Ground squirrel model for cholelithiasis: role of epithelial glycoproteins.
The cholesterol-fed Richardson's ground squirrel (Spermophilus richardsonii) has proven to be an effective animal model in which to study factors that influence cholesterol gallstone formation and associated alterations in the gallbladder epithelium. Ground squirrels of either sex, fed a 2% cholesterol-enriched diet, exhibit cholesterol monohydrate crystal precipitation within 24 hours and macroscopically visible cholesterol stones by 3 weeks. Data on bile chemistry, biliary cholesterol precipitation, and various mucosal alterations occurring prior to, during, and after stone formation were collected using sampling intervals from 6 hours to 20 weeks on the diet. The results indicate that mucin hypersecretion appears to be more closely related to the initiation of nucleation than does either bile calcium of pH. Mucus hypersecretion begins within 18 hours of diet initiation and continues throughout the 20 week experimental period. Apical excrescences became more common and were larger in size during the early stages of cholelithiasis. Administration of aspirin during the experimental period demonstrated an inhibition of mucin synthesis and release. Gallstones were not formed in these aspirin-treated animals. A lectin-binding panel for 10 epithelial glycoprotein-related sugars indicated the mucin secreted by the gallbladder epithelium of 7 day experimental animals differed from that of controls. The most obvious difference was the abolition of WGA binding in the experimental animals, suggesting an absence of sialic acid expression in the mucin during the lithogenic process. Ultrastructural histochemistry indicated that both sulphomucin and sialomucin were present in the secretory granules and within the surface mucus layer of both experimental and control animals. Experimental animals, however, exhibited a significant predominance for sulphomucin. This pattern varies from that typically seen in other regions of the gastrointestinal tract where sialomucins predominate during pathologic processes.