LDL and HDL transfer rates across peripheral microvascular endothelium agree with those predicted for passive ultrafiltration in humans.

The mechanisms by which LDLs and HDLs cross the vascular endothelium from plasma into interstitial fluid are not understood, and have never been studied in humans in vivo. We determined whether the plasma-to-lymph clearance rates of LDL and HDL conform with those predicted by passive ultrafiltration through intercellular pores, or if it is necessary to invoke an active process such as receptor-mediated transcytosis. Plasma and afferent peripheral lymph were collected under steady-state conditions from 30 healthy men, and assayed for seven globular proteins of molecular radii 2.89-8.95 nm, complement C3, and apo AI, apo AII, and apo B. Plasma-to-lymph clearance rates of the seven proteins fitted the relation expected for molecules of their size when transported through two populations of pores of radius 4.95 and 20.1 nm. The same model parameters were then found to accurately predict the clearance rates of both HDL and LDL. The apparent clearance of complement C3, previously shown to be secreted by cultured endothelium, exceeded that predicted by the model. We conclude that the transport of HDL and LDL from plasma into interstitial fluid across the peripheral vascular endothelium in healthy humans can be explained by ultrafiltration without invoking an additional active process such as transcytosis.

Lipoprotein remodeling generates lipid-poor apolipoprotein A-I particles in human interstitial fluid.

Although much is known about the remodeling of high density lipoproteins (HDLs) in blood, there is no information on that in interstitial fluid, where it might have a major impact on the transport of cholesterol from cells. We incubated plasma and afferent (prenodal) peripheral lymph from 10 healthy men at 37°C in vitro and followed the changes in HDL subclasses by nondenaturing two-dimensional crossed immunoelectrophoresis and size-exclusion chromatography. In plasma, there was always initially a net conversion of small pre-ß-HDLs to cholesteryl ester (CE)-rich α-HDLs. By contrast, in lymph, there was only net production of pre-ß-HDLs from α-HDLs. Endogenous cholesterol esterification rate, cholesteryl ester transfer protein (CETP) concentration, CE transfer activity, phospholipid transfer protein (PLTP) concentration, and phospholipid transfer activity in lymph averaged 5.0, 10.4, 8.2, 25.0, and 82.0% of those in plasma, respectively (all P < 0.02). Lymph PLTP concentration, but not phospholipid transfer activity, was positively correlated with that in plasma (r = +0.63, P = 0.05). Mean PLTP-specific activity was 3.5-fold greater in lymph, reflecting a greater proportion of the high-activity form of PLTP. These findings suggest that cholesterol esterification rate and PLTP specific activity are differentially regulated in the two matrices in accordance with the requirements of reverse cholesterol transport, generating lipid-poor pre-ß-HDLs in the extracellular matrix for cholesterol uptake from neighboring cells and converting pre-ß-HDLs to α-HDLs in plasma for the delivery of cell-derived CEs to the liver.

Secretion of adipokines by human adipose tissue in vivo: partitioning between capillary and lymphatic transport.

Peptides secreted by adipose tissue (adipokines) may enter blood via capillaries or lymph. The relative importance of these pathways for a given adipokine might influence its biological effects. Because this has not been studied in any species, we measured the concentrations of seven adipokines and eight nonsecreted proteins in afferent peripheral lymph and venous plasma from 12 healthy men. Data for nonsecreted proteins were used to derive indices of microvascular permeability, which in conjunction with the molecular radii of the adipokines were used to estimate the amounts leaving the tissue via capillaries. Transport rates via lymph were estimated from the lymph adipokine concentrations and lymph flow rates and total transport (secretion) as the sum of this and capillary transport. Concentrations of nonsecreted proteins were always lower in lymph than in plasma. With the exception of adiponectin, adipokine concentrations were always higher in lymph (P < 0.01). Leptin and MCP-1 were secreted at the highest rates (means: 43 µg/h or 2.7 nmol/h and 32 µg/h or 2.4 nmol/h, respectively). IL-6 and MCP-1 secretion rates varied greatly between subjects. The proportion of an adipokine transported via lymph was directly related to its molecular radius (r(s) = +0.94, P = 0.025, n = 6), increasing from 14 to 100% as the radius increased from 1.18 (IL-8) to 3.24 nm (TNFα). We conclude that the lymph/capillary partitioning of adipokines is a function of molecular size, which may affect both their regional and systemic effects in vivo. This finding may have implications for the physiology of peptides secreted by other tissues.

