Steady-state and closed-state inactivation properties of inactivating BK channels

Calcium-dependent potassium (BK-type) Ca²⁺ and voltage-dependent K⁺ channels in chromaffin cells exhibit an inactivation that probably arises from coassembly of Slo1 α subunits with auxiliary β subunits. One goal of this work was to determine whether the Ca²⁺ dependence of inactivation arises from any mechanism other than coupling of inactivation to the Ca²⁺ dependence of activation. Steady-state inactivation and the onset of inactivation were studied in inside-out patches and whole-cell recordings from rat adrenal chromaffin cells with parallel experiments on inactivating BK channels resulting from cloned α + β2 subunits. In both cases, steady-state inactivation was shifted to more negative potentials by increases in submembrane [Ca²⁺] from 1 to 60 μM. At 10 and 60 μM Ca²⁺, the maximal channel availability at negative potentials was similar despite a shift in the voltage of half availability, suggesting there is no strictly Ca²⁺-dependent inactivation. In contrast, in the absence of Ca²⁺, depolarization to potentials positive to +20 mV induces channel inactivation. Thus, voltage-dependent, but not solely Ca²⁺-dependent, kinetic steps are required for inactivation to occur. Finally, under some conditions, BK channels are shown to inactivate as readily from closed states as from open states, indicative that a key conformational change required for inactivation precedes channel opening.