Mechanistic details of BK channel inhibition by the intermediate conductance, Ca2+-activated K channel.

Salivary gland acinar cells have two types of Ca(2+)-activated K channels required for fluid secretion: the intermediate conductance (IK1) channel and the large conductance (BK) channel. Activation of IK1 inhibits BK channels including in small, cell-free, excised membrane patches. As a first step toward understanding the mechanism underlying this interaction, we examined its voltage sensitivity. We found that the IK1-induced inhibition of BK channels was only weakly voltage dependent and not accompanied by alteration in BK gating kinetics. These actions of IK1 on BK channels are not consistent with a mechanism whereby activation of IK1 causes a shift of the BK channel's voltage dependence as occurs for many BK modulatory processes. In a search for other clues about the interaction mechanism, we noted that the N-terminus of the IK1 channel shares some chemical features with the N-terminal regions of two BK subunits known to inhibit BK activity by blocking the cytoplasmic end of the BK pore. Thus, we tested the idea that the N-terminus of IK1 channels may act similarly. We found that a peptide derived from the N-terminal region of the IK1 protein blocked BK channels. Significantly, we also found that the activation of IK1 channels competed with block by the N-terminus peptide. Thus, the activation of IK1 channels inhibits BK channels by a mechanism that involves block of the cytoplasmic pore, not an alteration in the voltage dependence of BK gating. The mediator of this cytoplasmic pore block may be the IK1 N-terminus.