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Charge movement in the sodium channel

The Journal of General Physiology
The Rockefeller University Press
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1051.tif Commentary Charge Movement in the Sodium Channel FREDJ. SIGWORTH From the Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520 In this issue of The Journal of General Physiology, Hirschberg, Rovner, Lieberman, and Patlak (1995) present a new estimate of the charge displacement that occurs in the activation of voltage-gated sodium channels. This kind of estimate is not new- Hodgkin and Huxley (1952) pointed out that the movement of at least six elementary charges (e0) must somehow accompany the activation of the sodium conductance. The surprise from the new result of Hirschberg et al. (1995) is that the estimated charge movement, ~'-'12 e0, is very much larger than previously thought, presenting some puzzles about the identity of the mobile charges. Recent studies have presented two kinds of evidence for charge movements of ~12 e0, in voltage-gated Shaker potassium channels. One set of evidence comes from quantitative measurements of gating currents. In these experiments the charge movement is measured as a displacement current, induced by membrane potential changes, that flows even when the ionic current through the channels is blocked by toxins or by the elimination of permeant ions. To normalize the mea- surement to charge movement in a single channel, the number of channels is esti- mated either through analysis of the ionic currents (e.g., before blocking the chan- nels; Schoppa et al., 1992) or through quantitative toxin binding (Aggarwal and MacKinnon, 1994). From these measurements the total charge movement per channel is estimated to be 12 to 14 elementary charges. Voltage-sensitivity Measurements The other line of evidence involves the measurement of the voltage sensitivity of channel activation, as Hodgkin and Huxley (1952) did. The merits and limitations of this approach have been known for some time (Almers, 1978), but are summa- rized briefly here. Consider a channel w

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