The extracellular loop of the auxiliary ß1-subunit is involved in the regulation of BKCa channel mechanosensitivity.

ABSTRACT

The large conductance Ca2+-activated potassium (BKCa) channel is activated by stretch. The stress-regulated exon (STREX) in α-subunits is known to affect the mechanosensitivity of BKCa channels. However, in human colonic smooth muscle cells (HCoSMCs), we found that the α-subunits without STREX (ZERO-BK) and ß1-subunits could be detected yet the cells were mechanosensitive. Whether the ß1-subunit is involved in the regulation of BKCa mechanosensitivity is unclear. In the present study, ZERO-BK and ß1-subunits were individually expressed or coexpressed in HEK293 cells and cell-attached patch-clamp techniques were used to measure BKCa currents defining mechanosensitivity. Single-channel patch-clamp recordings from HEK293 cells cotransfected with ZERO-BK and ß1-subunits showed stretch sensitivity, but there was no mechanosensitivity in HEK293 cells transfected only with ZERO-BK. We also showed that expression of the ß1-subunit could increase mechanosensitivity of the STREX-type α-subunits (STREX-BK). To identify the domain in ß1-subunits responsible for affecting stretch sensitivity, we expressed ß1- LoopDel (chimeric ß1-subunits without the extracellular loop) or ß1- C TermDel (chimeric ß1-subunits without COOH terminus) with ZERO-BK in HEK293 cells. The data showed that deletion of the extracellular loop but not the COOH terminus of ß1-subunits virtually abolished the mechanosensitivity. These results suggest that the extracellular loop of the ß1-subunit is involved in the regulation of BKCa channel mechanosensitivity and that role is independent of STREX.