A methodological study of an intravenous fat tolerance test with Intralipid R emulsion

State of the art review: Intravenous fat emulsions: Current applications, safety profile, and clinical implications

OBJECTIVE

To review the current state of the science regarding intravenous fat emulsions (IVFEs), with an emphasis on their safety profile.

DATA SOURCES

Articles were identified via a search of the MEDLINE database, including publications from 1979 to December 2009, using a search string that included the terms parenteral nutrition, lipid emulsion, fat emulsion, IVFE, safety, adverse effect, neonate intralipid, and terms describing a range of specific adverse events (AEs) such as pancreatitis.

STUDY SELECTION AND DATA EXTRACTION

We selected articles that allowed us to compare the results of clinical trials involving delivery of medications via IVFEs with the historical use and effects of IVFEs in parenteral nutrition, with an emphasis on AEs. We focused on 2 drugs in current use that are administered intravenously in lipid emulsions: propofol and clevidipine.

DATA SYNTHESIS

Clearance of the fat particles in IVFEs is mediated by the enzyme lipoprotein lipase. AEs are more likely if the rate or duration of IVFE administration exceeds the enzyme's clearance capacity. AEs are also more likely after administration of a 10% IVFE formulation than a 20% formulation, because the higher concentration of free phospholipid in the 10% formulation interferes with lipoprotein lipase activity. AEs can be reduced by administering IVFEs at a dosage < or = 2.5 g/kg/day and at a rate < or = 0.11 g/kg/h. The anesthetic agent propofol, which is formulated in a 10% IVFE, has been used clinically for 25 years. Typical AEs associated with propofol use include infection, high plasma triglyceride concentrations, and pancreatitis. Recent clinical trials involving clevidipine, which is formulated in a 20% IVFE, have demonstrated a low rate of lipid-related AEs.

CONCLUSIONS

The results of this review demonstrate that IVFEs are well tolerated when administered in accordance with guideline recommendations.

Removal of exogenous triglycerides in subjects with massive obesity before and after jejunoileal shunt operation

The fractional removal rate of exogenous triglycerides (TG) in 12 obese subjects with a mean body weight of 132+/-5 kg (S.E.M.) has been studied before a jejunoileal shunt operation. The study was repeated postoperatively at stable body weights 30-54 months later. The weight loss ranged from 13 to 65 kg (mean weight reduction 27%). In 8 patients, 0.3-6.0 kg of skin and subcutaneous tissue were removed surgically for cosmetic reasons during the weight reduction period. Serum cholesterol fell from 220+/-8 to 141+/-11 mg/100 ml (p less than 0.001). The serum TG reduction was not significant, 1.80+/-0.25 before and 1.50+/-0.19 mmol/l after operation. The fractional removal rate of exogenous TG, determined by the intravenous fat tolerance test (IVFTT) with the Intralipid fat emulsion, was not significantly changed. The IVFFT k2 value was 3.5+/-0.5%/min before and 4.3+/-0.6 after surgery (p greater than 0.05). Previous studies have shown that a major part of Intralipid is removed initially in skeletal muscle. The unchanged fractional removal rate of Intralipid, despite the marked reduction in the amount of adipose tissue, supports the concept that the adipose tissue may play a minor role in the initial elimination of exogenous TG from the bloodstream.

Effects of long-term therapy with labetalol on lipoprotein metabolism in patients with mild hypertension

The effects of labetalol on serum lipoproteins, the intravenous fat tolerance test (IVFTT) and lipoprotein lipase (LPL) and hepatic lipase (HL) activities were studied in 16 patients with mild hypertension before and after 6 months of therapy. Most patients were found to be normotensive on 200 mg labetalol/day. Before therapy the mean concentration of serum TG was 0.75 +/- 0.21 (SD) mmol/l, of total cholesterol 5.41 +/- 1.25 mmol/l and of HDL cholesterol 1.67 +/- 0.61 mmol/l. After labetalol no significant changes were found in the concentrations of TG and cholesterol in the VLDL, LDL and HDL fractions. The mean values for the IVFTT and for LPL and HL activities were in the normal range and remained unchanged during therapy.

The effect of D,L-carnitine supplementation on muscle metabolism, neuropathy, cardiac and hepatic function in hemodialysis patients. A pilot study

Three patients on regular hemodialysis treatment were perorally supplemented with D.L-carnitine, 900 mg/d, during four weeks. Before treatment, muscle carnitine concentrations were low and the patients showed signs of hypometabolism in skeletal muscle, cardiomyopathy, impaired hepatic function and a moderate hyperlipoproteinemia. After treatment, the skeletal muscle metabolism normalized. The results further indicate improvement of the cardiac and hepatic function and of the neuropathy on the sensory side. No improvement was found, however, in plasma lipoprotein pattern or in variables reflecting plasma lipid transport (lipoprotein lipase activity and intravenous fat tolerance).

Lack of effect of exogenous or endogenous gastric inhibitory polypeptide on the elimination rate of Intralipid in man

Six healthy subjects were studied by means of an intravenous fat tolerance test on three occasions, in the fasting state, in the postprandial state, and during an intravenous infusion of gastric inhibitory polypeptide (GIP). No effects on the elimination rate of Intralipid were seen either by endogenous or exogenous GIP.

Intravenous fat tolerance test with Intralipid in chronic renal failure

The intravenous fat tolerance test with Intralipid has been used to evaluate the triglyceride (TG) removal capacity in 34 male patients with varying degree of chronic renal failure (CRF). The investigations were repeated in 10 patients after 2-3 years when the renal function had deteriorated further. The fractional removal rate of Intralipid (k2) was low compared to controls, regardless of degree of renal function, etiology of renal disease or treatment with antihypertensive drugs. The k2 value did not change in patients investigated twice, although there was a considerable decrease in renal function. Both in patients and controls there was a strong negative correlation between k2 and the very low density lipoprotein (VLDL) TG concentration. However, at the same k2 value, the VLDL TG concentration was higher in patients than in controls. These data suggest that the hypertriglyceridemia commonly seen in patients with CRF is caused both by an increased VLDL production and a decreased VLDL removal from the circulation. Furthermore, these changes in TG metabolism are induced early in the course of renal insufficiency.

