We have recently shown that a maxi-K+ channel from vas deferens epithelial cells contains two Ba2+-binding sites accessible from the external side: a "flickering" site located deep in the channel pore and a "slow" site located close to the extracellular mouth of the channel. Using the patch-clamp technique, we have now studied the effect of internal Ba2+ on this channel. Cytoplasmic Ba2+ produced a voltage- and concentration-dependent "slow" type of block with a dissociation constant of approximately 100 microM. However, based on its voltage dependence and sensitivity to K+ concentration, this block was clearly different from the external "slow" Ba2+ block previously described. Kinetic analysis also revealed a novel "fast flickering" block restricted to channel bursts, with an unblocking rate of approximately 310 s(-1), some 10-fold faster than the external "flickering" block. Taken together, these results show that this channel contains multiple Ba2+-binding sites within the conduction pore. We have incorporated this information into a new model of Ba2+ block, a novel feature of which is that internal "slow" block results from the binding of at least two Ba2+ ions. Our results suggest that current models for Ba2+ block of maxi-K+ channels need to be revised.

Original publication

DOI

10.1016/S0006-3495(98)77780-X

Type

Journal article

Journal

Biophys J

Publication Date

01/1998

Volume

74

Pages

199 - 209

Keywords

Barium, Binding Sites, Cells, Cultured, Epithelial Cells, Humans, Ion Channel Gating, Kinetics, Large-Conductance Calcium-Activated Potassium Channels, Male, Membrane Potentials, Models, Biological, Potassium, Potassium Channel Blockers, Potassium Channels, Potassium Channels, Calcium-Activated, Time Factors, Vas Deferens