Potassium ions, transport

Fig. 1. The relative selectivity of two mobile carriers. In both parts of the figure, the circles and squares represent the sodium and potassium ions transported, respectively. (Cited from Ref. 8))...

An essential requirement for diffusion of Na+ ions is the creation of a concentration gradient for sodium between the filtrate and intracellular fluid of the epithelial cells. This is accomplished by the active transport ofNa+ ions through the basolateral membrane of the epithelial cells (see Figure 19.4). Sodium is moved across this basolateral membrane and into the interstitial fluid surrounding the tubule by the Na+, K+-ATPase pump. As a result, the concentration of Na+ ions within the epithelial cells is reduced, facilitating the diffusion of Na+ ions into the cells across the luminal membrane. Potassium ions transported into the epithelial cells as a result of this pump diffuse back into the interstitial fluid (proximal tubule and Loop of Henle) or into the tubular lumen for excretion in the urine (distal tubule and collecting duct). 

Post, R.L., Hegyvary, C., Kume, S. (1972). Activation by adenosine triphosphate in the phosphorylation kinetics of sodium and potassium ion transport adenosine triphosphatase. J. Biol. Chem. 247, 6530-6540. 

The hydrated thallous ion is similar in size to the hydrated potassium ion, and early literature reported that the uptake of T1 cations in muscle cells made use of the specific uptake mechanism developed for potassium. However, later studies, taking account of the complexity of potassium transport, and the different types of potassium channels, have found some differences between the cellular T1 uptake and the potassium uptake. Thus, digoxin that inhibits the Na/K ATP-ase enzyme system as well as the potassium ion-transport, did not affect the ° T1 transport. 

Vibrations characteristic of Sb—O stretches and deformations have been discussed for solid antimonates such as M Sb03, MnSb206, and M2Sb207.770 Preparation of a new family of mixed oxides with structures related to that of cubic KSb03 has been announced, and the detailed structure of one such compound, Bi3GaSb2On, has been obtained.771 Potassium ion transport can occur through two-dimensional tunnels that occur in the structures of K3Sb5014 and KjSb.On.772... 

Effect On potassium ion transport in hypertensive patients R. Garay et al, J. Hypertension 4, Suppl. 5, S208... 

Briskin dp (1986) Plasma membrane transporting AT Pase role in potassium ion transport Physiol Plant 68 159-163. 

Dobler. M. Dunitz, J.D. Krajewski, J. Structure of the potassium ion complex with enniatin B, a macrocyclic antibiotic with potassium ion transport properties. J. Mol. Biol. 1969, 42. 603-606. 

T. Hill and Y. Chen, Cooperative Effects in Models of Steady-State Transport Across Membranes Simulation of Potassium Ion Transport in Nerve, Proc. Natl Acad. Scl USA 66(3), 607-614 (1970). 

Figure 5. Comparison of potassium ion transport between 18-crown-6 and cyclic pentamer of 6,8 dioxabicyclo[3.2.1]-octan-7-one., 18-Crown-6 O cyclic pentamer.

The change in the absorption spectrum and the molecular structure on photoisomerization of azobenzenes can be used for practical applications. The literature on possible applications in photoresponsive materials was reviewed up to 1988 by Rau. Important contributions by Japanese workers, for example, on light-driven potassium ion transport through membranes by means of crown ethers with a photofunctional azobenzene cap, were recently summarized by Shinkai. Moreover, light-manipulation of other super molecules based on azobenzene photoisomerization is described in the book by Feringa. " The E— Z photoisomerization for the preparation of thermally unstable Z-isomers for use as kinetic probes in microheterogeneous media was described in Section 89.5. 

Sodium and potassium ions are actively absorbed from the intestine. As a consequence of the electrical potential caused by transport of these ions, an equivalent quantity of Cf is absorbed. The resulting osmotic effect causes absorption of water (56). 

With respect to the carrier mechanism, the phenomenology of the carrier transport of ions is discussed in terms of the criteria and kinetic scheme for the carrier mechanism the molecular structure of the Valinomycin-potassium ion complex is considered in terms of the polar core wherein the ion resides and comparison is made to the Enniatin B complexation of ions it is seen again that anion vs cation selectivity is the result of chemical structure and conformation lipid proximity and polar component of the polar core are discussed relative to monovalent vs multivalent cation selectivity and the dramatic monovalent cation selectivity of Valinomycin is demonstrated to be the result of the conformational energetics of forming polar cores of sizes suitable for different sized monovalent cations. 

Neutral carriers are organic complexing agents which are capable of sequestering and transporting ionic species in a hydrophobic organic phase. The antibiotics, valino-mycin and nonactin were the first neutral carriers to be incorporated in an ISE These macrocyclic neutral carriers contain a polar internal cavity and an outer hydro-phobic shell. The excellent selectivity exhibited by valinomycin for potassium ions is... 

A well-known example of active transport is the sodium-potassium pump that maintains the imbalance of Na and ions across cytoplasmic membranes. Flere, the movement of ions is coupled to the hydrolysis of ATP to ADP and phosphate by the ATPase enzyme, liberating three Na+ out of the cell and pumping in two K [21-23]. Bacteria, mitochondria, and chloroplasts have a similar ion-driven uptake mechanism, but it works in reverse. Instead of ATP hydrolysis driving ion transport, H gradients across the membranes generate the synthesis of ATP from ADP and phosphate [24-27]. 

Fig. 6.24 A hypothetic scheme of the time behaviour of the spike linked to the opening and closing of sodium and potassium channels. After longer time intervals a temporary hyperpolarization of the membrane is induced by reversed transport of potassium ions inside the nerve cell. Nernst potentials for Na+ and K+ are also indicated in the figure. 

Potassium ion secretion. Potassium ions are secreted in the distal tubule and the collecting duct. These ions diffuse down their concentration gradient from the peritubular capillaries into the interstitial fluid. They are then actively transported up their concentration gradient into the tubular epithelial cells by way of the Na+, K+ pump in the basolateral membrane. Finally, potassium ions exit the epithelial cells by passive diffusion through K+ channels in the luminal membrane and enter tubular fluid to be excreted in the urine. 

An intracellular to extracellular difference in sodium and potassium ion concentrations is essential to the function of nerves, transport of important nutrients into the cell, and maintenance of proper cell volume. 

The transport of hydrated sodium and potassium ions through the cell membrane is slow, and this transport requires an expenditure of energy by the cell. 

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