Single-channel current

Calculation of Single Channel Currents and the Equivalent Free Energy... 

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.

In the previous section was given the experimental demonstration of two sites. Here the steady state scheme and equations necessary to calculate the single channel currents are given. The elemental rate constants are thereby defined and related to experimentally determinable rate constants. Eyring rate theory is then used to introduce the voltage dependence to these rate constants. Having identified the experimentally required quantities, these are then derived from nuclear magnetic resonance and dielectric relaxation studies on channel incorporated into lipid bilayers. 

Utilizing the above five experimentally derived rate constants and Eyring rate theory, the ten rate constants of Eq. 6 are all obtained. With the rate constants known, the probability of each occupancy state, /(ox) for example, can be calculated and finally the single channel current can be calculated as a function of molal activity of sodium ion. This is done for a 100 mV transmembrane potential in Fig. 9. It should be emphasized that Fig. 9 represents a calculation of single channel currents... 

Fig. 10. Calculated sodium ion single channel currents for the malonyl Gramicidin channel and comparison with experimental data points using four different models all of which fit the data well but only one of which, B., is correct. The point to be made is that both the independent determination of rate constants and of the binding site locations are required.

A general expression for the single channel current equation may be obtained by rewriting Eq. 4 as... 

FROM SINGLE-CHANNEL CURRENTS TO WHOLE-CELL CURRENTS... 

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)...

Neher, E., The charge carried by single-channel currents of rat cultured muscle cells in the presence of local anaesthetics. J. Physiol., 339, 663-678, 1983. 

Fig. 21.5. Levamisole-activated single-channel currents activated by 30 pM levamisole in a cell-attached patch and current voltage relationship. The rectangular current pulses were recorded at different patch potentials to determine the relationship between channel current and potential. The slope was linear with a conductance of 34 pS.

SENS and LEVR isolates. Each plot was obtained at +75 mV and demonstrates multiple conductance levels (levels I, II and III for SENS and IV and V for LEVR). Inset examples of single-channel currents corresponding to each of the conductance levels I, II and III level are shown. (B) Amplitude histograms obtained from the data illustrated in (A). The plots confirm the presence of open-channel currents with multiple conductance levels. 

Fig. 21.15. GABA and piperazine single-channel currents recorded from cell-attached patches of Ascaris bag muscle membrane. Trans-patch potential, -75 mV GABA channel mean open-time, 32 ms piperazine channel open-time, 18 ms GABA 3 pM in the pipette piperazine 500 pM in the pipette.

Robertson, S.J. and Martin, R.J. (1993) Levamisole-activated single-channel currents from muscle of the nematode parasite Ascaris suum. British Journal of Pharmacology 108, 170-178. 

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