Gating current harmonics. I. Sodium channel activation gating in dynamic steady states

Abstract

Internally perfused and pronase-treated giant axons were prepared for gating current measurements. Gating current records were obtained under large-amplitude sinusoidal voltage clamp after allowing for settling times into dynamic steady states. The current records were analyzed as functions of the mean membrane potential of the test sinusoid for which the amplitude and frequency were held constant. The nonlinear analysis consisted of determining the harmonic content (amplitudes and phases) of the distorted periodic current records. The most pronounced feature found in the analysis is a dominant second harmonic centered at Emean = +10 mV. A number of other characteristic harmonic behaviors were also observed. The harmonics tend to die away for very small (less than -60 mV) and very large (greater than +72 mV) values of Emean. The harmonic behavior seen in the axonal data is basically different from that seen in gating current simulations generated by the sodium-activation kinetics of standard models, including the Hodgkin-Huxley model. Some of the differences can be reconciled without requiring fundamental changes in the model kinetic schemes. However, the dominant harmonic feature seen in the axonal data cannot be reconciled with the model kinetics without a fundamental change in the models. The axonal data suggest two moving molecular components with independent degrees of freedom whose properties are outlined on the basis of the data presented herein.

A Resonance Model gives the Response to Membrane Potential for an Ion Channel: II. Simplification of the Calculation, and Prediction of Stochastic Resonance

“…At either end of the range, double peaks are not found, although the curve shows considerable distortion of the original sinusoidal form (the second harmonic would not be zero near the ends of the range). With the center potential in the range in which the open probability is between about 0.2 and 0.8, there is a double peak, and this roughly corresponds to the range in which Fohlmeister & Adelman (1985) found their double peaks. The second adjustable parameter here, the time delay, is chosen so that the curves look qualitatively reasonable.…”

“…A time delay is still required to reproduce the asymmetry, but it is simpler, adding a declining exponential fraction of the preceding current steps. Qualitatively, the results are similar; we made no attempt, either with the old form of the model or the new, to translate from the time domain to the frequency domain by calculating the second and third harmonics; therefore, it was not possible to try to "t the Fohlmeister & Adelman (1985) results quantitatively. It is not clear that the Fohlmeister and Adelman data were presented in such a way that we could have attempted a quantitative "t. However, we did "nd that we could not even obtain a qualitative "t with the Boltzmann model; no double peak could be found.…”

“…A second experimental "nding (after the current}voltage behavior) was the principal focus of the preceding paper on the model. Fohlmeister & Adelman (1985) studied the response of the gating current of a voltage gated sodium channel to an imposed 70 mV peak to peak sinusoidal potential. They found an asymmetric double peak in the rising phase of the sine wave, and a deep single trough in the falling phase.…”

Applications of Nonequilibrium Response Spectroscopy to the Study of Channel Gating. Experimental Design and Optimization

“…The idea of using such fluctuating voltages is not entirely new. Various frequency sine waves have been applied to sodium channels (Fohlmeister & Adelman, 1985a, b, 1986, 1987 and the presence of harmonics in gating current records was tested. This paper is attempted as a first systematic study of the various voltage waveforms and their applications to particular goal.…”

“…Although there have been a few exceptions (Takashima, 1978;Taylor & Bezanilla, 1979;Fernandez et al, 1982;Fohlmeister & Adelman, 1985a, b, 1986, 1987, conventional studies of gating have been based almost exclusively on stepped-potential type experiments ever since the very first investigations (Marmont, 1949;Cole, 1949;Hodgkin et al, 1949. Although these methods have been applied with increasing sophistication and cleverness, the resulting data sets only allow for a rather ambiguous determination of the underlying molecular-kinetic details (Armstrong, 1981;Hille, 1992, pp.…”