Low Density Lipoprotein Turnover Research Articles

HYPOCHOLESTEROLAEMIA AND INCREASED ELIMINATION OF LOW-DENSITY LIPOPROTEINS IN METASTATIC CANCER OF THE PROSTATE

To study the influence of tumour mass on lipid metabolism, the lipoprotein pattern in untreated patients with newly diagnosed cancer of the prostate was examined. Total cholesterol levels were reduced in patients with evidence of metastasis (n=30) compared with those without metastasis (n = 73). Since the major fraction of serum cholesterol is contained in low-density lipoproteins (LDL), turnover of LDL was studied in detail in 8 patients compared with 12 age-matched healthy men. LDL were cleared faster in the 3 patients with metastatic disease than in the patients without metastasis and in controls, indicating faster catabolism of LDL. Thus in prostatic cancer an increased tumour burden is associated with increased elimination of LDL, which contributes to reduced serum cholesterol levels.

Estimation of turnover of low density lipoproteins by simplified methods

This study was carried out to evaluate simplified methods for estimating parameters of kinetics of low density lipoproteins (LDL). Initially, LDL turnover studies were carried out in 78 patients, and fractional catabolic rates (FCRs) for LDL were determined from die-away curves of plasma radioactivity and from urine-to-plasma ( U P ) ratios of radioactivity. When U P ratios were calculated on a daily basis and throughout the study and averaged by a weighting procedure, the weighted U P ratios were highly correlated with plasma decay FCRs ( r = 0.92, P < 0.001). One simplified method involved calculation of FCR for LDL from the U P ratio on the seventh day after injection of radioactivity. Seventh-day U P ratios also were correlated significantly with plasma decay FCRs ( P < 0.001), but the strength of correlation between the two was relatively weak ( r = 0.52). Finally, another correlation was found to be significant. This was the plasma decay FCR vs. the fraction of injected dose disappearing from plasma during the first 24 h ( r = 0.86). This comparison was extended to 140 turnover studies, and the correlation remained high ( r = 0.90, P < 0.0001). Thus, estimation of FCR for LDL from the fraction of dose disappearing at 24 h appears superior to that determined from the 7th-day U P ratio.

Colestipol-induced changes in LDL composition and metabolism. II. Studies in humans.

We investigated the effect of the bile acid sequestrant, colestipol hydrochloride, on the composition and metabolism of human low density lipoprotein (LDL). Colestipol treatment produced a disproportionate decrease in LDL cholesterol compared to LDL apoB, resulting in a significant decrease in the LDL cholesterol/apoB ratio. Electron microscopy revealed that LDL particles were smaller in size and analytical ultracentrifugation demonstrated that colestipol therapy selectively depleted larger, more buoyant LDL particles of Sf degrees 6-7. Thus, colestipol therapy produced LDL that were smaller in size, more dense, and characterized by a decreased cholesterol to protein ratio. To determine whether the altered LDL had different metabolic properties, autologous LDL was isolated from subjects before and during colestipol therapy and their fractional catabolic rates (FCR) were then simultaneously determined in the same patient while on therapy. Eight LDL turnover studies comparing the catabolism of LDL isolated during therapy (Rx-LDL) and LDL isolated off therapy (Con-LDL) were performed in six subjects. All subjects responded to colestipol treatment, with an average 29% fall in LDL cholesterol. In four of six subjects, and in six of eight studies, the FCR of Rx-LDL was substantially slower than that of Con-LDL. These studies demonstrate that a drug intervention may alter subpopulations of LDL particles in such a way that overall LDL composition is changed. This alteration may independently affect the intrinsic metabolic behavior of the LDL. We suggest that such drug- (or dietary-) induced changes in LDL composition need to be considered in kinetic studies designed to assess the overall impact of the perturbation being studied.

Divergent effects of d-norgestrel on the metabolism of rat very low density and low density apolipoprotein B.

