Medical

Hypotheses

5: 533-548,

1979

PRIMARILY ATHEROSCLEROSIS : A PROCESS DETERMINED PLASMA LIPID THAT HAS ENTERED THE ARTERIAL WALL. L.H. Krut, Department of the Witwatersrand,

of Medicine, P.O. Bertshsm.

BY THE PHYSICAL

Earagwanath Hospital 2013. Johannesburg,

STATE

OF

and the University South Africa.

ABSTRACT [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 huTlan 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 (LOLI are elevated. [ii.1 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 n-en. (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. (VI The fact that LDL cholesterol does not gain access to cells of subjects with familial hyperoholesterolaemia, 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. (vi1 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 lumen and this, together with increased endothelial injury, makes it liable to the atherosclerotic process,

Key Words: Endotheliun, mucopolysaccharides, coronary heart disease.

cholesterol,

lipids,

533

glucose,

sorbitol,

phospho-

INTRODUCTION

occurs

Atherosclerosis extent and munity and

severity marked

implicated

in

of

universally lesions

in man. There are differences in the between different members of the same comin prevalence of significant lesions between

differences different communities (1 I, Differences between peoples are largely quantitative rather than qualitative in nature. The most important clinical expression of atherosclerosis is coronary heart disease (CHDI (21. In the 1950's epidemiological studies showed that differences in prevalence of CHD were directly related to both serum cholesterol concentration and to consunption of dietary fats (31. These associations were soon confirmed and more precisely defined (41, enhancing the view that dietary fat was its

pathogenesis.

In

the

intervening

decades

a host

of

environmental

and other factors have been implicated (see Krut 51. Despite an enormous accumulation of facts, the "lipid hypothesis" of this disease (21, on which most research has centred and on which preventive efforts have been primarily based, remains unproven (61. It would seem that we might gain more insight into approaches at prevention by first defining the pathogenesis of the atherosclerotic lesion which is commonly held to be the direct cause of this disease. For several adherents

decades the only concepts of atherogenesis which had their were the thrombogenic theory of Duguid (71 and the lipid imbibition theory of Page (81. These theories served primarily to draw attention to different features of the atherosclerotic plaque which seemed to their respective proponents to be most relevant in its genesis. Finer details of this lesion have since been described, in terms of which Ross and Glomset (91 have postulated a mechanism in its genesis. Their hypothesis is based on concepts of this disease and have limited clinically significant lesions based on evidence gleaned from

which

is

not

which

do

not

fit the epidemiological data to spontaneously occurring in man. The concepts presented here are the literature on this subject, much of

relevance

new. THE LOCALISATION

OF ATHEROSCLEROTIC

LESIONS

prior to the development of lesions, to some of the sites at which atheroma does form provides opportunity for studying events peculiar to these sites which might be relevant to atherogenesis. To be relevant these events must at least be such that they can, and presumably do, also occur at other than “typical” sites for lesions are not confined to the latter sites. The fact identify

that with the

it

is

possible,

precision

Factors

implicated in the localisation of lesions in the arterial tree have included analogies with knowledge from the science of fluid mechanics. When fluid flows in pipes there are alterations in the velocity of flow around bends, at bifurcations and at points of branching, with a decrease in lateral pressure and decrease in wall shear rate at these sites: sites which are indeed remarkably similar to those at which lesions in the arterial tree do form, This analogy has led Texon(lO] to postulate that because of decreased lateral pressure at these sites there is proliferation of endotheliun and of fibroblasts from deeper layers which ultimately result in advanced atherosclerotic lesions. It is however difficult to reconcile this interpretation with, for example, the absence of similar

534

lesions in venous pulmonary artery monsry hypertension

channels becomes with

with low susceptible an inevitable

lateral pressure to atherosclerosis

and the if

fact

there

that is

5he

pul-

in lateral pressure. have proposed that decreased in a decreased rate of egress of cholesterol synthesised in for a number of reasons, not fact that synthesis of cholesterol in for its accumulation in the atheroincrease

Caro et al (111, from similar considerations, velocity of flow at the above sites results from the arterial wall to the vessel lumen This concept also seems remote the artery. least

of

which

is

the arterial wall sclerotic lesion It has been atherosclerosis turnover of

the established cannot account

(see Adams

121.

