to polar cellular constituents renders the xenobiotic more water-soluble. While the lipophilic parent xenobiotics could readily diffuse into the cells, the increase in polarity associated with conjugation greatly reduces the ability of the compound to diffuse across the lipid bilayer of the cell membrane thus trapping the compound within the cell. The polar conjugates must therefore rely upon active transport processes to facilitate efflux from the cell. Hepatocytes, as well as other cells involved in chemical detoxification, are rich with members of the ATP-binding cassette superfamily of active transport proteins (ABC transporters). Cellular efflux of xenobiotics by these transporters is often referred to as Phase III elimination because Phase I or II detoxification processes often precede and are a requirement of Phase III elimination. A detailed description and discussion of elimination and transporters is presented in Chapter 15. 

Therefore, it is advisable to choose a cellular model according to its representativeness only for some or at least one epithelial function rather than a model representative for the entire airway epithelium. Permanent cell lines have been used to investigate individual epithelial functions like transepithelial Na+, Cl", and water transport, barrier function, or secretory function. Those models are predictive in regard only to the tested function. However, they are quite valuable especially in approaches with a larger throughput, since a sufficient amount of cells with an appropriate and reproducible quality are relatively easy to obtain. The use of human cell lines holds the advantage that toxicants can be tested on cells of the relevant host species and that results from these experiments need not be scrutinized for interspecies compatibility. 

All of the authors imply that separation of water phases probably occurs at the cellular level. The semi-permeable nature of the cell membrane towards ions and solutes which are capable of relaxing water protons provides compartments in which relaxation rates can be significantly different, even when water transport across the membrane is very rapid. Indeed this property of whole tissue has been used in the development of an NMR method of determining water transport across erythrocyte membranes (11). 

While water transports osmotically across the cell membrane to dilute the cellular contents, the cell grows. The membrane stretches to accommodate the newly accumulated mass. The geometry of the cell remains constant, that is it grows in every direction through the whole of the solid angle by the same amount in the same time period. The physical situation with this cell is sketched in Figure 3 ... 

In each sheath, there are three distinct layers. Xhe outer layer, including the epidermis, contains the bundles of fibers dispersed in a soft tissue matrix. Xhe middle layer consists of water-transporting fibrovascular tissue. Xhe inner layer consists of soft, cellular tissue. Xhe quantity and quality of fiber in each sheath depends on its width and its location in the stem. Xhe sheaths, about 20 per stem, are separated into four groups outside sheaths, adjacent to the... 

Various enzymes that are involved in the respiratory chain have long been demonstrable in tissue sections. It has been shown that mitochondria contain these enzymes, but otherwise these methods have yielded limited information since all cells contain the enzymes. Recently, however, electron microscopy has provided evidence concerning the mode of electrolyte and water transport in secretory epithelia. Energy-produdng enzymes (adenosine triphosphatase. glucose-6>phospha-tase. and others as well as carbonic anhydrase. occur at the cellular interfoces this fact supports the hypothesis that active transport takes place at these sites rather than diffusely through the cytoplasm... 

What is important here is that prostaglandins and eicosanoids appear to be produced in all kinds of vertebrates and invertebrates. In insects they affect egg-laying, salt and water transport and cellular immune defenses. Prostaglandin GA2 (PGA2) (Figure 3.11) seems to be important in insect immune response. 

The ready reversibility of this reaction is essential to the role that qumones play in cellular respiration the process by which an organism uses molecular oxygen to convert Its food to carbon dioxide water and energy Electrons are not transferred directly from the substrate molecule to oxygen but instead are transferred by way of an electron trans port chain involving a succession of oxidation-reduction reactions A key component of this electron transport chain is the substance known as ubiquinone or coenzyme Q... 

AQPO, formerly known as the Major Intrinsic Protein of 26 kDa (MDP26), is specifically expressed in the plasma membrane of eye lens fiber cells. It transports water to a low degree, but has also been implicated in cell adhesion and gap junction formation. Its main role is to maintain the transparency of the lens by maintaining a tight cellular connection to neighboring cells and/or by controlling the fluid circulation. 

Bone is a porous tissue composite material containing a fluid phase, a calcified bone mineral, hydroxyapatite (HA), and organic components (mainly, collagen type). The variety of cellular and noncellular components consist of approximately 69% organic and 22% inorganic material and 9% water. The principal constiments of bone tissue are calcium (Ca ), phosphate (PO ), and hydroxyl (OH ) ions and calcium carbonate. There are smaller quantities of sodium, magnesium, and fluoride. The major compound, HA, has the formula Caio(P04)g(OH)2 in its unit cell. The porosity of bone includes membrane-lined capillary blood vessels, which function to transport nutrients and ions in bone, canaliculi, and the lacunae occupied in vivo by bone cells (osteoblasts), and the micropores present in the matrix. 

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