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%.