demonstrated in the aorta at sites predisposed to there is both an increase in permeability and increased endothelial cells (see Ross and Glomset 91. This increased permeability has been shown in several animal species with a number cf Increased turnover markers and seems to be constantly present in the aorta. of endotheliun has been inferred from studies on the uptake of labelled thymidine by nuclei of endothelial cells at these sites and, if sought, this will also be found at other than the "typical" sites of lesions in the Since the endothelium remains a single layer cl-f aorta (unpublishedl. cells in the normal aorta, replication of an endothelial cell presumably occurs in response to loss or impending loss of a defunct adjacent cell. Unlike fluid to changes

directly that

in

circmference

flow in velocity

of the

changes in tension its longitudinal

pipes of

(and blood

arterial

veins1

every

flow,

wall

is

cardiac

cycle, in addition by alterations in

accompanied

the

from Laplace’s Law, inevitable the wall, plus transmission of the pulse wave along These bi-directional forces (and possibly other exert considerable shear on the endothelial layer, with,

in axis. relevant forces1 must among their other effects. It is conceivable that the endothelium is less able to accomnodate to these forces at points of branching and thus more prone to in jury at these sites, It would indeed be surprising if these forces did not result in damage to the endothelium at other sites from time to time, The outer curve of the aortic arch is known to be more prone to atherosclerosis than the inner curve. Other than changes in the velocity of blood flow that must occur around this curve, we can again infer from Laplace that the tension in the wall along the larger radius must be greater than that along the smaller radius, The endothelium on the outer wall would therefore be more liable to injury than that cn the J,nner

wali. Relevance

of

endothelial

injury

to the

development

of

atherosclerosis

Ross and Glomset (9) have reviewed numerous elegant studies of events following experimental injury to arterial Pndothelium. Injury to the endothelium, however it is produced, will bring blood into direct contact with basement membrane and other elements of the vascular ground substance including collagen and mucopolysaccharides [NPS).

535

As part wall at

of the normal these sites,

response to After their

injury, adhesion,

platelets platelets

adhere

to

the

vessel

release a number of macromolecules which, among their other effects, nay alter permeability of the vessel wall and could account for the increased permeability at sites prone to injury, Following this there is a proliferation of smooth muscle cells together with the generation of MPS, collagen fibrils and elastic fibres; elements which are indeed present in the atherosclerotic One may add that fibrinogen from plasma is also likely to be plaque. converted to fibrin at these sites as part of the normal sequence of events following platelet adhesion 1131 and/or through activation of Hageman factor by collagen (141. This could account for the presence of fibrin subendothelially after endothelium has regenerated, Fibrin is a well recognised constituent of the atherosclerotic plaque in rran (151; one which is often overlooked, The burden

of the thesis propounded by Ross and Glomset (91 is that injury is primarily responsible for the evolution of atheroaided to varying degrees by a number of other plasma constisclerosis, tuents. including plasma low density lipoproteins (LOLI, when their concentration is high. It is however difficult to accept that endothelial injury alone can lead to the development of clinically significant atherosclerosis since it occurs in normal animals under conditions where atherome is not developing. Even after experimental injury to endothelium, where the damage done is likely to exceed considerably that occurring spontaneously, the substantial cellular response which follows regresses in time. The fact that certain sites in the arterial tree are indeed liable to recurrent injury seems nevertheless to be benign in itself. The demonstrable changes (161 in the arterial wall at atherorra-prone sites in the aortas of animals not naturally prone to this disease are not such that there is significant encroachment on the vessel lumen, or likely to lead it seems likely that the to clinically significant events. Furthermore, arterial tree in all human subjects (and other mammals with a similar circulation) is subject to similar degrees of inevitable endothelial injury, yet there are marked differences in the degree of atherosclerosis between human populat ions (11. endothelial

The view that high plasma LOL is inherently injurious to endothelium also seems unlikely for it would be difficult to reconcile the characteristic It would focal nature of atherosclerotic lesions with this suggestion, also be difficult to understand why the endothelium of venous channels should be immune to such injury. If we are to accept that endothelial injury is relevant to the development and there are indeed very good reasons for believing of at herosclerosis, it is, there must be processes other than, or in addition to, this which determine the development of clinically significant lesions in n-an.

