Very-Low-Density Lipoprotein of Metabolic Syndrome Modulates Gap Junctions and Slows Cardiac Conduction

Very-low-density lipoproteins (VLDL) is a hallmark of metabolic syndrome (MetS) and each manifestation of MetS is related to atrial fibrillation (AF) risks. Slowed atrial conduction is a mechanism of AF in MetS. We hypothesized that VLDL can modulate and reduce atrial gap junctions. VLDLs were separated from normal (Normal-VLDL) and MetS (MetS-VLDL) individuals. VLDLs (15 µg/g) and equivalent volumes of saline (CTL) were injected respectively to C57BL/6 mice for 6 weeks. Electrocardiograms demonstrated that MetS-VLDL induced prolongation of P wave (P = 0.041), PR intervals (P = 0.014), QRS duration and QTc interval (both P = 0.003), but Normal-VLDL did not. Optical mapping of perfused hearts confirmed slowed conduction on atria and ventricles of MetS-VLDL mice. Slowed cardiac conduction was associated with significant atrial and ventricular remodeling, along with systolic dysfunction and comparable intra-cardiac fibrosis. MetS-VLDL induced downregulation of Cx40 and Cx43 at transcriptional, translational and tissue levels, and it also enhanced O-GlcNAcylation of Cx40 and Cx43. Protein structure analyses predicted O-GlcNAcylation at serine 18 of Cx40 and Cx43 which may impair stability of gap junctions. In conclusion, MetS-VLDL modulates gap junctions and delays both atrial and ventricular conduction. VLDL may contribute to the pathophysiology of atrial fibrillation and ventricular arrhythmias in MetS.

VLDL from Metabolic Syndrome Individuals Enhanced Lipid Accumulation in Atria with Association of Susceptibility to Atrial Fibrillation

Metabolic syndrome (MetS) represents a cluster of metabolic derangements. Dyslipidemia is an important factor in MetS and is related to atrial fibrillation (AF). We hypothesized that very low density lipoproteins (VLDL) in MetS (MetS-VLDL) may induce atrial dilatation and vulnerability to AF. VLDL was therefore separated from normal (normal-VLDL) and MetS individuals. Wild type C57BL/6 male mice were divided into control, normal-VLDL (nVLDL), and MetS-VLDL (msVLDL) groups. VLDL (15 µg/g) and equivalent volumes of saline were injected via tail vein three times a week for six consecutive weeks. Cardiac chamber size and function were measured by echocardiography. MetS-VLDL significantly caused left atrial dilation (control, n = 10, 1.64 ± 0.23 mm; nVLDL, n = 7, 1.84 ± 0.13 mm; msVLDL, n = 10, 2.18 ± 0.24 mm; p < 0.0001) at week 6, associated with decreased ejection fraction (control, n = 10, 62.5% ± 7.7%, vs. msVLDL, n = 10, 52.9% ± 9.6%; p < 0.05). Isoproterenol-challenge experiment resulted in AF in young msVLDL mice. Unprovoked AF occurred only in elderly msVLDL mice. Immunohistochemistry showed excess lipid accumulation and apoptosis in msVLDL mice atria. These findings suggest a pivotal role of VLDL in AF pathogenesis for MetS individuals.

VLDL-TG kinetics: a dual isotope study for quantifying VLDL-TG pool size, production rates, and fractional oxidation in humans

