Removal of exogenous plasma triglycerides in forearm muscle and subcutaneous tissue of hyper-and normotriglyceridaemic men

Clinical considerations and mechanistic determinants of postprandial lipemia in older adults

The typical diet of individuals in Western societies results in metabolic responses associated with fed-state fat metabolism for most of the daily life of the individual. This fat metabolism is characterized specifically by an increase in the concentration of plasma lipids, primarily triglycerides. Increased postprandial lipemia, which is typically observed in older individuals (i.e., >65 y old), has now emerged as an important correlate of cardiovascular disease risk. An understanding of the mechanisms contributing to the increased postprandial lipemia in older individuals becomes, therefore, of particular clinical importance in any effort to explain and address the well-documented increase in cardiovascular disease risk as individuals age. Current evidence points to an increase in the accumulation of ingested lipid in lipoprotein particles of hepatic origin, together with an overall accumulation of lipid in these lipoproteins during the postprandial period, as primary contributors to the postprandial lipemia in older persons. When this evidence is considered together with the evidence suggesting large atherogenic potential of lipoproteins of hepatic origin, this can, at least in part, explain the increased risk of cardiovascular disease in older individuals. Understanding changes in the metabolism of ingested fat in the immediate postprandial period with advancing age, and how lifestyle interventions such as diet and physical exercise can ameliorate the increase in postprandial lipemia in older individuals, is important in order to address the increased cardiovascular disease risk in this particularly affected and growing segment of the population.

Postprandial lipemia in the elderly involves increased incorporation of ingested fat in plasma free fatty acids and small (Sf 20-400) triglyceride-rich lipoproteins

In the elderly, the rise in postprandial plasma triglyceride (TG) concentrations is increased, contributing to their increased risk of cardiovascular disease. We sought to determine the incorporation of ingested fat (whipping cream enriched with [1,1,1-(13)C]triolein) into plasma lipids during the postprandial period in six healthy elderly (67 ± 1 yr old) and six healthy young (23 ± 2 yr old) subjects. Blood and expired air samples were taken before and at 2-h intervals during the 8-h postprandial period. As expected, the area under the curve of postprandial plasma TG concentrations was larger in the elderly compared with the young subjects (152 ± 38 vs. 66 ± 27 mg·dl(-1)·h, P < 0.05). The incorporation of [(13)C]oleate in plasma free fatty acids (FFAs) and TG of the small (S(f) = 20-400) triglyceride-rich lipoprotein (TRL) fraction was significantly higher in the elderly compared with the young subjects, resulting in increased postprandial contributions of the ingested lipid to plasma FFAs (41 ± 3 vs. 26 ± 6%, P < 0.05) and the small TRL fraction (36 ± 5 vs. 21 ± 3%, P < 0.05) in elderly. Plasma apoB-100 concentration was higher, whereas the rate of oxidation of the ingested lipid was lower (P < 0.05) in the elderly. We conclude that increased postprandial lipemia in the elderly involves increased contribution of ingested lipid to the plasma small TRLs. This appears to be driven at least in part by increased appearance of the ingested fat as plasma FFA and increased availability of apo B-100 lipoproteins in the elderly.

Induced mutant mice expressing lipoprotein lipase exclusively in muscle have subnormal triglycerides yet reduced high density lipoprotein cholesterol levels in plasma

To determine the contribution of muscle lipoprotein lipase (LPL) to lipoprotein metabolism, induced mutant mice were generated that express human LPL exclusively in muscle. By cross-breeding heterozygous LPL knockout mice with transgenic mice expressing human LPL only in muscle, animals were obtained that express human LPL primarily in skeletal muscle on either the null (L0-MCK) or normal (L2-MCK) LPL backgrounds, and these were compared with control littermates (L2). Fed and fasted post-heparin plasma (PHP) LPL activities were increased 1.4- and 2.3-fold, respectively, in L2-MCK mice and were normal in L0-MCK mice compared with controls. The specific enzyme activities of human LPL in mouse plasma was comparable to human LPL in human PHP. Skeletal muscle LPL activity was increased in both L2-MCK and L0-MCK mice in the fed (6.6-fold) and fasted (4.2-fold in L2-MCK; and 3.4-fold in L0-MCK) states. Adipose tissue LPL mRNA and activity were not detectable in L0-MCK mice. Growth and body mass composition were similar among all groups. In the fasted and fed state, L2-MCK mice had 31% and 53% reductions, respectively, in plasma triglycerides (TG), compatible with increased PHP LPL activity. Unexpectedly, both in the fasted and fed state the L0-MCK mice also had reduced TG (22%), despite normal PHP LPL activities. Very low density lipoprotein (VLDL) turnover studies revealed that the decreased TG were due to increased particle fractional catabolic rate in both L2-MCK and L0-MCK mice. Despite reduced TG, both L2-MCK and L0-MCK mice showed reduced high density lipoprotein (HDL) cholesterol levels (16% and 19%, respectively). HDL turnover studies indicated increased HDL cholesteryl ester fractional catabolic rate in the L2-MCK and L0-MCK compared with control mice. In summary, these studies suggest that muscle LPL is particularly potent with regard to VLDL metabolism and is sufficient to compensate for the lack of LPL in other tissues with regard to lipolyzing VLDL particles. With regard to HDL, muscle LPL expression does not result in normal levels due to enhanced breakdown either by mediating accelerated HDL clearance or by failing to establish normal HDL particles that are then cleared more quickly than normal. These studies provide new insights on the tissue-specific effects of LPL on lipoprotein metabolism.

