Biophysical Journal 74: 199-209 (1998)
© 1998 the Biophysical Society

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Biophys J, January 1998, p. 199-209, Vol. 74, No. 1

A Novel Type of Internal Barium Block of a Maxi-K⁺ Channel from Human Vas Deferens Epithelial Cells

 ^*Department of Physiological Sciences, University Medical School, Newcastle upon Tyne NE2 4HH, and  ^#Paediatric Molecular Genetics, Institute of Molecular Medicine, Oxford University, John Radcliffe Hospital, Oxford OX3 9DU, England

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 ~100 µM. 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 ~310 s¹, 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.

Biophys J, January 1998, p. 199-209, Vol. 74, No. 1
© 1998 by the Biophysical Society   0006-3495/98/01/199/11  $2.00

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