Very-low-density lipoproteins (VLDLs) are large, complex particles containing both surface proteins (e.g., ApoB100) and core lipids, e.g., cholesterol and triglycerides (TG). Whereas ApoB100 kinetics have been thoroughly studied, accurate measurement of VLDL-TG kinetics have proven difficult due to either complex mathematics or laborious procedures. The present study was therefore designed to measure VLDL-TG kinetics by dual isotope ex vivo labeled VLDL-TG tracers and well-established kinetics equations (bolus injection or the primed continuous infusion). Ten healthy Caucasian men [age, 23 +/- 3 yr old (mean +/- SD); body mass index, 24.7 +/- 1.3 kg/m(2)] were included in the study. VLDL-TG rate of appearance (Ra) was measured using a dual-tracer technique ([9,10-(3)H]-labeled VLDL-TG and [1-(14)C]-labeled VLDL-TG) to allow comparison of various bolus decay curve fits with the Ra obtained by the primed continuous infusion (PCI; considered the gold standard). In addition, VLDL-TG fatty acid oxidation was measured as (14)CO(2) in exhaled breath, using the hyamine trapping technique. Following a bolus injection, tracer decay was better described by a biexponential than a monoexponential fit (r(2) = 0.99 +/- 0.01 vs. 0.97 +/- 0.04, respectively, P = 0.01). VLDL-TG Ra calculated using the PCI correlated significantly with the biexponential fit (rho = 0.62, P < 0.05), whereas this was not the case for the monoexponential fit (rho = -0.18, P = not significant). VLDL-TG Ra using the best fit of the bolus injection method (biexponential) was less than values obtained by the constant infusion technique [biexponential, 34.3 (range, 27.1-69.6) vs. PCI, 44.4 (range, 33.0-72.7), P < 0.05]. Fractional oxidation of VLDL-TG was 37.2 +/- 8.8% at 240 min corresponding to 198.8 +/- 55.9 kcal/day or 10.6 +/- 3.3% of resting energy expenditure (REE). Our data demonstrate that VLDL-TG Ra measured by a biexponential fit to a bolus decay curve correlates well with VLDL-TG Ra measured by a primed continuous infusion, and therefore that a "second" peripheral VLDL-TG compartment with rapid exchange of TG exists. VLDL-TG volume of distribution is therefore greater than previously anticipated. Finally our data supports that VLDL-TG contributes quantitatively to REE.

Generation and characterization of two novel mouse models exhibiting the phenotypes of the metabolic syndrome: Apob48-/-Lepob/ob mice devoid of ApoE or Ldlr

The metabolic syndrome is a group of disorders including obesity, insulin resistance, atherogenic dyslipidemia, hyperglycemia, and hypertension. To date, few animal models have been described to recapitulate the phenotypes of the syndrome. In this study, we generated and characterized two lines of triple-knockout mice that are deficient in either apolipoprotein E (Apoe(-/-)) or low-density lipoprotein receptor (Ldlr(-/-)) and express no leptin (Lep(ob/ob)) or apolipoprotein B-48 but exclusively apolipoprotein B-100 (Apob(100/100)). These two lines are referred to as Apoe triple-knockout-Apoe 3KO (Apoe(-/-)Apob(100/100)Lep(ob/ob)) and Ldlr triple-knockout-Ldlr 3KO (Ldlr(-/-)Apob(100/100)Lep(ob/ob)) mice. Both lines develop obesity, hyperinsulinemia, hyperlipidemia, hypertension, and atherosclerosis. However, only Apoe 3KO mice are hyperglycemic and glucose intolerant and are more obese than Ldlr 3KO mice. To evaluate the utility of these lines as pharmacological models, we treated both with leptin and found that leptin therapy ameliorated most metabolic derangements. Leptin was more effective in improving glucose tolerance in Ldlr 3KO than Apoe 3KO animals. The reduction of plasma cholesterol by leptin in Ldlr 3KO mice can be accounted for by its suppressive effect on food intake. However, in Apoe 3KO mice, leptin further reduced plasma cholesterol independently of its effect on food intake, and this improvement correlated with a smaller plaque lesion area. These effects suggest a direct role of leptin in modulating VLDL levels and, likewise, the lesion areas in VLDL-enriched animals. These two lines of mice represent new models with features of the metabolic syndrome and will be useful in testing therapies targeted for combating the human condition.

