Geriatric-Cardiology Conference
75
LIPID METABOLISM AND ISCHAEMIC HEART DISEASE IN THE ELDERLY JENNIFER HORSEY AND BRIAN LIVESLEY
Clinical Gerontology Unit, St Francis' Hospital, London SE 22
While the pathogenesis of ischaemic heart disease remains controversial, several 'risk factors' have been identified in association with the incidence of this disease in populations. These include stress, hypertension, physical inactivity, diet, cigarette smoking, and raised serum lipid levels. Interest in hyperlipidaemia as a major risk factor has grown rapidly during the past ten years; the lipids of particular clinical importance being cholesterol and triglyceride. An association between serum cholesterol levels and the clinical effects of atherosclerosis has been demonstrated in most epidemiological studies. Moreover, the investigation of patients with arteriographically proven obstructive coronary disease has shown a high incidence of lipid abnormalities (Falsetti et al., 1968; Heinle et al., 1969; Barboriak et al., 1974). In Framingham, Massachusetts (Kannel et al., 1971), the risk of developing ischaemic heart disease was examined prospectively in more than 5000 men and women during a 14-year period. This demonstrated an increased risk proportional to antecedent cholesterol levels. In a nine-year follow-up of over 3000 men in the Stockholm prospective study (Carlson & Bottiger, 1972) elevation of plasma cholesterol and elevation of triglyceride levels were both found to be risk factors for ischaemic heart disease, with an effect independent of each other. The risk was increased further when both lipid levels were elevated. Cholesterol, and particularly its ester, is the predominant lipid to accumulate in atherosclerotic lesions, while triglyceride is only a minor component. It is possible that some protection is offered against triglyceride accumulation within the plaque by the presence of an enzyme, endothelial lipase, which retains activity in old age (Adams et al., 1969). At present, it is controversial as to whether atheromatous cholesterol is metabolically inert, or accessible for exchange with plasma cholesterol. Animal-feeding experiments
Downloaded from http://ageing.oxfordjournals.org/ at University of California, San Diego on December 28, 2015
tenance digoxin and analysed in relation to digoxin dose and renal function. In patients with normal renal function (serum creatinine less than 2 mg/100 ml, or serum urea less than 70 mg/100 ml), daily doses of 0.25 mg and 0.125 mg, but not 0.0625 mg of digoxin result in the majority of serum levels falling within the therapeutic range. When renal function is impaired, the higher doses give rise to serum levels in the toxic range, but not 0.0625 mg. In the steady state, with biliary clearance of digoxin around a mean of 18 ml/min, there is little likelihood of serum digoxin levels associated with toxicity being attained until creatinine clearance is grossly impaired, provided that the daily digoxin dosage does not exceed 0.25 mg.
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have not provided certain evidence for regression of atheroma. However, a study of 13 patients with obstructive atherosclerosis (Jagannathan et al., 1974) has demonstrated a definite, though slow, exchange of isotope-labelled cholesterol between the plasma and severely atherosclerotic arteries. If this mobility of atheromatous cholesterol is confirmed, then regression of atheroma in man, with its obvious therapeutic implications, becomes a possibility. NORMAL LIPID METABOLISM
Cholesterol is a structural element of cell membranes, and a precursor of bile acids and steroid hormones. Dietary cholesterol is absorbed in the jejunum in association with chylomicrons, and carried via the lymph to the liver, from where, like cholesterol synthesized endogenously, it is released mainly in the form of a-and /3-lipoprotetns. Almost all animal tissues can synthesize cholesterol from acetyl coenzyme A, but over 90 per cent of the total in the body is supplied by the liver and intestinal wall (Myant, 1973). The normal human body contains about 2 g cholesterol per kg body weight Fluctuations in the body total are minimized by regulatory mechanisms which differ between animal species; in man, increased absorption of cholesterol is followed by a rise in its faecal elimination from the exchangeable pool. Dietary cholesterol has, therefore, little immediate effect on plasma concentrations.
Uver Cholesterol —*-bile acids
-»~Gall bladder
> Fat absorption
Faeces M 2% d a i l y 1 lleum Jejunum Duodenum Fig. 1. The enterohepatic circulation of bile acids.
The liver is unique in its ability to degrade cholesterol to bile acids. These are released in the upper intestine in response to a fatty meal, and reabsorbed mainly in the ileum by active transport, returning to the liver via the portal vein (Fig. 1). About 95-98 per cent of the total circulating bile acids are reabsorbed daily, and only small quantities are eliminated in the faeces. In the steady state, bile-acid synthesis equals loss, since bile salts returning to the liver repress further degradation of cholesterol. However, interruption of the enterohepatic circulation stimulates synthesis of bile acids, and exploitation of this phenomenon forms the basis for some treatments of hypercholesterolaemia.
Downloaded from http://ageing.oxfordjournals.org/ at University of California, San Diego on December 28, 2015
The regulation of plasma cholesterol
Geriatric-Cardiology Conference
77
FACTORS AFFECTING PLASMA TRIGLYCERIDE LEVELS
TRANSPORT OF LIPIDS AS LIPOPROTEINS AND THEIR IDENTIFICATION
Lipids cannot circulate freely in aqueous solution unless they interact with proteins. The resulting polar complexes are termed lipoproteins. Four major classes of normal lipoprotein have been identified, each of which contains triglyceride, free and esterified cholesterol, phospholipid and protein in specific proportions (Fig. 2). The major lipid carried by a- and /?-lipoproteins is cholesterol, while triglyceride is the main constituent of pre-/?-lipoproteins and chylomicrons.
