State-dependent barium block of wild-type and inactivation-deficient HERG channels in Xenopus oocytes

1. The effects of Ba2+ on current resulting from the heterologous expression of the human ether-a-go-go related gene (HERG) (I-HERG) was studied with two-electrode voltage clamp techniques in Xenopus oocytes. 2, Ba2+ produced time- and voltage-dependent block of I-HERG. Significant inhibition was seen at concentrations as low as 1 mu M. Inhibition was greatest at step potentials between -40 and 0 mV; at mure positive potentials, inhibition decreased in association with time-dependent unblocking of channels. 3. An inactivation-atteauated mutant of HERG (S631A) was prepared and expressed in Xenopus oocytes. Ba2+ block of X631A differed from that of HERG in that extensive unblocking was no longer seen at positive potentials and the voltage dependence of step current block was greatly attenuated. 4. A mathematical model was applied to analyse quantitatively the inhibitory effects of Ba2+ on I-HERG. The model suggested similar voltage-dependent affinity of Ba2+ for the open and closed states, along with absence of binding to the inactivated state, and accounted well for Ba2+ effects on both wild-ty-pe and S631A channels. 5. We conclude that B2+ potently inhibits I-HERG in a characteristic state-dependent fashion, with strong unblocking at positive potentials related to the presence of an intact C-type inactivation mechanism.