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Permeation of Na+ through open and Zn(2+)-occupied conductance states of cardiac sodium channels modified by batrachotoxin: exploring ion-ion interactions in a multi-ion channel
- Source :
- Biophysical Journal. (3):654-666
- Publisher :
- The Biophysical Society. Published by Elsevier Inc.
-
Abstract
- Mammalian heart sodium channels inserted into planar bilayers exhibit a distinctive subconductance state when single batrachotoxin-modified channels are exposed to external Zn2+. The current-voltage behavior of the open state and the Zn(2+)-induced substate was characterized in the presence of symmetrical Na+ ranging from 2 to 3000 mM. The unitary conductance of the open state follows a biphasic dependence on [Na+] that can be accounted for by a 3-barrier-2-site model of Na+ permeation that includes double occupancy and Na(+)-Na+ repulsion. The unitary conductance of the Zn2+ substate follows a monophasic dependence on [Na+] that can be explained by a similar 3-barrier-2-site model with low affinity for Na+ and single occupancy due to repulsive interaction with a Zn2+ ion bound near the external entrance to the pore. The apparent association rate of Zn2+ derived from dwell-time analysis of flickering events is strongly reduced as [Na+] is raised from 50 to 500 mM. The apparent dissociation rate of Zn2+ is also enhanced as [Na+] is increased. While not excluding surface charge effects, such behavior is consistent with two types of ion-ion interactions: 1) A competitive binding interaction between Zn2+ and Na+ due to mutual competition for high affinity sites in close proximity. 2) A noncompetitive, destabilizing interaction resulting from simultaneous occupancy by Zn2+ and Na+. The repulsive influence of Zn2+ on Na+ binding in the cardiac Na+ channel is similar to that which has been proposed to occur between Ca2+ and Na+ in structurally related calcium channels. Based on recent mutagenesis data, a schematic model of functionally important residues in the external cation binding sites of calcium channels and cardiac sodium channels is proposed. In this model, the Zn(2+)-induced subconductance state results from Zn2+ binding to a site in the external vestibule that is close to the entrance of the pore but does not occlude it.
- Subjects :
- inorganic chemicals
Cation binding
Sodium
Lipid Bilayers
Molecular Sequence Data
Analytical chemistry
Biophysics
chemistry.chemical_element
In Vitro Techniques
Binding, Competitive
Models, Biological
Biophysical Phenomena
Permeability
Sodium Channels
chemistry.chemical_compound
Dogs
Animals
Amino Acid Sequence
Batrachotoxins
Lipid bilayer
Ion channel
Binding Sites
Voltage-dependent calcium channel
Sequence Homology, Amino Acid
Chemistry
Sodium channel
Myocardium
Electric Conductivity
Conductance
Crystallography
Kinetics
Zinc
Thermodynamics
Batrachotoxin
Cattle
Calcium Channels
Research Article
Subjects
Details
- Language :
- English
- ISSN :
- 00063495
- Issue :
- 3
- Database :
- OpenAIRE
- Journal :
- Biophysical Journal
- Accession number :
- edsair.doi.dedup.....fd7526cc1a1dcbaa2a80d91d13aa4923
- Full Text :
- https://doi.org/10.1016/S0006-3495(94)80839-2