Synergism of Thyroid Hormone and High Carbohydrate Diet in the Induction of Lipogenic Enzymes in the Rat MECHANISMS AND IMPLICATIONS

between the administration of triiodothyronine (T3) and a high carbohydrate (CHO) fat-free diet in the in-duction of lipogenic enzymes in two rat tissues, liver, and fat. Male thyroidectomized rats were treated with graded daily doses ofT3 and either supplemented with a high CHO diet or left on a regular diet. Enzymes stud-ied included malic enzyme (ME), fatty acid synthetase, glucose-6-phosphate dehydrogenase, and 6-phospho-gluconate dehydrogenase. In the liver, all four lipo-genic enzymes showed a synergistic response between T3 administration and high CHO feeding. In fat, ME also responded synergistically. The interaction was re-flected in ani increased sensitivity to T3. The dose of T3 required to achieve 50%Y maximal response was re-duced three- to seven-fold by the high CHO diet. This phenomenon could not be attributed to a dietary-in-duced alteration either in T3 metabolism or in number or affinity of the T3-nuclear receptors. Moreover, stud-ies of the relative rate of synthesis of ME suggested a simultaneous time of onset in the induction of ME, within 2 h after the application of either T3 or CHO. Thus, it is unlikely that either stimulus is secondary to the other. Since parallel experiments from this labora-tory (Towle, Mariash, and Oppenheimer. 1980. Changes in hepatic levels of messenger ribonucleic acid for malic enzylme during induction by thyroid hormone or diet. Biochemnistry. 19: 579-585.) show that ME induction both by CHO and T3 is mediated by an in-crease in specific messenger RNA for ME, the inter-action of T3 and the dietary factor occurs at a pretrans-lational level. Dr. Mariash is a recipient of National Institutes of Health Research Fellowship 1-F32-AM05880. Address reprint request