Serum lipids in alcoholics

Fasting serum lipid values (cholesterol, triglycerides and phospholipids) have been analysed in a group of 85 male and 10 female alcoholics of various ages in connection with an acute drinking bout and compared to the values of twice as many control subjects. The most prominent finding was an increase in the mean concentration of triglycerides and phospholipids, most marked in the younger age groups. The elevations, however, were moderate and most alcoholics had the same serum lipid values as the controls. Serum triglyceride values above 2 and 3 mmol/1, respectively, occurred in 23% and 4% in controls and in 28% and 13% in alcoholics. It is suggested that excessive intake of alcohol induces hypertriglyceridaemia only when other factors are present. One such factor may be a reduced i.v. fat tolerance.

Oxidation of intravenous lipid in infants and children with systemic inflammatory response syndrome and sepsis

During sepsis in adults, fat becomes a preferred fuel; however, oxidation may be impaired relative to the circulating fatty acid levels. Little is known about the ability of infants and children to oxidize lipids during systemic inflammation (SIRS) and sepsis. The aim of this study was to examine the oxidation of exogenous lipid in these patients. Sixteen patients with SIRS/sepsis and eight controls with no evidence of sepsis were studied by indirect calorimetry during an i.v. lipid utilization test (1 h of 0.3 g/kg/h glucose followed by 3 h of 0.1 g/kg/h glucose plus 0.15 g/kg/h lipid). The respiratory quotient (RQ) (1.0 for carbohydrate utilization and 0.7 for fat utilization) was measured. Results were compared by repeated-measures analysis of variance (ANOVA), paired or unpaired t tests. There was no difference in baseline RQ between controls and patients with SIRS/sepsis (mean +/- SD; 0.82 +/- 0.08 versus 0.82 +/- 0.04). The RQ of controls dropped significantly to 0.78 +/- 0.08 at 240 min (p < 0.001). The RQ of patients with SIRS/sepsis also fell to 0.78 +/- 0.06 (p < 0.01). Infants and children with SIRS/sepsis are able to oxidize i.v. lipid.

The hypertriglyceridemic clamp technique. Studies using long-chain and structured triglyceride emulsions in healthy subjects

In a randomized crossover study, plasma kinetics of 2 different types of fat emulsions were studied in 8 healthy volunteers by using a hypertriglyceridemic clamp technique. The method involves the stabilization of serum triglyceride (TG) concentration during 180 minutes at a predetermined level (4 mmol/L) by adjustment of TG infusion rate by repeated online measurements of serum TG concentration. The fat emulsions under study were a long-chain fatty acid triglyceride (LCT) emulsion (Intralipid 20%, Fresenius Kabi, Sweden) and a structured triglyceride (STG) emulsion (Structolipid 20%, Fresenius Kabi) where medium- and long-chain fatty acids have been interesterified within a TG molecule. The hypertriglyceridemic clamp was found to have acceptable reproducibility when tested in 3 healthy individuals on 2 different occasions, as similar steady-state TG levels were obtained by infusing similar amounts of fat. The average (+/-SEM) TG concentration during the 180-minute clamp was similar for STGs and LCTs (4.0 +/- 0.1 vs 3.9 +/- 0.1 mmol/L; not significant), but the amount of fat that had to be infused was significantly higher during STG than during LCT clamping (0.31 +/- 0.04 vs 0.21 +/- 0.02 g TG per minute; P < .05). Higher serum levels of free fatty acids (1.80 +/- 0.13 vs 0.96 +/- 0.09 mmol/L; P < .05), free glycerol (1.30 +/- 0.07 vs 0.76 +/- 0.08 mmol/L; P < .001), and beta-OH butyrate (1.61 +/- 0.44 vs 1.17 +/- 0.23 mmol/L; not significant) were obtained at the end of the clamp during infusion of STGs compared with LCTs. During infusion of STGs the medium-chain fatty acids octanoic (C:8) and decanoic acid (C:10) constituted approximately half of circulating fatty acids that correspond to the compositional ratio of the emulsion. Plasma lipoprotein lipase (LPL) concentration was higher during STG than during LCT clamping (6.06 +/- 0.62 vs 3.15 +/- 0.40 mU/mL; P < .05), and there was a positive correlation between the mean LPL concentration and the amount of infused TG during the steady-state period (r = 0.58; P < .05). In conclusion, the hypertriglyceridemic clamp method was found to give reproducible results and could be considered for comparison of metabolic clearance properties of different types of fat emulsions. The capacity of healthy subjects to eliminate STGs from blood was greater than for LCTs. An increased LPL activity induced by the higher TG infusion rate may have contributed to the increased metabolic clearance of STGs.

Removal of intravenous Intralipid in patients with familial hypercholesterolemia during inhibition of cholesterol absorption and synthesis

BACKGROUND

While plant stanols are known to upregulate low density lipoprotein (LDL) receptors, we studied the effects of plant stanol (STA) and sterol (STE) ester spreads on triglyceride-rich lipoprotein (TRL) removal in statin-treated patients with familial hypercholesterolemia (FH) using intravenous Intralipid-squalene fat tolerance test.

METHODS

Five patients consumed STA and STE in a randomized, crossover study for 4 weeks. TRL removal was studied at baseline and at the end of both periods. Serum, chylomicron (CM), and very low density lipoprotein lipids, squalene, and plant sterols were measured.

RESULTS

LDL cholesterol was decreased by both spreads (15-16%, p<0.05). Plant sterol concentrations were doubled in serum and CM by STE vs. STA. After the injection of Intralipid, CM squalene and sitosterol, but not triglycerides (TG), reached higher peak levels (and area under the incremental curve (AUIC) of squalene) by both spreads than at baseline. Despite different plant sterol concentrations by STE vs. STA, the incremental curves for plant sterols were similar by the spreads.

CONCLUSIONS

Despite the retarded removal of TRL lipids by STA and STE in the statin-treated subjects with FH, improvement of the fasting lipid profile was suggested important in consideration of combination of cholesterol absorption inhibitor with statins even in FH.

Obesity and post-prandial lipid metabolism. Feast or famine?

Both in Western countries and in third world countries there is an increasing incidence of obesity. Obesity per se or insulin resistance associated with obesity may increase cardiovascular risk factors including dyslipidemia, hypertension and Type 2 diabetes. Over the past decade the understanding has increased of specific mediators in the hypothalamus that are involved in regulating food intake and body weight. In obese humans fasting plasma lipids can be normal but postprandial lipid metabolism is abnormal with an accumulation of triglyceride-rich remnant lipoproteins. In viscerally obese men chylomicron remnant catabolism was markedly decreased when compared with lean individuals. The decreased clearance of chylomicron remnants in viscerally obese subjects may be explained by competition between chylomicron remnants and the increased hepatic production of VLDL for clearance by low density lipoprotein receptors. Increased food intake in rodent models of obesity was shown to be associated with a delay in the catabolism of remnant lipoprotein particles. Prevention of hyperphagia was found to correct the impairment in the metabolism of remnant lipoproteins. Under fasting and food restricted conditions the improvement of remnant metabolism was associated with an increased oxidation of remnant lipids as determined by a novel stable isotope breath test. Anti-obesity and lipid lowering drugs have been used for the treatment of obesity. Inhibitors of cholesterol synthesis inhibitors (statins) have been shown to be effective in treating dyslipidemia. Inhibition of cholesterol synthesis with Atorvastatin was shown to improve chylomicron metabolism by increasing chylomicron remnant catabolism in obese subjects as assessed by the newly developed stable isotope breath test.

