Sea anemone toxin (ATX II) modulation of heart and skeletal muscle sodium channel alpha-subunits expressed in tsA201 cells

We have expressed recombinant oc-subunits of hH1 (human heart subtype 1), rSkM1 (rat skeletal muscle subtype 1) and hSkM1 (human skeletal muscle) sodium channels in human embryonic kidney cell line, namely the tsA201 cells and compared the effects of ATX II on these sodium channel subtypes. ATX II slows the inactivation phase of hH1 with little or no effect on activation. At intermediate concentrations of ATX II the time course of inactivation is biexponential due to the mixture of free (fast component, tau(h)(fast)) and toxin-bound (slow component, tau(h)(slow)) channels. The relative amplitude of component, tau(h)(slow) allows an estimate of the IC50 values similar to 11 nM. The h slowing of inactivation in the presence of ATX II is consistent with destabilization of the inactivated state by toxin binding. Further evidence for this conclusion is: (i) The voltage-dependence of the current decay time constants (tau(h)) is lost or possibly reversed (time constants plateau or increase at more positive voltages in contrast to these of untreated channels). (ii) The single channel mean open times are increased by a factor of two in the presence of ATX II. (iii) The recovery from inactivation is faster in the presence of ATX II. Similar effects of ATX II on rSkM1 channel behavior occur, but only at higher concentrations of toxin (IC50 = 51 nM). The slowing of inactivation on hSkM1 is comparable to the one seen with rSkM1. A residual or window current appears in the presence of ATX II that is similar to that observed in channels containing mutations associated with some of the familial periodic paralyses.