Effect of Triton WR-1339 on lecithin-cholesterol acyltransferase in the rat

Fixed-time and continuous assays of very-low-density lipoprotein secretion rate from rat liver: mean vs. instantaneous velocity

Aim of the study

The secretion rate of triglyceride from rat liver is assayed by the measurement of triglyceride accumulation in plasma when its clearance is inhibited. The aim of the study was to measure and compare the secretion rate of triglyceride from rat liver by two methods of fixed-time and continuous assays.

Material and methods

A single dose of 200 mg of poloxamer-407 (P-407) was injected i.p. into starved male rats. The secretion rate of triglyceride was measured by fixed-time and continuous assays.

Results

The time course for the changes of serum triglyceride following injection of P-407 showed three distinct phases: a lag period of about 30 minutes, a linear increase in serum triglyceride that lasted more than 4 hours, and a slight decline of triglyceride accumulation that lasted about 24 hours. The mean rate of triglyceride secretion was 234.1 ±9.6 mg/dl/h during the linear phase. The linear phase was divided into five time protocols of 240, 180, 120, 60, and 30 minutes and the secretion rate was measured at three points of time in each protocol. The mean rate of triglyceride secretion was 3.91 ±0.15, 3.83 ±0.16, 3.76 ±0.29, 3.57 ±0.43 and 3.13 ±0.34 mg/dl/min in these protocols respectively. In the kinetic assay, the change in the absorbance per three successive five minutes (ΔA/Δt) was measured and the secretion rate was calculated as 3.82 ±0.11 mg/dl/min.

Conclusions

The rate of triglyceride secretion can be measured by both fixed-time and kinetic assays and was about 3.82 ±0.11 mg/dl/min. The results of the two methods are more corresponded as the mean and instantaneous velocity respectively.

The Long Term Kinetic of Plasma Lipids and Lipoproteins in Tyloxapol Injected Rats

INTRODUCTION

The level of plasma triglyceride is balanced by the rate of secretion into and clearance from the plasma. Tyloxapol (Triton WR1339) is a nonionic detergent that inhibits lipoprotein lipase and hence clearance of triglyceride from the plasma.

AIM

To determine the kinetic of plasma lipids and lipoproteins following injection of tyloxapol over a period of two weeks.

MATERIALS AND METHODS

Fifteen male rats were starved over-night and injected intravenously with tyloxapol (400mg/kg). Blood samples were taken in three steps as, the early (1-6 hours), the middle (1-2 days) and the third (3-9 days) phase. Plasma total cholesterol and triglyceride were measured by enzymatic methods and total phospholipids were analysed as molybdenum blue. Serum lipoproteins were fractionated by electrophoresis on agarose gel (Sebia Inc).

RESULTS

The changes of plasma lipids following tyloxapol injection showed three distinctive phases. The early phase lasts at least 6 hours, and the concentrations of triglyceride, total cholesterol and phospholipids increased linearly. The rate of triglyceride secretion was 259.7 ± 8.1 mg/h.dl in this phase, which was comparable to the mean rate of 250.6 ± 37.0 mg/h.dl or 102.8 ± 15.2 mg/h.kg body in starved male rat. During the next 48 hour the lipids continued to accumulate but at a lower rate, and the levels of triglyceride, cholesterol and total phospholipids rose up to about 3200, 586 and 715 mg/dl respectively. In the last phase, the levels of plasma lipids decreased toward the basal levels after 5 days. In serum lipoprotein electrophoresis, the VLDL and LDL increased and HDL fraction disappeared simultaneously during the initial 2 hours of tyloxapol injection. The VLDL fell down toward the normal range, preceded to the reappearance of HDL during 5 days.

CONCLUSION

A single intravenous injection of tyloxapol shows three distinctive phases. In the early phase, triglyceride accumulates linearly and the rate of its increment in plasma is a good estimate of the rate of VLDL secretion from the liver.

