Formation of cholesteryl ester-rich particulate lipid during metabolism of chylomicrons.

The metabolism of chylomicrons doubly labeled with cholesterol-(3)H and triglyceride-(14)C was studied in unanesthetized rats which were absorbing a fatty test meal. 10 min after intravenous injection, 80% of chylomicron cholesteryl ester, but only 20% of chylomicron triglyceride, was found in the liver. Treatment of recipient rats with puromycin to block hepatic triglyceride release did not increase the proportion of chylomicron triglyceride found in the liver. Rapid partition of chylomicron triglyceride from cholesterol ester also occurred in rats in which the liver had been excluded from the circulation. However, now the cholesteryl ester accumulated in the plasma, whereas triglyceride was cleared by peripheral tissues. Residual labeled cholesterol in the plasma of such rats was shown to be present in particulate form, together with some residual triglyceride. The remnant particles contained about 13% cholesteryl ester. When injected into other rats the remnant particles appeared in the liver more rapidly than did chylomicrons.These observations were consistent with the hypothesis that the first step in chylomicron metabolism occurred in extrahepatic tissues where a large portion of the triglyceride was removed by the action of lipoprotein lipase. The remnant particles so produced contained the chylomicron cholesteryl ester and residual triglyceride, and they were removed from the plasma by the liver.

Quantitation of the transfer of surface phospholipid of chylomicrons to the high density lipoprotein fraction during the catabolism of chylomicrons in the rat.

Small chylomicrons (CM) labeled with cholesterol, cholesterol ester, phospholipid, and, in some cases, protein, were used to study the fate of these constituents as the CM are catabolized in the circulations of the hepatectomized and intact rat. In the hepatectomized animal after (1/2) h, CM are greatly reduced in volume, surface area, and diameter. During this period, the CM lost >92% of the mass of their triacylglycerol, >77% of the mass of their phospholipid, and >39% of their protein. Compared to the injected CM, the chemically altered particles, called CM "remnants," have a reduction in volume of 96% and in surface area of 88%. The labeled cholesterol esters remain with the CM remnants but, strikingly, a major fraction of the labeled phospholipids and labeled soluble apoproteins leave the CM and are found in the high density lipoprotein (HDL) fraction. The chemical composition of this HDL fraction contains relatively more phospholipid and less cholesterol ester than normal rat HDL. Because of the difference in composition of HDL between normal rats and those given CM, we estimate that the HDL phospholipid pool increased by congruent with25% by the infusion of congruent with 4-5 mg of CM phospholipid. Approximately 5 mg of phospholipid is secreted on CM by a fed rat in 1 h. The findings in hepatectomized rats indicate that a major fraction of the phospholipid and a minor fraction of the protein (soluble non-B apoproteins) of newly secreted CM are transferred from the CM to the HDL fraction during remnant formation. The same process probably occurs in intact rats except that the remnant particles are rapidly removed from the plasma by the liver and a smaller fraction of the surface of the CM enters the HDL fraction.

Absorption of oleic and palmitic acids from emulsions and micellar solutions.

A lipid mixture (monoolein, oleic acid-1-(14)C, and palmitic acid-9,10-(3)H) was infused intraduodenally at a steady rate for 8 hr in fasted, unanesthetized rats. The same dose of lipid was given together with pure conjugated bile salts either as an emulsion, 2.5 mM bile salts, or as a micellar solution, 10 mM bile salts. The emulsion contained very little or no micellar lipid. Thoracic duct lymph was collected and in some experiments bile and pancreatic juice were drained to the exterior. After 4-5 hr infusion the same steady lymphatic output of radioactive fatty acids was obtained with emulsion as with micellar solution. It was concluded that absorption of fatty acid could proceed efficiently in the virtual absence of micellar solubilization. In rats with biliary plus pancreatic fistulae, labeled triglyceride was absorbed poorly relative to free fatty acids in the same emulsified particles. This suggested that fatty acids were transferred to the absorptive cells in monomolecular solution and not as emulsion particles. Substitution of a synthetic nonionic detergent for bile salts in lipid mixtures given to rats with biliary and pancreatic fistulae did not affect the lymphatic output of radioactive fatty acids. This indicated that mucosal esterification of labeled free fatty acids was normal in the absence of bile salts. The physical state of the lipid did not affect the pathway of absorption. Finally, comparison of the increased output of esterified fat in the lymph with the output of labeled fat suggested that fat absorption did not greatly affect the turnover of endogenous, unlabeled fat. Results were consistent with the view that most of the endogenous lymph fat comes from reabsorbed biliary lipid.

