From Big Medical Encyclopedia

TRANSPORT of IONS — one of the main functions of biological membranes providing in the course of active and passive transfer of ions implementation of such manifestations of life activity as maintenance of osmotic pressure, generation of bioelectric potential, carrying out excitement etc.

Distinguish two types of T. and. — active and passive. Transfer of ions against a gradient of electrochemical potentials (a difference of electrochemical potentials on both sides biol. membranes), carried out by a cell due to energy of a metabolism, call to active T. and.; spontaneous transfer of ions through biol. a membrane in the direction of decrease in electrochemical potential, i.e. on a gradient, occurring without energy consumptions, released during metabolism, the name passive carries T. and. (see the Gradient).

The ionic composition of cytoplasm of a cell significantly differs from structure of the extracellular environment. The difference of electrochemical potentials arising at the same time provides diffusion of ions through cellular membranes and the related phenomena of osmosis (see. Osmotic pressure), generation and carrying out nervous impulse (see Excitement, Nervous impulse). In addition to transmembrane transport of ions, distinguish transcellular T. and. in cells of an epithelium of renal tubules (see Kidneys) and an epithelium of intestines (see), being one of the most important mechanisms of a homeostasis of an organism in general (see the Water salt metabolism, the Homeostasis).

Active T. and. through cellular membranes provides maintenance of ionic gradients on both sides of a membrane (see Membranes biological). Participation in active transport of ions of specialized fermental systems — ATFAZ, carrying out hydrolysis of the main power source of living cell of molecule ATP is proved (see Adeno-zinfosforny acids). Distinguish Na+, To +-Atfazu («sodium pompe»), to Sa2+-ATFAZ («the calcic pump»), N +-Atfazu («the proton pump»). Na+, To +-Atfaza is found in cells of all animals, and also in cells of plants and microorganisms. An exception only erythrocytes of nek-ry animals, in particular make dogs. Sa2+-Atfaza is most widely presented in muscle cells (see Muscles, Muscular contraction]), N +-Atfaza — in membranes of mitochondrions (see).

+, To +-Atfaza — membrane protein, a pier. the weight (weight) to-rogo varies from 190 Ltd companies to 560 Ltd companies. The molecule consists as assume, of polypeptide chains of two types about a pier. it is powerful (weighing) respectively over 100 Ltd companies (a-chain) and 50 000 ((3 chain). Polypeptide chains can create tetrameric and hexa-dimensional structures. The molecule has two centers of binding of ions, one of to-rykh (sodium) is located on an inner surface of a cellular membrane, and second (potassium) — on its exterior surface. Specific inhibitor of enzyme is the cordial glycoside the strophanthin G (ouabain) blocking work of a sodium pompe; it suppresses activity of the pump in concentration 10 ~ 7 — 10 ~ 4 mol! l.

Hydrolysis of one molecule ATP is followed by removal from a cell of 3 ions of sodium and absorption of 2 potassium ions. As at the same time transfer of charges is not compensated, as a result of Atfaza's functioning on a cell membrane there is electric potential difference with the sign «minus» in a cell (see. Bioelectric phenomena, Bioelectric potential).

Passive T. and. is defined by different types of diffusion (see

Diffusion, Permeability). Distinguish diffusion with participation specific a carriers-ionofo-ditch and diffusion through (ion channels) filled with time water. To ionophores (see) many antibiotics belong: valinomitsin, being a specific carrier of potassium ions, X — 537 (a specific carrier of calcium ions), etc. The substances forming ion channels received the name «kanaloformer». Such antibiotics as Amphotericinum In and nystatin, the channels specific to ions of chlorine forming with participation of membrane cholesterol concern to them; the gramicidin A creating single ion channels for cations of potassium and sodium. In excitable membranes the specialized natural natrium, potassium and calcium channels having selective permeability for ions of sodium, potassium and calcium are found. Natrium channels are blocked by toxins of sea fishes (tetrodotoksiny, saksitoksi-number), and potassium — to tetraethyls-moniyem. Distinguish three conditions of ion channels: opened, closed and inactivated. These states are controlled by membrane potential and are the cornerstone of development of action potentials (see Bioelectric potential).

Disturbance active and passive T. and. in a cell takes place at many diseases of the person and animals and, in turn, is followed by various cellular pathology. So, e.g., at hemolitic anemia disturbance of functioning of Na+, To +-Atfazy erythrocytes is noted. An inborn muscular myatonia (see) connect with disturbance of system of passive transport of ions of chlorine. At treatment of maniac-depressive psychoses (see. Maniac-depressive psychosis) ions of lithium effectively use, to-rykh connect the mechanism of action with suppression of activity of Na+, To +-Atfazy.

Value T. and. especially visually in membranes of cells of an epithelium of renal tubules. In these cells transcellular active transport of ions of sodium results from work of Na+, To +-Atfazy, localized in a basal membrane. As a result of secondary active transport after ions of sodium from primary urine pass ions of chlorine into plasma. After their reabsorption on an osmotic gradient water is absorbed.

For studying of mechanisms T. and. use various biophysical. and biochemical methods: a tracer technique, electric methods with use of microelectrodes and ionochuvstvitelny electrodes (see.

Ion-selective electrodes), osmometriya, etc.; the combined use of such methods with use various pharmakol was the most successful. the means including blockers ionotransportny ATFAZ and ion channels. Besides, began to use actively methods of nuclear magnetic resonance (see) and an electronic paramagnetic resonance (see), and also modeling on the basis of bisloyny lipidic membranes.

Bibliography: Antonov V. F. Lipids and ion permeability of membranes, M., 1982; Konev S. V., Aksentsev S. JT. and Bolotovsky I. D, Revelations of the two-dimensional world, Minsk, 1981; Membranes and a disease, under the editorship of JI. Bo ~ a fox, etc., the lane with English, M., 1980; With to at - l and h e in In, P. and Kozlov I. A. Proton adenozintrifosfataza, M., 1977; Hodorov B. I. General physiology of excitable membranes, M., 1975.

V. F. Antonov.