Studies on the pathogenesis of diet-induced dog gallstones.

Experimental diet-induced dog gallstones contained mainly protein, mucous substances, bile salts, bilirubin, an insoluble pigment which formed an insoluble black residue after acid hydrolysis, and only traces of cholesterol. Added dietary cholesterol was necessary to pigmented gallstone production and led to hypercholesterolemia. In bile, the ratio of cholesterol to bile salts was increased, but phospholipids were increased and cholesterol insolubility was not found. Dry weight, osmolality, and concentration of sodium and potassium in bile were reduced, but were not considered sufficient to influence micelle formation or lipid-pigment solubility. Taurine was reduced in serum and bile and unconjugated bile acids appeared in gallbladder bile; the pKa of these acids is near the pH of bile in these animals and may have caused precipitation of bile acids, accounting for their presence in the stones. Bile cultures were sterile. Total bilirubin content was unaltered but the methods used did not exclude the presence of unconjugated bilirubin as a potential cause of pigment precipitation in aqueous bile. Increased numbers of secretory vesicles occurred in gallbladder epithelium and large amounts of mucus were in the epithelial crypts. These observations suggest that bile proteins or mucous substances are important to lithogenesis in this model.