536

THE

ROLE THE

The CHD They

epidemiological has recently have also of developing

risk however, despite elevated

OF

LIPID

IN

DEVELOPMENT

ATHEROSCLEROTIC

evidence linking high plasma cholesterol level to been sumrnsrised by Glueck, Mattson and Bierman (171. indicated that there are multiple factors affecting the It is notable, this disease in high risk groups. in a country like Japan prevalence of CH3 remains low

that exposure plasma

to major

risk

cholesterol

factors

level

with

(181.

the Data

assign a primary role to plasma cholesterol this setting that other risk factors could expressions of this disease. Ever since arterial

Page

wall,

speculation the mechanisms

OF

LESION

and

(8) implying

which

notable this

it

exception would

influence

and it

the

clinical

was there

deposited

lipid the

lipid

to ii

in %he been muc'i in this lesion. means by which it is now gocd

evidence and

of

is

has

There the development of atherosclerosis. that this lipid is indeed derived from beta-lipoproteins very low density lipoproteins - VLDLI. It would be more say that plaque lipid is derived primarily from this plasma since deposited lipid is subject to some change by processes arterial wall (see Adams 121.

may promote

Mechanism

of seem

sort

concentration

that beta-lipoprotein this was atherogenic, on the source of the might be deposited and

suggested that

controversy by

of

(LDL;

accurate fraction in the

to

deposition

There

have been numerous studies showing by several methods that lipid plasma gains access to the arterial wall at its lumenal surface in normal and in atheromatous vessels (see Adams 121. For a variety of reasons, many of which have been cited by Adams, investigators have questioned whether such transfer could account for the lipid that is found in the plaque, Confusion appears to have arisen from the tacit assumption that plasma lipid, however it gains access to arterial wall, has relevance to atherogenesis (e.g. see Stein and Stein In studies 191, such as those referred to above, transfer of plasma lipid occurs throughout the arterial wall in normal anirnsls under conditions in which atheroma is not forming. Sue h transfer therefore seems likely to reflect normal events which need have no relevance to deposition of lipid occurring as part of atherogenesis. Even in athemmatous arteries, this transfer need reflect from both

no more than

normal

transport

processes,

Although our knowledge is rudimentary, there is evidence endothelium limits the transfer of plasma macromolecules cules above about 40,000 daltons traverse the endothelial lemma1 vesicles (191. The upper limit in molecular size thelium by this route has not been determined. It seems that the molecules of LDL and/or VLDL ordinarily traverse thelium in any quantity. To be deposited in the arterial VLDL would therefore need to gain access to the subendothelial

extraordinary means, the arterial wall only and where permeability

There

is

evidence sites

at

those

is

increased,

that

these endothelium

where corresponding

537

vascular by size. Mole-

that

cells

plasrra-

endohowever

unlikely

vascular wall,

LDL

space readily

molecules has with

in

traversing

those

been

endoand

by enter

damaged(20)

regions

of

the arterial wall as described above. The adhesion of ths platelets to sites of endothelial injury (91 does not provide an immediate seal. Low density lipoproteins, as well as other plasma constituents, are likely to continue to gain access to the subendothelial space at these sites, at least until the platelet aggregate becomes consolidated or endothelium regsnerates. By this mechanism the net transfer of plasma low density lipoproteins to the subendothelial space in unit time will vary directly with their concentration in plasma.

Burstein and Samaille L211 showed some years ago that the mucopolysaccharide heparin selectively precipitates low density lipoproteins from plasma in vitro and this formed the basis of a routine laboratory method for estimating the concentration of this plasma fraction. The possible relevance of an interaction between MPS in the artery and the trapping of plasma cholesterol in its wall appears to have been first Suggested by Faber [221, since when many others have found evidence in support of this (see Adams 121. Amenta and Waters (231 made observations which seem particularly pertinent to this discussion. They found that MPS extracted from hunan aorta precipitated low density lipoproteins of hypercholest8rolaemic rabbit plasms and hunan plasma, Using normal rabbit plasma, with its exceedingly low lipid levels relative to those in man 1241, precipitation did not occur. This may have relevance to the fact that the rabbit lipid in its arterial plasma lipid levels

does

not

accumulate

significant

amounts

of

plasms

tree, despite recurrent endothelial injury, unless are elevated. Bordjers and BjUrkeru3 (201 have inferred that plasma LOL which enters the arterial wall at sites of endothelial injury in normo-lipidemic rabbits may be rapidly eliminated; unlike the continued accumulation of lipid in the arterial wall of hyperlipidaemic rabbits. Amenta and Waters (231 did not determine the concentration of plasma lipids below which precipitation with MPS would not occur. It seems likely however that in man this level is always exC8eded since the method of Burstein and Samaille 1211 has been used successfully in populations with markedly different lavels of plasma LOL. It is conceivable, however, that the higher the concentiation of LDL in the arterial wall the n-ore likely is it to be bound to MPS in the subendothelial space. Another

suggestion

to

in the subendothelial suggested that these

process in this region

the meshwork of MPS located of the arterial wall.