Downloaded from http://ageing.oxfordjournals.org/ at University of California, San Diego on December 28, 2015
Plasma triglyceride concentration varies with diet. If the diet contains little or no fat, lipid concentrations tend to be constant throughout the day. Fat ingestion, however, produces a rise in triglyceride concentration, although levels of cholesterol and phospholipids remain relatively constant. Transport of triglyceride in blood depends on whether its immediate source is endogenous, or from dietary fat. Dietary triglyceride is digested and absorbed mainly as unesterified fatty acids and monoglycerides, and is resynthesized in the intestinal cell, from where it is released in the lymph as chylomicrons. The plasma concentration of chylomicrons varies with the timing of fat ingestion, but influx into the plasma is balanced by the rate of removal by the tissues. Isotopic studies have shown that plasma chylomicrons have a short half-life of 5-15 min. When no fat is ingested, plasma glycerides are exclusively of endogenous origin and are transported predominantly on pre-^lipoproteins produced in the liver. In the fasting state, fatty acids are mobilized from adipose tissue and are carried in the blood, bound to albumin, in unesterified form. Some of these are directly oxidized for energy by tissues such as muscle; a proportion enters the liver, where fatty acids in excess of energy requirements are re-esterified to triglyceride and released as pre-/J-lipoproteins. Dietary carbohydrate is a second source of endogenous triglyceride production. This lipogenesis occurs both in adipose tissue, where the fatty acids formed are directly esterified and stored as triglyceride, and in the liver, from where, after esterification, triglyceride is released as lipoprotein. It is important to note that this hepatic release is continuous, and, except during absorption of dietary fat, the liver is the main source of triglyceride in the blood. Triglyceride removal from the blood stream normally balances the rate of entry. This involves hydrolysis of the triglyceride moeity by lipoprotein lipase, or 'clearing factor' lipase. The site of action of this enzyme appears to be at the endothelial surface of the extrahepatic capillaries, and the uptake of triglyceride fatty acids by a particular tissue thus depends upon the activity of clearing-^factor lipase in its capillary walls. For example, in a state of calorie excess, a proportion of triglyceride fatty acid is taken up by adipose tissue, whereas, in the fasting state, fatty acids are mobilized from adipose tissue and are directly oxidized for energy by other tissues. Under these circumstances, pre^3-triglycerides are directed away from adipose tissue. It is probable that, in simple terms, these changes in the direction of metabolism are achieved through specific alterations in clearing-factor lipase activity, whose effects appear to be under hormonal control. Insulin, for example, favours an increase in clearing-factor lipase in adipose tissue, while others, such as catecholamines, ACTH and glucagon, inhibit this rise in activity.
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Chylomicron
Cholesterol
Phospholipid
Triglyceride
Fig. 2. The relative proportions of lipids in the lipoprotein particles.
A third classification is possible using the electrophoretic mobility of lipoproteins, determined by the charges of their protein component (Fig. 3). HDL tend to migrate with the a-globulins, producing an a-lipoprotein band. LDL migrate with the /J-globulins and form a ^3-lipoprotein band. The VLDL migrate just in front of the /?-lipoproteins, hence the pre-/3 nomenclature. Chylomicrons remain at the origin during electrophoresis on paper and agarose gel.
I
Origin
)3
Pre-/3
a
Fig. 3. A normal electrophoretic lipoprotein pattern.
Downloaded from http://ageing.oxfordjournals.org/ at University of California, San Diego on December 28, 2015
The classification of the complexes is based upon their separation utilizing their differing physical properties. They can be separated by density, particle size or their electrophoretic mobility. Lipoprotein isolation by density depends upon the proportion of lipid to protein in the molecule, and this determines their rate of flotation (Sf) in the ultracentrifuge. The four main fractions are the chylomicrons, the very low density lipoproteins (VLDL), the low density lipoproteins (LDL) and the high density lipoproteins (HDL). Classification by particle size is based on the nephelometric procedure of Stone & Thorp (1966). This separates the lipoproteins into only three groups—small (S) corresponding to the HDL and LDL particles; medium (M), and large (L) corresponding to the VLDL and chylomicrons, respectively. The reliability of this method as a diagnostic procedure, however, is questionable (Cramp, 1973).
