Development and application of a novel method to determine large very low density lipoprotein (VLDL1) kinetics

• Main Author: Alshayji, Iqbal
• Published: University of Glasgow 2008
• Subjects: 572.68; Q Science (General)
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.499434

High concentrations of large very low density lipoproteins (VLDL1) give rise to atherogenic dyslipidaemia, which is usually associated with insulin resistant conditions (e.g. obesity) and increases the risk for cardiovascular disease (CVD). Isotopic tracer methods for determining VLDL1 kinetics are costly, time-consuming, labour intensive and need experience and skill to calculate the kinetic parameters. The aim of this thesis was to develop a simpler and cost-effective method of obtaining triglyceride-rich lipoproteins (TRL) kinetic data, based on the fact that chylomicrons (CM) or CM-like particles (e.g. Intralipid) compete with large VLDL1 for the same lipoprotein lipase (LPL)-mediated catalytic pathway. From this method, it was possible to determine VLDL1-triglyceride (TG) and -apolipoprotein (apo) B production rates and the Intralipid-TG clearance rate (as a surrogate measure of CM clearance). Kinetic data obtained from this method agreed with values and relationships obtained from stable isotope methods. The protocol is relatively quick, inexpensive, and transferable to non-specialist laboratories. As a first application, the ‘Intralipid method’ was used to investigate the effects of hyperinsulinaemia and hyperglycaemia due to glucose ingestion on VLDL1-TG and -apoB production rates and Intralipid-TG clearance rate. This showed that hepatic VLDL1 production is suppressed in response to hyperinsulinaemia and that the change in Intralipid-TG clearance rate with hyperinsulinaemia correlated significantly with HOMA-estimated insulin resistance (HOMA-IR). In addition, alanine aminotranferase (ALT) concentrations (a marker for liver fat), within normal range, predicted the extent of hepatic VLDL1 suppression. Secondly, the Intralipid method was used to investigate the mechanisms responsible for the hypotriglyceridaemic effect of a moderate exercise session (120 min walking at 50% VO2max) in overweight/obese middle-aged men; the section of the population at high risk of CVD in whom exercise-for-health guidelines are targeted. This showed that the exercise-induced reduction in plasma TG was due to increased VLDL1-TG and -apoB clearance, rather than decreased hepatic production. Exercise also increased Intralipid-TG clearance rate, but to a lesser extent than VLDL1, suggesting an increased affinity of VLDL1 for LPL-mediated lipolysis post-exercise. Taken together, the Intralipid method is a relatively simple, safe and cost-effective method to determine in VLDL1-TG and -apoB production rates and Intralipid-TG clearance rates. It is also sensitive enough to detect physiological changes in TRL kinetics.