INTRAVENOUS LIPID EMULSIONS - REMOVAL MECHANISMS AS COMPARED TO CHYLOMICRONS

We have compared the metabolism of chylomicrons and a labeled emulsion , similar to those used for parenteral nutrition. Both were labeled in their triglyceride moieties and by a core label. It is known that chy lomicron triglycerides are cleared by two processes: removal of trigly cerides from the particles through lipolysis and removal of whole or p artly lipolyzed particles. It has been proposed that emulsion droplets are cleared by the same pathways. After intravenous injection to post prandial rats, triglycerides were cleared less rapidly from the emulsi on than from the chylomicrons (half-lives of 6.4 and 4.0 min), whereas the core labels were cleared at the same rate (half-lives around 7.5 min). This suggests that there was less lipolysis of the emulsion drop lets which was further supported by the finding that less label appear ed in the plasma free fatty acids (FFA). In adipose tissue of fed rats given chylomicrons, the ratio between fatty acid and core label was a bove 6, showing that fatty acids had been taken up after lipoprotein l ipase-mediated hydrolysis. In contrast, for rats given emulsion, that ratio was only 1.2 showing that nearly as much emulsion droplets as em ulsion-derived fatty acids were present in the tissue. In the liver th e ratio was 0.55 after chylomicrons but 0.93 after emulsion. In furthe r support of more lipolysis, fatty acids were oxidized more rapidly fr om chylomicrons than from emulsion. These data suggest that a large fr action of the emulsion droplets was removed from plasma with little or no preceding lipolysis. A substantial proportion, more than 50%, of t his uptake occurred in extrahepatic tissues.