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Pflugers Arch. 2010 Jun;460(1):131-52. doi: 10.1007/s00424-010-0792-6. Epub 2010 Mar 3.

External divalent cations increase anion-cation permeability ratio in glycine receptor channels.

Author information

1
Department of Physiology, School of Medical Sciences, University of New South Wales, Sydney, NSW, 2052, Australia.

Abstract

The functional role of ligand-gated ion channels in the central nervous system depends on their relative anion-cation permeability. Using standard whole-cell patch clamp measurements and NaCl dilution potential measurements, we explored the effect of external divalent ions on anion-cation selectivity in alpha1-homomeric wild-type glycine receptor channels. We show that increasing external Ca(2+) from 0 to 4 mM resulted in a sigmoidal increase in anion-cation permeability by 37%, reaching a maximum above about 2 mM. Our accurate quantification of this effect required rigorous correction for liquid junction potentials (LJPs) using ion activities, and allowing for an initial offset potential. Failure to do this results in a considerable overestimation of the Ca(2+)-induced increase in anion-cation permeability by almost three-fold at 4 mM external Ca(2+). Calculations of LJPs (using activities)_ were validated by precise agreement with direct experimental measurements. External SO (4) (2-) was found to decrease anion-cation permeability. Single-channel conductance measurements indicated that external Ca(2+) both decreased Na(+) permeability and increased Cl(-) permeability. There was no evidence of Ca(2+) changing channel pore diameter. Theoretical modeling indicates that the effect is not surface charge related. Rather, we propose that, under dilution conditions, the presence of an impermeant Ca(2+) ion in the channel pore region just external to the selectivity filter tends to electrostatically retard outward movement of Na(+) ions and to enhance movement of Cl(-) ions down their energy gradients.

PMID:
20198385
DOI:
10.1007/s00424-010-0792-6
[Indexed for MEDLINE]

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