Potential dependence of the admittance ofChara plasmalemma

Summary A computer-controlled apparatus is described, which combines the two powerful methods of voltage-clamping and admittance measurement. The 5-Hz admittance ofChara plasmalemma is obtained for transmembrane PD from −400 mV to 0. DC conductance is also measured by the bipolar staircase method. Both the DC and 5-Hz conductances at steady state display a central maximum at ≈−250 mV. This feature is attributed to the conductance/voltage characteristics of the H⁺ pump. The steady-state capacitance does not show any trend throughout the potential interval. At the time of the delay, before excitation commences, the 5-Hz conductance is smaller than after excitation. At the time of excitation the 5-Hz conductance echoes the time-course of the ionic current, while the capacitance undergoes a sharp decrease followed by an increase. A possible explanation of the capacitance behavior is attempted involving transport number effects and reactances associated with the Hodgkin-Huxley gating mechanism. At punchthrough the membrane becomes inductive.