Zn2(+)-induced subconductance events in cardiac Na+ channels prolonged by batrachotoxin. Current-voltage behavior and single-channel kinetics.
The mechanism of voltage-dependent substate production by external Zn2+ in batrachotoxin-modified Na+ channels from canine heart was investigated by analysis of the current-voltage behavior and single-channel kinetics of substate events. At the single-channel level the addition of external Zn2+ results in an increasing frequency of substate events with a mean duration of approximately 15-25 ms for the substate dwell time observed in the range of -70 to +70 mV. Under conditions of symmetrical 0.2 M NaCl, the open state of cardiac Na+ channels displays ohmic current-voltage behavior in the range of -90 to +100 mV, with a slope conductance of 21 pS. In contrast, the Zn2(+)-induced substate exhibits significant outward rectification with a slope conductance of 3.1 pS in the range of -100 to -50 mV and 5.1 pS in the range of +50 to +100 mV. Analysis of dwell-time histograms of substate events as a function of Zn2+ concentration and voltage led to the consideration of two types of models that may explain this behavior. Using a simple one-site blocking model, the apparent association rate for Zn2+ binding is more strongly voltage dependent (decreasing e-fold per +60 mV) than the Zn2+ dissociation rate (increasing e-fold per +420 mV). However, this simple blocking model cannot account for the dependence of the apparent dissociation rate on Zn2+ concentration. To explain this result, a four-state kinetic scheme involving a Zn2(+)-induced conformational change from a high conductance conformation to a substate conformation is proposed. This model, similar to one introduced by Pietrobon et al. (1989. J. Gen. Physiol. 94:1-24) for H(+)-induced substate behavior in L-type Ca2+ channels, is able to simulate the kinetic and equilibrium behavior of the primary Zn2(+)-induced substate process in heart Na+ channels. This model implies that binding of Zn2+ greatly enhances conversion of the open, ohmic channel to a low conductance conformation with an asymmetric energy profile for Na+ permeation.
Schild
L
L
Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06510.
Ravindran
A
A
Moczydlowski
E
E
D000818:Animals; D001496:Batrachotoxins / Q000494:pharmacology*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004285:Dogs; D004594:Electrophysiology; D004734:Energy Metabolism / Q000187:drug effects; D066298:In Vitro Techniques; D007700:Kinetics; D008954:Models, Biological; D008968:Molecular Conformation; D009132:Muscles / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D015222:Sodium Channels / Q000187:drug effects / Q000378:metabolism*; D015032:Zinc / Q000494:pharmacology*
D001496:Batrachotoxins; D015222:Sodium Channels; D015032:Zinc