External divalent cations increase anion-cation permeability ratio in glycine receptor channels.
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.
Sugiharto
Silas
S
Department of Physiology, School of Medical Sciences, University of New South Wales, Sydney, NSW, 2052, Australia.
Carland
Jane E
JE
Lewis
Trevor M
TM
Moorhouse
Andrew J
AJ
Barry
Peter H
PH
D002118:Calcium / Q000378:metabolism; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism*; D002463:Cell Membrane Permeability; D002712:Chlorides / Q000378:metabolism; D006801:Humans; D015640:Ion Channel Gating; D008564:Membrane Potentials; D008962:Models, Theoretical; D018408:Patch-Clamp Techniques; D018009:Receptors, Glycine / Q000235:genetics / Q000378:metabolism*; D015203:Reproducibility of Results; D012964:Sodium / Q000378:metabolism; D013431:Sulfates / Q000378:metabolism; D014162:Transfection
D002712:Chlorides; D018009:Receptors, Glycine; D013431:Sulfates; D012964:Sodium; D002118:Calcium