Metabolic effects of indinavir in healthy HIV-seronegative men

BACKGROUND

Therapy with HIV protease inhibitors (PI) has been associated with hyperglycemia, hyperlipidemia and changes in body composition. It is unclear whether these adverse effects are drug related, involve an interaction with the host response to HIV or reflect changes in body composition.

METHODS

Indinavir 800 mg twice daily was given to 10 HIV-seronegative healthy men to distinguish direct metabolic effects of a PI from those related to HIV infection. Fasting glucose and insulin, lipid and lipoprotein profiles, oral glucose tolerance (OGTT), insulin sensitivity by hyperinsulinemic euglycemic clamp, and body composition were measured prior to and after 4 weeks of indinavir therapy.

RESULTS

Fasting glucose (4.9 +/- 0.1 versus 5.2 +/- 0.2 mmol/l; P = 0.05) insulin concentrations (61.7 +/- 12.2 versus 83.9 +/- 12.2 pmol/l; P < 0.05), insulin : glucose ratio (12.6 +/- 1.7 versus 15.9 +/- 1.9 pmol/mmol; P < 0.05) and insulin resistance index by homeostasis model assessment (1.9 +/- 0.3 versus 2.8 +/- 0.5;P < 0.05) all increased significantly. During OGTT, 2 h glucose (5.1 +/- 0.4 versus 6.5 +/- 0.6 mmol/l; P < 0.05) and insulin levels (223.1 +/- 48.8 versus 390.3 +/- 108.8 pmol/l;P =0.05) also increased significantly. Insulin-mediated glucose disposal decreased significantly (10.4 +/- 1.4 versus 8.6 +/- 1.2 mg/kg x min per microU/ml insulin; 95% confidence interval 0.6--.0;P < 0.01). There was no significant change in lipoprotein, triglycerides or free fatty acid levels. There was a small loss of total body fat (15.8 +/- 1.4 versus 15.2 +/- 1.4 kg;P = 0.01) by X-ray absorptiometry without significant changes in weight, waist : hip ratio, and visceral or subcutaneous adipose tissue by computed tomography.

CONCLUSIONS

In the absence of HIV infection, treatment with indinavir for 4 weeks causes insulin resistance independent of increases in visceral adipose tissue or lipid and lipoprotein levels.

The effects of tolbutamide on lipoproteins, lipoprotein lipase and hormone-sensitive lipase

Type 2 diabetic patients are at increased risk to develop atherosclerotic vascular disease. These patients are often treated with sulphonylurea derivatives, and it has been suggested that this treatment might contribute to the increased atherosclerotic process. The aim of the present study was therefore to investigate whether tolbutamide influences lipid metabolism in such a way that the atherosclerotic process may be promoted. Addition of tolbutamide (5-500 mg/l) to isolated rat fat adipocytes inhibited the lipoprotein lipase (LPL) activity in a dose-dependent manner to levels about 50% of those registered in the absence of tolbutamide. This effect was due to inhibition of the activation of the enzyme in the tissue and not to interference with the interaction of enzyme with its substrate. Addition of tolbutamide (500 mg/l) also inhibited noradrenaline (100 nM) and isoprenaline (40 nM)-induced lipolysis by 48.1 +/- 7.4% (mean +/- S.E.M.) and 47.3 +/- 5.5%, respectively. The decreased lipolysis in tolbutamide preincubated adipocytes was shown to be the result of an inhibition of the phosphorylation of hormone sensitive lipase (HSL). Three months of tolbutamide treatment (0.5 g t.i.d.) in diet treated type 2 diabetic patients did not influence the plasma concentrations of cholesterol, triglycerides, LDL cholesterol, HDL cholesterol as well as HDL triglycerides and HDL phospholipids, and there were no differences compared to placebo treated patients. There was a tendency towards a decrement in the elimination rate of exogenous triglycerides in the tolbutamide group (P = 0.0801). No differences between the groups and no treatment effects were seen on LPL and hepatic lipase activities. In conclusion, our in vitro data show that tolbutamide has dual effects on lipid transport, with impairment of the LPL system, which would tend to decrease plasma lipoproteins by reducing hepatic production of lipoproteins. In vivo, these two effects seem to balance each other and plasma lipoprotein levels remain unaffected.

Fractional removal rate of fat emulsion (K2) remains to be low in APOE3/3 phenotype subjects with serum triglyceride level above 180mg/dl

The relation between fractional catabolic rate (K2) of an intravenously injected fat emulsion, Intralipid, and the level of serum triglyceride (TG) was evaluated to cast light on TG-rich lipoprotein metabolism in 182 subjects who were homozygotes for the most common form of apoE3/3. Both normolipidemic individuals and primary hyperlipidemic patients were included. To assess the influence of variation in the apoE phenotype on fat emulsion metabolism, 25 subjects with the apoE 4/3 phenotype and 21 with apoE 3/2 phenotype were also evaluated. In the apoE 3/3 subjects, K2 decreased with increasing TG level up to 180mg/dl, but above 180mg/dl, K2 remained at a constant level. This TG value was therefore concluded to be a cut off beyond which the TG-dependent decrease in K2 disappeared. No apparent correlation between K2 and serum TG was observed in subjects with apoE 4/3 or 3/2 phenotypes. In subjects with TG above 180mg/dl, presumably both accelerated synthesis and limited removal are involved in the development of their hypertriglyceridemia. Since K2 demonstrated little change with increase of TG over 180mg/dl in apoE 3/3 subjects, we concluded that the capacity to catabolize fat emulsion reaches a kinetic saturation. The TG value of 180mg/dl may be a physiological significance. The relation between K2 and TG was specific in apoE 3/3, as it was not observed in subjects having apoE 4/3 or 3/2 phenotypes.

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.