Oxidised lipoproteins may promote inflammation through the selective delay of engulfment but not binding of apoptotic cells by macrophages

During normal tissue homeostasis apoptotic cells (AC) are rapidly recognised and engulfed by neighbouring cells or macrophages (Mphi), thus preventing an inflammatory response. Conversely, in chronically inflamed tissues, including the atherosclerotic artery and rheumatoid joint, removal of AC is defective despite the co-localisation of seemingly adequate numbers of Mphi. Mechanisms preventing removal of AC in vivo remain obscure, but might include oxidised low-density lipoprotein (ox-LDL), which is abundant in chronic inflammatory lesions. Although implicated in their pathogenesis, defining the role of ox-LDL on inflammatory processes has proved complicated. In fact, seemingly contradictory results have previously been described, though these may in part reflect the heterogeneous nature of ox-LDL applied in these studies. We wished to investigate the effect of physiologically representative ox-LDL on the binding and engulfment of apoptotic vascular smooth muscle cells (VSMC) and fibroblasts, as these have previously been shown to co-localise with Mphi in chronically inflamed tissues in vivo. We show that Mphi recognition of AC in vitro is not affected at physiological levels of ox-LDL. However, engulfment of intact AC is dramatically reduced/delayed. Importantly, in the absence of ox-LDL rapid phagocytosis of intact AC suppresses Mphi inflammatory cytokine release. In striking contrast, in the presence of ox-LDL, despite binding of AC to Mphi, release of IL-6 and MCP-1 is no longer suppressed. We propose that ox-LDL could maintain an inflammatory response by inhibiting the engulfment of AC, required for Mphi de-activation. This mechanism may contribute to chronic persisting inflammation in the atherosclerotic artery and rheumatoid joint.

Maternal loading with very low-density lipoproteins stimulates fetal surfactant synthesis

We examined whether administration of very low-density lipoproteins (VLDL) to pregnant rats increases surfactant phosphatidylcholine (PtdCho) content in fetal pre-type II alveolar epithelial cells. VLDL-triglycerides are hydrolyzed to fatty acids by lipoprotein lipase (LPL), an enzyme activated by heparin. Fatty acids released by LPL can incorporate into the PtdCho molecule or activate the key biosynthetic enzyme cytidylyltransferase (CCT). Dams were given BSA, heparin, VLDL, or VLDL with heparin intravenously. Radiolabeled VLDL given to the pregnant rat crossed the placenta and was distributed systemically in the fetus and incorporated into disaturated PtdCho (DSPtdCho) in pre-type II cells. Maternal administration of VLDL with heparin increased DSPtdCho content in cells by 45% compared with control (P < 0.05). VLDL produced a dose-dependent, saturable, and selective increase in CCT activity. VLDL did not significantly alter immunoreactive CCT content but increased palmitic, stearic, and oleic acids in pre-type II cells. Furthermore, hypertriglyceridemic apolipoprotein E knockout mice contained significantly greater levels of DSPtdCho content in alveolar lavage and CCT activity compared with either LDL receptor knockout mice or wild-type controls that have normal serum triglycerides. Thus the nutritional or genetic modulation of serum VLDL-triglycerides provides specific fatty acids that stimulate PtdCho synthesis and CCT activity thereby increasing surfactant content.

LDL immunization induces T-cell-dependent antibody formation and protection against atherosclerosis

Atherosclerosis is an inflammatory disease, and the involvement of immune mechanisms in disease progression is increasingly recognized. Immunization with oxidized low density lipoprotein (LDL) decreases atherosclerosis in several animal models. To explore humoral and cellular immune reactions involved in this protection, we immunized apolipoprotein E knockout mice with either homologous plaque homogenates or homologous malondialdehyde (MDA)-LDL. Immunization with both these antigen preparations reduced lesion development. The plaques contained immunogen(s) sharing epitopes on MDA-LDL, MDA-very low density lipoprotein, and oxidized cardiolipin. This shows that a T-cell-dependent antibody response was associated with protection against atherosclerosis. The protection was associated with specific T-cell-dependent elevation of IgG antibodies against MDA-LDL and oxidized phospholipids, and the increased titers of IgG antibodies were correlated with decreased lesion formation and lower serum cholesterol levels.

