Effect of depolarization on binding kinetics of scorpion α-toxin highlights conformational changes of rat brain sodium channels

Binding of scorpion a-toxins to receptor site 3 on voltage-gated sodium channels inhibits,sodium current inactivation and is voltage-dependent. To reveal the direct effect of depolarization, we analyzed binding, kinetics of the a-toxin Lqh-II (from Leiurus quinquestriatus hebraeus) to rat brain Synaptosomes and effects on rat brain II (rBII) channels expressed in mammalian cells. Our results indicated that the 33-fold decrease in toxin affinity for depolarized (0 mV, 90 mM [K+](out), K-d = 5.85 +/- 0.5 nM) versus polarized (-55 mV. 5 mM [K+](out), K-d = 0.18 +/- 0.04 nM) synaptosomes at steady state results from a 48-fold reduction in the association rate (k(on) at 5 mM [K+] = (12.0 +/- 4) x 10(6) M-1 s(-1) and (0.25 +/- 0.03) x 10(6) M-1 s(-1) at 90 mM [K+](out)) with nearly no change in the dissociation rate. Electrophysiological analyses of rBII channels expressed in mammalian cells revealed that approximately 75% and 40% of rBII occupied fast- and slow-inactivated states, respectively, at resting membrane potential of synaptosomes (-55 mV), and Lqh-II markedly increased the steady-state fast and slow inactivation. To mimic electrophysiological conditions we induced fast depolarization of toxin-bound synaptosomes, which generated a biphasic unbinding of Lqh-II from toxin-receptor complexes. The first fast off rate closely resembled values determined electrophysiologically for rBII in mammalian cells. The second off rate was similar to the voltage-independent steady-state value, attributed to binding to the slow-inactivated channel states. Thus. the Lqh-II voltage-dependent affinity highlights two independent mechanisms representing conformational changes of sodium channels associated with transitions among electrically visible and invisible inactivated states.