Phospholipid-rich particles in commercial parenteral fat emulsions. An overview

In parenteral nutrition, the infusion of a fat EMU supplies both concentrated energy and covers the essential fatty acid requirements, the basic objective being to mimic as well as possible the input of chylomicrons into the blood. This objective is well met by the TAGRP of the EMU, which behave as true chylomicrons. However, commercial EMU also contain an excess of emulsifier in the form of PLRP. The number of these PLRP depends directly on the PL/TAG ratio of the EMU. They differ from the TAGRP by their composition (PL vs TAG and PL), their structure (PL in bilayer versus monolayer), and their granulometry (mean diameter 70-100 nm for PL vs 200-500 nm). The metabolic fate of the PLRP is similar in several ways to that of the TAGRP: exchanges of PL with the PL of the different cellular membranes and of the lipoproteins; captation of free CH from these same structures; and enrichment in apolipoproteins. However, because the TAGRP are the preferred substrates of the lipolytic enzymes, their clearance is much more rapid (half-life < 1 h) than that of the PLRP. As the infusion is continued, the PLRP end up accumulating and being transformed into LP-X (free CH/PL = 1; half-life of several days). As soon as the EMU is infused, the PLRP enter into competition with the TAGRP, in the lipolysis process as well as for sites of binding and for catabolism. The sites for catabolism of the two types of PAR are not the same: adipose tissues and muscles utilize the fatty acids and monoacylglycerols released by the lipolysis of the TAGRP; hepatocytes take up their remnants; the RES and the hepatocytes participate in the catabolism of the PLRP and the LP-X. Thus, prolonged infusion of EMU rich in PLRP leads to a hypercholesterolemia, or at least a dyslipoproteinemia, due to elevated LP-X, associated with a depletion of cells in CH, stimulating thus tissue cholesterogenesis. However, parenteral nutrition has evolved towards the utilization of EMU with a low PL/TAG ratio (availability of 30% formula) and less rapid delivery. For these reasons, the hypercholesterolemias that used to be observed with the 10% EMU have become much less spectacular or have even disappeared. It is interesting to note that patients on prolonged TPN, in particular those with a short small intestine, have weak cholesterolemia, reflecting a lowering of HDL and LDL not masked by elevated LP-X. At present, it seems difficult to produce sufficiently stable parenteral EMU devoid of PLRP. Notwithstanding, all the observations made since the introduction of the EMU in TPN are in favour of the use of PLRP-poor EMU. It is clear that the 10% formulas, and generally those with a PL/TAG ratio of 12/100, are ill-advised, especially in patients with a retarded clearance of circulating lipids.

Post-prandial lipaemia

Post-prandial lipaemia represents the state of absorption during which TG metabolic capacity is under challenge. Low TG metabolic capacity imparts the risk of development of atherosclerosis. TG-intolerance has been shown to be an independent risk factor for CAD and impaired TG metabolic capacity could underlie a common high risk lipoprotein constellation of low HDL cholesterol and small sized HDL and LDL. Magnitude and duration of post-prandial lipaemia determine how much cholesterol is diverted from LDL and HDL into TG-rich lipoproteins through which it causes atherosclerosis. Potential means of intervention are improvement of TG metabolic capacity by reducing obesity, prescription of aerobic exercise, reduction of oxidizability of post-prandial lipoproteins by antioxidants and TG-lowering drugs.

Effects of an acute increase in plasma triglyceride levels on glucose metabolism in man

The aim of the study was to evaluate the effects of an acute increase in triglyceride levels induced by Intralipid (Kabivitrum, Stockholm, Sweden) infusion on forearm glucose uptake, glucose oxidative metabolism, and hepatic glucose production independent of circulating free fatty acid (FFA) levels in man. Six normal subjects underwent three different tests in random order. Each test consisted of a control period of 120 minutes followed by a euglycemic, hyperinsulinemic clamp lasting 120 minutes. In test 1, a high-dose intravenous Intralipid infusion was performed to increase triglyceride and FFA levels. In test 2, heparin (30 U/min) plus low-dose Intralipid infusions were performed to maintain triglyceride at normal levels and increase only FFA levels. Test 3 was performed as a control study. During the 120-minute control period, forearm glucose uptake and hepatic glucose production were not affected by increasing only FFA levels (test 2) or FFA and triglyceride levels (test 1) as compared with the control study. On the contrary, glucose oxidation was significantly decreased as compared with the control study during tests 1 and 2, without a further significant decrease during simultaneously increased FFA and triglyceride levels. Concomitantly, lipid oxidation was similar in tests 1 and 2, at values significantly greater than in test 3. During the euglycemic clamp, forearm glucose uptake and glucose oxidation were significantly lower during tests 1 and 2 than test 3. At variance with the control period, the increase of triglyceride levels during test 1 caused a significant 30% to 40% decrease of both parameters as compared with test 2.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of intravenously infused egg phospholipids on lipid and lipoprotein metabolism in postoperative trauma

Lipid emulsions contain not only triglyceride (TG)-rich particles but also phospholipid (PL)-rich particles that are believed to trap free cholesterol and apoprotein E, when they are infused in excess. The present study was designed to evaluate the effects of such abnormal PL-rich particles on lipid metabolism during a 5-day infusion in man. Eighteen patients undergoing esophagectomy were evenly randomized to receive intravenously during 5 days 1.75 g.kg-1.d-1 long-chain TG from either a 10% lipid emulsion with a PL/TG weight ratio of 0.12 (group A), a 10% emulsion with a PL/TG weight ratio of 0.06 (group B), or a 20% emulsion with a PL/TG weight ratio of 0.06 (group C). Plasma PL, free cholesterol, and apoprotein E increased progressively in group A (4.1 +/- 0.3 mmol/L, 2.4 +/- 0.3 mmol/L, and 0.089 +/- 0.012 g/L on day 5, respectively) but not in groups B (2.7 +/- 0.3 mmol/L, 1.3 +/- 0.2 mmol/L, and 0.048 +/- 0.007 g/L) and C (2.4 +/- 0.2 mmol/L, 1.2 +/- 0.1 mmol/L, and 0.050 +/- 0.006 g/L). Free fatty acids and TGs remained constant and similar in each group postoperatively. After fat infusion had been stopped at the end of the fifth day, the elimination of plasma TGs over the next 4 hours was comparable in the three groups. We conclude that excess egg PLs induce alterations of plasma lipids even within a few days.

