Interaction of scorpion α-toxins with cardiac sodium channels:: Binding properties and enhancement of slow inactivation

The effects of the scorpion alpha -toxins Lqh II, Lqh III, and Lqh alpha IT on human cardiac sodium channels (hH1), which were expressed in human embryonic kidney (HEK) 293 cells, were investigated. The toxins removed fast inactivation with EC50 Values of <2.5 nM (Lqh III), 12 nM (Lqh II), and 33 nM (Lqh alpha IT). Association and dissociation rates of Lqh III were much slower than those of Lqh II and Lqh alpha IT, such that Lqh III would not dissociate from the channel during a cardiac activation potential. The voltage dependence of toxin dissociation from hi-II channels was nearly the same for all toxins tested, but it nas different from that found for skeletal muscle sodium channels (muI; Chen et al., 2000). These results indicate that the voltage dependence of toxin binding is a property of the channel protein. Toxin dissociation remained voltage dependent even at high voltages where activation and fast inactivation is saturated, indicating that the voltage dependence originates from other sources. Slow inactivation of hH1 and muI channels was significantly enhanced by Lqh II and Lqh III. The half-maximal voltage of steady-state slow inactivation was shifted to negative values, the voltage dependence was increased, and, in particular for hH1, slow inactivation at high voltages became more complete. This effect exceeded an expected augmentation of slow inactivation owing to the loss of fast inactivation and, therefore, shows that slow sodium channel inactivation may be directly modulated by scorpion a-toxins.