FIGURE 5.

Fast application of Ba²⁺ during a long depolarizing pulse reveals very weak open-channel block by 20 mM Ba²⁺. Current traces in response to depolarizing pulses to +50 mV for 10 s in 0 mM Ba²⁺-free solution (black trace) and with a rapid application and withdrawal of a 0 mM 20 mM Ba²⁺ solution during a subsequent pulse (gray trace). Down and up arrows indicate the start and end of the Ba²⁺ application, respectively. Exposure to Ba²⁺ caused a rapid fall in current amplitude of ∼10%. A similar effect was observed in six other cells.