Origin and fate of rat plasma cholesterol in vivo. Modelling of cholesterol movements between plasma and organs

Selective uptake of high density lipoproteins cholesteryl ester in the dog, a species lacking in cholesteryl ester transfer protein activity

Amongst the processes involved in the reverse cholesterol transport (RCT) from organs to liver, including high density lipoproteins-apolipoprotein AI (HDL-apoAI) dependent tissue uptake and cholesteryl ester transfer protein (CETP)-mediated transfers, the selective uptake of cholesteryl ester (CE) is of increasing interest through its antiatherogenic role. The purpose of this report is to develop a simple protocol allowing study of this process in an animal model with easier quantification of CE selective uptake. The dog was chosen essentially because this animal has a low CETP activity and an appropriate size to conduce a kinetic study. Tracer kinetics were performed to estimate in vivo the contributions of the pathways involved in HDL-CE turnover in dogs. Stable isotopes, 13C-acetate and D3-leucine as labeled precursors of CE and apoAI, were infused to fasting dogs. Isotopic enrichments were monitored in plasma unesterified cholesterol and in HDL-CE and apoAI by mass spectrometry. Kinetics were analyzed using compartmental modeling. Results concerned the measurement of the activity of cholesterol esterification (0.13+/-0.032 h(-1)), rate of HDL-apoAI catabolism (0.024+/-0.012 h(-1)), HDL-CE turnover (0.062+/-0.010 h(-1)) and CE selective uptake (0.038+/-0.014 h(-1)). Our results show that CE in dogs is mainly eliminated by selective uptake of HDL-CE (60% of HDL-CE turnover), unlike in other species studied by similar methods in our laboratory. This study shows that among species used to analyze cholesterol metabolism, the dog appears to be the animal in whom HDL-CE selective uptake represents the largest part of HDL-CE turnover.

Plasma kinetics of a chylomicron-like emulsion in patients with coronary artery disease

Chylomicron catabolism in the bloodstream consists of lipolysis by lipoprotein lipase and uptake of remnants by the liver. In rats, triglyceride-rich emulsions can mimic chylomicron metabolism. To further validate this model in man, the emulsion was injected intravenously into fasting and into subjects previously fed a test fatty meal. The plasma kinetic curves of the emulsion 3H-triglyceride and 14C-cholesteryl ester were determined. The fractional clearance rate (FCR) of both labels was markedly reduced in the fed subjects (triglycerides: fed = 0.018 +/- 0.007; fasting = 0.105 +/- 0.013 min-1, P < 0.001; cholesteryl ester: fed = 0.016 +/- 0.001; fasting = 0.040 +/- 0.006 min-1; P < 0.05) indicating that the emulsion and chylomicrons generated from the testinal lipid absorption compete for the same catabolic processes, confirming the validity of the method. The emulsion was injected into 11 patients with CAD and into 11 controls. All had plasma cholesterol < 240 and triglycerides < 250 mg/dl. FCR of triglycerides was 5-fold smaller in CAD compared to controls (0.028 +/- 0.004 and 0.141 +/- 0.069 min-1, respectively, P < 0.01). FCR of cholesteryl ester was 4-fold smaller in CAD than in controls (0.015 +/- 0.004 and 0.056 +/- 0.067 min-1 respectively, P < 0.05). These results indicate that both chylomicron lipolysis and remnant removal are diminished in CAD.

Crilvastatin, a new 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor, inhibits cholesterol absorption in genetically hypercholesterolemic rats

Crilvastatin is a new drug from the pyrrolidone family, which acts as a non-competitive inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase. The long-term effects of oral crilvastatin treatment (200 mg per day per kg body weight for 4 and 10 weeks) were investigated on in vivo cholesterogenesis in male adult normocholesterolemic (SW) and genetically hypercholesterolemic (RICO) rats. In both strains of rats, the treatment had no effect on the plasma cholesterol level, but efficiently inhibited cholesterol synthesis in liver and intestine, as shown by the decreased incorporation of exogenous [14C]acetate into hepatic (3.5-fold in SW, 1.7-fold in RICO rats) and intestinal (2.5-fold in SW, 3.3-fold in RICO rats) sterols. In RICO rats in which the dietary cholesterol absorption coefficient was two-fold lower in treated (38%) than in untreated (78%) rats, this drug reduced intestinal cholesterol absorption. As a result, the total plasma cholesterol input (absorption + synthesis), measured by isotope analysis in RICO rats, was markedly lower in treated (11.3 mg per day) than in untreated animals (28.8 mg per day).

Human cholesterol synthesis measurement using deuterated water. Theoretical and procedural considerations

Human cholesterogenesis is measurable as the rate of incorporation of deuterium derived from deuterium oxide (D2O) within the body water pool into plasma or erythrocyte cholesterol pools. Oral D2O equilibrates across body water, thus enabling extracellular sampling of pools (such as urine) to serve as accurate indicators of intracellular deuterium enrichments at the point of synthesis. Required doses of D2O fall below the threshold associated with negative side effects. Deuterium/carbon incorporation ratios into cholesterol during biosynthesis have been established that are applicable in humans. Models using unconstrained and constrained curve fitting permit improved flexibility in interpretation of deuterium-uptake kinetics. However, sample-size restrictions presently limit the ability of the technique to examine the kinetics within individual lipoprotein species. Correction of enrichment data for proton exchange during combustion and reduction phases of sample preparation is an additional important procedural concern. In summary, the deuterated-water procedure is a useful tool in studies of human cholesterol synthesis that offers the advantages of short measurement interval, relative noninvasiveness, and provision of a direct index of synthesis in comparison with other available techniques.

Modification of erythrocyte membrane lipid composition induced by a single intravenous infusion of phospholipid-triacylglycerol emulsions in man

The study aimed to investigate whether, during short term infusion of lipid emulsions in man, red blood cell (RBC) membrane lipid composition was altered and RBC-free cholesterol (FC) could serve as a source of FC accumulated in the plasma. 3 normal subjects were infused intravenously with either 10% Intralipid [10% IL; PL:triglyceride (TG) weight ratio of 0.12] at the rates of 0.1, 0.2 and 0.3 g TG.kg(-1).h(-1) (providing PL intakes of 12, 24, 36 mg.kg(-1).h(-1), respectively) or with 30% Intralipid (30% IL; PL:TG ratio of 0.04) at the rate of 0.3 g TG.kg(-1).h(-1) (providing 12 mg PL.kg(-1).h(-1)). Infusion of 10% IL at a slow rate and 30% IL at a high rate caused no change in RBC and plasma FC content. However, 10% IL infusion at intermediate and high rates induced a significant decrease in RBC-FC: PL ratio. This change was still present at 18 h after the cessation of high rate infusion. RBC-FC: PL ratio and plasma PL measured during infusion were significantly correlated (r = -0.87, p < 0.001). FC efflux from RBC appears to contribute to the rise in plasma FC. This study indicates that the excessive amount of PL present as liposomes in some intravenous lipid emulsions can alter erythrocyte membrane lipid composition.