Rats treated with the contraceptive steroid d-norgestrel have lower plasma very low density lipoprotein (VLDL)-triglycerides and higher low density lipoprotein (LDL)-cholesterol than controls. To explain these results, the kinetics of VLDL and LDL turnover were studied by injecting 125I-labeled rat-VLDL and 131I-labeled rat-LDL simultaneously into rats treated with a small dose of d-norgestrel (4 micrograms per day per kg body weight0.75 for 18 days, n = 22) and their untreated controls (n = 22). VLDL- and LDL-apoB specific activity-time curves obtained over 50 hr best conformed to a three-pool model. VLDL-apoB clearance expressed as irreversible catabolic rate (k01) was markedly enhanced in the treated versus control rats (0.57 vs. 0.34 pools hr-1), leading to a marked reduction in VLDL-apoB pool size (270 vs. 420 micrograms). However, VLDL-apoB production rates were similar in the two groups (160 vs. 140 micrograms/hr, respectively). The 125I-labeled apoB specific activity-time curve derived from the catabolism of 125I-labeled VLDL-apoB also showed enhanced clearance in d-norgestrel-treated rats. 125I-Labeled IDL-apoB and 125I-labeled LDL-apoB specific activity-time curves failed to intersect the VLDL-apoB curve at maximal heights, suggesting input of intermediate density lipoprotein (IDL) and LDL independent of VLDL catabolism in both groups. However, the extent of independent LDL-apoB production was similar in both groups. Clearance of 131I-labeled LDL-apoB following injection of 131I-labeled rat-LDL was delayed in the d-norgestrel-treated versus control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Low density lipoprotein kinetics in African Green monkeys showing variable cholesterolaemic responses to diets realistic for westernised people

African Green (vervet) monkeys were fed either an atherogenic Western diet (WD), a prudent diet (PD), or a high carbohydrate diet (HCD) for various lengths of time. Monkeys differed greatly in their response to the WD, and a strong negative correlation was observed between the fractional catabolic rates (FCR) of low density lipoprotein (LDL) and the total plasma cholesterol concentrations ( r = −0.83 P = 0.0029). Similarly the individual synthetic rates (SR) of LDL plotted against the total plasma cholesterol concentrations showed a correlation coefficient of r = 0.94 P = 0.01 for WD animals. There was no correlation for PD or HCD animals for either SR or FCR and the total plasma cholesterol concentrations. The aortic lesion scores correlated with the SRs ( r = 0.66 P = 0.040) but not with the FCRs. LDL turnover was also studied using 125I-labelled native LDL and methylated 131I-labelled LDL in African Green monkeys showing high or low cholesterolaemic response to an atherogenic Western diet. Measurements after 4 weeks and again after 6 months on the diet showed a decrease in both total and non-receptor mediated clearance of LDL. Receptor-mediated catabolism was significantly lowered in the hyperresponding group ( P = 0.0011) and not in the hyporesponding group with a resultant significant difference between the two groups being P = 0.0355 after 6 months. Non-receptor mediated catabolism of LDL was similarly affected in that there was no significant difference between the two groups at 4 weeks but the clearance of methylated LDL was markedly lowered at 6 months, for the hyporesponding group P = 0.0003 and for the hyperresponding group P = 0.0184. Furthermore the hyperresponding group was depressed to a significantly greater extent when compared to the hyporesponding group ( P = 0.0241).

Regulation of plasma levels of low-density lipoprotein cholesterol: interpretation of data on low-density lipoprotein turnover in man.

At the present time the most useful technique with which to examine the kinetics of low-density lipoprotein (LDL) cholesterol in vivo is the labeled LDL turnover study. However, a major limitation of this method is that, despite its ability to accurately measure both the plasma LDL concentration and LDL production rate, it cannot directly quantify LDL receptor activity. The present study defines the equations that describe the relationship between LDL cholesterol production rate, LDL receptor number, and plasma LDL cholesterol concentration. These equations provide a method that allows calculation of total LDL receptor activity based on the results of an LDL turnover study. With the use of this technique and data from previously published series, the effects of the genetic absence of receptors, aging, and the treatment of hypercholesterolemia with mevinolin on LDL cholesterol kinetics were analyzed.

Low density lipoprotein metabolism in familial combined hyperlipidemia and familial hypercholesterolemia: Kinetic analysis using an integrated model