Feeding premature infants

We have tried to offer a rational approach to the methods of premature infant feeding. Most of our information is incomplete, and many of the presumptions used in deciding feeding technique are not based on reliable scientific data. The calculations presented in this discussion are valuable as teaching methods in the nursery. The complexity of the questions about premature infant feeding makes it necessary for personnel in the neonatal unit to focus regularly on infant feeding and to allow for time dedicated to considering their suggestions.

Evaluation of lipid metabolism during antihypertensive treatment with nicardipine sr

The antihypertensive and metabolic effects of oral nicardipine SR 40 mg b.i.d. have been studied in 18 (15 m, 3 f; age 52.7 y) hypertensives with mild hypercholesterolaemia, treated for 3 months after a 2 week period on placebo. An iv Fat Tolerance Test (FTT) was also performed in 8 patients following placebo, treatment with acute nicardipine 20 mg and chronic administration of nicardipine SR. There was a significant fall in BP from 160/97 on placebo to 147/87 after 3 months on nicardipine SR with no change in heart rate. Blood lipids did not change significantly. The disappearance rate of the lipid emulsion in the ivFTT showed no significant change (K2 was 1.93%/min after placebo, 1.84 after nicardipine 20 mg and 1.71 after chronic treatment). The results suggest that nicardipine is an effective antihypertensive drug and that it is devoid of untoward effects on lipid metabolism.

Safety and efficacy of glycerol and amino acids in combination with lipid emulsion for peripheral parenteral nutrition support

The safety and efficacy of administering lipid emulsion with ProcalAmine, a glycerol-based parenteral nutrition solution, for peripheral nutrition has not been previously studied. Thirty-four patients recovering from major trauma or surgery were studied while receiving a peripheral parenteral nutrition regimen of either ProcalAmine with 10% lipid emulsion (group 1) or ProcalAmine with 20% lipid emulsion (group 2) for up to 5 days postinjury. Daily dose was 45 mL/kg ProcalAmine, providing 1.35 g of amino acids/kg and 1.35 g of glycero/kg, and 500 mL/day lipid emulsion. The mean daily nitrogen balance was -0.3 g/day in group 1 and -4.1 g/day in group 2. There was no progressive accumulation of circulating glycerol, and urinary glycerol excretion was minimal (less than 0.2 g/day), indicating effective utilization of glycerol as an energy substrate. Our finding that nitrogen balance was better with 10% fat emulsion suggests a limitation in fat utilization in this setting. Both regimens were well tolerated; there were no adverse clinical reactions and no occurrences of phlebitis in either group.

Metabolic clearance of a fat emulsion containing medium-chain triglycerides in cirrhotic patients

Medium-chain triglycerides (MCTs) may be advantageous in cirrhotic patients, but data on the clearing capacity are lacking. Intravenous fat tolerance test (IVFTT) was performed on 28 cirrhotic patients (Child's A, n = 9; Child's B, n = 13; Child's C, n = 6) to determine the clearance rate of Lipofundin MCT 20%, which contains a physical mixture of MCT and long-chain triglycerides (LCT). The mean fractional elimination rate (k2) of MCT/LCT physical blend in the cirrhotic patients (7.72%/min) was comparable with that of the healthy control subjects (n = 9, k2 = 5.43%/min). Significantly higher serum free fatty acid (FFA) levels were found in the cirrhotic patients during IVFTT. The difference could be accounted for by their higher basal levels. In all patients, serum FFA concentrations returned to basal value at the end of 1 hour. Prompt appearance of glycerol and beta-hydroxybutyrate was seen during IVFTT, indicating that hydrolysis of triglycerides and release of ketone bodies from the liver were rapid. It was concluded that the ability to eliminate MCT/LCT physical blend was not impaired in the cirrhotic patients.

Effect of medium- and long-chain triglyceride infusion on lipoprotein and hepatic lipase in healthy subjects

Plasma lipolytic activity and hydrolysis of intravenous fat were studied in six healthy subjects during infusion of a long-chain triglyceride (LCT) fat emulsion (Intralipid 20%) or of a medium-chain triglyceride (MCT)/LCT emulsion (Lipofundin MCT 20%). The fat emulsions were infused continuously at a rate of 0.17 g triglyceride kg-1 body weight (BW)h-1 for 6 h in random order at 7-day intervals. A continuous infusion of glucose (0.18 g kg-1 BW h-1) was administered for a period of 7 h and was started 1 h before the lipid infusion. Infusions of both types of fat increased plasma triglyceride (TG), free fatty acid (FFA) and lipoprotein lipase (LPL) levels and steady-state values were present during the 3rd to 5th h of infusion. MCT/LCT infusion resulted in higher plasma levels at steady-state of TG (3.63 +/- 0.45 [SEM] vs 2.73 +/- 0.45 mmol l-1; P less than 0.05), FFA (1.05 +/- 0.08 vs 0.54 +/- 0.04 mmol l-1; P less than 0.01) and LPL (4.6 +/- 0.6 vs 2.6 +/- 0.5 mU ml-1; P less than 0.05) in comparison with LCT administration. There was a positive correlation between plasma LPL activity and TG concentration (r = 0.77; P less than 0.001) when data for the two infusions were combined. Although the same amount of fat was infused on a weight basis, the molar infusion rate was 40% higher with MCT/LCT than with LCT infusion, due to differences in molecular weights (634 vs 885 Da).(ABSTRACT TRUNCATED AT 250 WORDS)

Lipoprotein lipase activity in skeletal muscle is related to insulin sensitivity

The relative effects of obesity, alone or in combination with insulin resistance and hyperinsulinemia (with or without diabetes), on lipoprotein concentrations, blood pressure, and other risk factors for cardiovascular disease were investigated in 28 men (mean age, 63 years). Special attention was given to lipoprotein lipase (LPL) activity in tissues and to postheparin plasma LPL activity and hepatic lipase activity and their relation to insulin resistance. The 28 men fulfilled the entrance criteria of the study so that they could be allocated to one of the four groups (seven in each group): 1) normal body weight, normal fasting insulin level, and normal glucose tolerance (controls); 2) the same as in group 1 but with moderate obesity; 3) the same as in group 2 but with fasting hyperinsulinemia; 4) the same as in group 3 but with non-insulin-dependent diabetes mellitus. Glucose infusion rate for the control group was 8.1 +/- 2.1 mg/kg body wt/min (mean +/- SD) at an insulin infusion rate of 56 milliunits/m2/min. The average values in groups 2, 3, and 4 were 6.0 +/- 0.7, 3.2 +/- 0.5, and 1.9 +/- 1.0 mg/kg body wt/min, respectively. Concentrations of very low density lipoproteins as well as blood pressure and urate concentrations were highest and those of high density lipoproteins were lowest in the two hyperinsulinemic groups (groups 3 and 4). Skeletal muscle LPL activity was 46 +/- 23, 41 +/- 25, 23 +/- 6, and 31 +/- 13 milliunits/g wet wt (mean +/- SD) in the four groups, respectively. There was a positive correlation between glucose infusion rate and muscle LPL activity (r = 0.58, p less than 0.0001). The hepatic lipase activity was positively correlated with the insulin area under the curve of the intravenous glucose tolerance test (r = 0.35, p = 0.02). Furthermore, blood pressure, free fatty acid concentration, liver enzymes, and urate concentrations were significantly correlated with glucose infusion rate at the clamp test. These data give further support for insulin resistance as an important factor behind the observed lipoprotein abnormalities and blood pressure elevations as part of the insulin resistance syndrome characteristic for obese and diabetic patients.