Metabolism of protein-free lipid emulsion models of chylomicrons in rats.

Emulsions were prepared by ultrasonication of mixtures of triolein, cholesteryl oleate, phosphatidylcholine and cholesterol in aqueous dispersions, then purified by ultracentrifugation. After injection into rats, the metabolism of the artificial, protein-free emulsions was comparable to the metabolism of chylomicrons collected from rat intestinal lymph during the absorption of fat. Like chylomicrons, the emulsion triacylglycerol was removed from the plasma more quickly than emulsion cholesteryl ester. Also like chylomicrons, much more emulsion cholesteryl ester than triacylglycerol appeared in the liver 10 min after injection, and only trace amounts appeared in the spleen. Because the artificial emulsions gained apolipoproteins when incubated with plasma, their metabolism was probably facilitated by the recipient rat plasma apolipoproteins and so, in rats made apolipoprotein-deficient by treatment with estrogen, the removal of emulsions from the plasma was slowed. Removal was also slowed in hyperlipidemic rats fed a high-fat, high-cholesterol diet to expand the plasma pools of the triacylglycerol-rich lipoproteins and remnants. The results indicate that the metabolism of lymph chylomicrons can be modeled by artificial, protein-free lipid emulsions not only in the initial partial hydrolysis by lipoprotein lipase, but also in the delivery of a remnant-like particle to the liver.

Isoforms of rat apolipoproteins and changes induced by insulin deficiency and fasting.

Reasons for the disordered lipoprotein metabolism in insulin deficiency are not completely understood. In this study the apolipoproteins from plasma of fed and fasted streptozotocin-induced insulin-deficient rats were compared with normal control rats. Analysis of the apolipoprotein isoforms by two-dimensional electrophoresis revealed increased proportions of sialylated apo E and of sialylated apo C-III in diabetic rats compared with control rats. Fasting increased the proportion of sialylated apo E but not the proportion of sialylated apo C-III. 3H-labeled leucine was injected into normal and insulin-deficient rats, followed by a chase of unlabeled leucine after 30 min. Blood samples were collected at intervals over 24 h and the apolipoprotein components were separated by two-dimensional electrophoresis. The relative specific activities of sialylated isoforms of apo E were less than the relative specific activities of non-sialylated apo E isoforms. In contrast, sialylated isoforms of apo C-III had higher relative specific activities than non-sialylated apo C-III. No interconversions of apo E or apo C-III isoforms were found within the lipoprotein fractions. In insulin-deficient diabetic rats the relative specific activities of sialylated apo E and apo C-III isoforms were both increased relative to non-sialylated isoforms when compared with control rats. The results of this study suggest that the isoforms of apo E and apo C-III associated with the plasma lipoproteins of diabetic rats are changed in parallel with changes in synthesis of the isoforms. The changes in association with the isoforms of the apolipoproteins possibly contribute to abnormal metabolism of plasma lipoproteins in insulin deficiency.

The condensing effects of egg lecithin and cholesterol on triolein monolayers are inhibited by substitution of one saturated acyl chain in the triacylglycerol.

Previous work showed that the clearance from plasma of chylomicron-like emulsions injected intravenously was affected by the acyl chains of the constituent triacylglycerols. Compared with emulsions containing triolein (OOO) as the only triacylglycerol, clearances were decreased by a single saturated chain in emulsions containing 1,3-dioleoyl-2-stearoyl-sn-glycerol (OSO), 1,2-dioleoyl-3-stearoyl-sn-glycerol (OOS) or 1-stearoyl-2,3-dioleoyl-sn-glycerol (SOO). The differences in clearance may reflect physical differences at the oil-water interface related to chain interactions of the triacylglycerol structures with other lipid components. In the present work lipid monomolecular films at the air-water interface were used to establish the capacity of OOO to interact with the pure synthetic triacylglycerols OOS and SOO, and the capacity of OOS and SOO to co-exist in monolayers of lecithin and of cholesterol was compared with OOO. Substituting one oleoyl chain by a stearoyl chain induced a 20% condensation in monomolecular films of the pure triacylglycerols. Mixtures of OOO with either pure egg yolk phosphatidylcholine or cholesterol also showed substantial condensing effects. In contrast substituting one oleoyl chain by a stearoyl chain substantially lessened the condensing effects. At surface pressures above the collapse pressure of the pure triacylglycerols, substantially more OOO than OOS or SOO was retained in mixed monolayers with phosphatidylcholine. These differences could underlie the effects on metabolism of saturated chains in emulsion triacylglycerols.

