Title

Limits and contributing mechanisms for big single-channel currents in BK channels

Date of Award

2003

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Physiology and Biophysics

First Committee Member

Karl L. Magleby, Committee Chair

Abstract

BK channels have a larger conductance than any other K + channels. Although such a large unitary conductance would decrease the number of BK channels required to control cellular excitability, the limits of the single-channel currents through BK channels and mechanisms underlying the large conductance are not known. The first section of my study is concerned with the voltage and K+ dependent limits of single-channel currents through BK channels. The second and third sections are concerned with mechanisms underlying the large conductance.In the first section I report single-channel currents through wild type BK channels obtained over a range of voltages and K+i . The maximal observed single-channel currents were ∼150 pA in 3.4 M K+i corresponding to a transfer rate of ∼ one K+ ion per ns. For all examined K+ i there was an indication that the single-channel currents become sublinear at large voltages. Single-channel currents at extreme positive voltages increased sublinearly with increase in K+i, suggesting that the sublinearity of single channel currents through BK channels at high voltage could not be explained solely by diffusion limited entry of K+ ions from the bulk solution into the intracellular vestibule of the channel.In the second section I show by sequence comparison of BK channels to lower conductance K+ channels, that BK channels have a ring of eight negatively charged glutamate residues at the entrance to the intracellular vestibule that is absent in lower conductance K+ channels. I found that this ring of charge doubles the conductance of BK channels by increasing the concentration of K+ in the intracellular vestibule through an electrostatic mechanism. Thus, a simple electrostatic mechanism contributes to the large conductance of BK channels.In the third section I show that intracellular protons (H+ i) block BK channels. The H+i block of BK channels could not be described by the Woodhull equation commonly used to model voltage-dependent non-competitive block. Data over a range of K +i and pHi indicates that H+ i block of BK channels is relieved by increasing K+ i, consistent with a competitive interaction between H+ i and K+i. Removing the ring of negative charge decreases proton block by 33%.

Keywords

Biology, Neuroscience; Biophysics, General

Link to Full Text

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