Solute Transport across Membranes:- Ion-Channel Function Is Measured Electrically

Because a single ion channel typically remains open for only a few milliseconds, monitoring this process is beyond the limit of most biochemical measurements. Ion fluxes must therefore be measured electrically, either as changes in Vm (in the millivolt range) or as electric cur rents I (in the microampere or picoampere range), using microelectrodes and appropriate amplifiers. In patch-clamping, a technique developed by Erwin Neher and Bert Sakmann in 1976, very small currents are measured through a tiny region of the membrane sur face containing only one or a few ion-channel molecules (Fig. 11–47). The researcher can measure the size and duration of the current that flows during one opening of an ion channel and can determine how often a channel opens and how that frequency is affected by trans membrane potential, regulatory ligands, toxins, and other agents. Patch-clamp studies have revealed that as many as 10⁴ ions can move through a single ion channel in 1 ms. Such an ion flux represents a huge amplification of the initial signal; for example, only two acetyl choline molecules are needed to open an acetylcholine receptor channel (as described below).

FIGURE 11–47 Electrical measurements of ion-channel function. The “activity” of an ion channel is estimated by measuring the flow of ions through it, using the patch-clamp technique. A finely drawn-out pipette (micropipette) is pressed against the cell surface, and negative pressure in the pipette forms a pressure seal between pipette and mem brane. As the pipette is pulled away from the cell, it pulls off a tiny patch of membrane (which may contain one or a few ion channels). After placing the pipette and attached patch in an aqueous solution, the researcher can measure channel activity as the electric current that flows between the contents of the pipette and the aqueous solution. In practice, a circuit is set up that “clamps” the transmembrane potential at a given value and measures the current that must flow to maintain this voltage. With highly sensitive current detectors, re searchers can measure the current flowing through a single ion channel, typically a few picoamperes. The trace showing the current as a function of time (in milliseconds) reveals how fast the channel opens and closes, how frequently it opens, and how long it stays open. Clamping the Vm at different values permits determination of the effect of membrane potential on these parameters of channel function.

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Solute Transport across Membranes:- The Acetylcholine Receptor Is a Ligand-Gated Ion Channel

Solute Transport across Membranes:- The Neuronal Na+ Channel Is a Voltage-Gated Ion Channel

Solute Transport across Membranes: -The Structure of a K+ Channel Reveals the Basis for Its Specificity

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