Voltage-Dependent Membrane Capacitance in Rat Pituitary Nerve Terminals Due to Gating Currents

Abstract

We investigated the voltage dependence of membrane capacitance of pituitary nerve terminals in the whole-terminal patch-clamp configuration using a lock-in amplifier. Under conditions where secretion was abolished and voltage-gated channels were blocked or completely inactivated, changes in membrane potential still produced capacitance changes. In terminals with significant sodium currents, the membrane capacitance showed a bell-shaped dependence on membrane potential with a peak at approximately -40 mV as expected for sodium channel gating currents. The voltage-dependent part of the capacitance showed a strong correlation with the amplitude of voltage-gated Na+ currents and was markedly reduced by dibucaine, which blocks sodium channel current and gating charge movement. The frequency dependence of the voltage-dependent capacitance was consistent with sodium channel kinetics. This is the first demonstration of sodium channel gating currents in single pituitary nerve terminals. The gating currents lead to a voltage- and frequency-dependent capacitance, which can be well resolved by measurements with a lock-in amplifier. The properties of the gating currents are in excellent agreement with the properties of ionic Na+ currents of pituitary nerve terminals.

Voltage-dependent capacitance of human embryonic kidney cells

“…We can conclude from our results with mammalian cells, and from the reported observations with sodium channels in neurons [22,36] that the charge contributions from the collective response of membrane proteins (e.g., transporters, channels, and pumps) and lipids in the field is minimal relative to that observed with prestin in the membrane. The enormity of charge may result from chloride movement within prestin [61].…”

“…Finally, there is a possibility that this additional differential capacitance arose from charge distribution in K and Ca channel proteins that occurred during transitions between conformational states (i.e., gating charges), these transitions may occur even though the channels are blocked. This is the explanation suggested to explain the voltage dependent capacitance observed in rat pituitary nerve terminals in the absence of sodium channels [36] in this case the voltage capacitance function was similar to that shown in Fig. 5(d) where the capacitance decreased as the holding potential became more positive decreasing by up to 61 fF at 0.05 V.…”

“…When HEK cells are transfected with prestin, the capacitance-voltage signal is also a convolution of at least two functions, but the total charge varies from cell to cell, and is at most 30% (5×10 6 electrons) of that observed in OHCs. This is generally greater than measured from gating charges of sodium channels in neurons at about 6×10 4 electrons [36]. It is also generally much greater than that measured for HEK cells where the charge from the parabolic and linear components is about the same at 5×10 4 electrons (Fig.…”

A large pool of releasable vesicles in a cortical glutamatergic synapse

“…⌬C m was measured as the difference of C m 50 ms before and 200-300 ms after the step, except in the experiments in which sine-wave and square-pulse techniques were compared (difference of C m 2 s before and 2 s after the step). With these settings, the contribution of Na ϩ channel gating charge movements is likely to be small (43,44). First, the high sine-wave frequency will minimize gating charge effects (44).…”

“…With these settings, the contribution of Na ϩ channel gating charge movements is likely to be small (43,44). First, the high sine-wave frequency will minimize gating charge effects (44). Second, the observed ⌬C m is unlikely to be generated directly by gating charge movements, which are much faster.…”