Geriatric-Cardiology Conference
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HYPERLIPIDAEMIA
It is difficult to give a precise definition of hyperlipidaemia since, at present, the plasma levels of cholesterol and triglyceride which can be considered safe are not known. Indeed, there are no well-established average values for different age groups, particularly for the very young and for the elderly. Lipid levels vary widely between populations and one approach to defining normal
Wi
2a
2b
IS;
3
Fig. 4. Electrophoretic patterns of Types 1-5 hyperlipoproteinaemias compared with normal (N)-
Downloaded from http://ageing.oxfordjournals.org/ at University of California, San Diego on December 28, 2015
Separation in the ultracentrifuge is the reference method of choice, because quantitative values for each fraction are obtainable. However, the equipment is expensive and beyond the scope of many laboratories. Excellent separation of the fractions can be obtained with agarose gel electrophoresis (Noble, 1968). This provides only a qualitative identification which may be difficult to interpret unless serum cholesterol and triglyceride values are simultaneously measured. Three different protein components, or apoproteins, of the lipoproteins have been identified. These may be designated A, B and C apoproteins (Fredrickson et al., 1967). They differ in their terminal residues, total amino acid content, and immuno-chemical behaviour. The A apoprotein is normally the only protein constituent of the a-(highdensity) lipoprotein, and the B apoprotein that of the /?-(low-density) lipoprotein. The pre-^S-(very-low-density) lipoproteins and chylomicrons contain a mixture of apoproteins. It is probable that the a- and /?- lipoproteins in plasma originate from the pre-j3-lipoproteins and chylomicrons after they have disposed of their triglyceride load.
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MODIFIED FREDRICKSON CLASSIFICATION
It should be remembered that, after an overnight fast, chylomicrons will be absent from normal serum. The Type 1 pattern is usually found in children, and is distinguished by the presence of fasting chylomicronaemia. In these subjects, the plasma or serum has a supernatant 'cream layer' after overnight storage at 4°C. In Type 2a, cholesterol and hence the jS-lipoproteins are increased, while pre-/?lipoprotein levels are normal. The plasma is clear. In Type 2b hyperlipoproteinaemia, both cholesterol and triglyceride levels are elevated so that the /3- and pre-jS-lipoprotein fractions are increased. The plasma may be clear or slightly turbid. Type 3 is a rare disorder, and is characterized by an abnormal /3-lipoprotein, which contains an unusually high ratio of triglyceride, and floats with the pre^S fraction in the ultracentrifuge—hence the term 'floating j3' lipoprotein. Both cholesterol and triglyceride values are increased. The diagnosis of this disorder depends upon the discovery of a lipoprotein of /? electrophoretdc mobility in the pre-/J density fraction. Type 4 is indicated by an increase of pre-j3-lipoproteins and serum triglyceride while cholesterol levels and the j3 fraction are normal, or slightly elevated. The plasma is turbid. The Type 5 pattern is a variation of Type 4 with the addition of fasting chylomicronaemia. It is distinguished from Type 1 in that it usually occurs in adults. Furthermore, the pre-/? fraction is always elevated in Type 5, but may be normal, increased or decreased in Type 1. Types 2a, 2b, 3 and 4 are recognized as being associated with an increased risk of ischaemic heart disease.
Fredrickson's classification has been widely accepted and has contributed to the understanding of hyperlipidaemia. However, it should be remembered that it is an arbitrary classification based on genetic disorders and the usefulness of lipoprotein phenotyping
Downloaded from http://ageing.oxfordjournals.org/ at University of California, San Diego on December 28, 2015
limits has been to compare lipid levels in a community with 'high risk' for ischaemic heart disease with those in a community with 'low risk' (Keys, 1966). Another difficulty in interpreting the significance of lipid levels is that normal ranges differ according to the various standard laboratory procedures which are used to determine cholesterol and triglyceride concentrations. To assist in solving this problem Wynder & Hill (1972), circulated a questionnaire to established investigators asking what should be accepted as optimum, or 'normal' lipid levels. They concluded that presently accepted values (generally 250 mg/100 ml for cholesterol, and 150 mg/100 ml for triglyceride) were too high, and that, in men of 50 years of age, upper limits should be 185/100 ml for cholesterol and 100 mg/100 ml for fasting triglyceride. As an alternative to defining lipid abnormalities in terms of raised cholesterol and triglyceride levels, Frederickson and colleagues (1967) using paper electrophoresis, devised a convenient system of lipoprotein phenotyping which was originally intended to define inherited lipid disorders. The five types of hyperlipoproteinaemia classified by Fredrickson were subsequently revised by a WHO committee (Beaumont et al., 1970) who subdivided the Type 2 disorder into Types 2a and 2b (Fig. 4).
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Table. Some causes of secondary hyperlipidaemia Diabetes
Obstructive liver disease
Gout Alcoholism Chronic renal failure Nephrotic syndrome Hypothyroidism
Cushing's syndrome Addison's disease Glycogen storage disease Idiopathic hypercalcaemia
LIPID LEVELS m THE ELDERLY
Despite the correlation between serum lipid levels and cardiovascular disease documented by most authors, there is a diversity of opinion regarding the presence of this correlation in the elderly. However, few population studies have included subjects over the age of 65. Brown & Ritzmann (1967) examined 233 hospitalized male patients over 65 years of age in an attempt to correlate factors associated with an absence of ischaemic heart disease. Mean serum cholesterol levels were significantly higher in the 100 patients with known ischaemic heart disease than in those without (212 mg/100 ml and 181 mg/100 ml respectively). Furthermore, evidence of degenerative peripheral arterial disease was four times more common in the patients with IHD. This suggests that similar pathological processes are responsible for both conditions. Vavrik and colleagues (1974a, b) found a statistically significant positive correlation between the prevalence of atherosclerotic cardiovascular disease and cholesterol concentration. A frequency distribution curve showed that two-thirds of the subjects had serum cholesterol in the range 170-250 mg/100 ml. However, in the individual elderly patient a low cholesterol level, i.e. below 170 mg/100 ml, did not exclude the presence of atherosclerosis. Unfortunately, triglycerides and lipoproteins were not examined. Other reports have produced contrary evidence. The results of the Stockholm prospective study (Carlson & BSttiger, 1972) did not show a linear relationship between the rate of ischaemic heart disease and initial cholesterol and triglyceride values in the group over 60 years of age. Similarly, Schadel et al. (1971) in an investigation of over 600 elderly subjects, concluded that there was no definite relationship between serum cholesterol levels and atherosclerotic cardiovascular disease.
