Current oscillations under voltage-clamp conditions: an interplay of series resistance and negative slope conductance

Using the patch-clamp technique, we observed profound oscillations of the whole-vacuole outward current across the tonoplast of Mesembryanthemum crystallinum L. (common ice plant). These current oscillations showed a clear voltage dependence and appeared at membrane potentials more positive than 90-100 mV. This paper describes the oscillations in terms of two separate mechanisms. First, the Mesembryanthemum vacuolar membrane shows a negative slope conductance at membrane potentials more positive than 100-120 mV. The fact that the oscillations and the negative slope conductance show a similar threshold potential suggests that (part of) the same mechanism is involved in both phenomena. The second mechanism involved is the voltage drop across the series resistance. As a result, the potential actually experienced by the vacuolar membrane deviates from the command potential defined by the patch-clamp amplifier. This deviation depends in an Ohmic manner on the current magnitude. We suggest that the interplay of the negative slope conductance and the voltage drop across the series resistance can cause a positive feedback which is responsible for the current oscillations.