Serum apolipoprotein j in health, coronary heart disease and type 2 diabetes mellitus.

Apolipoprotein (apo) J, clusterin, is ubiquitously expressed in many tissues, and is a component of high-density lipoproteins (HDLs). There is experimental evidence that it may be anti-atherogenic through its effects on cholesterol transport, smooth muscle cell proliferation and lipid peroxidation. HDLs containing apo J and apo A-I carry paraoxonase (PON1), which protects low-density lipoproteins from oxidative modification; however, the extent to which apo J affects coronary heart disease (CHD) is not known. We have developed a sandwich ELISA that enables apo J to be assayed in the range of 13-200 microg/mL. Serum apo J was 52.8+/-0.8 microg/mL (mean+/-SEM; range, 36.0-84.3 microg/mL; n=92) in healthy Japanese men, and 49.3+/-0.5 microg/mL (34.5-72.8; n=241) in healthy Japanese women. Multiple regression of these data and results from 67 men with CHD showed that apo J concentration was unrelated to age, sex or body mass index, but was positively related to serum PON1 (p<0.001) and apo B (p<0.02) concentrations. In women, it was also positively related to blood glucose (p<0.02). After adjusting for its associations with covariates, serum apo J averaged 5.4 microg/mL, lower in CHD men than in controls (p<0.003). Type 2 diabetics had higher apo J concentrations (men, 83.1+/-3.4 microg/mL, n=64; women, 64.0+/-2.3 microg/mL, n=46) than healthy men and women (p<0.001). In these Type 2 diabetics, apo J concentration was unrelated to PON1 concentration, but was positively related to blood glucose (p<0.01). After adjustment for its relation to blood glucose, the mean apo J concentration was similar in diabetics and healthy subjects. These findings suggest that apo J may be anti-atherogenic in humans, and that its concentration is raised by Type 2 diabetes.

Mechanism and Physiologic Significance of the Suppression of Cholesterol Esterification in Human Interstitial Fluid.

Cholesterol esterification in high density lipoproteins (HDLs) by lecithin:cholesterol acyltransferase (LCAT) promotes unesterified cholesterol (UC) transfer from red cell membranes to plasma in vitro. However, it does not explain the transfer of UC from most peripheral cells to interstitial fluid in vivo, as HDLs in afferent peripheral lymph are enriched in UC. Having already reported that the endogenous cholesterol esterification rate (ECER) in lymph is only 5% of that in plasma, we have now explored the underlying mechanism. In peripheral lymph from 20 healthy men, LCAT concentration, LCAT activity (assayed using an optimized substrate), and LCAT specific activity averaged, respectively, 11.8, 10.3, and 84.9% of plasma values. When recombinant human LCAT was added to lymph, the increments in enzyme activity were similar to those when LCAT was added to plasma. Addition of apolipoprotein AI (apo AI), fatty acid-free albumin, Intralipid, or the d < 1.006 g/ml plasma fraction had no effect on ECER. During incubation of lymph plus plasma, the ECER was similar to that observed with buffer plus plasma. When lymph was added to heat-inactivated plasma, the ECER was 11-fold greater than with lymph plus buffer. Addition of discoidal proteoliposomes of apo AI and phosphatidycholine (PC) to lymph increased ECER 10-fold, while addition of apo AI/PC/UC disks did so by only six-fold. We conclude that the low ECER in lymph is due to a property of the HDLs, seemingly substrate inhibition of LCAT by excess cell-derived UC. This is reversed when lymph enters plasma, consequent upon redistribution of UC from lymph HDLs to plasma lipoproteins.

