Gastrin-releasing peptide stimulates gallbladder motility but not sphincter of Oddi motility in Australian brush-tailed possum

The neural distribution and action of gastrin-releasing peptide in the extrahepatic biliary tree of the Australian brush-tailed possum was investigated. Immunohistochemical staining of fixed specimens demonstrated gastrin-releasing peptide-containing nerves throughout the neural plexuses of the gallbladder, sphincter of Oddi, and mucosa of the common bile duct. Gastrin-releasing peptide (5-2000 ng/kg) increased gallbladder tone to a level equivalent to that produced by cholecystokinin octapeptide (160 ng/kg). This action was tetrodotoxin-insensitive. Sphincter of Oddi motility and transsphincteric flow were not altered. Possible mediation of the gallbladder response by gastrin was examined. Gastrin (50-2500 ng/kg) stimulated gastric acid secretion, elevated gallbladder motility to 64% of that produced by gastrin-releasing peptide, and did not alter sphincter of Oddi motility. In conclusion, gastrin-releasing peptide-containing nerves are found in the neural plexus of the possum extrahepatic biliary tree. Gastrin-releasing peptide induces gallbladder contraction in part by a direct action on gallbladder smooth muscle and also via release of gastrin.