Several models for low density lipoprotein (LDL) apo B metabolism were applied to LDL turnover data from subjects with two distinct genetic forms of hyperlipidemia, familial hypercholesterolemia (FH), and familial combined hyperlipidemia (FCHL). Of the first two models tested, there was good agreement between the observed and predicted data for FH in one (model A), and for FCHL in the other (model B). The major difference between these two models is that LDL is kinetically homogenous in model A and heterogeneous in model B, raising the possibility that LDL subspecies differences may occur between these two disorders. The findings are consistent with LDL homogeneity in FH and LDL heterogeneity in FCHL. Two other integrated models (models C and D) provided good agreement between observed and predicted data in both disorders. Although neither could be rejected outright on the basis of known physiology, parameter estimates were more variable with model D. Analysis of the data using model C was consistent with the known pathophysiologic defect in LDL catabolism in FH and suggests that individuals with FH as well as FCHL have more than one LDL subpopulation in plasma. The urine/plasma ( U P ) ratio was shown to be constant from day 4 to day 14 of the study in FH, while in FCHL this value declined in all cases. Thus, determination of LDL fractional catabolic rates (FCR) by the U P ratio method may be invalid in certain groups of patients. The other traditional method for calculating LDL FCR, the Matthews' analysis, overestimated FCR in some instances, and could lead to systematic errors when used to determine LDL FCR and production rates. These studies point out some of the complexities of LDL metabolism that may relate to heterogeneity of LDL and their subspecies, and underscore the difficulty of accurately measuring LDL turnover parameters using either the U P ratio method or a simple model in which only plasma LDL decay curves are analyzed. These studies also point out the need to use an integrated model for LDL metabolism that utilizes both plasma and urine radioactivity data.

Turnover of low-density lipoprotein isolated from interstitial inflammatory fluid of the rabbit.

Utilizing the polyvinyl sponge-implant model in the rabbit, we have described in vivo modification of low-density lipoproteins (LDL) in interstitial inflammatory fluid (IF). In the present studies, plasma clearance rates of IF-LDL were determined and compared with normal whole plasma LDL (WP-LDL) as well as plasma LDL modified by chemical reaction with acetic anhydride (Ac-LDL). Lipoproteins were labeled with 125I and injected into the lateral ear vein of recipient rabbits. At 10 min after injection, only 51.5% of IF-LDL could be accounted for in recipient plasma, as compared to 91.9% for WP-LDL, and 2.4% of Ac-LDL. Subsequent log-linear decay rates were similar for IF-LDL and WP-LDL (t 1/2 = 9.5 vs. 11.0 h). Autoradiography of plasma obtained from recipient animals at 15 min and 1 h after injection revealed a return to normal electrophoretic mobility of [125I]IF-LDL. These results indicate that IF-LDL is a mixture of both modified and essentially unmodified particles. We propose that the modified particles may be removed from the circulation by mechanisms described for Ac-LDL.

Rapid method for measuring plasma low-density lipoprotein turnover using high-performance gel exclusion chromatography

High-performance gel exclusion chromatography using flow-through radioactivity monitoring was employed for rapid measurement of low-density lipoprotein (LDL) turnover. Iodinated LDL was injected into five fasting rabbits. Serial blood samples were obtained and small aliquots (0.1–0.2 ml) were injected into the chromatographic system using 0.2 M Tris—acetate buffer (pH 7.0). Radioactivity in lipoproteins was measured by an on-line gamma counter (Flo-One) attached to the high-performance liquid chromatograph and in a regular gamma counter after collecting the fractions. Sequential ultracentrifugation was also used to separate lipoproteins, and the radioactivity in each fraction was measured. The Flo-One method was faster; however, the efficiency of Flo-One varied with the amount of radioactivity and therefore it was necessary to inject the same amount of radioactivity to get comparable values.

Gemfibrozil Therapy in Primary Hypertriglyceridemia Associated With Coronary Heart Disease

Certain primary hypertriglyceridemias cause abnormalities in lipoproteins that seemingly predispose patients to coronary heart disease. We examined metabolism of low-density lipoproteins (LDL) in 11 men with both hypertriglyceridemia and coronary heart disease and compared them with that of controls. The LDL turnover was measured during placebo and gemfibrozil therapy. With placebo, LDL-cholesterol level usually was normal, but production and fractional clearance of LDL were high. The LDL composition also was abnormal. Gemfibrozil reduced triglycerides, lowered production and fractional clearance of LDL, and normalized LDL composition. The LDL-cholesterol level usually rose, but generally not to abnormally high levels. Therefore, normalization of LDL metabolism and marked reduction of triglycerides by gemfibrozil suggest benefit to hypertriglyceridemic patients who are at high risk for coronary heart disease. However, when LDL-cholesterol level rises excessively, gemfibrozil may not be sufficient therapy.

Cholestyramine-induced changes in low density lipoprotein composition and metabolism. I. Studies in the guinea pig.