Downloaded from http://ageing.oxfordjournals.org/ at University of California, San Diego on December 28, 2015
in clinical practice remains debatable. This is particularly important since although a familial trait of hyperlipoproteinaemia is found in approximately one-third of patients with premature ischaemic heart disease (Clinical review, 1974) relatives of a patient with, for example, a Type 2b disorder may show Type 2a, 4 or 5 lipoprotein patterns. This indicates that there is only a limited relationship between the phenotype and the aetiology of the disorder. Furthermore, it is not certain how environmental factors contribute to Types 2a, 2b, 4 and 5, since a subject's phenotype can change in response to weight reduction, therapeutic diets and drug therapy. It is likely that more than one aetiological factor can produce a particular lipoprotein pattern. It is important to consider whether the hyperlipidaemia is a primary or a secondary disorder since the lipid abnormality may respond to treatment of the underlying disease (Chait, 1973). There is no specific lipoprotein pattern for any one of the disorders associated with secondary hyperlipidaemia (Table), and the pattern can vary even in the same patient.
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COMMENT
There is a need to establish a normal lipid range for all age groups, if the relationship between lipid abnormalities and the development of ischaemic heart disease is to be fully understood. It is possible that hyperlipidaemia has its most important role in the pathogenesis of obstructive coronary arterial lesions in the initial stages of the disease, and that later, when the condition is clinically apparent, the relationship is less obvious. Current evidence points to a drop in hpid levels after the age of 60 years in men, which may be a result of changes in metabolism owing to ageing processes, environmental factors such as dietary modification, or to the earlier deaths of those with severe hyperlipidaemia. However, since two-thirds of the total deaths registered as being due to ischaemic heart disease occur in the over-65s, what are the pathogenetic mechanisms leading to these events, and what is the role which can be ascribed to abnormal lipid metabolism? REFERENCES
C. W. M., BAYLIS, O. B., ABDULLA, Y. H., MAHLER, F. R. & ROTT, M. A. (1969) Lipase, esterase and triglyceride in the ageing human aorta. J. Atheroscler. Res. 9, 87-102.
ADAMS,
BARBORIAK, J. J., RIMM, A. A., ANDERSON, A. J., TRISTANI, F. E., WALKER, J. A. & FLEMMER, R. J.
(1974) Coronary artery occlusion and blood lipids. Am. Hearty. 87, 716-21. BEAUMONT, J. L., CARLSON, L. A., COOPER, G. R., FEJFAR, Z., FRBDRICKSON, D. S. & STRASSER, T.
(1970) Classification of hyperlipidaemias and hyperlipoproteinaemias. Bull. World Health Organ. 43, 891-915. BROWN, R. C. & RTTZMANN, L. (1967) Some factors associated with absence of coronary heart disease in persons aged 65 or older. J. Am. Geriatr. Soc. 15, 239—49. CARLSON, L. A. & BOTTIGER, L. E. (1972) Ischaemic heart disease in relation to fasting values of plasma triglyceride and cholesterol. Stockholm Prospective Study. Lancet i, 865—8.
Downloaded from http://ageing.oxfordjournals.org/ at University of California, San Diego on December 28, 2015
Wood et al. (1972) studied the prevalence of lipid and lipoprotein abnormalities in a Califomian population between the ages of 25 and 79 years. In men, mean triglyceride levels reached a peak in the sixth decade after which they fell. Similarly, mean cholesterol levels increased with age until the seventh decade and then decreased. No such peaks were observed in the women studied, and mean lipid levels continued to rise with age. This fall in mean cholesterol concentration in the eighth decade in men has been ascribed to the selective removal from the population of those with the highest cholesterol levels, which phenomenon, apparently, does not occur in women. Similarly, higher glyceride concentrations in men of all ages up to 59 years suggest that the predominant incidence of IHD in men may be related to the higher glyceride concentrations present when atherosclerosis is developing, and that the drop in mean levels after the age of 60 years may be the result of earlier deaths of those men with the highest levels. In the same study, lipoprotein electrophoresis showed that a-lipoproteins tended to be higher in women than in men, suggesting the possibility that a-lipoprotein cholesterol is less atherogenic than ^-lipoprotein cholesterol, to account for these sex differences. The effect of ageing upon the serum lipids of normal subjects was investigated by Lavietes et al. (1973) who followed-up 28 subjects over a 30-year period. No significant change was found between mean cholesterol and triglyceride levels in weight-stable men at the end of the study; but the five men who had gained over ten pounds in weight showed a significant rise in serum cholesterol. The twelve women showed increases in cholesterol and triglycerides regardless of weight changes.
