The Functional Surface Charge Density of a Fast K Channel in the Myelinated Axon of Xenopus laevis |
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Authors: | F Elinder P Århem |
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Institution: | (1) The Nobel Institute for Neurophysiology, Department of Neuroscience, Karolinska Institutet, S-171 77 Stockholm, Sweden, SE |
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Abstract: | The action of Mg2+ on the putative xKv1.1 channel in the myelinated axon of Xenopus laevis was analyzed in voltage clamp experiments. The main effect was a shift in positive direction of the open probability curve
(16 mV at 20 mm Mg2+), calculated from measurements of the instantaneous current at Na reversal potential after 50–100 msec steps to different
potentials. The shift was measured at an open probability level of 25% to separate it from shifts of other K channel populations
in the nodal region. The results could be explained in terms of screening effects on fixed charges located on the surface
of the channel protein. Using the Grahame equation the functional charge density was estimated to −0.45 e nm−2. Analyzing this value, together with previously estimated values from other K channels, with reference to the charge of different
extracellular loops of the channel protein, we conclude that the loop between the transmembrane S5 segment and the pore forming
P segment determines the functional charge density of voltage-gated K channels.
Received: 11 December 1997/Revised: 24 April 1998 |
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Keywords: | : Myelinated axon — Voltage clamp — K channels — Surface charges — Magnesium |
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