Deficiency of the very low-density lipoprotein (VLDL) receptors in streptozotocin-induced diabetic rats: insulin dependency of the VLDL receptor.

Hyperlipidemia is a common feature of diabetes and is related to cardiovascular disease. The very low-density lipoprotein receptor (VLDL-R) is a member of the low-density lipoprotein receptor (LDL-R) family. It binds and internalizes triglyceride-rich lipoproteins with high specificity. We examined the etiology of hyperlipidemia in the insulin-deficient state. VLDL-R expression in heart and skeletal muscle were measured in rats with streptozotocin (STZ)-induced diabetes. STZ rats showed severe hyperlipidemia on d 21 and 28, with a dramatic decline in VLDL-R protein in skeletal muscle (>90%), heart (approximately 50%) and a loss of adipose tissues itself on d 28. The reduction of VLDL-R protein in skeletal muscle could not be explained simply by a decrease at the transcriptional level, because a dissociation between VLDL-R protein and mRNA expression was observed. The expression of LDL-R and LDL-R-related protein in liver showed no consistent changes. Furthermore, no effect on VLDL-triglyceride production in liver was observed in STZ rats. A decrease in postheparin plasma lipoprotein lipase activity started on d 7 and continued to d 28 at the 50% level even though severe hyperlipidemia was detected only on d 21 and 28. In rat myoblast cells, serum deprivation for 24 h induced a reduction in VLDL-R proteins. Insulin (10(-6) m), but not IGF-I (10 ng/ml), restored the decreased VLDL-R proteins by serum deprivation. These results suggest that the combination of VLDL-R deficiency and reduced plasma lipoprotein lipase activity may be responsible for severe hyperlipidemia in insulin-deficient diabetes.

Effects of intravenous apolipoprotein A-I/phosphatidylcholine discs on LCAT, PLTP, and CETP in plasma and peripheral lymph in humans.

OBJECTIVE: We have previously shown that intravenous apolipoprotein A-I/phosphatidylcholine (apoA-I/PC) discs increase plasma pre-beta HDL concentration and stimulate reverse cholesterol transport (RCT) in humans. We have now investigated the associated changes in the following 3 HDL components that play key roles in RCT: lecithin:cholesterol acyltransferase (LCAT), cholesteryl ester transfer protein (CETP), and phospholipid transfer protein (PLTP). METHODS AND RESULTS: apoA-I/PC discs (40 mg/kg over 4 hours) were infused into 8 healthy men. Samples of blood and prenodal peripheral lymph were collected for 24 to 48 hours. At 12 hours, plasma LCAT concentration had increased by 0.40+/-0.90 mg/L (+7.8%; mean+/-SD; P<0.05), plasma cholesterol esterification rate by 29.0+/-9.0 nmol/mL per h (+69.5%; P<0.01), plasma CETP concentration by 0.5+/-0.2 mg/L (+29.7%; P<0.01), and plasma PLTP activity by 1.45+/-0.67 micromol/mL per h (+23.9%; P<0.01). In contrast, plasma PLTP concentration had decreased by 4.4+/-2.7 mg/L (-44.8%; P<0.01). The changes in PLTP were accompanied by alterations in the relative proportions of large lipoproteins containing inactive PLTP and small particles containing PLTP of high specific activity. No changes were detected in peripheral lymph. CONCLUSIONS: Nascent HDL secretion may induce changes in PLTP, LCAT, and CETP that promote RCT by catalyzing pre-beta HDL production, cholesterol esterification in HDLs, and cholesteryl ester transfer from HDLs to other lipoproteins.

Human apoA-I/C-III/A-IV gene cluster transgenic rabbits: effects of a high-cholesterol diet.

