Thyroid status alters structure of VLDL secreted by perfused rat liver

The interaction of thyroid status and oleic acid infusion rate on the thermal behavior of the very low density lipoprotein (VLDL) secreted by isolated perfused rat liver was examined. The livers were infused at 37 degrees C with oleate at rates of 0, 83, 166, or 332 mumoles/hr for 4 hours and VLDL was isolated from the perfusate at 12 degrees C. The lipid composition of the VLDL secreted by the perfused liver from hyperthyroid animals was dramatically different from controls at all infusion rates of oleate. Significant changes in the ratio of [phospholipid + cholesterol]/[triglyceride] and in fatty composition of secreted triglycerides occurred. Differential scanning calorimetry of the intact VLDL and extracted triglycerides secreted by euthyroid rats suggested the existence of four thermotropic endothermic transitions centered at -20.5, -14.0, -3.0, and 9.0 degrees C. Both the total enthalpies and the temperatures at which the phase alterations occurred in the triglyceride fraction from VLDL secreted by livers from euthyroid rats were highly dependent on the rate of infusion of oleate. Pretreatment of the rats with triiodothyronine and infusion of 332 mumoles oleate/hr abolished in the intact VLDL the temperature transitions centered at -20.8 and at 10.5 degrees C and decreased the total enthalpy from 8.13 to 38.5 cal/gm. Livers from rats pretreated with propylthiouracil and infused with oleate at 332 mumoles/hr secreted VLDL in which only one transition centered at -5.0 degrees C remained. The total enthalpy was unaffected. At all rates of infusion of oleate, the phase behavior of the intact VLDL or triglycerides extracted from the VLDL was altered by prior treatment of the rats with triiodothyronine or propylthiouracil. The thyroid state of the rat profoundly affected the thermal properties of the VLDL secreted by the perfused liver infused with the unsaturated fatty acid oleate.