Formation and fate of endogenous triglycerides in blood plasma of rabbits

The formation of hepatic triglyceride fatty acids from palmitate-l-C14 and their transport from the liver to the blood and from the blood to peripheral tissues were studied in rabbits. Isotopic equilibration of triglyceride fatty acids (TGFA) in subcellular compartments of the liver required up to 2 hr. Hepatic TGFA appear to be the immediate precursor of TGFA contained in very low-density lipoproteins of plasma. Isotopic transfer between hepatic TGFA and TGFA of very low-density lipoproteins occurred rapidly in relation to the turnover rate of TGFA in liver and plasma. Thus, after a few hours, the turnover rate of TGFA in these compartments can be considered as a unit. Part of the turnover of this pool is the result of extrahepatic transport of TGFA in very low-density lipoproteins, but in fasted animals most of it occurred within the liver itself. Experiments in which labeled palmitate or labeled TGFA in very low-density lipoproteins were injected intravenously showed that the distribution in tissues of FFA is not affected by the nutritional state, but that of TGFA is markedly altered. About 20 times as much TGFA radioactivity was deposited in the adipose tissue of re-fed rabbits as in that of fasted animals, but oxidation was considerably less. The rate of esterification of FFA, however, depended greatly on nutritional state in adipose tissue and, to a lesser degree, in skeletal muscle and lung. These findings are discussed in relation to the roles of lipoprotein lipase activity and esterifying capacity of tissue in the fate of circulating FFA and TGFA.