It would seem lial space do lipid droplets low magnification

recognised

account for the retention specifically of plasma space has been put forward by Iverius (251. He molecules are selectively retained by a sieving

that not in

feature

in

the

connective

tissue

matrix

WL

in

density lipoproteins if trapped in the subendotheExtracellular stab18 complexes in this milieu. the subendothelial space large enough to be visible at and which take up routine lipid stains are a well of the evolving atheromatous lesion in man L261. This the lipoprotein complexes have been disrupted and that low

remain

would suggest that the lipids freed of apolipoprotein

have coalesced.

538

Role

of

deposited

lipid

in

atherogenesis

The capacity of the arterial wall to clear lipids deposited in it would seem to be limited, Theoretically, some of the lipids, other than cholesterol, could be metabolised locally (see Adams High density 121. lipoproteins (H)Ll, it is thought, could readily traverse vascular endothelium (191, and thus conceivably the multiple cell layers of the artery. There has been much speculation recently that HDL may have a role in preventing atherosclerosis (see Glueck, Mattson and Bier-man 171. The pertinent question, however, would seem to be whether HOL could effect a significant net transfer of deposited UIL cholesterol out of the arterial wall. This has not been examined, It seems relevant to note that populations with marked differences in prevalence of CKI show little differercein plasma H3L levels (241. Another possible mechanism for egress of deposited lipid is that it could be taken up by macrophages. Cells in atheromatous lesions containing lipid droplets are well described. These cells are now widely held to oe smooth muscle cells which have migrated from the media or arise from replication of the occasional smooth muscle cell normally found subendothelially, or by both processes (see Ross and Glomset 91. It is suggested that these cells, whatever their origin, appear in these areas in response to previously deposited free lipid and some of the lipid drop.lets are phagocytosed by these cells. The migration of these cells into the vessel lumen appears not to have been recorded and capillaries L271 (and probably lymphatics) which they could conceivably enter are not normally present in this part of the vessel wall. These cells have the capacity to generate connective tissue elements (see Ross and Glomset 5, Adams 12) which seems likely to incorporate them in a matrix of their own making. Thus, in addition to producing the connective tissue elements of the atherosclerotic plaque, these cells seem more likely to ensure the trapping of lipid from low density lipoproteins in the arterial wall tnan to effect lipid clearance. It is suggested that the cellular and tissue reactions that have significance in atherogenesis occur in response to the presence of free lipid in the subendothelial space and not to endothellal

injury

per

se.

the above concept there is evidence that free lipid li,e. to apolipoprotein) is able to evoke a tissue reaction, Cholesterol several of its esters implanted subcutaneously in the rat produce a local granulcma (see Adams 121. Cholesterol acetate crystals placed in contact with aortic endothelium in the rabbit evokes and extsnIn keeping lipid not

sive

with bound and

‘fibrous’

Factors

affecting

reaction the

around physical

it

(281,

state

of

deposited

lipid

The physical state of the lipid in the subendothelial nificantly altered by phospholipid and the consequent affected profoundly. In an aqueous medium phospholipid, tidylcholine, can emulsify triglycerides and cholesterol micellise cholesterol (29-311. When the cholesterol pholipid and implanted subcutaneously both lipids are

is

little

tissue

reaction

(321.

