THYROID-HORMONE IS REQUIRED FOR DIETARY FISH-OIL TO INDUCE HYPERSECRETION OF BILIARY CHOLESTEROL IN THE RAT

In the rat, both fish oil diet and thyroid hormone replacement are rep orted to augment bile cholesterol secretion out of proportion to bile flow or secretion of other bile lipids. We sought common mechanisms fo r these effects and evaluated the role of phospholipid fatty acid comp osition in the process. Methimazole-treated hypothyroid rats were fed low-fat chow or chow supplemented with 10% corn oil or fish oil, and w ere studied before and after thyroid hormone treatment. Serum, hepatic , and bile lipids were measured, phospholipid fatty acid composition d etermined, and hepatic 3-hydroxy-3-methylglutaryl CoA reductase activi ty assayed. Fish oil diet stimulated cholesterol secretion into bile o nly after thyroid hormone was given, and this action was synergistic w ith that of thyroid hormone. Reduced serum cholesterol in fish oil-tre ated rats was associated with increased biliary cholesterol secretion and diminished hepatic cholesterol content. This suggests that augment ed biliary cholesterol secretion may contribute to the fish oil-induce d reduction of serum cholesterol. No definite relationship between hep atic or biliary phospholipid fatty acid composition and biliary secret ion was apparent, although high bile cholesterol secretion was associa ted with a low percentage of hepatic and bile phospholipid linoleic ac id.