Discontinuous single electrode voltage-clamp measurements: assessment of clamp accuracy in Vicia faba guard cells

The use of a discontinuous single electrode voltage-clamp (dSEVC) offers an attractive alternative to the patch-clamp technique, since whole-cell meas urements can be performed with a single sharp electrode. Comparison of curr ent-voltage relations, however, revealed a weaker voltage dependence of cha nnels measured with the dSEVC compared to patch clamp. The accuracy of the dSEVC was tested on Vicia faba guard cells impaled with double-barrelled el ectrodes. The actual clamp potential was measured independently of the dSEV C, at the second barrel. The weaker voltage dependence of ion channels appe ared to be due to an overestimation of the clamp potential by the dSEVC. Th e deviation between the intended and actual clamp potential showed a linear relationship with the injected current; on average a 126 mV deviation was found for a clamp current of 1 nA. The deviation was probably caused by a s low settling capacity at the electrode, not compensated by the dSEVC amplif ier. It is concluded that the dSEVC method in its current state is only sui ted for the study of small ion conductances in plant cells.