Modulation of cloned skeletal muscle sodium channels the scorpion toxins Lqh II, Lqh III, and LqhαIT

The scorpion α-toxins Lqh II, Lqh III, and LqhαIT from Leiurus quinquestriatus hebraeus are representatives of typical α-toxins, specific for either mammals (Lqh II) or insects (LqhαIT), and α-like toxins (Lqh III) which act on both mammals and insects. For a comparative study of the effects of these toxins on mammalian sodium channels we stably expressed rat skeletal muscle sodium channel α subunits (μI) in HEK 293 cells and measured Na⁺ currents in the whole-cell patch-clamp mode. The α- and α- like toxins strongly slowed down channel inactivation with a half-maximal effect at 1.4 nM (Lqh II), 5.4 nM (Lqh III), and 0.5 nM (LqhαIT). The recovery from fast inactivation was accelerated by all toxins with the potency sequence: Lqh II>LqhαIT> Lqh III. The voltage dependence of inactivation and recovery from inactivation were reduced while the threshold for activation was only slightly shifted by ≃10 mV without altering the slope factors, suggesting uncoupling of the impaired inactivation from the activation. The toxins induced an increase in peak inward current, which was accounted for by an increased maximal open-channel probability. Although all three toxins induced similar modifications of the channel properties, their kinetics of association and dissociation were very different. Between -140 and -80 mV toxin association was not voltage dependent. In 100 nM toxin the association time constants were: 1.3 s (Lqh II), 20 s (Lqh III), and 3.8 s (LqhαIT). At positive voltages the toxin dissociated from the channel; at ±100 mV the dissociation time constants were 30, 321, and 135 ms, respectively. In contrast to the association, dissociation was voltage dependent with a similar slope of about 12 mV per e-fold change for all three toxins. The strong differences in the association and dissociation kinetics of these toxins may identify them as members of different scorpion α-toxin subgroups.