Membrane Potential Voltage Clamping

Let us suppose that we need to raise the K concentration of a fibroblast cell to 200 X 10 M. The average fibroblast cell volume is 10 liters with an internal K concentration of 150 x 10 M. Therefore, we need to add (50 X 10 moles/liter) (10 liters) = 5 x 10 moles K . (This assumes, of course, no leakage from the cell during the process.) This quantity of corresponds to (5 x 10 moles) (96,488 coulombs/mole) = 4.82 x 10 coulombs of charge. Assuming a constant current d.c. power source is available to deliver 10 amps, current must be passed for 4.82 seconds to achieve the desired concentration (4.82 seconds) (10 amps) = 4.82 x 10 coulombs. 

The actual experiment requires the insertion of a micropipet, filled with KCl, into the cell. The positive lead of the power supply will be connected to a noble metal wire located in this micropipet. In most cases a second micropipet would be used to complete the circuit via the negative terminal of the power supply. 

Historically, iontophoretic delivery of medicines has been confined to rather superficial, epidermal applications. Ions of zinc and copper have been employed in the treatment of some skin infections and chronic infections of sinuses and cavities. Chloride ions can be utilized to loosen superficial scars. Many drugs used for general metabolic or specific hormonal effects can be driven into the circulating blood by iontophoresis, although currently most physicians use the more common forms of drug delivery. 

The electroencephalogram (EEG), electromyogram (EMG), and electrocardiogram (ECG) are all measurements of electrical signals generated in 

The EEG is routinely used to diagnose different types of epilepsy and to localize the area in the brain causing the epilepsy. In addition, it is used to localize brain tumors and diagnose certain types of psychopathic disorders. 

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A circuit that uses a differential amplifier to maintain constant membrane potential by electronically balancing the ion channel current. This method allows the experimenter to analyze action potentials of excitable membranes resulting from an initial transient rise in sodium ion permeability followed by a transient rise in potassium ion permeability The technique is especially valuable for studying kinetic properties of voltage-gated channels as well as voltage-dependent channels. See Membrane Potential Patch Clamp Methods... 

Voltage clamp—Electrophysiological technique for measuring ion currents across the cell membrane, while a constant membrane potential (voltage) is maintained by the apparatus. 

Figure 4. Effects of dihydro-brevetoxin B (H2BVTX-B) on Na currents in crayfish axon under voltage-clamp. (A) A family of Na currents in control solution each trace shows the current kinetics responding to a step depolarization (ranging from -90 to -I-100 mV in 10 mV increments). Incomplete inactivation at large depolarizations is normal in this preparation. (B) Na currents after internal perfusion with H2BVTX-B (1.2 a M). inactivation is slower and less complete than in the control, and the current amplitudes are reduced. (C) A plot of current amplitudes at their peak value (Ip o, o) and at steady-state (I A, A for long depolarizations) shows that toxin-mOdified channels (filled symbols) activate at more negative membrane potentials and correspond to a reduced peak Na conductance of the axon (Reproduced with permission from Ref. 31. Copyright 1984 American Society for Pharmacology and Experimental Therapeutics).

The study of processes at ITIES and in membrane electrochemistry requires elimination of two ohmic potential differences, achieved with a four-electrode potentiostat, voltage-clamp (Fig. 5.17). 

Fig. 6.21 Joint application of patch-clamp and voltage-clamp methods to the study of a single potassium channel present in the membrane of a spinal-cord neuron cultivated in the tissue culture. The values indicated before each curve are potential differences imposed on the membrane. The ion channel is either closed (C) or open (O). (A simplified drawing according to B. Hille)...

Bolton But these are single cells, they are not resting. If you look at the membrane potentials they are very likely to be active. If you don t voltage clamp them, they can be spontaneously active. 

Hirst If you take a piece of arteriole, stimulate the purinergic nerves to evoke an excitatory junction potential, measure the diameter of the arteriole, and apply a voltage clamp, you get no contraction. If you let the membrane potential run into levels where voltage-dependent Ca2+ channels are activated, there is a contraction. 

In order to confirm the possible interaction of ethanol and crocin on NMDA receptors, we also performed whole-cell patch recording with primary cultured hippocampal neurons and measured membrane currents induced by the application of NMDA in a voltage-clamped condition. Application of 100 pM NMDA induced an inward current of 100.2 9.8 pA (n=10) at a holding potential of -60 mV. The NMDA-induced inward current was not affected by 10 pM CNQX (data not shown), but was completely abolished by 30 pM APV, supporting the fact that the response was mediated by NMDA receptors. Ethanol inhibited NMDA-induced currents in a concentration-dependent manner. Crocin (10 pM) had no effect on NMDA-induced currents by itself (data not shown), but attenuated the inhibitory effect of ethanol on NMDA-induced currents. The concentration-effect curve for ethanol was shifted to the right by the presence of crocin [22]. 

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