Roles of LRRC26 as an auxiliary γ1-subunit of large-conductance Ca2+-activated K+ channels in bronchial smooth muscle cells

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 gamma-subunits of the BK channel (BK gamma) in several cell types, its physiological roles in SMCs are unclear. The BK gamma expression patterns in selected SM tissues were examined using real-time PCR analyses and Western blotting. The functional contribution of BK gamma 1 to BK channel activity was examined by whole cell patch-clamp in SMCs and heterologous expression systems. BK gamma 1 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 BK alpha and BK gamma 1 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 BK alpha/BK gamma 1 complex. However, this range was much more negative than in mouse aortic SMCs (mASMCs) or in HEK293 cells expressing BK alpha alone and beta-subunit (BK gamma 1). Mallotoxin, a selective activator of BK channel that lacks BK gamma 1, dose-dependently activated BK currents in mASMCs but not in mBSMCs. The abundant expression of BK gamma 1 in mBSMCs extensively facilitates BK channel activity to keep the resting membrane potential at negative values and prevents contraction under physiological conditions.