The magnitude of decrease in hepatic very low density lipoprotein apolipoprotein B secretion is determined by the extent of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibition in miniature pigs

It has been postulated that the rate of hepatic very low density lipoprotein (VLDL) apolipoprotein (apo) B secretion is dependent upon the activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. To test this hypothesis in vivo, apoB kinetic studies were carried out in miniature pigs before and after 21 days treatment with high-dose (10 mg/kg/day), atorvastatin (A) or simvastatin (S) (n = 5). Pigs were fed a diet containing fat (34% of calories) and cholesterol (400 mg/day; 0.1%). Statin treatment decreased plasma total cholesterol [31 (A) vs. 20% (S)] and low density lipoprotein (LDL) cholesterol concentrations [42 (A) vs. 24% (S)]. Significant reductions in plasma total triglyceride (46%) and VLDL triglyceride (50%) concentrations were only observed with (A). Autologous [131I]VLDL, [125I]LDL, and [3H]leucine were injected simultaneously, and apoB kinetic parameters were determined by triple-isotope multicompartmental analysis using SAAM II. Statin treatment decreased the VLDL apoB pool size [49 (A) vs. 24% (S)] and the hepatic VLDL apoB secretion rate [50 (A) vs. 33% (S)], with no change in the fractional catabolic rate (FCR). LDL apoB pool size decreased [39 (A) vs. 26% (S)], due to reductions in both the total LDL apoB production rate [30 (A) vs. 21% (S)] and LDL direct synthesis [32 (A) vs. 23% (S)]. A significant increase in the LDL apoB FCR (15%) was only seen with (A). Neither plasma VLDL nor LDL lipoprotein compositions were significantly altered. Hepatic HMG-CoA reductase was inhibited to a greater extent with (A), when compared with (S), as evidenced by 1) a greater induction in hepatic mRNA abundances for HMG-CoA reductase (105%) and the LDL receptor (40%) (both P < 0.05); and 2) a greater decrease in hepatic free (9%) and esterified cholesterol (25%) (both P < 0.05). We conclude that both (A) and (S) decrease hepatic VLDL apoB secretion, in vivo, but that the magnitude is determined by the extent of HMG-CoA reductase inhibition.

[Effects of purified VLDL injection on experimental hyperlipidaemia]

Forty-two neonate NZW rabbits were randomly allocated to two experimental groups (II and III) and a control group (I). Groups II and III were intraperitoneally injected with purified VLDL 0.5mg and 1.0mg, respectively within 12 hours after their birth, and then were repeated with same doses twice on the 7th and 15th day, respectively. From the 18th day of their life, all the rabbits were fed with high cholesterol forage to establish atherosclerosis models. Serum levels of TG, TCh, and HDL-C were determined in the 17th, 30th, and 40th week of their life. Serum levels of TG in Groups II and III were much lower than those in Group I in the 17th week. Serum levels of TG and ratio of LDL-C to HDL-C were significantly lower and of HDL-C much higher in Groups II and III than those in Group I in the 30th week. The results determined in the 40th week were the same as above, and serum levels of TCh in Groups II and III were significantly lower than those in Group I. There Were no significant differences in serum levels of LDL-C and TCh among the three groups. The results suggested peritoneal injection of purified VLDL into rabbits in their earliest stage of life had effects on the formation of hyperlipidaemia, and could Prevent them from atherosclerosis. Its mechanism was briefly discussed in the paper.

Infusion of atherogenic lipoprotein particles increases hepatic lipase activity in the rabbit

Hepatic lipase plays a key role in the turnover of potentially atherogenic lipoprotein remnants and in determining the relative distribution of high density lipoprotein (HDL) particle size subclasses. Rabbits fed a cholesterol-enriched diet have been found to accumulate potentially atherogenic chylomicron remnants and beta-very low density lipoprotein (beta-VLDL) and show a rapid increase in liver and postheparin plasma hepatic lipase activity. To determine whether the particles that accumulate during cholesterol feeding are a stimulus for this increase in hepatic lipase activity, we infused normal chow-fed rabbits with a chylomicron remnant plus beta-VLDL-enriched plasma fraction isolated from rabbits fed 0.5% cholesterol-supplemented chow. The infusion of this plasma fraction for 4 h increased hepatic lipase activity up to 2.9-fold over control rabbits and resulted in a loss of larger sized HDL particles consistent with the action of hepatic lipase. The increase in activity was significantly correlated with the concentration of infusate phospholipid, unesterified cholesterol, and esterified cholesterol, but not with the infusate triglyceride concentration. The change in the plasma cholesterol concentration of recipient rabbits, which reflects the degree of lipoprotein accumulation in these rabbits, was also significantly correlated with the change in hepatic lipase activity. However, a chylomicron remnant and beta-VLDL-depleted fraction of plasma from cholesterol-fed rabbits did not increase hepatic lipase activity. Furthermore, triglyceride presented as an artificial lipid emulsion (Intralipid) was not able to stimulate hepatic lipase activity, although triglyceride is a substrate for hepatic lipase.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipoprotein-induced modulation of cyclosporine-A-mediated immunosuppression

