Membrane conductance, single-channel

Fig. 4. A, Histogram of single channel conductances, y, of Gramicidin A in diphytanoyl L-a-lecithin/n-decane membranes for 1 M KC1, 130 mV potential and 40 °C. Note that single channel conductance is simply the single channel current, i , divided by the applied potential. Reproduced from reference 12) with permission.

C. Histogram of single channel conductances of Gramicidin A in glyceryl monoolein/hexadecane membranes for 1 M KC1, 103 mV applied potential and 23 °C. Reproduced with permission from Ref. 13>... 

CFTR has a single-channel conductance of about 8 pS. It is present in the apical membranes of many epithelia. Its mutation leads to the potentially lethal disease cystic fibrosis. In addition to acting as a chloride channel, CFTR is also thought to regulate, e.g., the epithelial sodium channel ENaC, a molecularly unknown outwardly-rectifying chloride channel, and possibly also potassium channels and water channels. Some of these potential regulatory processes, however, are controversial. CFTR also acts as a receptor for bacteria. 

At the level of a single channel, addition of ACh is followed by transient openings of the channel. The current i flowing through an open channel is 4 pA at a membrane potential Voi-l 00 mV. Since one ampere (A) represents the flow of 6.24-1018 charges per second, 2.5-107 Na+ ions per second flow through an open channel. The conductance g of a plasma membrane channel is the measure of the ease of flow of cuirent between the extracellular space and the cytosol or vice... 

Fig. 6.23 Single-channel currents flowing across the membrane between the protoplast and vacuole of Chara corallina. Among several channels with different conductivity the recordings of the 130 pS channel are recorded here. The zero line is at the top of each curve. (By courtesy of F. Homble)...

Solving the energy equation provides prediction of the temperature distribution and its effect on cell performance in a PEFC. Figure 12 presents a temperature distribution in the middle of the membrane for a single-channel PEFC. The maximum temperature rise in this case is 4 °C, which will only fect cell performance slightly. However, the temperature variation depends strongly on the thermal conductivities of the GDL and flow plate as well as thermal boundary conditions. 

Ionic conductivity of purified Na+ channels reincorporated into phospholipid bilayers, (a) The current through a single channel is measured as a function of time. Downward deflections of the traces indicate openings of the channel (increased current of Na+ across the membrane). The numbers on the right indicate the electric potential difference across the membrane (Atji). When Ai// is made less negative, the channel opens more frequently and stays open longer. 

A parallel development came from studies on artificial lipid bilayer membranes. Hladky and Hay don (1984) found that when very small amounts of the antibiotic gramicidin were introduced into such a membrane, its conductance to electrical current flow fluctuated in a stepwise fashion. It looked as though each gramicidin molecule contained an aqueous pore that would permit the flow of monovalent cations through it. Could the ion channels of natural cell membranes act in a similar way To answer this question, it was first necessary to solve the difficult technical problem of how to record the tiny currents that must pass through single channels. 

Another versatile mode is the cell-excised configuration (Hamill 1993). It is obtained by suddenly removing the patch-pipette from the cell, so that the membrane patch is pulled off the cell. This mode easily allows to expose the channel proteins to drugs by changing the bath solution. The single channel currents are recorded on a videotape and are analyzed off-line by a computer system. Various parameters are evaluated, such as the single channel conductance, open-and closed-times of the channel, and the open-state probability, which is the percentage of time the channel stays in its open state. 

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