Cholesterol is necessary for triacylglycerol-phospholipid emulsions to mimic the metabolism of lipoproteins.

Emulsions with lipid compositions similar to the triacylglycerol-rich lipoproteins were metabolized similarly to natural chylomicrons or very-low-density lipoproteins when injected intravenously in rats. Radioactive labels tracing the emulsion triacylglycerols and cholesteryl esters were both removed rapidly from the blood stream, but the removal rate of triacylglycerols was faster than that of cholesteryl ester. Most of the removed cholesteryl ester label was found in the liver, but only a small fraction of the triacylglycerol label was found in this organ, consistent with hepatic uptake of the remnants of the injected emulsion. Emulsions otherwise identical but excluding unesterified cholesterol were metabolized differently. The plasma removal of triacylglycerols remained fast, but the cholesteryl esters were removed very slowly. Heparin stimulated lipolysis, but failed to increase the rate of removal of cholesteryl esters from emulsions lacking cholesterol. Evidently, emulsions lacking cholesterol were acted on by the enzyme lipoprotein lipase, but the resultant triacylglycerol-depleted remnant particle remained in the plasma instead of being rapidly taken up by the liver. Therefore, the presence of emulsion cholesterol is a critical determinant of early metabolic events, and the findings point to a similar role for cholesterol in the natural triacylglycerol-rich lipoproteins.

Competition between chylomicrons and their remnants for plasma removal: a study with artificial emulsion models of chylomicrons.

In previous studies, protein-free emulsions of defined lipid composition were shown capable of simulating either the metabolism of chylomicrons (chylomicron-like emulsion) or their remnants (remnant-like emulsion), depending on the content of free, unesterified cholesterol. To validate further the assumption that remnant-like and chylomicron-like emulsion have metabolic pathways in common with their natural counterparts, studies of competition for plasma removal were undertaken: the remnant-like emulsion labeled with [3H]triolein was injected sequentially twice in the carotid arteries of rats to compare the clearance of remnant-like emulsion of the second injection with the first (control). Prior to the second injection, a large bolus of the chylomicron-like emulsion or rat lymph chylomicron was injected, to check the hypothesis that remnant generated from chylomicron-like emulsion or natural chylomicrons could compete with and displace remnant-like emulsion particles from their tissue receptor sites. Experiments were also performed in rats treated with Triton WR-1339, to block the generation of remnants. Results showed that remnants derived from either natural chylomicrons or chylomicron-like emulsion both strongly competed with the remnant-like emulsion. In contrast, when transformation of remnants was prevented by Triton, the undegraded particles of chylomicron-like emulsion or natural chylomicron were unable to compete with or displace remnant-like emulsion from its sites of removal from the plasma. In agreement with plasma clearance data, the hepatic uptake of the remnant-like emulsion was inhibited by the surplus dose of natural chylomicrons. In contrast, the spleen uptake was unaffected by it.

Effects of phospholipid composition on the metabolism of triacylglycerol, cholesteryl ester and phosphatidylcholine from lipid emulsions injected intravenously in rats.

