Malignant Hypertension Is Accompanied by Marked Alterations in Chylomicron Metabolism
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.
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.
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, β-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 α1 and α2 are the FCR of each compartment. The mean weighted FCR was calculated by the equation
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
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⇓).
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⇓).
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 β-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
|FCR||=||fractional clearance rate|
- Received July 18, 1995.
- Revision received August 22, 1995.
- Accepted September 21, 1995.
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