Effects of triiodothyronine on biosynthesis and secretion of triglyceride by livers perfused in vitro with [3H]oleate and [14C]glycerol.

Livers from euthyroid and hyperthyroid (triiodothyronine-treated) rats were perfused in uitro with tracer amounts of ['4C]glycerol and substrate quantities of r3H]oleate.Hepatic uptake of total free fatty acid, [3H] oleate and ['4C]glycerol were similar regardless of hormonal treatment.Output of triglyceride was 4.46 k 0.48 pmol/g and 2.14 k 0.36 by livers from euthyroid and hyperthyroid animals, respectively.Although total incorporation of ['4C]glycerol into perfusate triglyceride was similar in both groups, the specific radioactivity of triglyceride was approximately 30% higher in the hyperthyroid.The glycero-3-phosphate content of livers from triiodothyronine (T3)-treated rats was 52.3% that of livers from euthyroid rats (0.80 k 0.15 uersus 1.53 f 0.16 pmol/g, respectively).After 4 h of perfusion, the same relationship was observed.The specific radioactivity of glycero-3-phosphate following perfusion with ['4C]glycerol in livers from T3-treated rats was twice that of the euthyroid.Synthesis of triglyceride, calculated from glycero-3-phosphate specific radioactivity, was 4.62 f 0.44 pmol/g/4 h by livers from euthyroid rats and 1.95 f 0.27 after T3 treatment.The incorporation of r3H]oleate into perfusate triglyceride by livers from hyperthyroid rats was approximately 30% of the euthyroid, while the specific radioactivity was approximately equal in both groups.Triglyceride synthesized from r3H]oleate was calculated to be 2.00 k 0.50 pmol/ g of liver/$ h for livers from T3-treated animals and 4.29 f 0.57 for euthyroid controls.The rates of synthesis of perfusate and hepatic triglyceride, and output of perfusate triglyceride, by livers from hyperthyroid rats were lower than in the euthyroid, and similar values were obtained whether calculated by chemical assay, or from radioactive incorporation of [3H]oleate or ['"C] glycerol.Additionally, livers from T3-treated rats had higher rates of ketogenesis, secreted less glucose, and produced more 14C02 from glycerol than did livers from euthyroid rats.Clearly, livers from hyperthyroid rats synthesize and secrete less triglyceride (i.e.very low density lipoprotein) than do livers from euthyroid animals.These conclusions were reached whether glycerol