Ion transport pathways

Figure 14 Ion transport pathways responsible for water flux across intestinal epithelia. Sodium absorption in villus tip cells (left) stimulates water absorption, while chloride channel exit in crypt cells (right) stimulates water secretion.

In biological systems, therefore, the behavior of Li+ is predicted to be similar to that of Na+ and K+ in some cases, and to that of Mg2+ and Ca2+ in others [12]. Indeed, research has demonstrated numerous systems in which one or more of these cations is normally intrinsically involved, including ion transport pathways and enzyme activities, in which Li+ has mimicked the actions of these cations, sometimes producing inhibitory or stimulatory effects. For example, Li+ can replace Na+ in the ATP-dependent system which controls the transport of Na+ through the endoplasmic reticulum Li+ inhibits the activity of some Mg2+-dependent enzymes in vitro, such as pyruvate kinase and inositol monophosphate phosphatase Li+ affects the activity of some Ca2+-dependent enzymes— it increases the levels of activated Ca2+-ATPase in human erythrocyte membranes ex vivo and inhibits tryptophan hydroxylase. 

Ion transport pathways across the luminal and basolateral membranes of the thick ascending limb cell. The lumen positive electrical potential created by K+ back diffusion drives divalent (and monovalent) cation reabsorption via the paracellular pathway. NKCC2 is the primary transporter in the luminal membrane. 

Ion transport pathways across the luminal and basolateral membranes of collecting tubule and collecting duct cells. Inward diffusion of Na+ via the epithelial sodium channel (ENaC) leaves a lumen-negative potential, which drives reabsorption of and efflux of K+. (R, aldosterone receptor.)... 

Sarkadi, B. Parker, J.C. (1991). Activation of ion transport pathways by changes in cell volume. Biochim. Biophys. Acta 1071,407-427. 

Ussing chamber measurements are the most commonly used method to investigate transepithelial ion transport [164, 165], They can be performed on either functional epithelia from biopsies or on cultivated epithelia. The Ussing chamber itself is more or less a U-shaped chamber, which is compartmentalized by an epithelial cell layer and solute flux between both compartments can be measured. Electrogenic ion fluxes can be measured directly by short circuit measurements or calculated from open circuit measurements [165], Non-electrogenic solute flux needs continuous concentration measurements for each compartment separately [166]. Thus, Ussing chamber experiments enable to study permeation kinetics for secretion and resorption separately [166-169] and to evaluate effects on ion transport pathways [85, 87, 90, 170]. 

Schematic representation of the resting (left side) and stimulated (right side) state of the parietal cell. Basolateral membrane contains three major receptor classes gastrin (G), acetylcholine (ACh), and histamine (H). Their actions are mediated by cAMP responses, Ca changes, or both. In addition, there are a number of ion transport pathways. In the stimulated state, the apical membrane acquires H", K -ATPase contained in the tubulovesicles (tv) as well as the property of K+ and CI conductance, both of which are essential in the secretion of HCl. A change in cytoskeletal arrangement is also associated with stimulation. CaM = calmodulin SC -secretory canaliculus mf = microfilaments. [Reproduced with permission from D. H. Malinowska and G. Sachs, Cellular mechanisms of acid secretion, Clin. Gastroenterol. 13, 322 (1984).]...

Oxygen Anion Transport in Solid Oxides, Fig. 11 Refined stracture of NdBaCo205 s at 573 °C and p02 = 10 atm [29]. Atoms are represented by refined atomic displacement ellipsoids, partial shading indicates location of oxygen vacancies at the 03 site, and arrows are illustrative indications of the ion transport pathway... 

The membrane domain of the enzyme contains the ion-transport pathways. There must be a hydrophilic pathway in the membrane domain that the ions can traverse as the cytoplasmic domain changes conformation as a function of phosphorylation and dephosphorylation. 

Figure 1. A general model of the sodium and gastric acid pumps identifying their domains and their two subunits. The cytoplasmic domain binds ATP, becomes phosphorylated and releases ADP and Pi. The stalk transduces the conformational change due to phosphorylation/dephos-phorylation to the membrane domain which contains the ion transport pathway, binding Na or H with high affinity (ion in ) in one conformational state, and with low affinity in a second conformational state (ion out ) [7].

Ion Transport Pathways from Bond Valence Maps in Crystalline Cation... 

For screening purposes, bond valence or bond valence site energy (BVSE) pathway models derived from static structure models appear to be the most straightforward approach. In a range of earlier studies, it has been discussed how the bond valence method can be used to analyze ion transport pathways statistically, yielding predictions of ionic conductivity from crystal structure data and RMC- or MD-generated structure models [4, 8-10]. 

Given the correlation between bond valence and electron density, it appears tempting to compare also what electron density maps and maps of the BVSE predict as ion transport pathways. Hirshfeld surface analysis has been explored to characterize intermolecular interactions in molecular crystals [47,48]. This analysis is based on the procrystal, which is obtained from superposition of spherical atomic electron densities placed at the crystal structure positions, a quantity that can readily be calculated from the structure using software tools such as CrystalExplorer [49]. The approach was also explored as a tool to map out voids in porous crystals such as metal organic framework materials and zeolites [50]. 

Prasada Rao R, Tho TD, Adams S (2011) Ion transport pathways in molecular dynamics simulated alkali silicate glassy electrolytes. Solid State Ion 192 25-29... 

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