Very-low-density lipoprotein subfraction composition and metabolism by adipose tissue

Lipoprotein lipase (LPL) plays a pivotal role in very-low-density lipoprotein (VLDL) metabolism. Within the circulation, the VLDL population is heterogeneous with respect to both size and composition. Several studies have investigated the action of LPL in vitro on different VLDL subfractions, but little is known of the action of LPL in vivo. To investigate this, arterial and adipose tissue venous plasma samples were obtained from 16 normal male healthy volunteers (aged 24.4 +/- 1.8 years; body mass index, 23.5 +/- 0.7 kg.m-2) following an overnight fast. VLDL subfractions were isolated (VLDL1 of Sf 60 to 400 and VLDL2 of Sf 20 to 60) and characterized in terms of triacylglycarol (TAG) and apolipoprotein (apo) B, E, CI, CII, and CIII content. The apolipoprotein content of VLDL1 differed from that of VLDL2: the VLDL2 fraction contained significantly more apo B (0.018 +/- 0.004 v 0.011 +/- 0.003 mumol.L-1, p = .001) but the ratios of TAG:apo B and apo CI:B, and CII:B, and CIII:B were significantly higher in VLDL1 (48,200 +/- 7,980 v 13,860 +/- 2,420, 22.7 +/- 5.5 v 12.5 +/- 2.2, 45.0 +/- 6.3 v 14.9 +/- 2.0, and 0.434 +/- 0.077 v 0.357 +/- 0.054, respectively, molar ratios, all P < .05). The venous blood draining an adipose tissue depot contained less VLDL1-TAG than arterial blood (328 +/- 68 v 381 +/- 83 mumol.L-1, respectively, P < .01), whereas VLDL2-TAG exhibited an opposite tendency (199 +/- 46 v 172 +/- 31 mumol.L-1, NS). Concentrations of VLDL1-apo B, -apo CII, and -apo CIII were significantly less in adipose tissue venous blood compared with arterial blood (0.011 +/- 0.004 v 0.013 +/- 0.004, 0.38 +/- 0.08 v 0.43 +/- 0.10, and 1.33 +/- 0.35 v 1.58 +/- 0.38 mumol.L-1, respectively, all P < .05). These studies demonstrated novel differences in VLDL1 and VLDL2 in terms of composition and metabolism by human adipose tissue LPL in vivo.

Phenylalanine kinetics in human adipose tissue

Very little is known about the regulation of protein metabolism in adipose tissue. In this study systemic, adipose tissue, and forearm phenylalanine kinetics were determined in healthy postabsorptive volunteers before and during a 2-h glucose infusion (7 mg.kg-1.min-1). [3H]Phenylalanine was infused and blood was sampled from a radial artery, a subcutaneous abdominal vein, and a deep forearm vein. Adipose tissue and forearm blood flow were measured with 133Xe and plethysmography, respectively, and body fat mass was determined by dual energy x-ray absorptiometry. During glucose infusion, glucose concentration increased from 86 +/- 2 to 228 +/- 13 mg/dl and insulin concentration increased from 6.6 +/- 0.6 to 35.0 +/- 3.9 mU/liter, both P < 0.001. Systemic phenylalanine appearance decreased from 40.3 +/- 1.9 to 37.0 +/- 1.6 mumol/min during glucose infusion (P < 0.05). Baseline whole body adipose tissue phenylalanine release (5.2 +/- 1.4 mumol/min) was approximately 12% of systemic phenylalanine appearance and decreased (P < 0.05) to 2.3 +/- 0.9 mumol/min during glucose infusion. In contrast, phenylalanine release from the forearm did not change during glucose infusion. These results indicate that adipose tissue is a small but significant contributor to systemic phenylalanine appearance. Phenylalanine release from adipose tissue like lipolysis, is relatively sensitive to hyperinsulinemia.

