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.
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.
Institut de Pharmacologie, de l'Universite de Lausanne, Switzerland.
L
L
Schild
E
E
Moczydlowski
D000595:Amino Acid Sequence; D000818:Animals; D001496:Batrachotoxins / Q000494:pharmacology; D001665:Binding Sites; D001667:Binding, Competitive; D055592:Biophysical Phenomena; D001703:Biophysics; D015220:Calcium Channels / Q000235:genetics / Q000378:metabolism; D002417:Cattle; D004285:Dogs; D004553:Electric Conductivity; D066298:In Vitro Techniques; D007700:Kinetics; D008051:Lipid Bilayers / Q000378:metabolism; D008954:Models, Biological; D008969:Molecular Sequence Data; D009206:Myocardium / Q000378:metabolism*; D010539:Permeability; D017386:Sequence Homology, Amino Acid; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D013816:Thermodynamics; D015032:Zinc / Q000378:metabolism
D001496:Batrachotoxins; D015220:Calcium Channels; D008051:Lipid Bilayers; D015222:Sodium Channels; D012964:Sodium; D015032:Zinc