Glutathione accelerates sodium channel inactivation in excised rat axonal membrane patches

The effects of glutathione were studied on the gating behaviour of sodium channels in membrane patches of rat axons. Depolarizing pulses from –120 to –40 mV elicited sodium currents of up to 500 pA, indicating the simultaneous activation of up to 250 sodium channels. Inactivation of these channels in the excised, inside-out configuration was fitted by two time constants ( h1=0.81 ms; h2= 5.03 ms) and open time histograms at 0 mV revealed a biexponential distribution of channel openings ( short=0.28 ms; long=3.68 ms). Both, the slow time constant of inactivation and the long lasting single channel openings disappeared after addition of the reducing agent glutathione (2–5 mM) to the bathing solution. Sodium channels of excised patches with glutathione present on the cytoplasmatic face of the membrane had inactivation kinetics similar to channels recorded in the cell-attached configuration. These observations indicate that redox processes may contribute to the gating of axonal sodium channels

ATP-Sensitive Potassium Channels Exhibit Variance in the Number of Open Channels below the Limit Predicted for Identical and Independent Gating

In small cells containing small numbers of ion channels, noise due to stochastic channel opening and closing can introduce a substantial level of variability into the cell's membrane potential. Negatively cooperative interactions that couple a channel's gating conformational change to the conformation of its neighbor(s) provide a potential mechanism for mitigating this variability, but such interactions have not previously been directly observed. Here we show that heterologously expressed ATP-sensitive potassium channels generate noise (i.e., variance in the number of open channels) below the level possible for identical and independent channels. Kinetic analysis with single-molecule resolution supports the interpretation that interchannel negative cooperativity (specifically, the presence of an open channel making a closed channel less likely to open) contributes to the decrease in noise. Functional coupling between channels may be important in modulating stochastic fluctuations in cellular signaling pathways

Nonlinear Electrical Effects in Lipid Bilayer Membranes: II. Integration of the Generalized Nernst-Planck Equations

In this paper the ion transport across a thin lipid membrane is treated using a generalized form of the Nernst-Planck equations. An additional term is introduced into the flux equations to account for the image force acting on the ion. As the membrane thickness is of the same order of magnitude as the range of the image forces, the potential energy of the ion in the membrane is strongly dependent on position. The integration of the flux equations leads to a general expression for the integral membrane conductance λ as a function of the voltage u. The ratio λ(u)/λ(0) (λ(0) = membrane conductance in the limit u → 0) depends on the dielectric constant and the thickness of the membrane, but is independent of the ionic radius. When the numerical values of the potential energy function, as calculated by the method of electrical images, are inserted into the expression for λ(u)/λ(0), a strongly non-linear current-voltage characteristic is obtained. The theoretical current-voltage curve agrees satisfactorily with the experimental data at a low ionic strength and at low voltages; at higher voltages the observed membrane conductance exceeds the predicted value

Comparison of the effects of Anemonia toxin II on sodium and gating currents in frog myelinated nerve

Na+ and gating currents were measured in myelinated frog nerve fibres without and in the presence of 7 μM Anemonia toxin II in the extracellular solution. From the experiments, kinetic parameters of Na+ currents and of gating charge displacements during (‘on’ response) and after (‘off’ response) depolarizations were determined. The following parallel modifications of Na+ currents and charge displacements by Anemonia toxin II were observed: the toxin reduces the maximum Na+ permeability and the ‘on’ charge displacement; Na+ activation and ‘on’ charge displacement become faster; Na+ inactivation and the decline of the ‘off’ charge displacement with increasing pulse duration (charge immobilization) are prolonged; slow components of ‘on’ charge displacements are diminished. The observations support the notion that the fast ‘on’ charge displacement is connected with the process of Na+ activation, while Na+ inactivation is linked to charge immobilization. Our experiments suggest that slow ‘on’ charge displacements during longer depolarizations are correlated with the process of Na+ inactivation

Increased charge displacement in the membrane of myelinated nerve at reduced extracellular pH.

Asymmetry currents were measured in nodes of myelinated nerve fibers from Rana esculenta at extracellular pH values of 5.2, 7.0, and 8.1 by averaging the currents during and after 1-ms depolarizing and hyperpolarizing voltage pulses. The charge displacement in the nodal membrane was obtained by numerical integration of the asymmetry currents. Lowering the pH from 7.0 to 5.2 significantly slows down the kinetics of the fast charge displacement during depolarization but hardly affects the kinetics after repolarization. The pH reduction increases the maximum charge displacement during depolarization by 46%. No differences between asymmetry currents were found between pH 7.0 and 8.1. It is concluded that protonation by extracellular H+ ions may increase the net charge or the transition range of mobile subunits in the nerve membrane