Human serum lipoproteins form complexes with cyclosporine-A and act as a carrier of cyclosporine-A in vivo. We compared the immunosuppressive effects of free cyclosporine-A, a complex composed of cyclosporine-A and lipoproteins, free cyclosporine-A in the presence of each unbound lipoprotein, and each lipoprotein without cyclosporine-A with one another at concentrations comparable with in vivo conditions on PHA-stimulated peripheral blood mononuclear cells. Free cyclosporine-A reduced the proliferation of the PHA-stimulated mononuclear cells to 50% at a concentration of 300 ng ml-1 (SD +/- 30, n = 12) lipoprotein-deficient medium. Cyclosporine-A loaded into VLDL showed a 50% proliferation rate reduction at 60 micrograms VLDL ml-1 (SD +/- 10, n = 12) and 180 ng cyclosporine-A ml-1. In the presence of 100 ng ml-1 cyclosporine-A 180 micrograms ml-1 VLDL (SD +/- 25, n = 12) showed a proliferation rate reduction of 50%. In the same way VLDL without cyclosporine-A induced a reduction to 50% at 740 micrograms ml-1 (SD +/- 30, n = 12). Cyclosporine-A loaded into LDL showed a 50% proliferation rate reduction at 27 micrograms ml-1 LDL (SD +/- 5, n = 12) with 80 ng ml-1 cyclosporine-A. In the presence of 100 ng ml-1, cyclosporine-A 150 micrograms ml-1 LDL (SD +/- 25, n = 12) showed a proliferation rate reduction of 50%. In the same way, LDL without cyclosporine-A induced a reduction to 50% at 950 micrograms ml-1 (SD +/- 50, n = 12).(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the metabolism of apolipoprotein B in hypertriglyceridemic subjects using simultaneous administration of tritiated leucine and radioiodinated very low density lipoprotein

To study the metabolic pathways of apolipoprotein B (apoB), a series of studies were carried out in which both radioiodinated very low density lipoproteins (VLDL) and tritiated leucine were simultaneously injected into three hypertriglyceridemic subjects. The appearance and disappearance of tritium activity in VLDL apoB, intermediate density lipoprotein (IDL) apoB, and low density lipoprotein (LDL) apoB were followed as was the disappearance of iodine activity from VLDL and the appearance and disappearance of iodine activity in IDL apoB and LDL apoB. It was found that a delipidation chain could describe the kinetics of both endogenously and exogenously labeled VLDL. A slow component of VLDL was necessary to fit the VLDL 131I-labeled apoB data and was consistent with the observed VLDL [3H]apoB kinetics. In addition, the estimated rate of conversion of VLDL apoB to LDL exceeded that which appeared to pass through the measured IDL pools, suggesting that a fraction of the IDL was not directly observed. It was also found that a higher percentage of VLDL 131I-labeled apoB was converted to LDL apoB than was VLDL [3H]apoB. To evaluate possible causes of this apparent anomaly, simultaneous examination of all kinetic data was performed. This exercise resulted in the resolution of removal pathways from multiple compartments in the VLDL delipidation chain and the conversion of slowly metabolized VLDL to IDL and LDL. The wide spectrum of this loss pathway indicates that previous estimates of VLDL apoB production rate using the radioiodinated methodology probably represent lower bounds for the true physiologic variable. It is important to note that these direct losses were apparent only when the combination of endogenous and exogenous labeling was used.

