Extrapolating microdomain Ca²⁺ dynamics using BK channels as a Ca²⁺ sensor

Ca²⁺ ions play crucial roles in mediating physiological and pathophysiological processes, yet Ca²⁺ dynamics local to the Ca²⁺ source, either from influx via calcium permeable ion channels on plasmic membrane or release from internal Ca²⁺ stores, is difficult to delineate. Large-conductance calcium-activated K⁺ (BK-type) channels, abundantly distribute in excitable cells and often localize to the proximity of voltage-gated Ca²⁺ channels (VGCCs), spatially enabling the coupling of the intracellular Ca²⁺ signal to the channel gating to regulate membrane excitability and spike firing patterns. Here we utilized the sensitivity and dynamic range of BK to explore non-uniform Ca²⁺ local transients in the microdomain of VGCCs. Accordingly, we applied flash photolysis of caged Ca²⁺ to activate BK channels and determine their intrinsic sensitivity to Ca²⁺. We found that uncaging Ca²⁺ activated biphasic BK currents with fast and slow components (time constants being τ_f≈0.2 ms and τ_s≈10 ms), which can be accounted for by biphasic Ca²⁺ transients following light photolysis. We estimated the Ca²⁺-binding rate constant k_b (≈1.8×10⁸M^-1s^-1) for mSlo1 and further developed a model in which BK channels act as a calcium sensor capable of quantitatively predicting local microdomain Ca²⁺ transients in the vicinity of VGCCs during action potentials.