Blockade of HERG channels expressed in Xenopus laevis oocytes by external divalent cations

We have investigated actions of various divalent cations (Ba2+, Sr2+, Mn2Co2+, Ni2+, Zn2+) On human ether-a-go-go related gene (HERG) channels expre ssed in Xenopus laevis oocytes using the voltage clamp technique. All dival ent cations inhibited HERG current dose-dependently in a voltage-dependent manner. The concentration for half-maximum inhibition (K-i) decreased at mo re negative potentials, indicating block is facilitated by hyperpolarizatio n, K-i at 0 mV for Zn2+, Ni2+, Co2+ Ba2+, Mn2+, and Sr2+ was 0.19 0,36, 0.5 0, 0.58, 2.36, and 6.47 mM, respectively. The effects were manifested in fo ur ways: 1) right shift of voltage dependence of activation, 2) decrease of maximum conductance, 3) acceleration of current decay, and 4) slowing of a ctivation, However, each parameter was not affected by each cation to the s ame extent. The potency for the shift of voltage dependence of activation w as in the order Zn2+ > Ni2+ greater than or equal to Co2+ > Ba2+ > Mn2+ > S r2+, whereas the potency for the decrease of maximum conductance was Zn2+ > Ba2+ > SPC > Co2+ > Mn2+ The kinetics of activation and deactivation were also affected, but the two parameters are not affected to the same extent. Slowing of activation by Ba2+ was most distinct, causing a marked initial d elay of current onset. From these results we concluded that HERG channels a re nonselectively blocked by most divalent cations from the external side, and several different mechanism are involved in their actions, There exist at least two distinct binding sites for their action: one for the voltage-d ependent effect and the other for reducing maximum conductance.