A little humour relaxes the gallbladder

Diseases of the gallbladder are painful and usually associated with the presence of gallstones. However, obstruction of the gallbladder by stones and prevention of the emptying of bile does not wholly explain gallbladder diseases; there may be dysfunction in the sphincter of Oddi or dysmotility of the smooth muscle in the gallbladder. Cholesterol and bile acids in the bile have established effects on gallbladder contractility. Cholesterol alters cholecystokinin (CCK) receptor and voltage-dependent Ca²⁺ channel function but the mechanism(s) of action of bile acids on smooth muscle motility is not well defined.

In this issue of The Journal of Physiology, Lavoie et al. (2010) report one mechanism by which hydrophobic bile salts can alter contractility in gallbladder smooth muscle. The focus is on the signalling pathways downstream of the receptor Gpbar1 (also known as TGR5, M-Bar or GPR131) and the data support an increasing appreciation that the biology of bile is complex and has multiple facets.

The gallbladder develops from the embryonic gastrointestinal tract and retains the morphology of the intestinal wall with serosal, muscular and mucosal layers. Emptying of bile from the gallbladder is primarily activated post-prandially by the hormone CCK but motility is also regulated by intrinsic and extrinsic neuronal inputs. Furthermore, interstitial cells of Cajal, cells that generate pacemaker potentials and amplify neuronal signalling in other regions of the gut, are also present in the muscle layers of the gallbladder. These several cell types contribute to complex mechanisms for regulating gallbladder motility.

The formation of bile and cycling of bile acids through the liver, intestines and portal circulation serve the purposes of clearing lipid soluble toxins and aiding absorption of dietary lipids and fat soluble nutrients. This homeostatic regulation of bile is due to signalling pathways that alter gastrointestinal motility, nutrient absorption and cholangiocyte function. In addition bile acids play a role in the handling of nutrients once they are absorbed including effects on glucose homeostasis and insulin resistance. Bile acids activate Gpbar1 receptors on the plasma membrane and the farnesoid X and pregnane X receptors, which are intracellular proteins coupled to gene transcription. Both types of receptor are expressed throughout the body but the levels are highest in the gastrointestinal tract, liver and gallbladder, consistent with regulating bile salt homeostasis.

Epithelial cells in the mucosa of the gallbladder express the highest levels of Gpbar1 receptors in the body and functional studies have shown that Gpbar1 is coupled to the G_αS heterotrimeric G protein. Thus activation of Gpbar1 leads to cyclic adenosine monophosphate (cAMP) production. Mice homozygous for gene-targeted knockout of Gpbar1 expression do not produce gallstones when on a lithogenic diet and there are alterations in insulin sensitivity (Vassileva et al. 2010).

A role of Gpbar1 in smooth muscle of the gallbladder had not been described. However in their present paper, Lavoie et al. (2010) show that Gpbar1 mRNA and protein can be detected in the muscle layer of the gallbladder. In physiological studies, gallbladder smooth muscle cells respond to hydrophobic bile acid (agonists for Gpbar1) with a decrease in contractility, inhibition of spontaneous action potentials and suppression of transient rises in intracellular Ca²⁺ concentration. These effects for Gpbar1 activation are very similar to the effects reported by the same group (Zhang et al. 1994) of treatments that elevated intracellular cAMP in gallbladder smooth muscle.

Careful testing using compounds that block cAMP-dependent signalling and ATP-dependent K⁺ channels confirms that Gpbar1 activation reduces gallbladder smooth muscle contractility in the same way as other G_αS coupled receptors. Furthermore, there is no effect of hydrophobic bile acids on smooth muscle from Gpbar1 knockout mice whereas compounds that directly activate cAMP signalling do reduce the frequency of transient rises in intracellular Ca²⁺ concentration in the smooth muscle from knockout mice.

This study shows that Gpbar1 activation alters gallbladder motility by an effect on the smooth muscle cells. The ligand selectivity of the receptor has been determined and several groups have investigated the potential role of Gpbar1 activation in glucose homeostasis and insulin resistance. A synthetic agonist is available. The existence of at least one single nucleotide polymorphism in the Gpbar1 gene and the resistance of Gpbar1 mice to gallstone development raise the possibility that alterations in the function of the protein might lead to gallbladder and other diseases.

It is worth noting that Gpbar1 is not responsive to all bile acids and that activation of other signalling pathways or even other bile acid receptors might contribute to the function of gallbladder smooth muscle. Farnesoid X receptors have been identified in vascular smooth muscle and were found to be coupled to proliferation and survival of smooth muscle cells (Bishop-Bailey et al. 2004). Bile acids are also reported to modulate large conductance Ca²⁺-activated K⁺ channels by direct modulation of the channel protein (Dopico et al. 2002). Effects of Gpbar1 activation that are independent of changes in contractility and which might have longer term effects on gallbladder function are predicted based on the robust coupling of cAMP levels to gene transcription in other cells.

Finally, the concept that an inbalance of the four humours was the cause of all disease was favoured by physicians ever since the time of Hippocrates. Given that so much recent research has found broad and significant effects of bile acids throughout the body, perhaps modern medicine is finding that there is a contribution of yellow bile to the human condition after all.