Accuracy of plasma turbidity measurement for determining fat intolerance during total parenteral nutrition

The accuracy of plasma turbidity measurements in predicting ability to metabolise intravenous fat emulsions during total parenteral nutrition was studied in 35 adult surgical patients. Plasma turbidity, expressed as a light scattering index (LSI), was determined by nephelometry and compared with measured triglyceride (TG) concentrations. A poor coefficient of correlation was found between LSI and TG (r = 0.52). The sensitivity and specificity of LSI in predicting TG concentration were 19% and 96% respectively. This indicates that the measurement of LSI is more useful in ruling out hypertriglyceridaemia than in detecting it. Consequently, clinical tolerance of intravenous fat emulsion cannot be monitored by measuring plasma turbidity. In order to avoid metabolic complications which may occasionally occur during intravenous nutritional therapy including fat emulsion, determination of plasma TG levels at timed intervals are recommended.

Plasma clearance of intravenous fat emulsion in acute bacterial infections

A variety of abnormalities in lipid metabolism during infection have been reported and it has been questioned whether intravenous fat emulsion should be given in septic states. This study was designed to investigate the removal of a fat emulsion from plasma during septicaemia. An intravenous fat tolerance test (0.1 g of fat in the form of Intralipid 10% per kg b.w.) was performed in 28 adult patients who had been admitted for suspected septicaemia. All patients were febrile (mean body temperature 38.9 +/- 0.1 degrees C SEM), the majority had leucocytosis (mean value 10.5 +/- 1.0 10 (9)1 ) and all were haemodynamically stable. On the basis of bacterial cultures taken from various locations, the patients were classified as septic (n = 12; gram-negative sepsis, n = 8; gram-positive sepsis, n = 4), infected but without septicaemia (n = 9) or febrile of unknown cause (all cultures negative, n = 7). Plasma fractional removal rates of intravenous fat were similar in the different groups. The mean plasma clearance rate of intravenous fat was 6.6 +/- 0.5 (% minute), which does not differ from that reported for healthy controls. All patients with proven septicaemia and 5 non-septic patients were studied two weeks later when their infection had been treated and they were off antibiotic medication. The plasma clearance rate of i.v. fat was significantly lower in the recovery phase than during the acute febrile state (5.9 +/- 0.5 vs 4.2 +/- 0.4 %/min, n = 17; p < 0.05). It is concluded that clearance of intravenous fat emulsions from plasma is unimpaired in haemodynamically stable patients with septicaemia or other acute infections.

Intralipid removal from plasma of uraemic and intensive care patients

The removal rates from plasma of Intralipid and (125)I-albumin, simultaneously injected i.v., were analyzed in 10 chronic renal failure (CRF) patients. The results were compared with 9 intensive care unit (ICU) patients and five healthy subjects as controls. The fractional removal rates of Intralipid (k2) were significantly lower for the CRF patients compared to both ICU patients and healthy subjects. The initial plasma concentration of Intralipid, calculated by extrapolation of the elimination curve back to zero-time (y0), showed significantly higher values for CRF patients and healthy subjects compared to ICU patients. Removal curves expressed as ratios between Intralipid and (125)I-albumin paralleled the elimination curves for Intralipid, which suggests that the slope of the curves depends on Intralipid removal and not on leakage of macromolecules from the circulation. The very low initial plasma concentration of Intralipid in the ICU patients cannot be explained by a hyperdynamic circulation, but may be attributed to a rapid first passage disappearance of Intralipid from plasma into tissues such as the pulmonary vasculature.

Metabolism of intravenous fat emulsion in the surgical newborn

The metabolism of an intravenous (IV) fat emulsion was investigated by the combination of chemical balance and computerized indirect calorimetry techniques in 21 newborns (birth weight, 3.0 +/- 0.1 kg; mean +/- SE). All babies were appropriate for gestational age and received total parenteral nutrition after a major surgical procedure. The study was divided into two consecutive periods. Phase 1 consisted of infusion of 10% glucose and 2% amino acid solutions for 24 hours, and phase 2 involved the "Intralipid utilization test" (isocaloric and isovolemic infusion of Intralipid 10% for four hours). The caloric intake was 67.1 +/- 1.9 kcal/kg/d during both phases of the study. The resting energy expenditure was 44.8 +/- 1.6 and 46.5 +/- 1.8 kcal/kg/d during phases 1 and 2 respectively. During glucose/amino acid infusion, 12 patients oxidized endogenous fat, and de novo lipogenesis from glucose was observed in nine. During the Intralipid infusion, there was a significant and progressive decrease of carbon dioxide production, respiratory quotient, and carbohydrate utilization (oxidation plus conversion to fat). Net lipogenesis ended and fat utilization significantly increased. By the second hour of Intralipid infusion, 58% of energy expenditure was derived from fat oxidation. The drop in carbon dioxide production correlated positively with the decrease in carbohydrate utilization (r = .07; P less than .001). During the third and fourth hours of phase 2, the percentage of fat utilized was negatively correlated with the amount of fat given (r = -.07; P less than .01). The surgical neonate showed rapid metabolic adaptation to Intralipid infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in plasma lipids, lipoproteins, triglyceride secretion and removal in chicks with estrogen implants

