(Hypertension. 1995;26:1207-1210.)
© 1995 American Heart Association, Inc.
Articles |
Malignant Hypertension Is Accompanied by Marked Alterations in Chylomicron Metabolism
From the Heart Institute of the Medical School Hospital and Faculty of Pharmaceutical Sciences, University of São Paulo (Brazil).
Correspondence to Raul C. Maranhão, MD, PhD, Instituto do Coração do Hospital das Clínicas da FMUSP, Av Dr Enéas C. Aguiar 44, 1o Andar, São Paulo, SP, 05403-000, Brazil.
 |
Abstract |
|---|
 Top Abstract IntroductionMethodsResultsDiscussionReferences |
|---|
Abstract Malignant hypertension (MH) is a severe complication of untreated arterial hypertension that damages the vascular system. It is often accompanied by disturbances in lipid metabolism that could contribute to its pathophysiology. We examined chylomicron metabolism in MH patients using a triglyceride-rich emulsion known to mimic natural chylomicrons when injected into the bloodstream. The emulsion was labeled with [3H]triolein and [14C]cholesteryl oleate and injected intravenously into 15 normolipidemic MH patients aged 29 to 56 years (8 men) for comparison with 17 healthy control subjects. Consecutive plasma samples were taken at regular intervals during 1 hour for determination of the disappearance curves of the labels. The fractional clearance rate of the [3H]triolein emulsion in MH patients was twice as small as that of control subjects (0.061±0.012 and 0.141±0.074 min-1, respectively). On the other hand, [14C]cholesteryl oleate fractional clearance rate was not statistically different in MH patients and control subjects (0.032±0.004 and 0.056±0.014 min-1, respectively). These results indicate that in MH, lipolysis (measured by the fractional clearance rate of [3H]triolein) is pronouncedly diminished, whereas the removal of the remnant particles (measured by the fractional clearance rate of [14C]cholesteryl oleate) is not importantly affected. In conclusion, there is an alteration in the circulatory transport of dietary lipids that may be an important component in the vascular disease associated with MH.
Key Words: hypertension, malignant • chylomicron • emulsions
|
Introduction |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
The disease of MH leads to a shorter life expectancy, mainly because it elicits marked alterations in the vascular system. This condition is often accompanied by disturbances in lipid metabolism that could also contribute to vessel damage.1 2 Hypertriglyceridemia due to the accumulation of VLDL in the plasma is the characteristic feature of hyperlipidemia secondary to MH.3
VLDL is the triglyceride-rich lipoprotein synthesized by the liver. The intestine synthesizes another triglyceride-rich protein, the chylomicron, from absorbed dietary fat and cholesterol. The products of the intravascular degradation of chylomicrons, the chylomicron remnants, are considered atherogenic lipoproteins.4 5 However, for a number of reasons the metabolism of chylomicrons and their remnants is difficult to assess in human subjects and to date has not been evaluated in MH.
In the current investigation, aiming to verify whether chylomicron metabolism is altered in MH, we determined the plasma kinetics of a triglyceride-rich emulsion in 15 MH patients with plasma lipids and apolipoproteins within the normal range and in 17 control subjects. In several previous studies in experimental animals6 7 and human subjects,8 the lipid emulsion was shown to behave like chylomicrons obtained from the lymph. The injected emulsion was labeled with [3H]triolein and [14C]cholesteryl oleate, and the disappearance of the radioactive lipids from the plasma was determined from analysis of plasma samples collected at regular intervals during 60 minutes. The results showed that chylomicron metabolism is pronouncedly altered in MH patients.
|
Methods |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Study Subjects
The control group included 17 normotensive, normolipidemic healthy subjects (mean blood pressure, <130/90 mm Hg; serum creatinine, <1.5 mg/dL; total cholesterol, <6.2 mmol/L; triglycerides, <6.5 mmol/L; LDL <3.36 mmol/L; HDL >0.91 mmol/L; apoA-1 <2.10 g/L; apoB <1.60 g/L). Thirteen were men and 4 were women (15 whites, 2 blacks) aged 23 to 63 years (mean, 45.5±11.7 years). Control subjects did not have any history of coronary artery disease; diabetes; hyperuricemia; obesity; thyroid, gastrointestinal, kidney, or liver diseases; or history of excessive alcohol drinking. The MH group included 15 patients (total cholesterol, <6.2 mmol/L; triglycerides, <6.5 mmol/L) who had a diagnosis of MH based on the World Health Organization criteria, that is, the presence of retinal exudates and hemorrhages with or without papilledema. Eight were men and 7 were women (13 whites, 2 blacks) aged 24 to 61 years (mean, 40.8±8.2 years). Mean blood pressure at presentation was 233±52/160±35 mm Hg, and serum creatinine was 2.2±2.1 mg/dL. The patients received three or four drugs for adequate control of blood pressure, most commonly diuretics, ß-adrenergic blockers, and angiotensin-converting enzyme inhibitors (Table 1).