The effect of intravenously injected beta very low density lipoprotein on small and large arterial injuries

A series of daily injections of beta very low density lipoprotein (beta-VLDL) was administered over 4-5 days to rabbits whose arteries contained either experimental circumferential lesions or areas of intimal thickening. The circumferential lesions were similar to those that occur spontaneously and were produced by the application of longitudinal tension. The intimal thickening was produced by denuding the endothelium with a balloon catheter. Over the period of injection of beta-VLDL the plasma cholesterol levels rose in a pulse-like manner from 60 to 100 mg/dl. Following cessation of injections the cholesterol levels initially rose further and then decreased to normal levels within 4 weeks. Injections of beta-VLDL, commencing 1-2 days after production of the circumferential lesions, resulted in an increase in the number of mononuclear leukocytes (primarily macrophages) and in a moderate accumulation of lipid by these cells and the medial smooth muscle cells. If the injections were started 14 days postinjury there was some accumulation of lipid in the large lesions but none in small lesions. There was no lipid accumulation in any lesions if the beta-VLDL was administered 3 months postinjury or if the animals were injected 2 days after injury and examined 3 months later. A very slight accumulation of lipid occurred in the intimal thickening, or neo-intima, following a series of beta-VLDL injections given to rabbits 2 or 6 weeks after balloon catheter injury. The series of injections produced a significant increase in the number of mononuclear leukocyte profiles per area of the neo-intima, suggesting an increased infiltration of these cells into the injured artery. These results suggest that a small transient increase in the plasma concentration of cholesterol-carrying lipoproteins may lead to increased infiltration of mononuclear leukocytes into areas of intimal thickening or areas of "spontaneously occurring" injury.

Effect of plasma lipoproteins on cholesterol accumulation in macrophages: comparison of lipoproteins from normal and homozygous familial hypercholesterolemic subjects

Total cholesterol (TC) content of mouse peritoneal macrophages (MPM) increased when incubated with increasing concentrations of normal low density (N-LDL) or very low density (N-VLDL) lipoprotein. Incubation with increasing concentrations of normal high density lipoprotein (N-HDL) caused a decrement in cellular mass of TC in MPM. Incubation of MPM with serum from normal subjects as well as from subjects with homozygous familial hypercholesterolemia (HFH) resulted in a 25% increment in cellular mass of TC, due to an increment in both free cholesterol (FC) and cholesteryl ester (CE) fractions. Accumulation of TC in MPM, due mainly to elevation of CE, was observed when the macrophages were incubated in the presence of LDL or VLDL derived from either group of subjects. N-LDL caused a higher increment in cellular CE compared to HFH-LDL. However, the presence of HFH-VLDL in the medium caused elevation in the cellular TC and CE content to a higher level than did N-VLDL. The presence of N-HDL as well as of HFH-HDL in the medium resulted in a similar decrement in the cholesterol content of MPM. The decrement was expressed in both FC and CE fractions. The present study shows different abilities of normal and HFH plasma lipoproteins to cause cholesterol accumulation in MPM.

[The effect of hyperlipoproteinemia induced by the administration of an atherogenic lipoprotein fraction on various indices of lipid metabolism in rabbits]

Hyperlipoproteinemia, which was developed after intravenous administration of lipoprotein atherogenous fraction, caused the secondary metabolic alterations. These alterations were manifested as a decrease in catabolism of blood serum lipoproteins, accumulation of hydrophobic lipid components in the lipoproteins as well as in a decrease in relative content of high density lipoproteins responsible for elimination of cholesterol from tissues.

[Mechanism of the maintenance of experimental hyperlipoproteinemia in the rabbit]

A decrease in lipolytic and total esterase activities as well as an increase in the rate of cholesterol esterification were found in blood of rabbits with experimental hyperlipoproteinemia caused by intravenous administration of atherogenic lipoproteins. Simultaneously, content of free and esterified cholesterol was increased in liver tissue, content of triglycerides and phospholipids was decreased in mitochondria; the lipolytic activity was elevated.