Lipid emulsions were prepared with a similar size and lipid composition to natural lymph chylomicrons, but in which the surface phospholipid was either egg phosphatidylcholine, dioleoyl-, dimyristoyl-, dipalmitoyl- or 1-palmitoyl-2-oleoylphosphatidylcholine (EYPC, DOPC, DMPC, DPPC or POPC). When injected into the bloodstream of conscious rats, the emulsions containing EYPC or POPC were metabolized similarly to natural chylomicrons, consistent with rapid lipoprotein lipase-mediated hydrolysis of triacylglycerols, followed by hepatic uptake of the remnants derived from the emulsions. Phospholipids from the injected emulsions were removed more slowly and became associated with the high-density lipoprotein fractions of the plasma. Emulsions containing DPPC were metabolized differently. Triacylglycerols disappeared very slowly from plasma, indicating lack of hydrolysis by lipoprotein lipase, and phospholipid radioactivity did not transfer to high-density lipoprotein. With emulsions containing DMPC, the plasma removal rates for emulsion triacylglycerols and cholesteryl esters were fast, but phospholipid radioactivity failed to transfer to the high-density lipoprotein fractions of plasma. With DOPC emulsions, clearances were slower than EYPC or POPC emulsions, but transfer to high-density lipoproteins was efficient. Therefore, an unsaturated chain at the glycerol 2-position was necessary for rapid hydrolysis by lipoprotein lipase and for efficient transfer of phospholipids to high-density lipoproteins. With an unsaturated chain at the glycerol 2-position, a saturated chain at the glycerol 1-position optimized the rate of remnant removal from the plasma.

Hyperlipidemia in tumor-bearing rats.

Hyperlipidemia occurs in animals bearing tumors but the mechanism of its development is uncertain. We have measured triacylglycerol clearance and production rate in rats bearing a transplantable sarcoma. The plasma content of very-low-density lipoprotein triacylglycerol was increased in these tumor-bearing rats but our data excluded a primary clearance defect because the rate of triacylglycerol accumulation (mg/min) after Triton injection was equal to or greater than in normal control rats, except in cachectic rats with very large tumors. The fractional clearance of injected radioactive triacylglycerols was less in tumor-bearing rats than in controls, but the turnover (mg/min) was probably not decreased in the tumor-bearing rats because of their expanded plasma pool. Also inconsistent with a decreased turnover was our finding of a greater production of radioactive plasma triacylglycerols after injection of a tracer dose of radioactive free fatty acid, and unchanged production in Triton-treated rats. Therefore, in the fasted state, the hyperlipidemia of the tumor-bearing rats was associated with an unchanged or possibly an increased flux of hepatic triacylglycerols and a primary clearance defect was excluded. After fat-feeding, rats with tumors developed a higher post-prandial hyperlipidemia than control rats. Therefore, the clearance mechanism for the plasma triacylglycerols was close to saturation in the fasted state, and the added influx of exogenous triacylglycerols was removed less efficiently in the tumor-bearing rats.

Effects of sphingomyelin and phosphatidylcholine acyl chains on the clearance of triacylglycerol-rich lipoproteins from plasma. Studies with lipid emulsions in rats.

Series of lipid emulsions were prepared as physical models of lymph chylomicrons. The emulsion phospholipid was systematically varied with respect to sphingomyelin, in 0-100% mixtures with egg yolk phosphatidylcholine (EYPC). In other emulsions, the phospholipid was systematically varied with respect to dipalmitoylphosphatidylcholine (DPPC) in 0-100% mixtures with 1-palmitoyl-2-oleoylphosphatidylcholine (POPC). All emulsions contained unlabeled free cholesterol, radiolabeled triolein (TO) and radiolabeled cholesteryl oleate (CO). The emulsions were injected into conscious rats to measure the clearances of emulsion TO and CO and the capture of lipid radioactivity by selected organs. The emulsions containing EYPC or POPC were metabolized similarly to lymph chylomicrons, consistent with rapid lipoprotein lipase-mediated hydrolysis of emulsion TO followed by hepatic uptake of the CO in the triglyceride-depleted emulsion remnants. Emulsions stabilized with either 1-oleoyl-2-stearoyl- or 1-stearoyl-2-oleoylphosphatidylcholine (OSPC or SOPC) were metabolized similarly. Increasing amounts of sphingomyelin in EYPC emulsions progressively slowed the removal of TO and CO labels from plasma. With 50% sphingomyelin clearance was very slow, while emulsion clearance was negligible with 100% sphingomyelin. Emulsions containing 20% of DPPC in POPC were metabolized similarly to 100% POPC, but 40% or more of DPPC progressively slowed the removal from plasma of both TO and CO. With 100% DPPC clearance was characterized by a rapid initial removal of about 30% of the injected material, followed by a second phase when removal was negligible, suggesting lack of hydrolysis of triacylglycerols by lipoprotein lipase. Changes in the apolipoproteins associated with the emulsions probably mediated the observed changes in clearance.