In previous animal studies, bile acid sequestrant resins have been shown to increase the fractional catabolic rate (FCR) of a low density lipoprotein (LDL) tracer isolated from a normal donor animal and to increase hepatic LDL-receptor activity. In addition, in man, these resins are known to alter LDL composition such that low density lipoproteins are smaller, more dense, and have a decreased cholesterol:protein ratio. To determine whether metabolic consequences resulted from these changes in LDL composition, we fed cholestyramine chow (2% resin by weight) to guinea pigs, which lowered LDL cholesterol levels by 55%. LDL was isolated from control donors (C-LDL) and from cholestyramine-treated donors (CH-LDL). Compared to the C-LDL, the CH-LDL were smaller in size, depleted of cholesteryl ester and phospholipid, and had a marked decrease in their cholesterol:protein ratio. To determine whether the clearance of the altered CH-LDL was different from that of C-LDL, we labeled the two LDL preparations with 125I or 131I and simultaneously injected them into control and cholestyramine-treated guinea pigs. In 27/29 animals studied, the FCR of the CH-LDL was slower than that of C-LDL, demonstrating that the compositional changes alter the metabolism of CH-LDL. When C-LDL was used as the sole tracer in both control and treated animals, cholestyramine treatment increased the FCR by 41%; when CH-LDL was used as sole tracer, the increase in FCR on treatment was only 26%. This suggested that C-LDL was cleared more rapidly by the LDL-receptor pathway than was CH-LDL. Further support for this idea came from observations that C-LDL was degraded more readily by cultured fibroblasts and that nonenzymatic glucosylation abolished the difference in FCR between C-LDL and CH-LDL. These studies show that the effects of bile sequestration are complex and that the compositional changes produced have profound metabolic consequences. The implications of these observations for interpretation of LDL turnover studies are discussed.

Protocol for efficient plasma sampling for low density lipoprotein turnover studies.

Radiolabeled low density lipoprotein (LDL) is commonly used to study the turnover of LDL apolipoprotein B (apoB), the major protein component of LDL. Following an intravenous injection of radioiodinated LDL, typical sampling schedules have including 20-25 samples over a 14-day period with frequent sampling during the first 12 hr and daily samples thereafter. This is a burdensome task for subjects and investigators. To improve acceptance of the procedure, we have examined the effects of reduced sampling schedules upon the estimation of the fractional catabolic rate (FCR) for LDL apoB. Data from 36 different sets of LDL decay curves obtained from investigations of subjects with a variety of lipoprotein phenotypes have been used to test these schedules. Our results indicate that by choosing specific intervals over a 14-day period only 10 samples are sufficient to accurately determine the fractional catabolic rate for LDL in plasma. This reduced sampling schedule should facilitate the study of LDL turnover in large groups of subjects as outpatients.

Turnover of low density lipoproteins during inhibition of cholesterol absorption by neomycin.

The mechanisms for the hypocholesterolemic actin of neomycin were examined in six patients with various levels of plasma total cholesterol and triglycerides. All patients were studied on a metabolic ward. The first period of 6 weeks was for control. Thereafter, neomycin (1.5 g/day) was started, and the patients were readmitted for another 6-week period after 2 to 3 months of treatment with the drug. Cholesterol balance studies showed that neomycin increased fecal excretion of neutral steroids by an average of 45%; the drug also inhibited absorption of exogenous cholesterol by an average of 44%. During treatment with neomycin, the plasma total cholesterol fell by an average of 20%, low density lipoproteins (LDL) fell by 25%, and high density lipoproteins, by 16%. Neomycin did not change plasma triglyceride levels. Turnover of the apoprotein of LDL (apoLDL) was measured following injection of 125I-apoLDL. Neomycin decreased synthesis of apoLDL by 28%. The decrease in plasma apoLDL level was correlated positively with the decrease in apoLDL synthetic rate. The effect of the drug on clearance of LDL was less constant; four of six patients had an increase in fractional clearance rates of apoLDL, but the change for the whole group was not statistically significant. These data suggest that a decrease in production of LDL is a major factor in the lowering of LDL following inhibition of cholesterol absorption; however, an increase in clearance rates may occur in some patients.

Effect of dietary eggs on liver and plasma proteins of rats. I. Synthesis of serum low-density lipoproteins.

The total lipid content in the livers of rats consuming cooked whole eggs as the only source of food for 21 days was elevated to three times that of control animals. The incorporation of 14C-labelled amino acids into serum low-density lipoprotein apoproteins was increased by 52% in these rats, without affecting the incorporation into serum albumin and liver total proteins. The turnover of low-density lipoprotein in these egg-fed rats appears to be very high.

Plasma low density lipoprotein transport kinetics in noninsulin-dependent diabetes mellitus.