75
LIPID METABOLISM AND ISCHAEMIC HEART DISEASE IN THE ELDERLY JENNIFER HORSEY AND BRIAN LIVESLEY
Clinical Gerontology Unit, St Francis' Hospital, London SE 22
While the pathogenesis of ischaemic heart disease remains controversial, several 'risk factors' have been identified in association with the incidence of this disease in populations. These include stress, hypertension, physical inactivity, diet, cigarette smoking, and raised serum lipid levels. Interest in hyperlipidaemia as a major risk factor has grown rapidly during the past ten years; the lipids of particular clinical importance being cholesterol and triglyceride. An association between serum cholesterol levels and the clinical effects of atherosclerosis has been demonstrated in most epidemiological studies. Moreover, the investigation of patients with arteriographically proven obstructive coronary disease has shown a high incidence of lipid abnormalities (Falsetti et al., 1968; Heinle et al., 1969; Barboriak et al., 1974). In Framingham, Massachusetts (Kannel et al., 1971), the risk of developing ischaemic heart disease was examined prospectively in more than 5000 men and women during a 14-year period. This demonstrated an increased risk proportional to antecedent cholesterol levels. In a nine-year follow-up of over 3000 men in the Stockholm prospective study (Carlson & Bottiger, 1972) elevation of plasma cholesterol and elevation of triglyceride levels were both found to be risk factors for ischaemic heart disease, with an effect independent of each other. The risk was increased further when both lipid levels were elevated. Cholesterol, and particularly its ester, is the predominant lipid to accumulate in atherosclerotic lesions, while triglyceride is only a minor component. It is possible that some protection is offered against triglyceride accumulation within the plaque by the presence of an enzyme, endothelial lipase, which retains activity in old age (Adams et al., 1969). At present, it is controversial as to whether atheromatous cholesterol is metabolically inert, or accessible for exchange with plasma cholesterol. Animal-feeding experiments
Downloaded from http://ageing.oxfordjournals.org/ at University of California, San Diego on December 28, 2015
tenance digoxin and analysed in relation to digoxin dose and renal function. In patients with normal renal function (serum creatinine less than 2 mg/100 ml, or serum urea less than 70 mg/100 ml), daily doses of 0.25 mg and 0.125 mg, but not 0.0625 mg of digoxin result in the majority of serum levels falling within the therapeutic range. When renal function is impaired, the higher doses give rise to serum levels in the toxic range, but not 0.0625 mg. In the steady state, with biliary clearance of digoxin around a mean of 18 ml/min, there is little likelihood of serum digoxin levels associated with toxicity being attained until creatinine clearance is grossly impaired, provided that the daily digoxin dosage does not exceed 0.25 mg.
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have not provided certain evidence for regression of atheroma. However, a study of 13 patients with obstructive atherosclerosis (Jagannathan et al., 1974) has demonstrated a definite, though slow, exchange of isotope-labelled cholesterol between the plasma and severely atherosclerotic arteries. If this mobility of atheromatous cholesterol is confirmed, then regression of atheroma in man, with its obvious therapeutic implications, becomes a possibility. NORMAL LIPID METABOLISM
Cholesterol is a structural element of cell membranes, and a precursor of bile acids and steroid hormones. Dietary cholesterol is absorbed in the jejunum in association with chylomicrons, and carried via the lymph to the liver, from where, like cholesterol synthesized endogenously, it is released mainly in the form of a-and /3-lipoprotetns. Almost all animal tissues can synthesize cholesterol from acetyl coenzyme A, but over 90 per cent of the total in the body is supplied by the liver and intestinal wall (Myant, 1973). The normal human body contains about 2 g cholesterol per kg body weight Fluctuations in the body total are minimized by regulatory mechanisms which differ between animal species; in man, increased absorption of cholesterol is followed by a rise in its faecal elimination from the exchangeable pool. Dietary cholesterol has, therefore, little immediate effect on plasma concentrations.
Uver Cholesterol —*-bile acids
-»~Gall bladder
> Fat absorption
Faeces M 2% d a i l y 1 lleum Jejunum Duodenum Fig. 1. The enterohepatic circulation of bile acids.
The liver is unique in its ability to degrade cholesterol to bile acids. These are released in the upper intestine in response to a fatty meal, and reabsorbed mainly in the ileum by active transport, returning to the liver via the portal vein (Fig. 1). About 95-98 per cent of the total circulating bile acids are reabsorbed daily, and only small quantities are eliminated in the faeces. In the steady state, bile-acid synthesis equals loss, since bile salts returning to the liver repress further degradation of cholesterol. However, interruption of the enterohepatic circulation stimulates synthesis of bile acids, and exploitation of this phenomenon forms the basis for some treatments of hypercholesterolaemia.