Almonds inhibit dyslipidemia and vascular dysfunction in rats through multiple pathways

BACKGROUND

Almonds are reported to be protective against cardiovascular diseases (CVDs); however, the possible mode of action has only infrequently been explored.

OBJECTIVE

This study aimed at investigating the mechanistic basis for the benefits of almonds in atherosclerotic CVDs.

METHODS

Three studies in 3 groups of rats were designed with the use of tyloxapol (study 1), a high-fat diet (HFD; study 2), and white-flour fructose (WFF; study 3). In each of the studies, the first group acted as the control [administered saline in study 1 and fed a normal diet (ND) in studies 2 and 3]; the second and third groups were treated with tyloxapol in study 1, an HFD in study 2, and WFF in study 3. The third group in each study was also fed almonds (3 g/kg) for 4 wk, after which blood was collected for biochemical evaluation. Livers and aortas were isolated from the rats in studies 1 and 2 for enzyme assays and vascular analysis, respectively.

RESULTS

Almond supplementation significantly (P < 0.05) prevented hyperlipidemia in all of the rat models. Supplementation suppressed cholesterol synthesis, leading to a 65% inhibition of tyloxapol-induced activation of hepatic β-hydroxy-β-methylglutaryl coenzyme A reductase. The almond intervention inhibited by 56% the HFD-induced increase in serum concentrations of hepatic aminotransferases. Almonds also protected against an HFD-induced increase in uric acid (0.9-fold), phosphorus (1.1-fold), alkaline phosphatase (4.6-fold), and γ-glutamyltransferase (1-fold), with resultant concentrations that were not different from those in ND-fed rats (P > 0.05). Almonds partially restored the vascular reactivity of isolated aortas and prevented HFD-induced endothelial dysfunction by reducing inhibition of endothelial nitric oxide (NO) synthase and promoting NO release. The 70% decrease in HDL cholesterol that was observed in the WFF group was prevented by almond supplementation; serum and LDL cholesterol were also normalized.

CONCLUSIONS

The inhibition of de novo cholesterol synthesis, prevention of hepatic damage, and restoration of vascular function via the protection of endothelium and influence on the NO pathway are some of the mechanisms underlying the medicinal value of almonds in CVDs.

Apolipoprotein AII is a regulator of very low density lipoprotein metabolism and insulin resistance

Apolipoprotein AII (apoAII) transgenic (apoAIItg) mice exhibit several traits associated with the insulin resistance (IR) syndrome, including IR, obesity, and a marked hypertriglyceridemia. Because treatment of the apoAIItg mice with rosiglitazone ameliorated the IR and hypertriglyceridemia, we hypothesized that the hypertriglyceridemia was due largely to overproduction of very low density lipoprotein (VLDL) by the liver, a normal response to chronically elevated insulin and glucose. We now report in vivo and in vitro studies that indicate that hepatic fatty acid oxidation was reduced and lipogenesis increased, resulting in a 25% increase in triglyceride secretion in the apoAIItg mice. In addition, we observed that hydrolysis of triglycerides from both chylomicrons and VLDL was significantly reduced in the apoAIItg mice, further contributing to the hypertriglyceridemia. This is a direct, acute effect, because when mouse apoAII was injected into mice, plasma triglyceride concentrations were significantly increased within 4 h. VLDL from both control and apoAIItg mice contained significant amounts of apoAII, with approximately 4 times more apoAII on apoAIItg VLDL. ApoAII was shown to transfer spontaneously from high density lipoprotein (HDL) to VLDL in vitro, resulting in VLDL that was a poorer substrate for hydrolysis by lipoprotein lipase. These results indicate that one function of apoAII is to regulate the metabolism of triglyceride-rich lipoproteins, with HDL serving as a plasma reservoir of apoAII that is transferred to the triglyceride-rich lipoproteins in much the same way as VLDL and chylomicrons acquire most of their apoCs from HDL.