We have generated transgenic rabbits that express the entire human apoA-I/C-III/A-IV gene cluster. As in humans, h-apoA-I and h-apoC-III were expressed in liver and intestine, whereas h-apoA-IV mRNA was detected in intestine only. Transgenic rabbits had significantly higher plasma total cholesterol, HDL-cholesterol and total phospholipid concentrations than non-transgenic littermates. In contrast to similar transgenic mice previously generated, which have gross hypertriglyceridemia, triglyceride concentrations were only moderately raised in transgenic rabbits. Plasma and HDL from transgenic rabbits were more effective than those from controls in promoting cholesterol efflux from cultured hepatoma cells. They had lower LCAT, lower CETP and higher PLTP activities than non-transgenic littermates. Cholesterol-feeding produced major increases in plasma lipids. The qualitative response to the diet was not modified by cluster expression. Human apoA-I concentration was halved by cholesterol-feeding, whereas h-apoC-III and h-apoA-IV concentrations were not significantly altered. Cholesterol efflux from hepatoma cells to plasma and HDL was not altered by the diet. Since lipoprotein metabolism of rabbits closely resembles that of humans, human apoA-I/C-III/A-IV transgenic rabbits may provide a reliable model for studies of the transcriptional regulation of the cluster, and for evaluating the effects of different agents on the expression of the three genes.

Variations in lipid and apolipoprotein concentrations in human leg lymph: effects of posture and physical exercise.

We studied the variations in the concentrations of cholesterol, triglycerides, phospholipids, apolipoproteins (apos) (A-I, A-II, B, C-III, E), free glycerol and albumin in human prenodal leg lymph during the 24 h cycle. Lymph was collected continuously for up to 96 h from nine healthy males on a low-fat isocaloric diet. In three free-living subjects, all lipid and apolipoprotein concentrations underwent synchronous variations, rising during the night and decreasing during the day. In three subjects who remained in supine rest for 48 h, the amplitude of circadian variation was much smaller. In three who alternated periods of supine rest with upright exercise, the highest concentrations occurred during rest. Lipid, apolipoprotein and albumin concentrations were inversely related to lymph flow rate. Free glycerol, much of which in tissue fluid is derived from local adipocytes, did not follow this pattern. On multiple regression, concentrations in lymph were related independently to the corresponding concentration in plasma (positive) and to lymph flow rate (negative) or lymph albumin concentration (positive). These results show that lipoprotein concentrations in human tissue fluid are determined only partly by their concentrations in plasma. They are also strongly affected by hemodynamic factors via their effects on fluid transport.

A novel LCAT mutation (Phe382-->Val) in a kindred with familial LCAT deficiency and defective apolipoprotein B-100.

We studied a four-generation family (17 subjects) with familial lecithin:cholesterol acyltransferase (LCAT) deficiency. A 30-year-old Caucasian male with corneal clouding and HDL cholesterol <0.1 mmol/l was a compound heterozygote for a novel mutation (Phe(382)-->Val), a previously reported mutation (Thr321-->Met) and a common variant (Thr208-->Ser) of the gene. Immunoreactive LCAT concentration (1.2 microg/ml), alpha-LCAT activity (13 nmol/ml per h) and cholesterol esterification rate (CER) (14 nmol/ml per h) in his plasma were, respectively, 14, 8 and 14% of the mean values in healthy subjects. The proband and 13 of his relatives also had familial defective apo B (FDB, Arg3500-->Gln). Six subjects had LCAT Phe382-->Val in combination with FDB. Plasma lipoprotein(a) (Lp(a)) was 24 nmol/l in the proband and 46-211 nmol/l in his father and siblings, consistent with expression of the 16 kringle 4 isoform. The proband had no signs of coronary heart disease (CHD), but his father, a paternal uncle and a female cousin had CHD before age 38 years.

Effects of intravenous apolipoprotein A-I/phosphatidylcholine discs on paraoxonase and platelet-activating factor acetylhydrolase in human plasma and tissue fluid.

