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Abstract:
Offset voltages of various origins have to be considered in patch-clamp experiments. Some of the offsets are constant during a typical experiment, such as amplifier input offsets; some are variable, such as liquid junction potentials, depending on ionic conditions. Some arise in the external circuit (i.e., in the patch pipette, in the experimental chamber, or at the silver chloride electrodes), and some arise in the patch-clamp amplifier. Typical magnitudes are ±30 mV for amplifier offsets, up to 100 mV (depending on Cl− concentrations) for electrode offsets, and up to ±15 mV for liquid junction potentials at interfaces between different solutions. It is standard practice to compensate amplifier and electrode offsets by performing a reference measurement before the pipette is sealed to a cell. This is done by adjusting a variable offset (V o), which, in the amplifier, is added to the command voltage (V c), such that there is zero current flow at V c = 0. This protocol is correct, provided that none of the offsets mentioned above changes during the experiment. If, however, the pipette solution is different in its composition from the bath solution (as is usually the case for whole-cell measurements), a liquid junction potential will be present at the pipette tip during the reference measurement. This will no longer be the case during the test measurement, when the liquid junction is replaced by the membrane under study.