Fatty acid turnover in the ischaemic compared to the non-ischaemic human heart

Cardiac extraction, oxidation and release of plasma free fatty acids (FFA) was measured by coronary sinus catheterization, utilizing infusions of 3H palmitate and 14C oleate, in patients with ischaemic heart disease (IHD) at rest and during pacing induced angina pectoris and, for comparison, in healthy men of similar and younger age and men with hypertriglyceridaemia (HTG). At rest IHD patients differed from healthy men only by greater cardiac fatty acid release, which correlated with a significant glycerol release. In IHD patients, unlike in healthy men, myocardial extraction of both palmitate and oleate decreased while fractional oxidation of oleate increased during pacing. Fatty acid release was unaltered. Men with HTG had at rest higher myocardial FFA extraction than IHD patients, which did not decrease during pacing, but like in the patients oleate fractional oxidation increased on pacing. It is concluded that, in the moderately ischaemic human heart, the restricted blood flow may contribute to limit the fatty acid flux into the myocardium. The augmented cardiac fatty acid release in IHD patients is not related to ischaemia per se but may derive from an increased amount of cardiac interstitial fat.

Arterial deepvenous difference of lipoproteins in skeletal muscle of patients in postoperative state: effects of medium chain triglyceride emulsion

The effect of fat infusion with medium chain triglycerides (MCT) and long chain triglycerides (LCT) on serum lipoproteins before and after passage through the skeletal muscle was investigated with the forearm technic in eight patients after abdominal operation. All lipoprotein fractions were enriched with triglycerides and phospholipids from infused artificial fat particles with the consequence of significantly increased ratios of TG/PL and TG/apo B in VLDL, of TG/apo B in LDL and TG/apo A-I in HDL. Uptake and release of lipoprotein components by skeletal muscle are given by arterial-deepvenous differences considering the blood flow rates. The positive arterial-deepvenous difference of VLDL triglycerides after 4-hr infusion is interpreted as cleavage and uptake of infused MCT by the muscle. The release of LDL is more pronounced after the fat infusion than before, suggesting a degradation and enhanced catabolism of artificial fat particles. HDL release may be also a consequence of catabolism of artificial TG/PL-particles. These results indicate an uptake of MCT/LCT emulsion by the skeletal muscle.

Heparin-released lipolytic activity in the forearm vascular bed in patients with hypertriglyceridaemia

Hypertriglyceridaemia (HTG) may be caused by increased production or decreased removal of very low density lipoproteins (VLDL)or by a combination of these mechanisms. A decreased lipoproteinlipase activity (LLA) in the vascular wall of the tissues could be the mechanism of impaired removal. Therefore, the release of lipase activity from the forearm musculature after regional arterial injection of heparin was studied in healthy volunteers and patients with HTG. Heparin was injected in the doses 1.5, 15 and 150 i.u. and arterial and deep venous blood was sampled simultaneously at short intervals and analysed for LLA using intralipid with triple 14C-oleate as substrate. There was no significant difference in release of lipolytic activity between the groups and there was no significant correlation between plasma triglyceride concentration and LLA release in the HTG group or the whole material. Thus, deficient skeletal muscle LLA does not seem to be a general causative mechanism in HTG.

Lipoprotein lipase activity of adipose tissue, skeletal muscle and post-heparin plasma in primary endogenous hypertriglyceridaemia: relation to lipoprotein pattern and to obesity

The lipoprotein lipase (LPL) activity was determined from heparin eluates of adipose tissue and skeletal muscle and from post-heparin plasma of sixty-five males with hypertriglyceridaemia and of seventy males with normal serum lipid levels. The patients were subgrouped by their lipoprotein concentrations into types 2b, 4 and 5. The mean LPL activity of adipose tissue (per tissue weight) of nonobese type 2b, 4 and 5 patients was reduced to 54%, 41% and 13%, respectively, of the corresponding value of normolipidaemic men. On the other hand, among obese hyperglyceridaemic men only those with type 5 showed a decreased LPL activity in adipose tissue (44%). The mean skeletal muscle LPL was subnormal in nonobese type 4 (55%) and in type 5 patients (34%) but was normal in type 2b and in obese type 4 patients. The post-heparin plasma LPL activity was significantly reduced in all nonobese hyperglyceridaemic groups but was normal in obese patients apart from cases with type 5 who had low values. One exceptional subject with type 5 had high post-heparin plasma LPL activity. It is concluded that a low LPL activity may be a crucial factor in the pathogenesis of hypertriglyceridaemia in nonobese subjects and in patients having type 5 disorder.

Fat emulsion catabolism in vitro and in vivo--sex related differences

The removal rates of an intravenously administered 10% fat emulsion (Intralipid) from plasma in male and female conscious rats are described. The plasma concentration of fat emulsion particles at various time intervals following a bolus administration (0.2 g/kg) was measured by nephelometry. At the dose employed, the removal of fat emulsion from the plasma followed first order kinetics, ie, a constant fraction was removed from the plasma per unit of time, K2 (%/min). Females exhibited a significantly greater fractional removal rate (K2) than comparably aged males (21.0 +/- 1.0 vs 15.0 +/- 1.4, p less than 0.05). Postheparin lipoprotein lipase, measured using fat emulsion as substrate, also was significantly greater in female rats compared with males. Our results demonstrate that, in rats, fat emulsion (Intralipid) is catabolized more rapidly in females than in males and a greater lipoprotein lipase activity in female rats may be the causative factor.