Sodium transport through cell membranes

A laboratory membrane brine electrolysis cell, designed for automated operation, was constructed ( 1,2). This system enables the measurement of the sodium ion transport number of a membrane under specific sets of conditions using a radiotracer method. In such an experiment, the sodium chloride anolyte solution is doped with 22Na radio-tracer, a timed electrolysis is performed, and the fraction of current carried by sodium ion through the membrane is determined by the amount of radioactivity that has transferred to the sodium hydroxide catholyte solution. The voltage drop across the membrane during electrolysis is simultaneously measured, so that the overall performance of the material can be evaluated. 

Numerous investigations on the transport of the lithium ion through cell membranes of erythrocytes have established five different pathways by which anion exchange and sodium-potassium co-transport control lithium uptake into the cell, whilst... 

Water and electrolytes. Each day in an average adult, about 5.51 of food and fluids move from the stomach to the small intestine as chyme. An additional 3.5 1 of pancreatic and intestinal secretions produce a total of 9 1 of material in the lumen. Most of this (>7.5 1) is absorbed from the small intestine. The absorption of nutrient molecules, which takes place primarily in the duodenum and jejunum, creates an osmotic gradient for the passive absorption of water. Sodium may be absorbed passively or actively. Passive absorption occurs when the electrochemical gradient favors the movement of Na+ between the absorptive cells through "leaky" tight junctions. Sodium is actively absorbed by way of transporters in the absorptive cell membrane. One type of transporter carries a Na+ ion and a Cl ion into the cell. Another carries a Na+ ion, a K+ ion, and two Cl ions into the cell. 

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. 

Tissue electrodes [2, 3, 4, 5, 45,57], In these biosensors, a thin layer of tissue is attached to the internal sensor. The enzymic reactions taking place in the tissue liberate products sensed by the internal sensor. In the glutamine electrode [5, 45], a thick layer (about 0.05 mm) of porcine liver is used and in the adenosine-5 -monophosphate electrode [4], a layer of rabbit muscle tissue. In both cases, the ammonia gas probe is the indicator electrode. Various types of enzyme, bacterial and tissue electrodes were compared [2]. In an adenosine electrode a mixture of cells obtained from the outer (mucosal) side of a mouse small intestine was used [3j. The stability of all these electrodes increases in the presence of sodium azide in the solution that prevents bacterial decomposition of the tissue. In an electrode specific for the antidiuretic hormone [57], toad bladder is placed over the membrane of a sodium-sensitive glass electrode. In the presence of the antidiuretic hormone, sodium ions are transported through the bladder and the sodium electrode response depends on the hormone concentration. 

There is reason to beheve that cardiac glycosides, like other inotropic substances, act on the contractibility of the heart by affecting the process of calcium ion transfer through the membrane of myocardiocytes. The effect of cellular membranes in electric conductivity is mediated by transport of sodium, calcium, and potassium ions, which is a result of indirect inhibitor action on the (Na+-K+) ATPase of cell membranes. 

Inorganic ions, such as sodium and potassium, move through the cell membrane by active transport. Unlike diffusion, energy is required for active transport as the chemical is moving from a lower concentration to a higher one. One example is the sodium-potassium ATPase pump, which transports sodium [Na ] ions out of the cell and potassium [K ] into the cell. 

The SR membrane contains a very efficient calcium uptake transporter, which maintains free cytoplasmic calcium at very low levels during diastole by pumping calcium into the SR. The amount of calcium sequestered in the SR is thus determined, in part, by the amount accessible to this transporter. This in turn is dependent on the balance of calcium influx (primarily through the voltage-gated membrane calcium channels) and calcium efflux, the amount removed from the cell (primarily via the sodium-calcium exchanger, a transporter in the cell membrane). 

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