Figure 5.

The change in evoked BK_Ca currents with time in HEK 293 cells expressing the α and β₁ subunit (hSloα + β₁) of the BK_Ca channel. HEK 293 cells were voltage clamped at −40 mV and currents evoked by changing the voltage to a range of test potentials (−40 to 60 mV). (Ai) illustrates evoked currents in a HEK 293 cell at the beginning of the recording period; (Aii) illustrates evoked currents after 350 s of recording. (B) The blocking properties of IBTX on BK_Ca channels formed from α + β₁ subunits. Currents were evoked at +60 mV from a holding potential of −40 mV. (n = 6). (C) The activation time constant for currents evoked by changing the potential to +60 mV, the activation of the BK_Ca current could be fitted to a single exponential (n = 5). The β₁ subunit has slowed macroscopic current kinetics at the subnanomolar [Ca⁺⁺] employed during these experiments. The free intracellular calcium ion concentration in panels Ai, Aii, B and C was 0.2 nM. (D) The effects of free [Ca⁺⁺] on the hSloα+β₁ G-V relationship. Conductance was calculated from current and voltage, normalized to the maximum, and plotted error bars represent SEM. Solid curves represent fits to the Boltzmann function. V(1/2) for 0.2 nM, 2.4 μM and 55 μM free calcium were 108.4 ± 4 mV, 62.6 ± 1.3 mV and 5.1 ± 3.1 mV respectively.