Definition of current components. (A) Traces on left show currents from a mouse CC in a slice with largely inactivating BK currents. The first two traces highlight the peak current activated by a direct step to 110 mV (red trace) and then the additional outward current activated by the step 110 mV after a 50-ms loading step to 10 mV (blue trace). Longer loading steps result in larger outward current at 100 mV. Black arrows show maximal total amount of current that activates as a consequence of the Ca²⁺ loading step to 10 mV above that activated by a direct step to 80 mV (red trace). Red arrows identify the fraction of that current that is inactivated by this protocol. On the right, the first two traces on an expanded time base highlight the peak current activated by a direct step to 110 mV (red trace; I_Kv) and the net inward current activated by a step to 10 mV (blue trace; I_in). (B) Traces show currents from a mouse CC with predominantly noninactivating BK current. The red arrow highlights the fraction that may inactivate (but which may also reflect Ca²⁺ clearance). (C) Frequency distribution of current decays from mouse (black) and rat (red) CCs recorded in slices is plotted as a function of decay time constant. Time constants were fit to the decaying phase of current after the longest loading step in the family of traces (as in A and B). The red arrow highlights an arbitrary point separating the more rapidly and more slowly decaying currents in mouse. In mouse, 45% of cells had BK currents decaying faster than 110 ms. In rat, 85% of cells decayed faster than 110 ms. (D) The decay time constants from cells as in C are plotted as a function of the fraction of sustained BK current in the same cell, as defined from the current that persists (red arrows in A and B). The red arrow in D defines the arbitrary separation of cells with largely inactivating current (BK_i) and largely noninactivating current (BK_s) used here.