Review article: control of gall-bladder motor function.

Muscular contraction of the gall-bladder is the primary determinant of bile delivery into the duodenum. Where bile goes following its secretion by the liver depends upon a co-ordinated series of pressure interrelations between the hepatic secretory pressure at the entrance to the biliary system, a low pressure conduit, and the pressure differences between the gall-bladder, cystic duct and sphincter of Oddi. During fasting, the relatively higher tone in the sphincter of Oddi fosters the entry of bile into the gall-bladder. The gall-bladder accommodates this influx without an increase in intravesicular pressure through its compliance or distensibility, which consists of active muscle relaxation and passive fibroelastic components. The concentrating function of the gall-bladder keeps the volume small. Once about every 120 min during the interdigestive period, gall-bladder emptying occurs coincident with intense duodenal contractions; all part of the migratory myoelectric complex. This helps maintain the enterohepatic circulation of bile salts. Motilin, which mediates these events during fasting, acts by stimulating intrinsic cholinergic nerves. Cholecystokinin is the major determinant of gallbladder emptying with eating. Cholecystokinin acts through pre-ganglionic cholinergic nerves, to initiate gall-bladder contraction. Agonists like cholecystokinin and acetylcholine cause contraction of gall-bladder smooth muscle through signal transduction, which increases intracellular calcium levels and so initiates the contractile machinery. Cholecystokinin also acts on the sphincter of Oddi via pre-ganglionic cholinergic nerves to release vasoactive intestinal polypeptide and nitric oxide, and so lower tone. These events are co-ordinated with motility and secretory events in the upper gastrointestinal tract, delivering bile at appropriate times into the duodenum.