|
Lipid and Apolipoprotein Analysis
Total cholesterol and
triglycerides were determined with the aid of enzymatic
test kits (CHOD-PAP [Boehringer] and Abbott, respectively)
from samples taken after a 12-hour fast. ApoA-1 and apoB were assayed
by radial immunodiffusion (Boehringer).
Emulsion Preparation and Kinetic Studies
The chylomicron-like emulsion was prepared by ultrasonic
irradiation of a mixture of lipids composed of 69% triolein, 6%
cholesteryl oleate, 23% egg lecithin, and 2% cholesterol
in an aqueous medium, with added 1-[14C]cholesteryl
oleate and [3H]triolein, followed by
ultracentrifugation in saline density gradients as
described previously9 and was sterilized by passage
through a 0.2-µm filter. Three to five milligrams of the emulsion
total lipid in a volume of approximately 100 to 200 µL with 2 µCi
of the 14C and 4 µCi of the 3H label were
injected intravenously in a bolus, and plasma samples were
collected at preestablished intervals during 60 minutes. Radioactivity
in aliquots (1 mL) of plasma was measured in a scintillation solution
(PPO [5 g]/DM-POPOP [0.5 g]/Triton X-100 [333 mL]/toluene [667
mL]) with a spectrometer (Packard 1660 TR).
Decay Curve Calculations
The experimental curves of the plasma decay of the
[14C]cholesteryl oleate and [3H]triolein
radioactivity showed a biexponential aspect. The first exponential
represents the process of rapid removal of the labeled lipids
from the plasma, and the second exponential was associated with the
delayed removal process. The results concerning the radioactivity of
the emulsion after injection into the bloodstream were subjected to
bicompartmental analysis.10 Fitting was carried
out based on the least-squares error criterion with the use of the
function
 |
The initial concentration of the radioactivity in the plasma at
t=0 is represented by the sum
a1+a2, whereas
1 and 2 are the FCR of each compartment.
The mean weighted FCR was calculated by the equation
 |
Statistics
The Mann-Whitney U test was used to analyze differences
between medians of the emulsion FCR values and for
triglyceride plasma levels. Student's t test
was used for comparisons of the other lipids and apolipoprotein plasma
levels.11
|
Results |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Plasma Lipids and Apolipoproteins
Plasma concentrations of triglycerides and of total, LDL, VLDL, and HDL cholesterol as well as apoA-1 and apoB were not statistically different between MH patients and control subjects (Table 2).
|
Emulsion Kinetics
In all the studied subjects the disappearance curve of the
emulsion triglycerides was faster than that of cholesteryl
ester. Fig 1B shows the plasma decaying curves of the
emulsion radioactive triglycerides obtained from the MH and
control groups. In the MH patients the triglyceride
disappearance curve was pronouncedly slowed compared with the control
group; FCR was reduced approximately 2.3 times (MH patients,
0.061±0.012 min-1; control subjects,
0.141±0.074 min-1; P<.05;
Fig 2). The [14C]cholesteryl oleate
disappearance curve of the MH patients tended to be slowed in relation
to that of the control subjects (Fig 1A), but
[14C]cholesteryl oleate FCR was not significantly
diminished (MH patients, 0.032±0.004
min-1; control subjects, 0.056±0.014
min-1; Fig 2).
