Smooth muscle relaxation and activation of the large conductance Ca⁺⁺ – activated K⁺ (BK_Ca) channel by novel oestrogens

Abstract

BACKGROUND AND PURPOSEOestrogens can interact directly with membrane receptors and channels and can activate vascular BKCa channels. We hypothesized that novel oestrogen derivatives could relax smooth muscle by an extracllular effect on the α and β1 subunits of the BKCa channel, rather than at an intracellular site. EXPERIMENTAL APPROACHWe studied the effects of novel oestrogens on the tension of pre-contracted isolated rat aortic rings, and on the electrophysiological properties of HEK 293 cells expressing the hSloα or hSloα+β1 subunits. Two of the derivatives incorporated a quaternary ammonium side-chain making them membrane impermeable. KEY RESULTSOestrone, oestrone oxime and Quat DME-oestradiol relaxed pre-contracted rat aorta, but only Quat DME-oestradiol-induced relaxation was iberiotoxin sensitive. However, only potassium currents recorded in HEK 293 cells over-expressing both hSloα and hSloβ1 were activated by oestrone, oestrone oxime and Quat DME-oestradiol. CONCLUSION AND IMPLICATIONSThe novel oestrogens were able to relax smooth muscle, but through different mechanisms. In particular, oestrone oxime required the presence of the endothelium to exert much of its effect, whilst Quat DME-oestradiol depended both on NO and BKCa channel activation. The activation of BKCa currents in HEK 293 cells expressing hSloα+β1 by Quat DME-oestradiol is consistent with an extracellular binding site between the two subunits. The binding site resides between the extracellular N terminal of the α subunit and the extracellular loop between TM1 and 2 of the β1 subunit. Membrane-impermeant Quat DME-oestradiol lacks an exchangeable hydrogen on the A ring obviating antioxidant activity. AbbreviationsBKCa, large conductance calcium-activated potassium channel; DMAEC HCl, 2-dimethylamine ethyl chloride; DMF, dimethylformamide; hSloα, the alpha subunit of the human BKCa channel; hSloβ1, the beta1 subunit of the human

Regulation of Guinea Pig Detrusor Smooth Muscle Excitability by 17β-Estradiol: The Role of the Large Conductance Voltage- and Ca2+-Activated K+ Channels

“…Directly confirming the 17β-estradiol-BK channel functional interactions, we found that 17β-estradiol (100 nM) significantly increased the single BK channel NP o of inside-out excised membrane patches of UBSM cells ( Fig 3 ). These results are in support of earlier electrophysiological reports in non-UBSM cell types showing activation of the BK channels by 17β-estradiol [ 13 – 16 , 26 ]. Our single BK channel recordings were performed using symmetrical K + solutions with fixed Ca 2+ concentration and in the absence of signaling pathways that may alter BK channel P o .…”

“…However, emerging evidence in UBSM and other cell types suggests the existence of several non-genomic mechanisms of 17β-estradiol, which may acutely influence cell excitability and contractility. These non-genomic mechanisms include: inhibition of L-type Ca V channels [ 9 , 20 , 21 ], endothelial-dependent release of nitric oxide [ 22 ], activation of protein kinases [ 23 ], and activation of the BK channels [ 13 , 24 ]. Involvement of the BK channels in the mechanism of 17β-estradiol-induced response is supported by studies from non-UBSM cell types and recombinant systems expressing BK channels, which indicate the activation of BK channels by 17β-estradiol is dependent on their regulatory β1-subunit [ 5 , 13 – 16 ].…”

“…These non-genomic mechanisms include: inhibition of L-type Ca V channels [ 9 , 20 , 21 ], endothelial-dependent release of nitric oxide [ 22 ], activation of protein kinases [ 23 ], and activation of the BK channels [ 13 , 24 ]. Involvement of the BK channels in the mechanism of 17β-estradiol-induced response is supported by studies from non-UBSM cell types and recombinant systems expressing BK channels, which indicate the activation of BK channels by 17β-estradiol is dependent on their regulatory β1-subunit [ 5 , 13 – 16 ]. The role of the regulatory β1-subunit was also supported by studies from freshly-isolated murine colonic myocytes, where the increase in BK channel P o by 17β-estradiol was not observed in excised membrane patches from β1-subunit knockout (β1 -/- ) mice [ 25 ].…”

“…The cellular mechanisms of these functional effects of 17β-estradiol in UBSM are not well understood, but several mechanisms have been suggested, including L-type voltage-gated Ca 2+ (Ca V ) channel inhibition [ 2 , 9 ] and K + channel activation [ 12 ]. Among the K + channel targets of 17β-estradiol are the large conductance voltage- and Ca 2+ -activated K + (BK) channels [ 13 – 16 ]. A previous study [ 12 ] suggested the possible involvement of the BK channels in 17β-estradiol-induced UBSM relaxation as the relaxant effects of 17β-estradiol were concentration-dependently blocked by the specific BK channel inhibitor iberiotoxin.…”

Keeping you healthy: BK channel activation by omega-3 fatty acids

“…In addition to their effects on channel gating, β subunits grant BK channels sensitivity to several physiologically important compounds, thus making these subunits targets for possible pharmacological interventions. For example, β1-containing BK channels but not channels formed by the α subunit alone appear to be the target of 17β-estradiol and other compounds such as estrogen analogues, anti-estrogens, and the bile salt component lithocholic acid (Valverde et al, 1999; Dick et al, 2001; Bukiya et al, 2009; Maher et al, 2013). The activation of BK channels by 17β-estradiol has been proposed as the possible mechanism that mediates the acute relaxation of vascular smooth muscle induced by the hormone (White et al, 1995; Ruehlmann et al, 1998).…”