We have previously shown that intravenous apolipoprotein (apo) A-I/phosphatidylcholine (apo A-I/PC) discs increase plasma high-density lipoprotein (HDL) concentration in humans. We have now studied the associated changes in two enzymes, paraoxonase (PON) and platelet-activating factor acetylhydrolase (PAF-AH) that are carried in whole or in part by HDLs, and are thought to influence atherogenesis by hydrolyzing oxidized phospholipids in lipoproteins. Apo A-I/PC discs (40 mg/kg over 4 h) were infused into eight healthy males. Although plasma apo A-I and HDL cholesterol increased on average by 178 and 158%, respectively, plasma total PON and total PAF-AH concentrations did not rise. By the end of the infusion, HDL-associated PAF-AH had increased by 0.56 +/- 0.14 microg/mL (mean +/- S.D., P < 0.01), and nonHDL-associated PAF-AH had decreased by 0.84 +/- 0.11 microg/mL (P < 0.05). These changes were accompanied by an increase in the HDL-associated PAF-AH/apo A-I ratio from 0.19 to 0.35 (P < 0.05), and by a decrease in the nonHDL-associated PAF-AH/apo B ratio from 2.1 to 1.4 (P < 0.05). No changes in PON or PAF-AH concentrations were detected in prenodal lymph (tissue fluid), collected continuously from the leg. Our results show that the total concentrations of PON and PAF-AH in plasma are uninfluenced by plasma HDL concentration. PAF-AH transfers readily between HDLs and LDLs in vivo, and its distribution between them is determined partly by their relative concentrations and partly by HDL composition.

Reconstituted high-density lipoprotein attenuates organ injury and adhesion molecule expression in a rodent model of endotoxic shock.

The salutary effects of high-density lipoproteins (HDLs) in animal and human models of endotoxic shock have in the past been attributed to the ability of this lipoprotein to bind to lipopolysaccharide. However, the precise mechanisms for the protective effect of HDL are unclear. The first objective of this study was to determine the effects of HDLs on the organ injury and dysfunction associated with acute severe endotoxemia. Second, to gain insight into the mechanism of action of HDL, we also investigated the effect of HDLs on 1) the expression of P-selectin and intercellular adhesion molecule-1 in the kidneys of rats treated with endotoxin and 2) the rise in the plasma levels of tumor necrosis factor-alpha (TNF-alpha). Rats were given Escherichia coli lipopolysaccharide (6 mg/kg i.v.), pretreated with either vehicle (n = 9) or reconstituted HDL (rHDL; apolipoprotein A-I/phosphatidylcholine proteoliposomes, n = 10), and were monitored for 6 h. Here we report that rHDL attenuates the renal injury and dysfunction caused by endotoxin in the rat. In addition, rHDL reduced the degree of histological tissue injury in the lung, liver and intestine and attenuated the expression of P-selectin and intercellular adhesion molecule-1 in the renal glomerulus. Interestingly, pretreatment of rats with rHDL did not prevent the hypotension nor the rise in plasma levels of TNF-alpha (at 90 min) caused by endotoxin. Thus, rHDL reduces the organ injury/dysfunction, but does not affect the circulatory failure, nor the rise in plasma levels of TNF-alpha caused by endotoxin in the rat. We propose that the mechanisms of these beneficial effects of HDL may be related to direct inhibition of adhesion molecule expression.

CETP inhibitors and cardiovascular disease: Time to think again.

Inhibition of cholesteryl ester transfer protein (CETP) lowers plasma low-density lipoprotein cholesterol concentration and raises high-density lipoprotein (HDL) cholesterol, suggesting it might prevent cardiovascular disease (CVD). From the outset, however, the concept has been controversial owing to uncertainty about its effects on HDL function and reverse cholesterol transport (RCT). Although there has long been good evidence that CETP inhibition reduces atherosclerosis in rabbits, the first information on CETP as a CVD risk factor in a prospectively followed cohort was not published until after the first Phase 3 trial of a CETP inhibitor had begun. The worrying finding that CVD incidence was related inversely to plasma CETP has since been reproduced in each of five further prospective cohort studies. Similar results were obtained in subjects on or off statin therapy, for first and second CVD events, and for mortality as well as CVD morbidity. Additionally, two recent studies have found alleles of the CETP gene that lower hepatic CETP secretion to be associated with an increased risk of myocardial infarction. Meanwhile, CETP gene transfer in mice was found to increase RCT from peripheral macrophages in vivo, and human plasma with high CETP activity was shown to have a greater capacity to remove cholesterol from cultured cells than plasma with low activity. This mounting evidence for a protective function of CETP has been given remarkably little attention, and indeed was not mentioned in several recent reviews.  It appears to show that CETP inhibition does not test the HDL hypothesis as originally hoped, and raises a pressing ethical issue regarding two Phase 3 trials of inhibitors, involving more than forty thousand subjects, which are currently in progress. As the weight of evidence now clearly supports an adverse effect of CETP inhibition on CVD, an urgent review is needed to determine if these trials should be discontinued.