Central role of high density lipoprotein in plasma free cholesterol metabolism

This study was designed to provide direct information on the in vivo metabolism in man of free (unesterified) cholesterol in the major lipoprotein classes. Five human subjects were administered one or two (simultaneous) of the following; [2-(14)C] mevalonic acid, high density lipoprotein (HDL)-free [(14)C] cholesterol, low density lipoprotein (LDL)-free [(14)C] cholesterol, and very low density lipoprotein (VLDL)-free [(3)H]cholesterol. Blood was then obtained at frequent intervals for at least 9 h, and the alpha(HDL) and beta(LDL + VLDL) lipoproteins were quickly separated by heparin-manganese precipitation to prevent ex vivo exchange of free cholesterol. After the administration of [(14)C]mevalonic acid the specific activity (disintegrations per minute/micromole) of free cholesterol in the alpha- and beta-lipoproteins increased for 3 h. During this period the alpha-free cholesterol specific activity was higher than the beta specific activity. After administration of VLDL and LDL labeled with free cholesterol, the alpha-free cholesterol specific activity reached a peak value within 20 min, at which time it was considerably lower than the beta-free cholesterol specific activity. When HDL labeled with free cholesterol was administered, a precursor product relationship was observed between the alpha-free cholesterol (precursor) and beta-free cholesterol (product) specific activities.A multicompartmental model was developed that contained the simplest structure necessary to fit all of the data obtained. The kinetic analysis revealed the presence of extensive exchange (20-85 mumol/min) of free cholesterol between HDL and a tissue pool(s) enriched with newly synthesized free cholesterol. It was found that virtually all (>95%) of the free cholesterol in the beta-lipoproteins (LDL+VLDL) cycles directly through HDL. The free cholesterol in LDL appears to behave in the same fashion as the free cholesterol in VLDL. The results show that there are marked differences in the kinetic behavior of the free cholesterol fractions of alpha- and beta-lipoproteins. There is extensive recycling of free cholesterol between HDL and tissue pools, and between HDL and the beta-lipoproteins; this recycling has been quantitated. The findings support the view that in vivo, the free cholesterol in HDL plays a central role in exchange reactions and in the vascular-tissue cholesterol transport system.

Splanchnic metabolism of plasma apolipoprotein B: studies of artery-hepatic vein differences of mass and radiolabel in fasted human subjects

The metabolism of apoprotein B-containing plasma lipoproteins by human splanchnic tissues has been studied in 29 men undergoing coronary angiography. Before catheterization autologous radio-iodinated lipoproteins were infused into a peripheral vein: 10 subjects received (125)I-labeled Sf 12-60 lipoproteins; 12 received (125)I-labeled Sf 12-60 plus (131)I-labeled Sf 100-400 lipoproteins; and 7 received (125)I-labeled Sf 12-60 plus (131)I-labeled Sf 0-12 lipoproteins. Paired arterial and hepatic vein blood samples were subsequently collected for replicate measurements of apoprotein B (apo B) mass, radioactivity and specific activity in each lipoprotein class. Splanchnic plasma flow was measured with indocyanine green. All studies were conducted after a 14-h overnight fast. Newly synthesized apo B was shown to be secreted by splanchnic tissues as a component of Sf 100-400 lipoproteins, with no detectable uptake of apo B from this class. Sf 12-60 apo B was extracted by the splanchnic bed, with no detectable secretion. After continuous intravenous infusion of (125)I-labeled Sf 12-60 for five or more hours, 41-67% (mean 55%) of extracted Sf 12-60 apo B radioactivity reappeared in hepatic vein Sf 0-12 apo B. There was no detectable splanchnic catabolism of Sf 0-12 apo B. The rates of Sf 100-400 apo B secretion, calculated as the product of artery-hepatic vein concentration difference and splanchnic plasma flow, were greater than the previously reported rates of very low density lipoprotein apo B turnover in fed subjects obtained by kinetic analysis of plasma specific radioactivity decay curves, suggesting that there may be a diurnal variation in hepatic apo B synthesis. They also exceeded the splanchnic extraction rates of Sf 12-60 apo B, suggesting there was some extrasplanchnic catabolism of the apo B of Sf > 60 lipoproteins.