A procedure for inducing sustained hyperlipemia in rats by administration of a surfactant

Tyloxapol (Triton WR 1339) is a non-ionic detergent that inhibits lipoprotein lipase and thereby raises levels of serum lipids. It is used frequently for acute studies on lipids in rats but not for subacute or chronic studies. In the present work, we found that tyloxapol must be injected intravenously three times each week in order to have high and sustained levels of serum cholesterol and triglycerides for 1, 2, or 3 weeks. These results make it possible to extend the use of tyloxapol into chronic studies of hyperlipemia and vascular disease.

Balance/excretion of 3H- and 14C-tyloxapol in the male rabbit after intratracheal administration

1. Tyloxapol, trace-labelled (50-100 microCi/animal) with 3H or 14C, was administered intratracheally in a surfactant formulation (EXOSURF Neonatal) to the male rabbit in a total tyloxapol dose of 5 mg/kg. Urine, faeces, expired air, and blood were collected for up to 10 days following tyloxapol administration. 2. Over 5 days, 3H-tyloxapol-related radioactivity in the urine (13.4%) and faeces (27.4%) accounted for a major fraction of the labelled dose. However, urine also contained an additional 13% of the dose as tritiated water. Expired air accounted for only 4.2% of the dose. At the end of the study, an additional 35.6% of the radioactive dose was found in tissues and the carcass, mainly in the lung (27.4%) and to a lesser extent in the liver (2.8%) and kidney (0.4%). Levels of radioactivity in other tissues, including whole blood, were low. 3. Over a separate 10-day study, faecal (30.4%) and renal (9.7%) elimination of 14C-tyloxapol accounted for 40% of the radioactive dose, with expired air accounting for much less (2.7%). At the end of the study, additional radioactivity was recovered from the lung (43.9%) and to a lesser extent from the liver (3.8%) and kidney (0.3%). The half-life for the elimination of total radioactivity from the lung was estimated to be 10-12 days. 4. These data indicate that, following intratracheal administration, tyloxapol and metabolites were retained by the lung, released slowly into the systemic circulation, and eliminated through faecal and renal excretion.

Lead effect on the oxidation resistance of erythrocyte membrane in rat triton-induced hyperlipidemia

The anemia observed in severe chronic lead poisoning is in part attributable to alterations in the erythrocyte physicochemical properties. Since they are partly related to the membrane lipid composition, the aim of the present study was to determine the effects of a triton-induced hyperlipidemia on the resistance to oxidation of erythrocyte membranes in lead-treated Wistar rats. Our results showed that triton administration to lead-treated rats induced an increase in erythrocyte choline phospholipid levels together with a significant decrease in the erythrocyte membrane lipid resistance to oxidation. These results led us to suggest that anemia in lead poisoning is linked to interactions between lead present in the membrane and plasma phospholipids. Their increase in rat hyperlipidemia induced by triton resulted in a decrease in the membrane resistance to oxidation and finally in an erythrocyte fragility leading to their destruction.

Effects of Triton WR 1339 and orotic acid on lipid metabolism in rats

In order to investigate the effect of hepatic cholesterol flux on biliary bile acids, Triton WR 1339 and orotic acid were administered to rats, and the biliary cholesterol, phospholipids and bile acids were analyzed together with serum lipoproteins and hepatic lipids. Triton, which raised serum very low density lipoprotein and lipid levels and decreased serum high density lipoprotein liver lipid levels, increase the biliary cholic acid group/chenodeoxycholic acid group ratio (CA/CDCA) in the bile without affecting the total amount of bile acids and the other biliary lipids. Orotic acid, which decreased serum lipid and lipoprotein concentrations and increased liver lipid levels, increased the biliary excretion of cholesterol and phospholipids, but produced no significant change in the total amount of bile acids and in the CA/CDCA ratio in bile.

