A simple K^{+} channel kinetic model predicts STX effects through shifting activation and inactivation gating. Simulation of STX action through modification of voltage-dependent rate constants. The model employed was that of Johnson et al. (1999a) as modified from Wang et al. (1997). The effects of STX were simulated by perturbing the voltage-dependent rate constants governing activation (α and β) and inactivation (κ and λ). A 30 mV (α and β) and a 10 mV (κ and λ) shift (ΔV) was added to the voltage-dependent term of the rate constant master equations to mimic the effect of STX. The top panel shows the voltage clamp protocol and the perturbation (ΔV) to the rate equations is indicated as a small step. As indicated schematically in the top of A, the rate constant equations were temporarily modified for 3 s during the voltage step, beginning 3 s after the step initiation. The applied voltage and hence driving force were not actually changed. The dotted lines in the P_{open} traces indicate the control behavior in the absence of the shift. The model is shown schematically where α, β, κ, and λ are voltage-dependent rate constants defined by the following equations. Closed, open, and inactivated (closed also) states are symbolized as C, O, and I, respectively. α = α_{o} exp ^{[z δ e (V + DVa) / k}_{B}^{/ T]} forward; β = β_{o} exp ^{[ z (1− δ) e (V + DVa)/k}_{B}^{/T]} reverse; κ = κ_{o} exp ^{[z δ e (V + DVi)/k}_{B}^{/T]} forward; λ = λ_{o} exp ^{[ z (1− δ) e (V + DVi) / k}_{B}^{/ T]} reverse. α is the forward rate constant (s^{−1}); α_{o} is value of α in the absence of an electric field. zδ the gating charge and the fraction of the field it senses, e the electron charge, k_{B} is Boltzmann's constant, T is the absolute temperature, V is membrane potential, and ΔV is a bias potential that can be applied to the equations. The effect is to shift the log linear rate constant relationship along the voltage axis.

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