Plasma low density lipoprotein (LDL) transport kinetics were determined from the disappearance of 125I-LDL injected into age- and weight-matched groups of 13 normal subjects, 20 mild diabetics, and 8 moderately severe diabetic patients (fasting plasma glucose less than 150 and greater than 150 mg/100 ml, respectively). In mild diabetics, LDL apo-lipoprotein-B (apo-B) synthetic rate (SR) was significantly greater than normal. The fractional catabolic rate (FCR), however, was also increased so that plasma LDL concentration remained normal. In moderately severe diabetics, LDL SR was normal but FCR was reduced resulting in increased plasma LDL cholesterol and apo-B concentrations. In normal subjects, moderate obesity was associated with increased LDL secretion. In diabetic subjects, however, changes in LDL turnover were of equal magnitude in obese and nonobese patients. In normolipemic and hyperlipemic mild diabetic subjects with equal degrees of glucose intolerance, both LDL apo-B SR and FCR were greater than normal. The magnitude of these increases, however, was lower in the hyperlipemic individuals. Stepwise regression analysis revealed that both LDL SR and FCR correlated positively and linearly with insulin response to glucose loading, but negatively and curvilinearly with fasting plasma glucose and glucose response. We propose that in noninsulin-dependent diabetes, mild hyperglycemia is accompanied by increased LDL turnover, despite normal plasma LDL levels, whereas moderately severe hyperglycemia is associated with decreased LDL catabolism, resulting in increased plasma LDL levels. These changes cannot be attributed to the presence of obesity or hypertriglyceridemia, and may relate to varying degrees of insulin resistance and decreased insulin secretion affecting plasma very low density lipoprotein (VLDL) secretion, VLDL conversion to LDL, and LDL catabolism. Both increased LDL turnover in mild diabetes and delayed removal of LDL in moderately severe diabetes could increase cholesterol ester availability to peripheral tissues, and may result in an increased risk of atherosclerosis.

Significance of low density lipoprotein production in the regulations of plasma cholesterol level in man.

To determine whether production or catabolism of low density lipoprotein (LDL) is the major factor controlling LDL concentrations in subjects with plasma cholesterol levels from low-normal to mildly elevated, measurements of apoprotein of LDL (apoLDL) turnover were performed in 16 patients with various plasma cholesterol concentrations. Cholesterol balance studies were done simultaneously in 13 of these patients. Plasma concentrations of apoLDL and LDL-cholesterol were positively correlated with synthetic rates of apoLDL (r = 0.74, P less than 0.001; r = 0.50, P less than 0.05, respectively). No correlation was noted between the fractional catabolic rate for apoLDL and apoLDL levels (or LDL-cholesterol). For further analysis, the patients were divided into three groups with stepwise increases in apoLDL concentrations. When apoLDL levels rose significantly, from 83 +/- 5 SEM to 122 +/- 2 to 149 +/- 5 mg/dl, synthetic rates for apoLDL also increased significantly from 11.6 +/- 12. to 17.0 +/- 0.9 to 23.8 +/- 1.8 mg/d/kg ideal weight. In contrast, the fractional catabolic rate of apoLDL was not different among the three groups (0.32 +/- 0.03 vs. 0.29 +/- 0.02 vs. 0.33 +/- 0.03/d). No relation was noted between synthesis of total body cholesterol (or bile acids) and concentrations, production rates, or removal of apoLDL. Thus, concentrations of apoLDL and LDL-cholesterol in these subjects with plasma cholesterol levels from low-normal to mildly elevated were regulated mainly by synthetic rates of apoLDL and not by LDL catabolism.

Tryptic modification of human serum low-density lipoprotein leads to altered rates of turnover and intravascular transformation in the guinea pig.

The metabolism of human serum low-density lipoprotein (LDL) and its trypsin-treated counterpart have been compared in the guinea pig in vivo. Removal of surface-exposed protein from the lipoprotein particle in this way resulted in significant modification of its metabolism in guinea pigs in vivo. Limited trypsinisation of LDL permitted removal of 20-25% of its protein moiety; trypsinised LDL was deficient in lysine and arginine residues (25-30% of each removed). The modified particle retained its basic structural features, such as internal molecular architecture, but displayed an elevated net negative surface charge and diminished immunological reactivity. Following intravascular injection of iodinated LDL (131I) and trypsinised LDL (125I) into the same animal, the two lipoproteins displayed biexponential decays; the rate constants for the plasma turnover of LDL and trypsinised LDL were significantly different (P less than 0.05), trypsinised LDL exhibiting a slower disappearance from the circulation. Density-gradient ultracentrifugation revealed marked elevation in the modal densities of both LDL and trypsinised LDL upon metabolism in vivo, although the rate of increase was greater for trypsinised LDL than LDL in each case (average increment 0.022 g/ml and 0.014 g/ml at 24 h respectively). The diminished plasma clearance of trypsinised LDL as compared to the native human and guinea pig LDL indicates that sites required for the cellular recognition and uptake of the LDL particle reside in its surface-exposed, trypsin-accessible protein. Furthermore, such protein appears to play a central role in regulating the intravascular processes by which the lipid content of LDL is diminished, and by which it is transformed to a particle of higher density.