Estradiol implants in chicks resulted in marked elevation of all major plasma lipids with greatest increase in triglyceride (TG) followed by phospholipid (PL) and cholesterol (C). During the two-wk period, plasma TG level in estrogen (E)-treated chicks increased to about 45 times that of controls (139.6 vs 6,368.3 mg/dl). The level of cholesterol also increased steadily during the same period, attaining nearly a six-fold increase in comparison with the control (150.7 vs 871.8 mg/dl), and the level of PL was markedly elevated from 209 to 2,861 mg/dl. Besides the induction of hyperlipidemia, E treatment also resulted in a notable alteration in the fatty acid composition of plasma lipids; there was an increase in oleic acid concomitant with a decrease in polyunsaturated fatty acids, particularly, linoleic acid. One day after implantation, the percentage of oleic acid in TG fraction increased from 39.2 to 43.7%, reaching 55.4% of the total fatty acids at day 14. In contrast, the levels of linoleic and arachidonic acid decreased significantly from 16.1 to 8.3% and 4.3 to 0.6%, respectively, during the same period. In cholesteryl ester (CE) and PL, the oleic acid level also increased from 25.2 to 47.3% in the former and from 11.9 to 29.6% in the latter, reflecting enhanced hepatic lipogenesis. Analysis of plasma lipoproteins in E-treated chicks revealed dramatic alterations in the concentrations of lipids and protein in individual lipoprotein fractions, especially very low density lipoprotein (VLDL) fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Parameters for evaluation of lipid metabolism

The influence of the clinical condition and the intravenous intake on parameters of fat metabolism has been analyzed. Compared to normal subjects, the correlation between plasma concentrations and the turnover rate of glycerol and free fatty acids varies in the opposite direction in nutritionally depleted and severely injured patients. The significance of plasma concentrations as an index of fat mobilization should be interpreted in relation to the clinical condition. Kinetic measurements are particularly interesting in hypermetabolic patients. Plasma triglyceride and cholesterol concentrations are markedly affected by surgical procedures. Any delay (in treatment) following the injury and the type of intravenous regimen used have an important influence on plasma lipid levels and should be taken into account when groups of patients are studied. The infusion of exogenous fat emulsions significantly affects not only plasma triglyceride levels but also phospholipid and cholesterol concentrations and will modify the plasma lipoprotein pattern. Measurements of plasma clearance and oxidation of fat can be used to evaluate individual tolerance and the metabolic utilization of lipids, but these procedures cannot be easily applied in routine clinical practice. Regular monitoring of plasma triglyceride, cholesterol, and phospholipid concentrations during and after cessation of fat infusion is recommended for each patient who is receiving daily fat infusions so that the safe rate of infusion for that individual can be determined.

Phosphatidylcholine composition of emulsions influences triacylglycerol lipolysis and clearance from plasma

Sonicated emulsions containing triolein, a specific phosphatidylcholine and cholesterol were prepared. Bolus doses were injected intravenously into rats and plasma clearance kinetics and organ uptakes were determined. Emulsion triacylglycerol lipolysis by rat heart lipoprotein lipase was measured in vitro. Phosphatidylcholine molecular species influenced emulsion metabolism in vivo and in vitro. Emulsions containing saturated phosphatidylcholines at temperatures below their melting points were poor substrates for lipoprotein lipase, compared with those stabilized by mixed chain phosphatidylcholines. Distearoylphosphatidylcholine stimulated hepatic uptake compared with emulsions made with egg yolk phosphatidylcholine, which modeled chylomicrons closely. Emulsion populations with the same surface compositions but with mean diameters of 700-800 A and 1100-1300 A were metabolized similarly, suggesting that, within the normal chylomicron size range, size alone does not determine the disposition of triacylglycerol-rich emulsions or lipoproteins.

Increased removal rate of exogenous triglycerides after prolonged exercise in man: time course and effect of exercise duration

Ten healthy young men exercised for 1.5 and 3 hours, respectively, 4 to 6 weeks apart at the same work intensity, corresponding to 77% of the individual maximal heart rate. In the fasting state 1 mL of 10% Intralipid/kg body weight was injected IV, the fractional removal rate was calculated (k2-value), and fasting lipid and apolipoprotein C-I, C-II, and CIII concentrations were determined one day before, immediately before, immediately after, and one day after the exercise: measurements were also made two and three days after the three-hour session. An increase of the k2-value was found only the day after the three-hour exercise (+66%, P less than .01), while after the 1.5-hour exercise the k2-value was not changed. One and two days after the three-hour exercise the fasting serum triglyceride concentration was significantly decreased by 33% and 16%, respectively. Serum triglyceride concentration was decreased also the day after the shorter session (-17%, P less than .05). Decreases in serum cholesterol concentrations were significant after both exercise bouts, but the significance disappeared if corrections for the changes in calculated plasma volume were made. Serum level of C apolipoproteins was decreased one day after the three-hour exercise, while it was not changed after the shorter exercise. This study shows that prolonged exercise produces an increase in the removal rate of Intralipid triglycerides from the bloodstream, that this change is related to the duration of exercise, and that some time is required following the exercise, before it is manifest.

Plasma clearance of fat emulsion during continuous heparin infusion

Intravenous fat tolerance was tested in two groups of patients given a continuous i.v. infusion of heparin for several days. One group of 11 patients with deep vein thrombosis (DVT) of the leg was given 25,000-35,000 IU heparin daily for 4-5 days. The other group comprised 10 patients who had central venous catheters (CVC) for total parenteral nutrition. These patients were given 20,000 IU heparin daily for 6 days as prophylaxis against CVC-related thrombosis. In the DVT group heparinization was associated with a 44% decrease in plasma fat removal capacity (P less than 0.05). This reduction persisted for 2 days after the discontinuation of heparin therapy. In the CVC group the plasma fat removal capacity decreased by 29% during heparinization (P greater than 0.05, NS). During heparinization activated partial thromboplastin time was more than three times the basal value in the DVT group but less than twice those in the CVC group. One week after the heparin therapy the serum triglyceride levels were higher in both groups compared with initial values (DVT group: 1.2 +/- 0.2 s.e. mean vs. 1.7 +/- 0.3 mmol/l; P less than 0.05. CVC group: 1.0 +/- 0.1 vs. 1.4 +/- 0.2 mmol/l; NS). The possibility that full-dose heparinization reduces plasma fat removal capacity and that this may be due to a partial depletion of lipoprotein lipase stores is discussed.