In the

rabbit

539

aorta,

space could be sigtissue response notably phosphaesters and is mixed with phos-

cleared and there cholesterol acetate

mixed with "macrophages".

phosphollpid and implanted subendothellally is taken up by Although it is questionable whether these lipid-laden cells ever leave the arterial wall, it seems possible that if the free lipid is phagocytosed the cellular response it evokes would cease, Thus if dispersed finely enough to be phagocytosed the tissue reaction to deposited free lipid could be limited. In human atherosclerotic lesions, however, the proportion of phospholipids to neutral lipids (331 Is less than the amount required for optimal dispersion of the neutral lipids

in

vitro

Of

all

(29-311. the

lipids,

its

clearance

the

most

from

cholesterol the arterial

effectively

"solubilised"

atherosclerotic

to present

seems

wall,

Although phospholipid,

by

the major cholesterol

problem in is potentially Its proportion in relative to the other a well recognised

shows the greatest increase plasma lipids (33,341. Crystals of cholesterol are feature of advanced atherosclerotic lesions in n-an. It has been postulated (351 that the mechanism of cholesterol deposition during atherogenesis may be due to a crystallisation process and that the cholesterol crystals This concept seems to have been borne out by more promote atherogenesis. recent studies in which it has been shown that cholesterol in evolving lesions

exists

lesions

in

supersaturated

solution

(see

Small

361

and

thus

may

The in vitro system in which crystallisation crystallise out of solution, of cholesterol has been studied (351 would therefore seem to bear a closer resemblance to the in vlvo situation than was originally apparent, In this system, phosphatidylcholine can prevent the crystallisatlon of Its capacity to do so, however, cholesterol from supersaturated solution. the rate of cholesterol crystallisation is markedly inhibited by glucose; These observations increasing directly with increasing glucose (37). could have relevance in vivo for the concentration of glucose in the arterial wall varies directly with that In plasma and transfer of plasma glucose into the artery is not dependent on insulin (see Winegrad, Morrison and Clements 361. Thus, by interfering with the capacity of phosphatidylcholine to solubilise cholesterol, increased plasma glucose levels could promote cholesterol crystallisation, decrease its “clearance” by phagocyThis function tosis and enhance its potential to evoke a tissue reaction. of glucose could have relevance to the accelerated atherosclerosis in Also noteworthy is the well documented diabetics (see Keen 39, 11. observation that subjects who have suffered a myocardial infarction commonly show a lesser degree of glucose tolerance than do matched control The decrease in the groups (see Wilkens and Krut 37 and Keen 391. capacity of post myocardial infarction patients to handle an oral glucose load is reflected directly by a decrease in the capacity of extracts of their serum “lipids” to maintain cholesterol in stable solution in vitro [371. be added to the above that the polyol pathway is active in aortlc and media with synthesis of both sorbitol and fructose In this [3i31. Fructose Las well as other sugars) were previously found like glucose in promoting the crystallisation of cholesterol (371. More recent studies have shown that sorbltol has the same potent effect There as glucose In promoting cholesterol crystalllsation in vitro L401. is then considerable circumstantial evidence In support of the view [351 that crystalllsatlon of cholesterol In the arterial wall may contribute It may intima tissue to act

significantly

to the

atherosclerotic

process.

540

EVOLUTION

OF THE ATHEROSCLEROTIC

PLAQUE

Smooth muscle cells which migrate to sites where atheroma is evolving do not show the ordered arrangement of smooth muscle cells in the media of This, together with tne the arterial wall (see Rosswd Glomset 9.411. generation of connective tissue elements by these cells, seems likely to decrease the compliance of the affected areas and render the endothelium in the vicinity of evolving lesions more liable to injury and the Increased turnover of endothelia: at herogenic process thereby promoted, cells and decreased platelet survival in animals fed cholesterol has indeed been demonstrated and this attributed to endothelial injury due

directly

low density lipoproteins [see Ross and drawn from the latter observations could be interpreted differently, Namely, that it is because lesions are evolving in these animals that endothelial turnover is increased and platelet survival shortened. Thus, it cwld be argued that, once initiated, atheromatous lesions are likely to develop at an accelerated rate. Glomset

to elevated

91.

The

plasma

conclusions

The intima and inner half, or so, of the media is ordinarily devoid of capillaries [271, The viability of the inner arterial wall is therefore dependent on the direct transfer of nutrients from the lumenal surface outwards across multiple layers of cells to reach the vasa vasorum in the outer media, As the atheromatous lesion develops, nutrients would need to be transferred across an increasing depth of tissue. Since the diffusion of oxygen through tissue is generally limited to about 1 mm, there is clearly a limit to the depth to which lesions could grow and remain viable, particularly as the fibrotic and connective tissue elements seem likely to impede diffusion, The cellular elements of the lesion at the greatest depth from the lumenal surface are most likely to be deprived of nutrients and it is at this site where necrosis typically occurs (15, 411. The characteristic finding of a loss of smooth muscle cells subjacent to atherosclerotic lesions (151 could also be attributed to deprivation of nutrients in these areas. In terms of these concepts atherosclerotic lesions must be limited in the depth to which they can develop and this could account for their plaque-like structure, The further endothelial

