External barium block of Shaker potassium channels: Evidence for two binding sites

Abstract

External barium ions inhibit K+ currents of Xenopus oocytes expressing ShH4 Δ646, the non-inactivating deletion of the Shaker K+ channel. At the macroscopic level, Ba2+ block comprises both a fast and a slow component. The fast component is less sensitive to Ba2+ (apparent dissociation constant at 0 mV, K((0)), ≃ 19.1 mM) than the slow component and is also less voltage dependent (apparent electrical distance, δ, ≃ 0.14). The slow component (K((0)), ≃ 9.4 mM, δ ≃ 0.25) is relieved by outward K+ current, which suggests that the corresponding binding site resides within the channel conduction pathway. At the single channel level, the fast component of block is evidenced as an apparent reduction in amplitude, suggesting an extremely rapid blocking and unblocking reaction. In contrast, the slow component appears to be associated with long blocked times that are present from the beginning of a depolarizing command. Installation of the slow component is much slower than a diffusion limited p