Effects of cholesterol content on the metabolism of protein-free emulsion models of lipoproteins.

After intravenous injection, emulsions with compositions similar to chylomicrons behaved metabolically as described for chylomicrons, with faster removals of triacylglycerols than cholesteryl esters from the blood after injection into rats, and with greater uptakes of cholesteryl esters than triacylglycerols by the liver. In contrast, emulsions with a high content of free cholesterol showed equal removal rates from the blood of triacylglycerols and cholesteryl esters; and similar uptakes by the liver. This pattern of metabolism was that expected for a chylomicron core remnant particle. Emulsions poor in cholesteryl ester but rich in free cholesterol showed remnant-like behavior, whereas emulsions rich in cholesteryl ester but poor in free cholesterol were metabolized like nascent chylomicron particles. The amount of free cholesterol appeared to regulate metabolism by affecting the binding of apolipoproteins to the particle surface. Emulsions with a high content of free cholesterol bound less A-I, A-IV and C apolipoproteins, and the relative amount of apolipoprotein E was increased. All of these effects are consistent with the metabolic differences between chylomicrons and remnant particles, suggesting that the amount of free cholesterol plays a regulatory role in chylomicron metabolism.

Defective plasma clearance of chylomicron-like lipid emulsions in Watanabe heritable hyperlipidemic rabbits.

Human patients with familial hypercholesterolemia (FH) and Watanabe heritable hyperlipidemic rabbits (WHHL), while lacking normal receptors recognizing low-density lipoproteins (LDL), are said to have normal clearance of chylomicrons. In the present study, emulsions with a similar lipid composition to chylomicrons were injected intravenously in homozygous WHHL rabbits and normal control rabbits fed diet with low or high cholesterol. Radioactive labels tracing emulsion triolein and cholesteryl oleate were both removed rapidly from the bloodstream, with the removal rate of triolein always faster than that of cholesteryl oleate. This pattern was similar to the clearance of normal chylomicrons in rabbits or rats, and was consistent with the formation of remnant lipoproteins after hydrolysis of emulsion triolein by lipoprotein lipase, followed by hepatic uptake of the remnants. The removal of cholesteryl oleate was significantly slower in WHHL rabbits than in normal controls, suggesting that the absence of LDL receptor function led to impaired remnant clearance. Measured in post-heparin plasma, the activity of lipoprotein lipase was decreased in WHHL rabbits, but this was not associated with clear evidence of defective lipolysis of emulsion triolein. Apolipoprotein E did not appear to be deficient in WHHL rabbits. Plasma devoid of lipoproteins less than 1.006 g/ml from WHHL and normal control rabbits transferred similar amounts of apolipoprotein E to chylomicron-like emulsions after incubation. Impaired clearance of chylomicron remnants possibly contributes to the hypertriglyceridemia of WHHL rabbits and to accelerated atherogenesis when the function of LDL receptors is defective.

Interactions between model triacylglycerol-rich lipoproteins and high-density lipoproteins in rat, rabbit and man.

There are inverse relationships between HDL cholesterol and plasma triacylglycerol concentrations in normal and in hypertriglyceridemic individuals. To investigate the interactions between triacylglycerol-rich lipid particles and HDL, a lipid emulsion model of the triacylglycerol-rich lipoproteins was prepared. When emulsion particles were incubated with rat high-density lipoproteins (HDL) in the presence of lipid transfer activity (d greater than 1.21 g/ml fractions) from rabbit or human plasma there was a rapid bi-directional exchange of cholesteryl oleate (CO) and phospholipid (PL) labels between lighter and heavier fractions of HDL and emulsion particles. The transfers of CO and PL labels between both light and heavy fractions of HDL and the emulsion particles were increased with increasing amounts of emulsion added to the incubations. Incubation with the d greater than 1.21 g/ml fraction from rat plasma resulted in only a small exchange of CO whereas PL exchange was similar to rabbit and human plasma. Retinyl palmitate label was not transferred from emulsion particles to the HDL fractions even in the presence of lipid transfer activity from rabbit or human plasma. The present study shows that the transfer protein-mediated exchanges of surface and core lipids between HDL and the triacylglycerol-rich lipoproteins are affected by the quantity of triacylglycerol-rich particles in the system. This mechanism may contribute to the inverse relationships between plasma triacylglycerol concentrations and HDL concentrations in normal and hypertriglyceridemic individuals.