Lymphatic transport of high-density lipoproteins and chylomicrons.

The life cycles of VLDLs and most LDLs occur within plasma. By contrast, the role of HDLs in cholesterol transport from cells requires that they readily gain access to and function within interstitial fluid. Studies of lymph derived from skin, connective tissue, and adipose tissue have demonstrated that particles as large as HDLs require transport through lymphatics to return to the bloodstream during reverse cholesterol transport. Targeting HDL for therapeutic purposes will require understanding its biology in the extravascular compartment, within the interstitium and lymph, in health and disease, and we herein review the processes that mediate the transport of HDLs and chylomicrons through the lymphatic vasculature.

HDL-targeted therapies: progress, failures and future.

Since the discovery in the 1970s that plasma levels of high-density lipoprotein cholesterol (HDL-C) are inversely associated with cardiovascular outcome, it has been postulated that HDL is anti-atherogenic and that increasing HDL-C levels is a promising therapeutic strategy. However, the recent failure of three orally active, HDL-C-raising agents has introduced considerable controversy, prompting the question of whether increasing the cholesterol cargo of HDL in a non-selective manner is an effective pharmacological approach for the translation of its atheroprotective and vasculoprotective activities. The interrelationships between HDL-C concentration, HDL particle number and levels of diverse HDL particle subpopulations of defined composition are complex, as are their relationships with reverse cholesterol transport and other anti-atherogenic functions. Such complexity highlights the incompleteness of our understanding of the biology of HDL particles. This article examines the HDL hypothesis in molecular and mechanistic terms, focusing on features that have been addressed, those that remain to be tested, and potential new targets for future pharmacological interventions.

Signaling proteins are represented in tissue fluid/lymph from soft tissues of normal human legs at concentrations different from serum.

BACKGROUND: The mobile intercellular fluid flowing to and in the lymphatics contains filtered plasma products and substances synthesized and excreted by tissue cells. Among them are signaling proteins such as cytokines, chemokines, enzymes, and growth factors. They act locally in autocrine and paracrine systems regulating cell metabolism, proliferation, and formation of the ground matrix. They play an immunoregulatory role in infections, wound healing, and tumor cell growth. METHODS AND RESULTS: In this study we measured the concentration of selected cytokines, chemokines, tissue enzymes, and growth factors in tissue fluid/lymph drained from normal human leg soft tissues. Legs exposed to infections and trauma often result in development of lymphedema. Lymph was drained from superficial calf lymphatics using microsurgical techniques. Our studies showed generally higher concentrations of cytokines, chemokines, enzymes, and growth factors in lymph than in serum. The total protein L/S ratio was 0.22, whereas that of various lymph signaling proteins ranged between 1 and 10. CONCLUSIONS: This indicates that in addition to proteins filtered from blood, local cells contribute to lymph concentration by own production, depending on the actual cell requirement. Moreover, there were major individual differences of lymph levels with simultaneous stable serum levels. This suggests existence of a local autonomous regulatory humoral mechanism in tissues, not reflected in serum.

An expanded self-antigen peptidome is carried by the human lymph as compared to the plasma.

