Abstract
In visceral smooth muscle cells (SMCs), the large-conductance Ca2+-activated K+ (BK) channel is one of the key elements underlying a negative feedback mechanism that is essential for the regulation of intracellular Ca2+ concentration. Although leucine-rich repeat-containing (LRRC) proteins have been identified as novel auxiliary γ-subunits of the BK channel (BKγ) in several cell types, its physiological roles in SMCs are unclear. The BKγ expression patterns in selected SM tissues were examined using real-time PCR analyses and Western blotting. The functional contribution of BKγ1 to BK channel activity was examined by whole cell patch-clamp in SMCs and heterologous expression systems. BKγ1 expression in mouse bronchial SMCs (mBSMCs) was higher than in other several SMC types. Coimmunoprecipitation and total internal reflection fluorescence imaging analyses revealed molecular interaction between BKα and BKγ1 in mBSMCs. Under voltage-clamp, steady-state activation of BK channel currents at pCa 8.0 in mBSMCs occurred in a voltage range comparable to that of reconstituted BKα/BKγ1 complex. However, this range was much more negative than in mouse aortic SMCs (mASMCs) or in HEK293 cells expressing BKα alone and β-subunit (BKβ1). Mallotoxin, a selective activator of BK channel that lacks BKγ1, dose-dependently activated BK currents in mASMCs but not in mBSMCs. The abundant expression of BKγ1 in mBSMCs extensively facilitates BK channel activity to keep the resting membrane potential at negative values and prevents contraction under physiological conditions.