Downloaded from http://ageing.oxfordjournals.org/ at University of California, San Diego on December 28, 2015
The regulation of plasma cholesterol
Geriatric-Cardiology Conference
77
FACTORS AFFECTING PLASMA TRIGLYCERIDE LEVELS
TRANSPORT OF LIPIDS AS LIPOPROTEINS AND THEIR IDENTIFICATION
Lipids cannot circulate freely in aqueous solution unless they interact with proteins. The resulting polar complexes are termed lipoproteins. Four major classes of normal lipoprotein have been identified, each of which contains triglyceride, free and esterified cholesterol, phospholipid and protein in specific proportions (Fig. 2). The major lipid carried by a- and /?-lipoproteins is cholesterol, while triglyceride is the main constituent of pre-/?-lipoproteins and chylomicrons.
Downloaded from http://ageing.oxfordjournals.org/ at University of California, San Diego on December 28, 2015
Plasma triglyceride concentration varies with diet. If the diet contains little or no fat, lipid concentrations tend to be constant throughout the day. Fat ingestion, however, produces a rise in triglyceride concentration, although levels of cholesterol and phospholipids remain relatively constant. Transport of triglyceride in blood depends on whether its immediate source is endogenous, or from dietary fat. Dietary triglyceride is digested and absorbed mainly as unesterified fatty acids and monoglycerides, and is resynthesized in the intestinal cell, from where it is released in the lymph as chylomicrons. The plasma concentration of chylomicrons varies with the timing of fat ingestion, but influx into the plasma is balanced by the rate of removal by the tissues. Isotopic studies have shown that plasma chylomicrons have a short half-life of 5-15 min. When no fat is ingested, plasma glycerides are exclusively of endogenous origin and are transported predominantly on pre-^lipoproteins produced in the liver. In the fasting state, fatty acids are mobilized from adipose tissue and are carried in the blood, bound to albumin, in unesterified form. Some of these are directly oxidized for energy by tissues such as muscle; a proportion enters the liver, where fatty acids in excess of energy requirements are re-esterified to triglyceride and released as pre-/J-lipoproteins. Dietary carbohydrate is a second source of endogenous triglyceride production. This lipogenesis occurs both in adipose tissue, where the fatty acids formed are directly esterified and stored as triglyceride, and in the liver, from where, after esterification, triglyceride is released as lipoprotein. It is important to note that this hepatic release is continuous, and, except during absorption of dietary fat, the liver is the main source of triglyceride in the blood. Triglyceride removal from the blood stream normally balances the rate of entry. This involves hydrolysis of the triglyceride moeity by lipoprotein lipase, or 'clearing factor' lipase. The site of action of this enzyme appears to be at the endothelial surface of the extrahepatic capillaries, and the uptake of triglyceride fatty acids by a particular tissue thus depends upon the activity of clearing-^factor lipase in its capillary walls. For example, in a state of calorie excess, a proportion of triglyceride fatty acid is taken up by adipose tissue, whereas, in the fasting state, fatty acids are mobilized from adipose tissue and are directly oxidized for energy by other tissues. Under these circumstances, pre^3-triglycerides are directed away from adipose tissue. It is probable that, in simple terms, these changes in the direction of metabolism are achieved through specific alterations in clearing-factor lipase activity, whose effects appear to be under hormonal control. Insulin, for example, favours an increase in clearing-factor lipase in adipose tissue, while others, such as catecholamines, ACTH and glucagon, inhibit this rise in activity.
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Geriatric-Cardiology Conference
Chylomicron
Cholesterol
Phospholipid
Triglyceride
Fig. 2. The relative proportions of lipids in the lipoprotein particles.
A third classification is possible using the electrophoretic mobility of lipoproteins, determined by the charges of their protein component (Fig. 3). HDL tend to migrate with the a-globulins, producing an a-lipoprotein band. LDL migrate with the /J-globulins and form a ^3-lipoprotein band. The VLDL migrate just in front of the /?-lipoproteins, hence the pre-/3 nomenclature. Chylomicrons remain at the origin during electrophoresis on paper and agarose gel.
I
Origin
)3
Pre-/3
a
Fig. 3. A normal electrophoretic lipoprotein pattern.
Downloaded from http://ageing.oxfordjournals.org/ at University of California, San Diego on December 28, 2015
The classification of the complexes is based upon their separation utilizing their differing physical properties. They can be separated by density, particle size or their electrophoretic mobility. Lipoprotein isolation by density depends upon the proportion of lipid to protein in the molecule, and this determines their rate of flotation (Sf) in the ultracentrifuge. The four main fractions are the chylomicrons, the very low density lipoproteins (VLDL), the low density lipoproteins (LDL) and the high density lipoproteins (HDL). Classification by particle size is based on the nephelometric procedure of Stone & Thorp (1966). This separates the lipoproteins into only three groups—small (S) corresponding to the HDL and LDL particles; medium (M), and large (L) corresponding to the VLDL and chylomicrons, respectively. The reliability of this method as a diagnostic procedure, however, is questionable (Cramp, 1973).