of the lesion shows a change in its character, As injury continues the thrombogenic processes and trapping of low density lipoproteins will continue. In areas where the lesion is no longer viable the cellular response which follows the deposition of free lipid in viable tissue is unlikely to occur, but the thrombotic elements seem likely to persist, This will, after repeated episodes, result in a necrotic gruel overlaid by a rather structureless cap of fibrous materia covered by regenerated endothelium which characterises the “fibrous” plaque. evolution

The lesion obtruding on the lumen, its decreased compliance and the loss the supporting affect through necrosis of its cellular elements, increase its susceptibility to injury by the physical forces generated in the arterial wall. This could account for the tears which ocour in these lesions and which makes them liable to ulceration of their surfaces. Massive intra-luminal occlusion could follow either of the above complications, if thrombogenic processes are adequately activated by exposure of

of

541

sufficient of the connective tissue elements of (42,431. This would be most likely to occur in the coronary arteries and intracranial arteries, on the lumen may slow blood flow enough to allow become consolidated at such a site. The platelet where blood injury to plaques in large vessels, are more likely to be dislodged than to slowed, luminal occlusion. FAMILIAL

the

plaque

to

the

blood

smaller where

vessels, such as a lesion obtruding a platelet aggregate to aggregates at sites of

flow cause

is

not significantly massive intra-

HYPERCHOLESTEROLAEMIA

A disorder peculiar to subjects with familial hypercholesterolaemia would seem to b8 highly relevant to some of the concepts presented here. Studies with cultured skin fibroblasts and with leucocytes from these subjects have demonstrated two types of genetic defect (44-461. EQth defects have in common the inability, or markedly reduced ability, of plasma low density lipoproteins to attach to cell membrane and consequently the cholesterol [and presumably the other lipids1 of these lipoproteins do not gain access to these cells or gain access at a markedly reduced rate, It has however been shown that cholesterol free of apolipoprotein readily gains access to fibroblasts from these subjects, It has never been suggested that the atherogenic process in these individuals differs fundamentally from that in persons not so afflicted. If the above findings with fibroblasts and leucocytes apply to other cells, as is implied, then it is evident that plasma low density lipoproteins could not traverse the endothelial cells of the arterial wall and could gain access to the subFurtherendothelial space only at sites where this barrier is defective. more, smooth muscle cells which appear in the subednothelial space during atherogenesis could not take up deposited lipid unless this lipid was free of apolipoprotein.

There

is evidence that the lipid which accumulates in tendon xanthoms of subjects with this disorder is derived from plasma low density lipoproteins (471. This has been attributed to a leak of plasma from capillaries traumatised in the course of normal activity as splinting an affected area decreases the rate at which plasma lipoproteins reach the xanthoma. Lipid deposition in tendon xanthomas could occur by a process similar to that in arteries since, once extravasated, the low density lipoproteins would come into contact with the MPS in this tissue, and lipid deposition is by no There is however a considerable rate means the only feature of xanthomas. of turnover of lipopsoteins in tendons and reduction in serum cholesterol level commonly results in a visible reduction in the size of xanthomas. It is frequently inferred from the latter observation that this indicates The validity a similar reduction in the size of atherosclerotic lesions, of this inference may be questioned. There is evidence that normal subjects (481,

plasma lipid is also extravasated in tendons of We have no reason to believe that capillaries in subjects with familial hypercholesterolaemia are more fragile than in normal subjects. The fact that normal subjects do not develop frank xanthomas must mean that the rate at which plasma lipid is extravasated does not exceed the capacity of this tissue to clear this lipid. Yet normal subjects with “normal” plasma cholesterol level do develop athero-

sclerosis.