Uptake and degradation of rat and human very low density (remnant) apolipoprotein by parenchymal and non-parenchymal rat liver cells

1. The relative contribution of parenchymal and non-parenchymal cells to the in vivo hepatic uptake of serum apolipoproteins was measured 30 min after intravenous injection of radioiodinated rat very low density lipoprotein (VLDL) remnants, rat and human VLDL, low density lipoprotein (LDL) and high density lipoprotein (HDL). Using rat VLDL, VLDL-remnants, LDL and HDL, respectively, the non-parenchymal cells contain 4.7, 4.9, 6.1 and 5.3 times the amount of trichloroacetic acid-precipitable radioactivity per mg cell protein as compared to parenchymal cells. For human VLDL, LDL and HDL these values are 5.1, 12.0 and 5.9 respectively. 2. The abilities of homogenates of human liver, rat liver parenchymal cells and rat liver non-parenchymal cells to hydrolyze human and rat iodinated VLDL apoprotein were determined by measuring the amount of trichloroacetic acid-soluble (non-iodide) radioactivity liberated upon incubation at the optimal pH of 4.2. Non-parenchymal cells possess a 8--21-fold higher maximal capacity to degrade VLDL apoprotein per mg of cell protein than parenchymal cells. This factor is 5--6 for VLDL-remnant apoprotein degradation measured at low (suboptimal) apolipoprotein concentrations. 3. It is concluded that, in addition to parenchymal cells, the non-parenchymal cells play an important role in the hepatic uptake and possibly degradation of VLDL-(remnant) apoprotein.

Hypertriglyceridemia in the diabetic rat. Defective removal of serum very low density

Streptozotocin-induced diabetic rats show a marked fasting hypertriglyceridemia. It appears that only the very low density lipoprotein (VLDL) fraction is increased. VLDL from either normal or diabetic rats was labelled in vivo in the triglyceride moiety with [3H]palmitate and isolated. Both preparations, if injected intravenously into recipient rats, are removed more slowly from the circulation of diabetic rats as compared to normal rats, resulting in a reduction of the fractional catabolic rate (F.C.R.) by 70%. However, the absolute catabolic rate (turnover) of VLDL triglycerides was not changed in diabetics. It is concluded that the hypertriglyceridemia of the the diabetic rat is caused by a defective removal mechanism of VLDL triglycerides. The F.C.R. of 125I-labelled low density lipoproteins and high density lipoproteins is only 1-3% of the value for VLDL triglycerides and unchanged in diabetic rats.

Endogenous triglyceride turnover in liver and plasma of the dog

Radioactive glycerol and S(f) > 20 lipoproteins labeled with it were used to study turnover of plasma S(f) > 20 and hepatic triglyceride in anesthetized dogs. From specific activity-time curves of these lipids after an injection of labeled material, a tentative and incomplete model for the kinetics of endogenous hepatic and plasma triglyceride was defined and partially validated. Pool sizes and turnover rates of triglyceride in liver and S(f) > 20 lipoproteins of plasma were then calculated in seven dogs. Hepatic triglyceride was composed of two compartments: 60% metabolically inert and 40% metabolically active. Although communication between these hepatic compartments surely occurred during the time course of these studies, it was not sufficient to be detected by our present methods. The metabolically active compartment turned over as a single pool but with two destinations: a quite variable proportion (an average of 61%) was secreted into plasma as S(f) > 20 triglyceride, and an average of 39% was presumably hydrolyzed within the liver. The fractional turnover rate of plasma S(f) > 20 triglyceride was 2-3 times that of hepatic triglyceride. This finding, and the parallel decline of specific activities of plasma S(f) > 20 and liver triglyceride after injection of labeled glycerol, confirm the rate-determining role of hepatic triglyceride. In this respect the dog differs importantly from man. Though turnover rates of plasma S(f) > 20 triglyceride fell in the same range in men and dogs, the relationship of turnover rate to plasma concentration of this lipid differed greatly between them. The model for the dog does resemble that previously reported for man, however, in the lack of major recycling of intact plasma triglyceride between the liver and plasma. Lack of such recycling, however, does not exclude return of plasma triglyceride into a hepatic triglyceride sink. The amount of such unidirectional uptake, if any, could not be determined by these techniques.