Divalent cations as probes for structure-function relationships of cloned voltage-dependent sodium channels |
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Authors: | M Pusch |
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Institution: | (1) Max-Planck-Institut für biophysikalische Chemie, Am Fassberg, D-3400 Göttingen, Federal Republic of Germany;(2) Present address: Istituto di Cibernetica e Biofisica, CNR, Via Dodecaneso 33, I-16146 Genova, Italy |
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Abstract: | 1. Several cloned sodium channels were expressed in oocytes and compared with respect to their sensitivity to internal Mg2+ concerning the open-channel block and to external Ca2+ concerning open-channel block and shifts in steady-state activation. 2. A quantitative comparison between wild-type II channels and a mutant with a positive charge in the S4 segment of repeat I neutralized (K226Q) revealed no significant differences in the Mg2+ block. 3. The blocking effect of extracellular Ca2+ ions on single-channel inward currents was studied for type II, mutant K226Q and type III. A quantitative comparison showed that all three channel types differ significantly in their Ca+ sensitivity. 4. The influence of extracellular Ca2+ on the voltage dependence of steady-state activation of macroscopic currents was compared for type II and K226Q channels. Extracellular Ca+ increases the voltage of half-maximal activation, V1/2, more for K226Q than for wild-type II channels; a plot of V
1/2 against Ca]
o
, is twice as steep for the mutant K226Q as for the wild-type on a logarithmic concentration scale. 5. The differential effects of extracellular Ca+ and intracellular Mg2+ on wild-type II and K226Q channels are discussed in terms of structural models of the Na+ channel protein.Abbreviations Na]
i
intracellular Na+ concentration
- Mg]
intracellular Mg2+ concentration
- Ca]
o
extracellular Ca2+ concentration |
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Keywords: | Sodium channel Divalent cations Mg2+ block Patch clamp Transmembrane topology |
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