Measuring VLDL-triglyceride turnover in humans using ex vivo-prepared VLDL tracer

Abstract

There has been more interest in VLDL-triglyceride (TG) kinetics during the last decade. Unfortunately, robust measurement methods are elaborate and not readily available. Here, we describe a method using unique, ex vivo labeling of the fatty acid moiety of VLDL-TG followed by intravenous bolus infusion in the same person. We found that plasma disappearance of ex vivo-labeled VLDL-TG was comparable to that of in vivo-labeled VLDL-TG and that turnover rates can be safely estimated from the log linear decay of VLDL-TG specific activity. We found minor labeling of the plasma FFA (oleate) pool, which was largely attributable to coinfusion of free [14 C]triolein; VLDL-TG did not contribute substantially to the plasma FFA pool. The plasma decay curve of VLDL-TG was not affected by the presence of tracer in the FFA pool, provided that the data from 2 h after the VLDL tracer bolus infusion was used. The FFA contamination problem was circumvented by minor modification of the VLDL-TG tracer preparation. The approach we describe should expand the opportunity to study processes that cannot be assessed if the FFA precursor pool is labeled. This method for VLDL-TG tracer preparation can allow measurement of VLDL turnover, tissue uptake of VLDL-TG, and oxidation of VLDL-TG.-Gormsen L. C., M. D. Jensen, and S. Nielsen. Measuring VLDLtriglyceride turnover in humans using ex vivo-prepared VLDL tracer. J. Lipid Res. 2006. 47: 99-106. Supplementary key words very low density lipoprotein . lipoproteins . free fatty acids Our present understanding of human VLDL-triglyceride (TG) kinetics and metabolism has been limited by the lack of easily available and robust methods. Although the assessment of VLDL production using splanchnic balances is thought to be accurate (1, 2), this procedure requires blood sampling from the arterial bed as well as the hepatic and portal veins in combination with measurement of splanchnic blood flow, which is impractical in human experimental settings. Traditional turnover measurement, based on isotope dilution techniques using constant infusion of labeled VLDL-TGs (3), has not been widely used in human studies because of the lack of appropriate tracers and problems with tracer recycling. Consequently, a variety of alternative approaches have been used, as recently reviewed by Magkos and Sidossis (4). Among the most widely used approaches are labeling of the TG precursor pool with FFA (1-3), glycerol (8-11), or acetate (12), which results in in vivo incorporation of the tracer into the VLDL-TG complex. Frequent blood sampling and mathematical modeling of the data are needed to calculate the plasma VLDL-TG decay. These methods do not allow one to determine the metabolic fate of the fatty acid moiety of the TG molecule and are dependent upon the validity of the mathematical model for the given experimental setting. Another approach is to collect and isolate in vivo-labeled VLDL particles and reinfuse them into the same subject at a later time (13). However, using this approach, only a small amount ...

Myocardial Uptake of Circulating Triglycerides in Nondiabetic Patients With Heart Disease

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Postprandial VLDL-triacylglycerol secretion is not suppressed in obese type 2 diabetic men

“…Furthermore, our tracer-labelling procedure could alter the composition of the VLDL particles, thereby affecting our kinetic estimates. However, we have previously shown a similar metabolic capacity of our tracer to native VLDL-TAG [16]. Third, as our study groups differed not only in the presence and absence of diabetes but also in body weight and body composition, as well as biochemical variables (e.g.…”

“…We have developed a method to produce a VLDL-TAG tracer based on ex vivo labelling of the TAG moiety of VLDL particles and subsequent reinfusion [8,15,16]. In addition to calculation of the VLDL-TAG kinetic variables of interest, the method allows, by tracing VLDL-TAGassociated fatty acids, the study of their metabolic fate (i.e.…”

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