Geriatric-Cardiology Conference
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HYPERLIPIDAEMIA
It is difficult to give a precise definition of hyperlipidaemia since, at present, the plasma levels of cholesterol and triglyceride which can be considered safe are not known. Indeed, there are no well-established average values for different age groups, particularly for the very young and for the elderly. Lipid levels vary widely between populations and one approach to defining normal
Wi
2a
2b
IS;
3
Fig. 4. Electrophoretic patterns of Types 1-5 hyperlipoproteinaemias compared with normal (N)-
Downloaded from http://ageing.oxfordjournals.org/ at University of California, San Diego on December 28, 2015
Separation in the ultracentrifuge is the reference method of choice, because quantitative values for each fraction are obtainable. However, the equipment is expensive and beyond the scope of many laboratories. Excellent separation of the fractions can be obtained with agarose gel electrophoresis (Noble, 1968). This provides only a qualitative identification which may be difficult to interpret unless serum cholesterol and triglyceride values are simultaneously measured. Three different protein components, or apoproteins, of the lipoproteins have been identified. These may be designated A, B and C apoproteins (Fredrickson et al., 1967). They differ in their terminal residues, total amino acid content, and immuno-chemical behaviour. The A apoprotein is normally the only protein constituent of the a-(highdensity) lipoprotein, and the B apoprotein that of the /?-(low-density) lipoprotein. The pre-^S-(very-low-density) lipoproteins and chylomicrons contain a mixture of apoproteins. It is probable that the a- and /?- lipoproteins in plasma originate from the pre-j3-lipoproteins and chylomicrons after they have disposed of their triglyceride load.
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Geriatric-Cardiology Conference
MODIFIED FREDRICKSON CLASSIFICATION
It should be remembered that, after an overnight fast, chylomicrons will be absent from normal serum. The Type 1 pattern is usually found in children, and is distinguished by the presence of fasting chylomicronaemia. In these subjects, the plasma or serum has a supernatant 'cream layer' after overnight storage at 4°C. In Type 2a, cholesterol and hence the jS-lipoproteins are increased, while pre-/?lipoprotein levels are normal. The plasma is clear. In Type 2b hyperlipoproteinaemia, both cholesterol and triglyceride levels are elevated so that the /3- and pre-jS-lipoprotein fractions are increased. The plasma may be clear or slightly turbid. Type 3 is a rare disorder, and is characterized by an abnormal /3-lipoprotein, which contains an unusually high ratio of triglyceride, and floats with the pre^S fraction in the ultracentrifuge—hence the term 'floating j3' lipoprotein. Both cholesterol and triglyceride values are increased. The diagnosis of this disorder depends upon the discovery of a lipoprotein of /? electrophoretdc mobility in the pre-/J density fraction. Type 4 is indicated by an increase of pre-j3-lipoproteins and serum triglyceride while cholesterol levels and the j3 fraction are normal, or slightly elevated. The plasma is turbid. The Type 5 pattern is a variation of Type 4 with the addition of fasting chylomicronaemia. It is distinguished from Type 1 in that it usually occurs in adults. Furthermore, the pre-/? fraction is always elevated in Type 5, but may be normal, increased or decreased in Type 1. Types 2a, 2b, 3 and 4 are recognized as being associated with an increased risk of ischaemic heart disease.
Fredrickson's classification has been widely accepted and has contributed to the understanding of hyperlipidaemia. However, it should be remembered that it is an arbitrary classification based on genetic disorders and the usefulness of lipoprotein phenotyping
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limits has been to compare lipid levels in a community with 'high risk' for ischaemic heart disease with those in a community with 'low risk' (Keys, 1966). Another difficulty in interpreting the significance of lipid levels is that normal ranges differ according to the various standard laboratory procedures which are used to determine cholesterol and triglyceride concentrations. To assist in solving this problem Wynder & Hill (1972), circulated a questionnaire to established investigators asking what should be accepted as optimum, or 'normal' lipid levels. They concluded that presently accepted values (generally 250 mg/100 ml for cholesterol, and 150 mg/100 ml for triglyceride) were too high, and that, in men of 50 years of age, upper limits should be 185/100 ml for cholesterol and 100 mg/100 ml for fasting triglyceride. As an alternative to defining lipid abnormalities in terms of raised cholesterol and triglyceride levels, Frederickson and colleagues (1967) using paper electrophoresis, devised a convenient system of lipoprotein phenotyping which was originally intended to define inherited lipid disorders. The five types of hyperlipoproteinaemia classified by Fredrickson were subsequently revised by a WHO committee (Beaumont et al., 1970) who subdivided the Type 2 disorder into Types 2a and 2b (Fig. 4).
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Table. Some causes of secondary hyperlipidaemia Diabetes
Obstructive liver disease
Gout Alcoholism Chronic renal failure Nephrotic syndrome Hypothyroidism
Cushing's syndrome Addison's disease Glycogen storage disease Idiopathic hypercalcaemia
LIPID LEVELS m THE ELDERLY
Despite the correlation between serum lipid levels and cardiovascular disease documented by most authors, there is a diversity of opinion regarding the presence of this correlation in the elderly. However, few population studies have included subjects over the age of 65. Brown & Ritzmann (1967) examined 233 hospitalized male patients over 65 years of age in an attempt to correlate factors associated with an absence of ischaemic heart disease. Mean serum cholesterol levels were significantly higher in the 100 patients with known ischaemic heart disease than in those without (212 mg/100 ml and 181 mg/100 ml respectively). Furthermore, evidence of degenerative peripheral arterial disease was four times more common in the patients with IHD. This suggests that similar pathological processes are responsible for both conditions. Vavrik and colleagues (1974a, b) found a statistically significant positive correlation between the prevalence of atherosclerotic cardiovascular disease and cholesterol concentration. A frequency distribution curve showed that two-thirds of the subjects had serum cholesterol in the range 170-250 mg/100 ml. However, in the individual elderly patient a low cholesterol level, i.e. below 170 mg/100 ml, did not exclude the presence of atherosclerosis. Unfortunately, triglycerides and lipoproteins were not examined. Other reports have produced contrary evidence. The results of the Stockholm prospective study (Carlson & BSttiger, 1972) did not show a linear relationship between the rate of ischaemic heart disease and initial cholesterol and triglyceride values in the group over 60 years of age. Similarly, Schadel et al. (1971) in an investigation of over 600 elderly subjects, concluded that there was no definite relationship between serum cholesterol levels and atherosclerotic cardiovascular disease.
