Triglycerides produced in the livers of fasting rabbits are predominantly stored as opposed to secreted into the plasma

ObjectiveThe liver plays a central role in regulating fat metabolism; however, it is not clear how the liver distributes the synthesized triglycerides (TGs) to storage and to the plasma.Materials and methodsWe have measured the relative distribution of TGs produced in the liver to storage and the plasma by means of U-13C16-palmitate infusion in anesthetized rabbits after an overnight fast.ResultsThe fractional synthesis rates of TGs stored in the liver and secreted into the plasma were not significantly different (stored vs. secreted: 31.9 ± 0.8 vs. 27.7 ± 2.6% ∙ h− 1, p > 0.05). However, the absolute synthesis rates of hepatic stored and secreted TGs were 543 ± 158 and 27 ± 7 nmol ∙ kg− 1 ∙ min− 1 respectively, indicating that in fasting rabbits the TGs produced in the liver were predominately stored (92 ± 3%) rather than secreted (8 ± 3%) into the plasma. This large difference was mainly due to the larger pool size of the hepatic TGs which was 21 ± 9-fold that of plasma TGs. Plasma free fatty acids (FFAs) contributed 47 ± 1% of the FA precursor for hepatic TG synthesis, and the remaining 53 ± 1% was derived from hepatic lipid breakdown and possibly plasma TGs depending on the activity of hepatic lipase. Plasma palmitate concentration significantly correlated with hepatic palmitoyl-CoA and TG synthesis.ConclusionIn rabbits, after an overnight fast, the absolute synthesis rate of hepatic stored TGs was significantly higher than that of secreted due to the larger pool size of hepatic TGs. The net synthesis rate of TG was approximately half the absolute rate. Plasma FFA is a major determinant of hepatic TG synthesis, and therefore hepatic TG storage.