Differential regulation of calcium-activated potassium channels by dynamic intracellular calcium signals.

Calcium (Ca(2+))-activated K(+) (K(Ca)) channels regulate membrane excitability and are activated by an increase in cytosolic Ca(2+) concentration ([Ca(2+)](i)), leading to membrane hyperpolarization. Most patch clamp experiments that measure K(Ca) currents use steady-state [Ca(2+)] buffered within the patch pipette. However, when cells are stimulated physiologically, [Ca(2+)](i) changes dynamically, for example during [Ca(2+)](i) oscillations. Therefore, the aim of the present study was to examine the effect of dynamic changes in [Ca(2+)](i) on small (SK3), intermediate (hIK1), and large conductance (BK) channels. HEK293 cells stably expressing each K(Ca) subtype in isolation were used to simultaneously measure agonist-evoked [Ca(2+)](i) signals, using indo-1 fluorescence, and current/voltage, using perforated patch clamp. Agonist-evoked [Ca(2+)](i) oscillations induced a corresponding K(Ca) current that faithfully followed the [Ca(2+)](i) in 13-50% of cells, suggesting a good synchronization. However, [Ca(2+)](i) and K(Ca) current was much less synchronized in 50-76% of cells that exhibited Ca(2+)-independent current events (55% of SK3-, 50% of hIK1-, and 53% of BK-expressing cells) and current-independent [Ca(2+)](i) events (18% SK3- and 33% of BK-expressing cells). Moreover, in BK-expressing cells, where [Ca(2+)](i) and K(Ca) current was least synchronized, 36% of total [Ca(2+)](i) spikes occurred without activating a corresponding K(Ca) current spike, suggesting that BK(Ca) channels were either inhibited or had become desensitized. This desynchronization between dynamic [Ca(2+)](i) and K(Ca) current suggests that this relationship is more complex than could be predicted from steady-state [Ca(2+)](i) and K(Ca) current. These phenomena may be important for encoding stimulus-response coupling in various cell types.