title: A novel type of internal barium block of a maxi-K+ channel from human vas deferens epithelial cells.
abstract: 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.
authors.affiliation: Department of Physiological Sciences, University Medical School, Newcastle upon Tyne, England.
authors.forename: Y
authors.initials: Y
authors.lastname: Sohma
authors.forename: A
authors.initials: A
authors.lastname: Harris
authors.forename: B E
authors.initials: BE
authors.lastname: Argent
authors.forename: M A
authors.initials: MA
authors.lastname: Gray
mesh_terms: D001464:Barium / Q000494:pharmacology*; D001665:Binding Sites; D002478:Cells, Cultured; D004847:Epithelial Cells / Q000502:physiology*; D006801:Humans; D015640:Ion Channel Gating; D007700:Kinetics; D051036:Large-Conductance Calcium-Activated Potassium Channels; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D008954:Models, Biological; D011188:Potassium / Q000494:pharmacology; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000737:chemistry / Q000502:physiology*; D024681:Potassium Channels, Calcium-Activated; D013997:Time Factors; D014649:Vas Deferens / Q000502:physiology*
chemical_list: D051036:Large-Conductance Calcium-Activated Potassium Channels; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D024681:Potassium Channels, Calcium-Activated; D001464:Barium; D011188:Potassium