Ligand-Gating by Ca2+ Is Rate Limiting for Physiological Operation of BKCa Channels

Large conductance Ca2+- and voltage-activated potassium channels (BKCa) shape neuronal excitability and signal transduction. This reflects the integrated influences of transmembrane voltage and intracellular calcium concentration ([Ca2+](i)) that gate the channels. This dual gating has been mainly studied as voltage-triggered gating modulated by defined steady-state [Ca2+](i), a paradigm that does not approximate native conditions. Here we use submillisecond changes of [Ca2+](i) to investigate the time course of the Ca2+-triggered gating of BKCa channels expressed in Chinese hamster ovary cells at distinct membrane potentials in the physiological range. The results show that Ca2+ can effectively gate BKCa channels and that Ca2+ gating is largely different from voltage-driven gating. Most prominently, Ca2+ gating displays a pronounced delay in the millisecond range between Ca2+ application and channel opening (pre-onset delay) and exhibits slower kinetics across the entire [Ca2+](i)-voltage plane. Both characteristics are selectively altered by co-assembled BK beta 4 or an epilepsy-causing mutation that either slows deactivation or speeds activation and reduces the pre-onset delay, respectively. Similarly, co-assembly of the BKCa channels with voltage-activated Ca2+ (Cav) channels, mirroring the native configuration, decreased the pre-onset delay to submillisecond values. In BKCa-Cav complexes, the time course of the hyperpolarizing K+-current response is dictated by the Ca2+ gating of the BKCa channels. Consistent with Cav-mediated Ca2+ influx, gating was fastest at hyperpolarized potentials, but decreased with depolarization of the membrane potential. Our results demonstrate that under experimental paradigms meant to approximate the physiological conditions BKCa channels primarily operate as ligand-activated channels gated by intracellular Ca2+ and that Ca2+ gating is tuned for fast responses in neuronal BKCa-Cav complexes.