Voltage clamp techniques

Much of what we know about the properties of ion channels in cell membranes has come from experiments using voltage clamp. In general, the method allows ion flow across a cell membrane to be measured as electric current, whilst the membrane voltage is held under experimental control with a feedback amplifier. The method was first developed by Cole (1949) and Hodgkin et al. (1952) for use with the squid giant axon. Since then, many variants of the technique have evolved and voltage clamp analysis has been extended to a wide range of tissues. The usefulness of the voltage clamp stems firstly from the fact that it allows the separation of membrane ionic and capacitative currents. Secondly, it is much easier to obtain information about channel behaviour using currents measured from an area of membrane with a uniform, controlled voltage, than when the voltage is changing freely with time and between different regions of membrane. This is especially so as the opening and closing (gating) of most ion channels is affected by the membrane potential. This chapter will concentrate on voltage clamp methods that are used to measure currents from whole cells or large areas of membrane containing at least a few hundred channels; such currents are usually called macroscopic currents. We shall give particular attention to the single and double microelectrode methods. The use of both these techniques is increasing; double microelectrode clamp is being used for studies of channels expressed in Xenopus oocytes, while the switched clamp circuitry used for single microelectrode clamp is sometimes used with whole cell patch pipettes because it provides a neat way of dealing with the series resistance problems that can arise with such electrodes. The patch clamp technique has extended the application of voltage clamp methods to the recording of ionic currents flowing through single channels, but in its whole cell configuration has also become the most