Separation of the dietary fat and cholesterol influences on plasma lipoproteins of rhesus monkeys

In order to isolate the effects of the type of dietary fat from those of dietary cholesterol on the circulating pool of cholesterol, plasma lipoproteins were characterized in juvenile rhesus monkeys fed semipurified diets containing 31% of calories as corn oil or coconut oil. Half the diets contained 300 mg of cholesterol per 1000 kcal. The fatty acids of cholesteryl esters and phospholipids varied significantly with dietary fat such that saturated and monounsaturated fatty acids replaced polyunsaturated fatty acids in low-density lipoproteins (LDL) and high density-lipoproteins of rhesus monkeys fed coconut oil. Dietary cholesterol alone induced small but significant increases in the cholesteryl ester: triglyceride ratio in both very low-density lipoproteins and LDL. Whereas neither saturated fat (coconut oil) nor cholesterol alone substantially altered the plasma cholesterol concentration or lipoprotein profile, together these dietary components interacted synergistically to produce a significant elevation in plasma cholesterol. This was due primarily to a significant rise in the cholesteryl ester fraction of LDL, disproportionate to any change in LDL protein concentration. The data are consistent with current hypotheses that relate parameters of LDL turnover and clearance to their macromolecular structure and physicochemical characteristics.

Proceedings of the Australian Society for Medical Research.

Proceedings of the Australian Society for Medical Research.

Atherosclerosis: A process determined primarily by the physical state of plasma lipid that has entered the arterial wall

(i) A consideration of the factors determining the localisation of atherosclerotic lesions, particularly the role of endothelial injury, suggests that this must occur to a similar degree in all human populations and thus cannot be the primary determinant of clinically significant disease. It does however have a permissive role in atherogenesis which becomes relevant when plasma low density lipoproteins (LDL) are elevated. (ii) LDL gain access to the subendothelial space of arteries in significant amounts at sites where the endothelial barrier is defective at a rate which varies directly with their concentration in plasma. At this site LDL are precipitated by mucopolysaccharides when the concentration of LDL exceeds a critical level. Disruption of the LDL complexes leaves free lipid in the subendothelial space and it is this free lipid which evokes the tissue reaction that has relevance to atherogenesis in man. (iii) Since the inner arterial wall lacks an adequate scavenging system, the extent of the tissue reaction to free lipid, particularly cholesterol, depends on the efficacy with which this lipid is phagocytosed by smooth muscle cells that appear in evolving lesions during their proliferative phase. Effective phagocytosis requires that the neutral lipids are adequately dispersed by phospholipids. The capacity of phospholipids to do so is limited and markedly inhibited by glucose and sorbitol. Thus suboptimal glucose homoeostasis promotes atherogenesis. (iv) The sites of evolving lesions have decreased compliance, increasing their liability to injury and this accelerates the above processes. The proliferative phase continues until the deeper layers of lesions are deprived of nutrients and necrose. Further trapping of lipid can no longer evoke a tissue response but coagulation products continue to accrete to these sites leading ultimately to development of fibrous plaques. (v) The fact that LDL cholesterol does not gain access to cells of subjects with familial hypercholesterolaemia, whereas cholesterol free of apolipoprotein does, lends support to the above concepts. The turnover of LDL in tendon xanthomas of these subjects is not analogous to that in arteries since extravasated LDL in tendons can be cleared by lymphatics and these are absent in the inner arterial wall. (vi) Veins and the pulmonary artery are free of atheroma because they are not liable to recurrent endothelial injury at the same sites and because LDL reaching the subendothelial space could be cleared by lymphatics which are present at this site in these vessels. The development of pulmonary hypertension would obliterate low pressure vessels near the artery luman and this, together with increased endothelial injury, makes it liable to the atherosclerotic process.