fig7: Reduction of peak tail currents suggests fast block at negative membrane potentials. Remaining fractions of steady-state currents shown in Fig. 3 B are replotted in gray (10 μM C10: open squares; 100 μM C10: filled squares), with the fitting of voltage dependence (gray lines) extrapolated into the negative membrane potentials. The black symbols are the remaining fractions of peak tail currents after the application of 10 μM (open squares) or 100 μM C10 (filled squares). Each data point represents the mean ± SEM determined from 6–7 patches. Note that error bars are often smaller than the symbols.

Mentions: Why does C10 speed up the deactivation process rather than slowing it down? It is known that extremely slow open-channel blockers can result in an apparent speeding of tail currents, because a very slow component due to unblock is too small to detect in this case (Colquhoun et al., 1979). However, several lines of evidence suggest that this mechanism doesn't underlie the effect of C10 on BK currents. First, based on the voltage dependence of the rate constants, the extrapolated values suggest that C10 remains a very fast blocker at negative membrane potentials in spite of the decrease in apparent affinity. Second, if C10 becomes an extremely slow blocker at negative potentials, then the beginning of the tail currents should have the same amount of block as in the steady-state currents before repolarization, because it takes a long time for the block to reach a new equilibrium. Fig. 7 clearly shows that it is not the case. Peak tail currents at negative potentials are much less blocked than the steady-state currents in the preceding depolarization (120 mV). Gray markers and lines are from Fig. 3 B, indicating levels of block in steady-state currents at more positive potentials. Interestingly, the fitting of voltage-dependent steady-state block at positive potentials can predict the level of block in peak tail currents at negative potentials. This suggests that C10 is an extremely fast blocker at negative potentials so that the block at the beginning of repolarization reaches its new equilibrium right away. This can explain the lack of a “hook” in the tail currents, because the process of developing a new equilibrium is too fast to resolve in our measurement. Expanded views at the beginning of tail currents show that they always peak at the same time point with or without C10 (unpublished data). Additionally, the fact that the voltage dependence holds across a wide range of voltage corroborates our measurement of voltage dependence in C10 block.

Li W, Aldrich RW - J. Gen. Physiol. (2004)

Bottom Line: We show that molecules as large as decyltriethylammonium (C(10)) and tetrabutylammonium (TBA) have much faster block and unblock rates in BK channels when compared with any other tested K(+) channel types.Based on these findings we propose that BK channels may differ from other K(+) channels in its geometrical design at the inner mouth, with an enlarged cavity and inner pore providing less spatially restricted access to the cytoplasmic solution.These features could potentially contribute to the large conductance of BK channels.

Affiliation: Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, CA 94305, USA. raldrich@stanford.edu

Abstract: Potassium channels have a very wide distribution of single-channel conductance, with BK type Ca(2+)-activated K(+) channels having by far the largest. Even though crystallographic views of K(+) channel pores have become available, the structural basis underlying BK channels' large conductance has not been completely understood. In this study we use intracellularly applied quaternary ammonium compounds to probe the pore of BK channels. We show that molecules as large as decyltriethylammonium (C(10)) and tetrabutylammonium (TBA) have much faster block and unblock rates in BK channels when compared with any other tested K(+) channel types. Additionally, our results suggest that at repolarization large QA molecules may be trapped inside blocked BK channels without slowing the overall process of deactivation. Based on these findings we propose that BK channels may differ from other K(+) channels in its geometrical design at the inner mouth, with an enlarged cavity and inner pore providing less spatially restricted access to the cytoplasmic solution. These features could potentially contribute to the large conductance of BK channels.