Monitoring transient Ca2+ dynamics with large-conductance Ca2+-dependent K+ channels at active zones in frog saccular hair cells

Neurotransmitter release from the basolateral surface of auditory and vestibular hair cells is mediated by Ca2+ influx through voltage-gated Ca2+ channels. Co-localization of large-conductance Ca2+-activated K+ (BK) channels at the active zones of these cells affords them with an optimal location to act as reporters of the Ca2+ concentration changes at active zones of transmitter release. In this report we use BK channels in frog (Rana pipiens) hair cells to monitor dynamic changes in intracellular Ca2+ concentration during transient influxes of Ca2+, showing that BK current magnitude and delay to onset are correlated with the rate and duration of Ca2+ entry through Ca2+ channels. We also show that BK channels exhibit a much higher Ca2+ binding affinity in the open state than in the closed state.