|
|
|
Discussion |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Chylomicrons are secreted into the lymph, and on entry into the blood circulation they promptly attach to LPL, an enzyme bound mostly to the capillary endothelium of muscle and adipose tissue. LPL breaks down most of the chylomicron triglycerides. ApoC-II present on the lipoprotein surface binds chylomicron particles to LPL and is also a cofactor for LPL action. Chylomicron remnants resulting from LPL action detach from the capillary wall and are rapidly taken up by liver receptors using apoE as the ligand.12
The clinical evaluation of the chylomicron pathway in humans is difficult because of some physical and metabolic features of those lipoproteins and because of the process of lipid absorption by the intestine. Lipidic emulsions devoid of protein and mimicking the structure and basic composition of chylomicrons behave like those lipoproteins when injected into the plasma of rats6 7 and human subjects,8 thus they are a useful tool for the study of chylomicron metabolism. In contact with plasma, the emulsions immediately acquire apolipoproteins from either the native lipoproteins or free, unassociated apolipoproteins.6 Then, the emulsion particles become ready to undergo the intravascular catabolic processes suffered by chylomicrons. Because cholesteryl ester is in the most part preserved into the core of both native chylomicrons and chylomicron-like emulsions during intravascular catabolism, its plasma clearance is characteristically smaller than that of the triglyceride moiety, as also observed in the current study. Therefore, [14C]cholesteryl oleate clearance represents the removal from plasma of the emulsion particles, whereas [3H]triolein clearance reflects chiefly the lipolysis process. When the emulsion test was performed in normolipidemic patients with coronary artery disease, delayed lipolysis and removal from the plasma of the emulsion remnants were observed compared with subjects without the disease.8 Similar trends were observed when oral fat load tests were carried out in coronary artery disease patients as reported in several studies.13 14 15 In those studies coronary artery disease patients showed increased triglyceride and retinyl palmitate levels after the ingestion of a standard fatty meal. This also indicates delayed processes of chylomicron removal.
MH is the most severe form of arterial hypertension, resulting in extreme pathological and clinical consequences.1 2 MH requires treatment regimens in which the largest number and doses of antihypertensive drugs are used. It is therefore worthwhile to point out that regardless of their being in this condition, the participants of the present study had normal plasma lipid and apolipoprotein profiles. This indicates that in the cases included in this study, the metabolism of the endogenous lipoproteins, ie, VLDL, LDL, and HDL, was not substantially affected by the disease and its therapy. Nonetheless, the results of the plasma kinetic study show that the metabolism of chylomicrons was independently altered. The process of degradation of the emulsion [3H]triolein was pronouncedly affected, as shown by the more than twofold decrease in [3H]triolein FCR. This occurred despite our subjects having normal fasting triglyceride levels and indicates that the emulsion and VLDL have independent metabolic pathways. Possibly because of diminished lipolysis, the removal of the emulsion particles tended to be retarded, as shown by the decaying curves and [14C]cholesteryl oleate FCR values, although this was not statistically significant. It is known that when lipolysis is diminished, the removal of the particles from the plasma is also diminished because lipolysis is a prerequisite for the receptors to take up the particles: nondegraded chylomicrons16 or chylomicron-like emulsions17 are not efficiently trapped by the liver.
An unanswered question remains in this study: Were the observed effects on chylomicron metabolism consequent to MH or the antihypertensive drugs administered to the patients? Because it is impossible for MH patients to discontinue therapy, it was difficult to devise a protocol to clarify this issue. Antihypertensive agents, especially diuretics and ß-blockers, have unfavorable effects on serum lipids, including elevation of triglycerides and total cholesterol as well as a reduction of HDL cholesterol.18 19 20 21 Therefore, the use of these drugs could have contributed to the alterations in the emulsion kinetics observed.
As mentioned above, a number of studies performed in subjects suggest that diminished chylomicron lipolysis and remnant removal from the plasma are associated with coronary artery disease.4 5 Those data confirm the former hypothesis that chylomicrons and their remnants are importantly involved in atherogenesis. By showing altered chylomicron metabolism in MH, our results may contribute to the understanding of the complex mechanisms by which the hypertensive state may damage the vasculature.
|
Selected Abbreviations and Acronyms |
|---|
|
Received July 18, 1995; first decision August 22, 1995; accepted September 21, 1995.
|
References |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
1. Pickering G. High Blood Pressure. 2nd ed. London, UK: Churchill; 1968.
2. Bernardes-Silva H, Bortolotto LA, Giorgi DMA, Frimm CC, Giorgi CP, Bellotti G, Pileggi F. Ventricular function by radionuclide ventriculography in malignant hypertension. Hypertension. 1992;19(suppl II):II-210-II-213.
3. Catalano M, Aronica A, Carzaniga G, Sergni R, Libretti A. Serum lipids and lipoproteins in patients with essential hypertension. Atherosclerosis.. 1991;87:17-22. [Medline] [Order article via Infotrieve]
4.
Zilversmit DB. Atherosclerosis:
a post-prandial phenomenon.
Circulation. 1979;60:473-485.
