Ba²⁺ blockade. On rate constant of Ba²⁺ blockade as a function of membrane potential. The product of barium block on rate constant times the local barium concentration, k_on[Ba], was calculated from the relaxation experiments. Since barium block is voltage dependent the kinetics of barium entry to the blocking site can be examined using voltage jump experiments and observing the decay of the current as channels are blocked. Constants k_on[Ba] and k_off were calculated from the extent of the blockade and the time constant according to Eqs. 3 and 4. In this figure, the logarithm of the on rate constant standardized to 1 μM bulk barium concentration is plotted as a function of membrane potential for the WT channels (black symbols) and the E386N/E389N double mutant BK channels. The vertical distance of the two straight lines is Δφ times 2F/RT according to Eq. 7, where Δφ is 17 ± 2 mV.