Cholesterol content of red blood cells and low-density lipoproteins in hypertriglyceridemia.

The red blood cells and the low-density lipoproteins in hypertriglyceridemia have a lower ratio of unesterified cholesterol to phospholipid than normal. The low-density lipoproteins are also smaller and more dense in hypertriglyceridemia, and contain only 45% of the normal unesterified cholesterol mass. The phase behavior of the lipids shows that normal red cells and low-density lipoproteins are close to saturation with cholesterol, whereas in hypertriglyceridemia less cholesterol is present. Because newly secreted triacylglycerol-rich lipoproteins are poor in cholesterol, their excess production and transport in hypertriglyceridemia may prevent maintenance of the normal cholesterol content of blood cells and low-density lipoproteins. Partitioning of cholesterol into triacylglycerol-rich lipoproteins is able to account for significant fluxes of unesterified cholesterol in the plasma compartment.

Lymphatic absorption of n - 3 polyunsaturated fatty acids from marine oils with different intramolecular fatty acid distributions.

Male Wistar rats were given 0.5 ml of either fish oil or seal oil intragastrically. The intramolecular fatty acid distributions of the triacylglycerols administered were determined by non-specific Grignard degradation followed by isolation and analysis of the 2-monoacylglycerols. The n - 3 polyunsaturated fatty acids (PUFAs), especially eicosapentaenoic acid (20:5(n - 3)) and docosahexaenoic acid (22:6(n - 3)), were located in outer positions (sn-1/3) in the seal oil triacylglycerols whereas the sn-2 position of fish oil triacylglycerols was enriched in 20:5(n - 3) and 22:6(n - 3). The mesenteric lymph was collected over the following 24 h and the absorption patterns of n-3 PUFAs were determined. In the lymph, the n - 3 fatty acids characteristic of the marine oils rapidly increased both with regard to mole percentage and transport (micrograms/min). There were, however, no overall significant differences in the absorption patterns over a 24 h period. The ratio between mole percentage in the oil and mole percentage in the lymph calculated at the steady-state period was significantly greater for both 20:5(n - 3) and 22:6(n - 3) following fish oil administration compared with seal oil. Initially, the recovery of n - 3 PUFAs as a percentage of the total amount transported over the experimental period was higher following injection of fish oil than seal oil but seal oil resulted in greater recovery in the last two fractions at 8 and 24 h post injection, respectively. This indicated that n - 3 PUFAs from fish oil may have been better absorbed in the initial period of digestion but overall the structure of dietary triacylglycerols had negligible effects on the assimilation of n - 3 PUFAs when these were administered as native marine oils.

Phosphatidylcholine metabolism after transfer from lipid emulsions injected intravenously in rats. Implications for high-density lipoprotein metabolism.

When injected intravenously in rats, emulsion models of triacylglycerol-rich lipoproteins were metabolized like natural lipoproteins and during the hydrolysis of emulsion triacylglycerols, a large fraction of the emulsion phosphatidylcholine was transferred to the plasma high-density lipoproteins. The removal from plasma of emulsion phosphatidylcholine was followed for 2 h in unanaesthetized rats. The half-lives for removal of phospholipid after injection of emulsions stabilized with dioleoylphosphatidylcholine or 1-palmitoyl-2-oleolyphosphatidylcholine were 58-63 min when traced with isologous label. In comparison, the published half-lives of HDL mixed phospholipids in rats are approx. 40 min, indicating that much of the clearance of the emulsion phospholipid could be accounted for by HDL catabolism. Measured LCAT activity was sufficient to account for not more than 2% of the catabolism of the HDL phospholipids labelled by this physiological procedure. Removal from plasma of label was more rapid when the same emulsions were labelled with tracer amounts of the heterologous dipalmitoylphosphatidylcholine, showing that individual phosphatidylcholine species were handled distinctly even when present only in tracer amounts in a bulk of another phosphatidylcholine differing in acyl chains.

Plasma clearance of model lipoproteins containing saturated and polyunsaturated monoacylglycerols injected intravenously in the rat.

