INFLUENCE OF CONDUCTANCE CHANGES ON PATCH-CLAMP CAPACITANCE MEASUREMENTS USING A LOCK-IN AMPLIFIER AND LIMITATIONS OF THE PHASE TRACKING TECHNIQUE

We characterized the influence of conductance changes on whole-cell pa tch clamp capacitance measurements with a lock-in amplifier and the li mitations of the phase-tracking method by numerical computer simulatio ns, error formulas, and experimental tests. At correct phase setting, the artifacts in the capacitance measurement due to activation of line ar conductances are small. The cross talk into the capacitance trace i s well approximated by the second-order term in the Taylor expansion o f the admittance. In the case of nonlinear current-voltage relationshi ps, the measured conductance corresponds to the slope conductance in t he range of the sine wave amplitude, and the cross talk into the capac itance trace corresponds to the second-order effect of the slope condu ctance. The finite gating kinetics of voltage-dependent channels gener ate phase-shifted currents. These lead to major artifacts in the capac itance measurements when the angular frequency of the sine wave is dos e to the kinetic rate constant of the channel. However, when the chann el kinetics are sufficiently slow, or sufficiently fast, the cross tal k is still close to the second-order effect of the measured conductanc e. The effects of activation of voltage-dependent currents on the capa citance measurements may be estimated, provided a detailed characteriz ation of the kinetics and voltage dependence is available. A phase err or of the lock-in amplifier of a few degrees leads to significant proj ections. The phase-tracking method can be used to keep the phase align ed only during periods of low membrane conductance. However, nonideal properties of the equivalent circuit, in particular the fast capacitan ce between the pipette and the bath solutions, may lead to large phase errors when the phase-tracking method is used, depending on the elect rical properties of the cell. in this article we provide practical val ues, setting the range where possible artifacts are below defined limi ts. For proper evaluation of capacitance measurements, the capacitance and conductance traces should always be displayed together.