Hydrophobic bile salts inhibit gallbladder smooth muscle function via stimulation of GPBAR1 receptors and activation of K_ATPchannels

Abstract

Hydrophobic bile salts are thought to contribute to the disruption of gallbladder smooth muscle (GBSM) function that occurs in gallstone disease, but their mechanism of action is unknown. The current study was undertaken to determine how hydrophobic bile salts interact with GBSM, and how they reduce GBSM activity. The effect of hydrophobic bile salts on the activity of GBSM was measured by intracellular recording and calcium imaging using wholemount preparations from guinea pig and mouse gallbladder. RT-PCR and immunohistochemistry were used to evaluate expression of the G protein-coupled bile acid receptor, GPBAR1. Application of tauro-chenodeoxycholate (CDC, 50-100 μm) to in situ GBSM rapidly reduced spontaneous Ca 2+ flashes and action potentials, and caused a membrane hyperpolarization. Immunoreactivity and transcript for GPBAR1 were detected in gallbladder muscularis. The GPBAR1 agonist, tauro-lithocholic acid (LCA, 10 μm) mimicked the effect of CDC on GBSM. The actions of LCA were blocked by the protein kinase A (PKA) inhibitor, KT5720 (0.5-1.0 μm) and the K ATP channel blocker, glibenclamide (10 μm). Furthermore, LCA failed to disrupt GBSM activity in Gpbar1 −/− mice. The findings of this study indicate that hydrophobic bile salts activate GPBAR1 on GBSM, and this leads to activation of the cyclic AMP-PKA pathway, and ultimately the opening of K ATP channels, thus hyperpolarizing the membrane and decreasing GBSM activity. This inhibitory effect of hydrophobic bile salt activation of GPBAR1 could be a contributing factor in the manifestation of gallstone disease.

GPBA: a GPCR for bile acids and an emerging therapeutic target for disorders of digestion and sensation

“…Hydrophobic bile salts activate GPBA on gall bladder smooth muscle, leading to stimulation of cAMP/PKA pathway, opening of KATP channels, and membrane hyperpolarization that decreases contractility (Lavoie et al, 2010). The treatment of mice with BAs and a GPBA-selective agonist increases gall bladder volume, an effect that is absent from GPBAdeficient mice (Li et al, 2011).…”

“…Immunoreactive GPBA is localized to the apical membrane and cilia of cholangiocytes and biliary epithelial cells as well as to gall bladder smooth muscle (Lavoie et al, 2010). In the epithelium, GPBA agonists promotes cAMP formation, which results in activation of the cAMP-dependent chloride channel cystic fibrosis transmembrane conductance regulator and resultant chloride secretion.…”

A Membrane-proximal, C-terminal α-Helix Is Required for Plasma Membrane Localization and Function of the G Protein-coupled Receptor (GPCR) TGR5

“…Both unconjugated and conjugated bile acids as well as various steroid hormones have been identified as potent TGR5 agonists (5,6). In the liver, TGR5 is localized in sinusoidal endothelial cells, Kupffer cells, cholangiocytes, gallbladder epithelial cells, and gallbladder smooth muscle cells (7)(8)(9)(10)(11)(12)(13). Here, TGR5 modulates hepatic microcirculation, exerts anti-inflammatory, anti-apoptotic and choleretic effects, and promotes gallbladder filling (7,(12)(13)(14)(15).…”

“…In the liver, TGR5 is localized in sinusoidal endothelial cells, Kupffer cells, cholangiocytes, gallbladder epithelial cells, and gallbladder smooth muscle cells (7)(8)(9)(10)(11)(12)(13). Here, TGR5 modulates hepatic microcirculation, exerts anti-inflammatory, anti-apoptotic and choleretic effects, and promotes gallbladder filling (7,(12)(13)(14)(15). In the intestine, TGR5 is expressed in enteroendocrine L-cells, in immune cells as well as in neurons and astrocytes of the enteric nervous system (16 -18).…”

Cholesterol gallstone disease: focusing on the role of gallbladder

“…This effect of hydrophobic bile salts will ultimately result in opening K ATP channels, cell membrane hyperpolarization and decreased GBSM contraction. 25,26 Recently, the discovery of interstitial Cajal-like cells (ICLCs) in the gallbladder provides new insight into the pathogenesis of impaired gallbladder motility. [27][28][29] Gallbladder ICLCs are located almost exclusively within the muscularis propria, where they are electrically coupled with GBSM cells.…”