Triacylglycerols, with a saturated long-chain fatty acid at the glycerol-2-position, slow the clearance from plasma of remnants derived from injected chylomicrons and chylomicron-like emulsions. Slowing of remnant clearance also occurs when about 1% of monostearoylglycerol is added to a triolein chylomicron-like emulsion. We have now found that addition of monoacylglycerols, containing a saturated acyl chain from 12 to 20 carbons, slowed the plasma clearance and decreased the liver uptake of the remnants. In contrast, monoacylglycerols with unsaturated acyl chains were inconsistent in their effects on the remnant clearance. Monoarachidonin (M20:4) slowed remnant clearance comparable to that of saturated monoacylglycerols, monolinolenin (M18:3) and monolinolein (M18:2) were less effective, while monoolein had the least effect on remnant clearance. We have confirmed the defective remnant clearance in rats of injected emulsions containing saturated acyl chain by the using the diester-2-ether analogues of triolein and 1,3-dioleoyl-2-stearoylglycerol (OSO). Chylomicron-like lipid emulsions made with the ether analogues had clearance rates similar to their triester counterparts. Preformed remnants derived from emulsions of OSO, its ether analogue, and triolein emulsions or emulsions of triolein with approximately 1% saturated monoacylglycerols were prepared in hepatectomized rats. After intravenous injection into conscious recipient rats, these remnants were cleared from plasma similar to remnants traced in situ by lipolysis of injected chylomicron-like emulsions.

The effects of Triton WR-1339, protamine sulfate and heparin on the plasma removal of emulsion models of chylomicrons and remnants in rats.

Protein-free lipid emulsions with compositions modelling chylomicrons (chylomicron-like emulsion) or chylomicron remnants (remnant-like emulsion) were injected intra-arterially into nonanesthetized rats. Compared with control untreated rats, treatment with Triton WR-1339, protamine sulfate or heparin strongly modified the plasma removal of triacylglycerols and cholesteryl ester moieties of chylomicron-like emulsions, but had little effect on removal rates of triacylglycerols or cholesteryl esters of remnant-like emulsions. The effects on chylomicron-like removal were similar to those on natural lymph chylomicrons. The relative lack of effects on remnant-like emulsion removal provides additional evidence that remnant-like emulsions are a metabolic model for natural chylomicron remnants.

Hypertriglyceridemia is exacerbated by slow lipolysis of triacylglycerol-rich lipoproteins in fed but not fasted streptozotocin diabetic rats.

Hydrolysis by endothelial lipases of triacylglycerol-rich lipoproteins of diabetic origin were compared to lipoproteins of non-diabetic origin. The plasma lipoprotein fraction of density < 1.006 g/ml, including chylomicrons and VLDL, were incubated in vitro with post-heparin plasma (PHP) lipases. The lipoproteins of diabetic origin were hydrolysed at a significantly slower rate than lipoproteins from normal rats by the lipoprotein lipase component of PHP. However, if rats were fasted for 16 h prior to lipoprotein recovery, no differences in rates of VLDL hydrolysis were observed. Slower hydrolysis of lipoproteins of diabetic origin reflected a decrease in the apolipoprotein CII/CIII ratio and other changes in the apolipoprotein profile. To assess whether diabetic rats were less able to clear triacylglycerol independent of changes in the nature of the lipoproteins, we monitored the clearance of chylomicron-like lipid emulsions in hepatectomized rats. In vivo, emulsion triacylglycerol hydrolysis was not slowed due to diabetes. However, control and diabetic rats, which had been fasted for 16 h, cleared triacylglycerol at about twice the rate of fed rats. Triacylglycerol secretion rates in diabetic and control rats were similar, whether fed or fasted. We conclude that in streptozocin diabetic rats, hypertriglyceridemia was not due to overproduction of chylomicron- or VLDL-triacylglycerol, nor to decreased endothelial lipase activities. Rather, in fed diabetic rats, the triacylglycerol-rich lipoproteins are poorer substrates for lipoprotein lipase. This may lead to slower formation of remnants which would exacerbate slow remnant removal. VLDL of diabetic origin were hydrolysed as efficiently as VLDL from control donors, suggesting that in the fed state the lipolytic defect may be specific for chylomicrons.