Early and late effects of weight loss on lipoprotein metabolism in severe obesity

In a group of grossly obese patients serum lipoproteins and the intravenous fat tolerance test were analysed before a weight reduction program (n = 98), after 6 weeks (n = 58) and 1 year (n = 15). At one year follow-up the mean weight had fallen from 120 to 105 to 100 kg, respectively. In spite of severe overweight, only a moderate hypertriglyceridaemia (males mean fasting concentration of plasma triglycerides (TG) 2.50 +/- 1.36, females 2.03 +/- 1.94 mmol/l) was found before treatment. After 6 weeks significant reductions were found in VLDL-TG, -cholesterol, LDL-cholesterol and HDL-cholesterol (P less than 0.05 at least). After one year VLDL-TG was still below pretreatment level, LDL-cholesterol had increased above pretreatment level, whereas HDL-cholesterol was significantly higher compared to pretreatment (P less than 0.05). Some previous studies suggest that weight loss may lead to HDL-cholesterol reductions, which would be undesirable. The present study indicates that after sustained weight loss and at a stable lower body weight, HDL-cholesterol levels may increase above pretreatment levels after an initial drop during the catabolic weight reduction phase.

Inhibition of hepatic triglyceride secretion and exogenous triglyceride clearance in the cholestatic rat

Extrahepatic biliary obstruction in humans and rats leads to hypertriglyceridemia. The observed hypertriglyceridemia could result from either a defect of plasma triglyceride (TG) catabolism or hepatic over-production of TG. To examine these questions we have used the rat model to determine hepatic TG secretion by the Triton WR-1339 methodology (inhibition of peripheral lipolysis) and exogenous TG clearance (after i.v. injection of Intralipid). Four groups of rats were studied: group OB--48 h post-operative--bile-duct obstructed; group DV--bile diverted; group SC--sham-operated controls; and group FC--48 h fasted, unoperated controls. The hepatic TG secretion rate for group OB rats was a factor of 7 lower than that of either group SC or FC, and 5 times lower than that for group DV. There were no differences between the hepatic TG secretion rates of groups DV and FC or SC. After i.v. injection of Intralipid, plasma TG decreased with first-order kinetics. The rate constant was taken as the exogenous TG clearance rate (ETGCR). Mean ETGCR for group OB was a factor of 3 lower than that for either control group; while the ETGCR for group DV was equivalent to the control groups. Thus biliary diversion does not affect hepatic TG secretion or the ETGCR. The apparent cause of the hypertriglyceridemia of cholestasis in the bile-obstructed rat is impaired plasma TG catabolism.

Exogenous lipid clearance in compensated liver cirrhosis

The tolerance to exogenous fats has been evaluated in patients with liver cirrhosis. A three-stage lipid clearance test with continuous infusion (3 hr) of a triglyceride emulsion, Intralipid, was performed on 10 patients with well compensated liver cirrhosis and 10 normolipidemic volunteers. During the infusion, blood samples were collected for the measurement of particulate triglycerides (TG) by nephelometry; samples were also collected for total TG, free fatty acids (FFA) and free tryptophan (TRP) determinations. Plasma endogenous triglycerides were calculated as the total minus exogenous, particulate, TG. The fractional removal rate (K2) and the maximal clearing capacity (K1) for exogenous TG were lower in patients than in controls, though a significant difference (p less than 0.05) was found only for K1. Endogenous TG and FFA showed a comparable rise in patients and controls during Intralipid infusion. A significant increase in free TRP was noted in cirrhotics upon maximal infusion rate. It is concluded that: in patients with well compensated liver cirrhosis the maximal clearing capacity (K1) for exogenous TG is impaired. Nonetheless, moderate amounts of fat may be removed at a normal rate from the bloodstream; a normal synthesis rate of exogenous TG may be maintained even in a severely damaged liver; considering the possible role of free TRP in the pathogenesis of hepatic encephalopathy (HE), the use of large amounts of lipids should be discouraged in patients with decompensated liver cirrhosis, or even avoided in those with impending or overt HE.

Inhibition of human and rat lipoprotein lipase by high-density lipoprotein

The hydrolysis in vitro of preactivated Intralipid (an artificial triacylglycerol-phospholipid emulsion) by rat adipose tissue lipoprotein lipase is inhibited by rat high-density lipoprotein (HDL). The aim of this work was to investigate whether human lipoprotein lipase was also inhibited, the mechanism of inhibition of the rat enzyme by HDL, and the role of the various individual apolipoproteins. Both human and rat lipoprotein lipase from post-heparin plasma are inhibited by HDL. This inhibition is considerably decreased if the HDL is first made 'apolipoprotein poor' by removal of some transferable apolipoproteins. In contrast, both native and apolipoprotein poor HDL inhibit the hydrolysis of Intralipid by rat hepatic lipase. Apolipoproteins C and E, either free in solution or attached to lipid vesicles, inhibit the hydrolysis of activated Intralipid by rat lipoprotein lipase to a maximum of 85% and 50%, respectively. Apolipoprotein A attached to vesicles gives little inhibition. HDL apolipoprotein and apolipoprotein C compete with the substrate for binding to lipoprotein lipase with apolipoprotein C having a higher affinity for the enzyme than HDL apolipoprotein. The inhibition of lipoprotein lipase by HDL can be explained by the association of the constituent apolipoproteins, in particular apolipoprotein C, with the enzyme so that there is less enzyme available to act on substrate.

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.

Clearance of triglycerides from the circulation and its relationship to serum lipoproteins: influence of age and sex

In this study of a randomly selected population from 20 to 70 years of age, the clearance of triglycerides from the blood was studied after i.v. injection of an artificial triglyceride emulsion Intralipid. In women, the triglyceride clearance from the blood decreases with increasing age, but for men there was no change with age. Women had faster fractional removal rates of i.v. injected triglycerides than men in all age-groups. The triglyceride clearance showed a negative correlation to Broca index (obesity), serum triglyceride, serum cholesterol, triglycerides in very low density lipoproteins, and to cholesterol in very low density lipoproteins and in low density lipoproteins, but showed a strong positive correlation to cholesterol in high density lipoproteins.

Lipolytic and anticoagulant activities of a low molecular weight fragment of heparin

Low molecular weight heparin (LMWH) and standard heparin were given intravenously to six healthy subjects receiving a continuous infusion of Intralipid. After infusion, antifactor Xa, antithrombin II and coagulation activity (Normotest) were the same for both heparins. Activated partial thromboplastin time increased significantly, but the increase was much higher after standard heparin (+473%) than after LMWH (+48%). The increase in lipoprotein lipase activity was less pronounced after LMWH infusion. This resulted in a smaller decrease in Intralipid-triglyceride concentration and a smaller increase in both plasma FFA concentration and Intralipid fractional removal rate compared to standard heparin. This study shows that the plasma lipolytic potential of LMWH is weaker than that of standard heparin when given in doses with equipotent anticoagulation. LMWH may therefore be preferable to standard heparin as an antithrombotic agent in clinical situations where a high plasma lipolytic activity may be disadvantageous.

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.