Digital Leak Current Subtraction > Introduction
Ionic currents recorded using the voltage clamp technique are usually composed of a variety of components, mediated by different ionic channels (e.g. Na, K, Ca, Cl etc.). In order to study a particular current in detail, it is usually necessary to eliminate all the other currents from the signal. This is often done using pharmacological agents, such as TTX to block Na currents, TEA to block K currents, etc. However, even when such blocking agents are used, there often still remains some residual current in addition to the one under study. This current, is known as the leak current. It usually displays linear time-independent properties. In some circumstances, the leak current is very small and can be ignored. However, in others it can be as large as the currents under study, complicating the analysis of the signal waveforms unless it is removed.
Although the leak current cannot be removed pharmacological, its linear properties permit a digital subtraction approach to be used. The current signal can be considered to consist of 3 components
where Ii(t) is the time-dependent, voltage-activated, ionic current under study, Ilk is the leak current, and Ic(t) is transient capacity current due to the charging and discharging of the membrane capacity. Ic(t) and Ilk are always present in the signal, and scale linearly with the size of the voltage step. However, Ii(t) only occurs for voltage steps to potentials which activate the voltage sensitive ion channels. The Na current, for instance, is only evoked by depolarising voltage steps to potentials more positive than -60mV. It is possible to obtain a record, containing only leak and capacity currents, by using a hyperpolarising voltage step (or a small depolarising step)
Scaling this record to account for the differences in the size and/or polarity of the voltage step, and subtracting it from the test record, effectively removes the leak and capacity currents.
Since the scaling up of small subtraction records also scales up the background noise, it is usual to average several subtraction records before, scaling and subtracting. It is also possible to average the test records. WinWCP uses the following general algorithm
where M is the number of test records averaged and N the number of subtraction records.