5. Hazzard WR, Bierman EL. Delayed clearance of chylomicron remnants following vitamin A-containing oral fat load in broad-B disease (type III hypercholesterolemia). Metab Clin Exp. 1976;25:777-801.
6. Redgrave TG, Maranhão RC. Metabolism of protein-free lipid emulsion models of chylomicrons in rats. Biochim Biophys Acta.. 1985;835:104-112. [Medline] [Order article via Infotrieve]
7. Maranhão RC, Tercyak AM, Redgrave TG. Effects of cholesterol content on the metabolism of protein-free emulsion models of lipoproteins. Biochim Biophys Acta.. 1986;836:247-255.
8. Martins MCT, Pileggi F, Maranhão RC. Clearance of a chylomicron-like emulsion from plasma is delayed in patients with coronary artery disease. Braz J Med Biol Res.. 1995;25:47-52.
9. Oliveira HCF, Hirata MH, Redgrave TG, Maranhão RC. Competition between chylomicrons and their remnants for plasma removal. Biochim Biophys Acta.. 1988;958:211-217. [Medline] [Order article via Infotrieve]
10. Matthews CME. The theory of tracer experiments with I-labeled plasma proteins. Phys Med Biol.. 1957;2:36-45. [Medline] [Order article via Infotrieve]
11. Snedecor GW, Cochran WG. Statistical Methods. 6th ed. Ames, Iowa: Iowa State University Press; 1967.
12. Mahley RW, Hui DY, Innerarity TL, Beisiegel U. Chylomicron remnant metabolism: role of hepatic lipoprotein receptors in mediating uptake. Arteriosclerosis. 1989;9(suppl I):I-1-I-18.
13.
Groot PHE, van Stiphout WAHJ, Krauss XH, Jansen J, van
Tol E, van Ramshorst E, Chin-On S, Hofman A, Cresswell SR.
Postprandial lipoprotein metabolism in normolipidemic men
with and without coronary artery disease.
Arterioscler Thromb.. 1991;11:653-662.
14.
Patsch JR, Miesenböck G, Hopferwieser T,
Mühlberger V, Knapp E, Dunn JK, Gotto AM Jr, Patsch W.
Relation of triglyceride metabolism and
coronary artery disease: studies in the postprandial
state. Arterioscler Thromb.. 1992;12:1336-1345.
15. Simons LA, Dwyer T, Simons J, Bernstein L, Mock P, Poonia NS, Balasubramaniam S, Bron D, Branson J, Morgan J, Roy P. Chylomicrons and chylomicron remnants in coronary artery disease: a case-control study. Atherosclerosis.. 1987;65:181-189. [Medline] [Order article via Infotrieve]
16. Borensztajn J, Getz GS, Kotlar TJ. Uptake of chylomicron remnants by the liver: further evidence of the modulating role of phospholipids. J Lipid Res. 1988;29:1087-1096. [Abstract]
17. Maranhão RC, Roland IA, Hirata MH. Effects of Triton WR 1339 and heparin on the transfer of surface lipids from triglyceride-rich emulsions to high density lipoproteins in rats. Lipids.. 1990;25:701-705. [Medline] [Order article via Infotrieve]
18.
Ladinois CK, Neuman SL. The effects of
antihypertensive agents on serum lipids and lipoproteins.
Arch Intern Med. 1988;148:1280-1288.
19.
Vyssoulis GP, Karpanou EA, Pitsavos CE, Skoumas JN,
Paleologs AA, Toutouzas PK. Differentiation of beta-blocker
effects on serum lipids and apolipoproteins in hypertensive patients
with normolipidaemic or dyslipidaemic profiles. Eur Heart
J. 1992;13:1506-1513.
20. Weinberger MH. Mechanisms of diuretic effects on carbohydrate tolerance, insulin sensitivity and lipid levels. Eur Heart J. 1992;13(suppl G):5-9.
21.
Mäntäri M, Tenkanen L, Manninen V, Alikoski
T, Frick MH. Antihypertensive therapy in
dyslipidemic men: effects on coronary heart disease
incidence and total mortality. Hypertension.. 1995;25:47-52.
This article has been cited by other articles:
 |
|
 |
|
  T. G. Redgrave, G. F. Watts, I. J. Martins, P. H. R. Barrett, J. C. L. Mamo, S. B. Dimmitt, and A. D. Marais Chylomicron remnant metabolism in familial dyslipidemias studied with a remnant-like emulsion breath test J. Lipid Res., May 1, 2001; 42(5): 710 - 715. [Abstract] [Full Text]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||