BACKGROUND: The pre-nodal afferent lymph is the fluid which directly derives from the extracellular milieu from every parenchymal organ and, as it continues to circulate between the cells, it collects products deriving from the organ metabolism/catabolism. A comprehensive qualitative and quantitative investigation of the self-antigenic repertoire transported by the human lymph is still missing. METHODOLOGY/PRINCIPAL FINDINGS: A major difference between lymph and plasma could be visualized by FPLC and 2D gel in the amount of low molecular weight products corresponding to peptide fragments. Naturally processed peptides in normal pre-nodal human lymph were then fractionated by HPLC and characterized by multidimensional mass spectrometry. Analysis of more then 300 sequences identified self-peptides derived from both intracellular and extracellular proteins revealing the variety of catabolic products transported by human lymph. Quantitative analysis established that at least some of these peptides are present in the circulating lymph in nanomolar concentration. CONCLUSIONS/SIGNIFICANCE: The peptidome, generated by physiological tissue catabolism and transported by the pre-nodal lymph, is in addition to the self-peptidome generated in endosomal compartment. Unlike self antigen processed by local or nodal APC, which mostly produce epitopes constrained by the endosomal processing activity, self antigens present in the lymph could derived from a wider variety of processing pathways; including caspases, involved in cellular apoptosis, and ADAM and other metalloproteinases involved in surface receptor editing, cytokines processing and matrix remodeling. Altogether, expanding the tissue-specific self-repertoire available for the maintenance of immunological tolerance.

Mass kinetics of apolipoprotein A-I in interstitial fluid after administration of intravenous apolipoprotein A-I/lecithin discs in humans.

Apolipoprotein kinetics are customarily determined by modeling time curves of specific radioactivity or isotopic enrichment in plasma after intravenous infusion of radiolabeled lipoproteins or stable isotope-enriched amino acids. However, this provides no information on the fractional rate of transfer of the apolipoprotein from plasma to interstitial fluid (k(p-if)) or its mean residence time in interstitial fluid (MRT(if)). To determine these parameters for a pharmacologic dose of exogenous apolipoprotein A-I (apoA-I) given intravenously as apoA-I/lecithin discs, we measured apoA-I in plasma and prenodal leg lymph in five healthy men before, during, and after a 4 h infusion at 10 mg/kg/h. ApoA-I concentrations in plasma and lymph were modeled by linear compartmental models (SAAM II version 1.1), using lymph albumin to adjust for the effects of variations in lymph flow rate. k(p-if) averaged 0.75%/h (range, 0.33-1.32), and MRT(if) averaged 29.1 h (14.1-40.0). Neither parameter was correlated with the distribution volume (57-105 ml/kg) or the fractional elimination rate (1.44-2.91%/h) of apoA-I, determined by modeling plasma apoA-I concentration alone. Although used here to study the mass kinetics of apoA-I, if combined with infusion of a tracer, analysis of lymph could also expand the modeling of endogenous apolipoprotein kinetics.

A sandwich enzyme-linked immunosorbent assay for human plasma apolipoprotein A-V concentration.

Apolipoprotein A-V (apoA-V) is a recently discovered apolipoprotein that appears to have a role in plasma triglyceride (TG) transport. We have developed an ELISA for apoA-V using monoclonal antibodies that has a lower limit of detection of 0.3 ng/ml and linearity up to 20 ng/ml. The ELISA was then used to quantify plasma apoA-V in 196 healthy subjects and 106 patients with insulin-resistant diabetes mellitus. In the healthy subjects, total apoA-V concentration was 179.2 +/- 74.8 ng/ml, and it was greater in females than in males (P < 0.005). It was correlated positively with the plasma HDL cholesterol (r = 0.32, P < 0.0001), apoA-I (r = 0.27, P = 0.0001), and apoE (r = 0.18, P = 0.011) concentrations and negatively with plasma TG concentration (r = -0.22, P = 0.021). In relation to single nucleotide polymorphism 3 (-1131C/T) of the apoA-V gene, apoA-V concentration was higher in the T/T type than in the C/C type (P < 0.01). Plasma TG concentration was lower in the T/T type than in the C/C or C/T type (P < 0.05). ApoA-V concentration was lower in the diabetic patients (69.4 +/- 44.3 ng/ml; P < 0.01) than in the healthy controls.