In vitro and in vivo interactions of Triton 1339 with plasma lipoproteins of normolipidemic rhesus monkeys. Preferential effects on high density lipoproteins

Triton WR-1339 was incubated in vitro in various proportions with plasma from normolipidemic rhesus monkeys or with ultracentrifugally purified lipoproteins, and the products were examined by isopycnic density gradient ultracentrifugation, agarose column chromatography, electrophoretic and immunochemical techniques, and electron microscopy. Some experiments used apo A-I, apo A-II, or Triton labeled with either 125I or 131I. At concentrations of less than 10 mg/ml plasma, Triton interacted preferentially with HDL, changing lipoprotein size and density; Triton was progressively incorporated into the HDL particles, displacing apo E, apo A-I, and apo A-II. At concentrations above 10 mg/ml plasma, Triton displaced all apo A-I from the particle, and much lipid was dissolved into the Triton micelles. When Triton-treated HDL particles were used as a substrate for the enzyme LCAT, enzyme activity decreased in parallel to the displacement of apo A-I. There was no displacement of apo B from LDL nor any loss of lipids; but the particles became deformed and formed rouleaux. A single intravenous dose of Triton WR-1339 administered to a normolipidemic monkey (N) and to a hypercholesterolemic monkey (H) resulted in concentration-dependent HDL changes similar to those observed in vitro. LDL was less affected by Triton, with changes occurring only at high doses. After these structural changes, intravenously injected 131I apo A-I disappeared rapidly from the circulation; 125I apo A-II disappeared less rapidly. These increased clearances were accompanied by a drop in apo A-I plasma levels and the disappearance of HDL particles from plasma. The lipoprotein and apolipoprotein patterns returned to normal 14 days after Triton. We conclude that Triton WR-1339, when exposed to rhesus plasma in vitro or in vivo, interacts preferentially with HDL in a dose-dependent manner. At low concentrations, Triton acts on surface components of the HDL particle; at higher concentrations, Triton penetrates the particle, causing structural disruption. Because of its high affinity for HDL, Triton WR-1339 is a useful reagent for study of HDL structure-function relationships.

Carnitine esters: novel inhibitors of plasma lecithin: cholesterol acyltransferase in experimental animals but not in man (Homo sapiens)

1. Long-chain fatty acid esters of carnitine were observed to inhibit lecithin: cholesterol acyltransferase (LCAT, EC 2.3.1.43) in plasma from the rat (Rattus sativa) and rabbit (Oryctolagus cuniculus) but not in man (Homo sapiens). At a level of 500 nmol/ml of plasma, L-palmitoylcarnitine, L-stearoylcarnitine, and L-oleoylcarnitine inhibit the formation of cholesteryl esters by LCAT by 25-30%. 2. The inhibition is concentration-dependent and is observed only with acylcarnitine esters with acyl chains of 12C or greater. 3. Equipotent activity is obtained using either the DL-acylcarnitine esters or the L-acylcarnitine esters. 4. Inhibition of LCAT by carnitine esters is greater than that achievable with sodium dodecylsulfate on a mol:mol basis and is not reversible with albumin. 5. The failure of the carnitine esters to inhibit plasma LCAT from man suggests the possibility of subtle differences in the structure of human LCAT compared with that in other species.

Hepatic secretion and turnover of serum phosphatidylcholine-cholesterol acyltransferase in male and female rats

The activity of serum phosphatidylcholine-cholesterol acyltransferase (LCAT), output of the enzyme by the perfused rat liver, and the effect of pretreatment with colchicine on LCAT were studied in male and female rats. It was observed that: 1. Serum LCAT activity in the female exceeded that of the male in fasted animals, whereas in fed animals, LCAT activity was higher in the male than the female. With both sexes, however, serum LCAT activity in fed animals was greater than that in fasted animals. Data are presented which suggest that the observed sex differences were due to concentration and/or composition of the substrate rather than to differences in the serum concentration of the enzyme. 2. The release of LCAT by perfused livers from fasted female rats exceeded that of the male animals. The output of LCAT was inhibited by pretreatment (male) with colchicine, which suggests that hepatic secretion of LCAT is dependent on vesicular transport. 3. The decay of serum LCAT activity in vivo following injection of colchicine was more rapid in fasted female rats than in male animals. These observations lead us to postulate that the turnover rate of LCAT is higher in female rats than in male animals.