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in clinical practice remains debatable. This is particularly important since although a familial trait of hyperlipoproteinaemia is found in approximately one-third of patients with premature ischaemic heart disease (Clinical review, 1974) relatives of a patient with, for example, a Type 2b disorder may show Type 2a, 4 or 5 lipoprotein patterns. This indicates that there is only a limited relationship between the phenotype and the aetiology of the disorder. Furthermore, it is not certain how environmental factors contribute to Types 2a, 2b, 4 and 5, since a subject's phenotype can change in response to weight reduction, therapeutic diets and drug therapy. It is likely that more than one aetiological factor can produce a particular lipoprotein pattern. It is important to consider whether the hyperlipidaemia is a primary or a secondary disorder since the lipid abnormality may respond to treatment of the underlying disease (Chait, 1973). There is no specific lipoprotein pattern for any one of the disorders associated with secondary hyperlipidaemia (Table), and the pattern can vary even in the same patient.
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COMMENT
There is a need to establish a normal lipid range for all age groups, if the relationship between lipid abnormalities and the development of ischaemic heart disease is to be fully understood. It is possible that hyperlipidaemia has its most important role in the pathogenesis of obstructive coronary arterial lesions in the initial stages of the disease, and that later, when the condition is clinically apparent, the relationship is less obvious. Current evidence points to a drop in hpid levels after the age of 60 years in men, which may be a result of changes in metabolism owing to ageing processes, environmental factors such as dietary modification, or to the earlier deaths of those with severe hyperlipidaemia. However, since two-thirds of the total deaths registered as being due to ischaemic heart disease occur in the over-65s, what are the pathogenetic mechanisms leading to these events, and what is the role which can be ascribed to abnormal lipid metabolism? REFERENCES
C. W. M., BAYLIS, O. B., ABDULLA, Y. H., MAHLER, F. R. & ROTT, M. A. (1969) Lipase, esterase and triglyceride in the ageing human aorta. J. Atheroscler. Res. 9, 87-102.
ADAMS,
BARBORIAK, J. J., RIMM, A. A., ANDERSON, A. J., TRISTANI, F. E., WALKER, J. A. & FLEMMER, R. J.
(1974) Coronary artery occlusion and blood lipids. Am. Hearty. 87, 716-21. BEAUMONT, J. L., CARLSON, L. A., COOPER, G. R., FEJFAR, Z., FRBDRICKSON, D. S. & STRASSER, T.
(1970) Classification of hyperlipidaemias and hyperlipoproteinaemias. Bull. World Health Organ. 43, 891-915. BROWN, R. C. & RTTZMANN, L. (1967) Some factors associated with absence of coronary heart disease in persons aged 65 or older. J. Am. Geriatr. Soc. 15, 239—49. CARLSON, L. A. & BOTTIGER, L. E. (1972) Ischaemic heart disease in relation to fasting values of plasma triglyceride and cholesterol. Stockholm Prospective Study. Lancet i, 865—8.
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Wood et al. (1972) studied the prevalence of lipid and lipoprotein abnormalities in a Califomian population between the ages of 25 and 79 years. In men, mean triglyceride levels reached a peak in the sixth decade after which they fell. Similarly, mean cholesterol levels increased with age until the seventh decade and then decreased. No such peaks were observed in the women studied, and mean lipid levels continued to rise with age. This fall in mean cholesterol concentration in the eighth decade in men has been ascribed to the selective removal from the population of those with the highest cholesterol levels, which phenomenon, apparently, does not occur in women. Similarly, higher glyceride concentrations in men of all ages up to 59 years suggest that the predominant incidence of IHD in men may be related to the higher glyceride concentrations present when atherosclerosis is developing, and that the drop in mean levels after the age of 60 years may be the result of earlier deaths of those men with the highest levels. In the same study, lipoprotein electrophoresis showed that a-lipoproteins tended to be higher in women than in men, suggesting the possibility that a-lipoprotein cholesterol is less atherogenic than ^-lipoprotein cholesterol, to account for these sex differences. The effect of ageing upon the serum lipids of normal subjects was investigated by Lavietes et al. (1973) who followed-up 28 subjects over a 30-year period. No significant change was found between mean cholesterol and triglyceride levels in weight-stable men at the end of the study; but the five men who had gained over ten pounds in weight showed a significant rise in serum cholesterol. The twelve women showed increases in cholesterol and triglycerides regardless of weight changes.
