Activation of BK channels in rat chromaffin cells requires summation of Ca²⁺ influx from multiple Ca²⁺ channels

Large-conductance Ca²⁺ and voltage-dependent K⁺ channels (BK channels) in many tissues require high Ca²⁺ concentrations for activation and therefore might be expected to be tightly coupled to Ca²⁺ channels. However, in most cases, little is known about the relative organization of the BK channels and the Ca²⁺ channels involved in their activation. We probed the nature of the organization of BK and Ca²⁺ channels in rat chromaffin cells by manipulating Ca²⁺ influx through Ca²⁺ channels and by altering cellular Ca²⁺ buffering using EGTA and bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA). The results were analyzed to determine the distance between Ca²⁺ and BK channels that would be most consistent with the experimental data. Most BK channels are close enough to Ca²⁺ channels to be resistant to the buffering action of millimolar of EGTA, but are far enough to be inhibited by BAPTA. Analysis of the EGTA/BAPTA results suggests that BK channels are at a distance of 50 to 160 nm from Ca²⁺ channels. A model that assumes random distribution of Ca²⁺ and BK channels fails to account for the observed [Ca²⁺](i) detected by BK channels, suggesting that a specific mechanism may exist to mediate the functional coupling between these channels. Importantly, the effects of EGTA and BAPTA cannot be explained by assuming a one-to-one coupling between Ca²⁺ and BK channels. Rather, Ca²⁺ influx through a number of Ca²⁺ channels appears to act in concert to regulate the behavior of any individual BK channel. Thus differences in BK channel open probabilities may be explained by differences in the extent of Ca²⁺ domain overlap at the sites of individual BK channels.