542

It has been shown in rabbits that injury to capillaries is followed by an increase in the concentration of plasma low density lipoproteins in the lymphatics draining the affected area and the higher their plasma concentration the greater their absolute rate of extravasation (49-511. It seems likely that plasma lipoproteins which leak from capillaries in tendons are also cleared by this route and that in normal subjects the lipoprotein load does not exceed the capacity of this clearing system. The subendothelial space of the arterial wall, however, is not equipped Capillaries are not normally present in this with this clearing system. region nor have lymphatic channels been demonstrated there [see HigginEven if they were present it is certain that these totham et al 52). low pressure vessels would be obliterated in a region of the artery subject to its high intra-luminal pressure. Capillaries have been demonstrated in developed atherosclerotic lesicns of normal subjects and this represents neovascularisation (see Woerner 27, Haust 411. These capillaries could remain patent because the fibrous content of the lesion protects them from the intraluminal pressure, just as capillaries in the outer media are presumably protected by their distance from the vessel lumen. It is conceivable that lymphatics may accompany these new capillaries. It is questionable however whether such a development could clear significant amounts of previously deposited lipid. Turnover of cholesterol in atherosclerotic plaques of subjects with terminal illness has been found to be exceedingly low (531, and even this may have reflected turnover of cholesterol other than that in the The inference from these observations is that the cholesterol in plaque. advanced lesions is not part of a metabolic pool, It would indeed be surprising if lipid entrapped in necrotic tissue was part of a metabolic pool. It is therefore difficult to conceive of a reduction in plasma cholesterol level affecting it, Reports claiming reduction in size of atherosclerotic plaques, assessed by angiography, after lowering of plasma cholesterol level might reflect processes other than lipid mobilisation. It may be that there is ectasia of the vessel wall or, like scar tissue elsewhere, the lesion shrinks in time, It would be of interest to know the variations that occur with time on repeated angiography in subjects in whom plasma cholesterol level has not been reduced, VEINS

AN0

THE PULMONARY

ARTERY

Endothelial damage in veins and in the pulmonary artery must occur from time to time, According to the concepts presented here, this must lead to the transfer of low density lipoproteins into the subendothelial space, yet atherosclerotic lesions do not ordinarily develop in these vessels. There are several possible reasons for this, In veins blood flow is not pulsatile and pressure is low. The frequency with which endothelial injury is likely to occur at the same sites in veins is therefore very much less than in arteries, Thus the accumulation of lipids at specific sites would not be great, Of equal importance is the fact that in veins the capillary network arising from the vasa vasorum extends into or close to the intims (see Higginbotham 521. It seems likely that functional lymphatic channels are also present in this region (521 and would therefore provide the subendothelial space with a clearing system for lipoproteins, as described above, which is not present in the arterial wall.

543

The pulmonary artery is different from other arteries, It has a va5a vasorum similar to that of veins L521 and conceivably also has lymphatics reaching its subendothelial space, The development of pulmonary hypertension could predispose to atherosclerosis of this vessel in two ways, Increased intra-luminal pressure would increase tension in the vessel wall, according to Laplace's law. The susceptibility to endothelial damage would therefore increase and transfer of low density lipoproteins to the subendothelial space would now occur, or occur in greater quantity, Possibly more important is that a high intra-luninal pressure would obliterate low pressure vessels like capillaries and lymphatics that are near the lunen and thus eliminate the means by which these plasma constituents could be cleared, should they gain access to the subendothelial space. CONCLUSIONS The arterial

serve

its

tree primary

may be considered function, namely, function it is subject to high intra-luminal

to be less than perfectly adapted that of a conduit for blood, In to inevitable endothelial injury. pressure requires that its walls from that of venous channels and, with

to

serving this Its adaptation be nourished in a way different this, loss of a clearance system for macromolecules which gain access to the subendothelial space at sites of endothelial injury. While both endothelial injury and loss of a clearance system for macromolecules are necessary for the development of atherosclerotic lesions, neither has more than a permissive role which becomes relevant only when plasma cholesterol level is elevated,

Granted initiated and once

undergo stresses further at these affected

that

lesions evolve as has been outlined, it is plain that once they are likely to continue to develop at an accelerated rate they are advanced it is not likely that they could be made to significant regression. Where lesions already exist, increased on the promote sites

areas

arterial wall, such as in hypertension, are likely to their development through even greater endot helial injury than occurs in normotensive subjects and/or predispose more than ordinarily to thrombotic occlusion,

It seems therefore that measures aimed at reducing significantly the prevalence of this disease should be instituted at an early age. Ahrens (61 has made the same recomnendation and has pointed out that in no clinical trial to date has plasma cholesterol concentration been reduced in youth and maintained at a low enough level for long enough to test the lipid Adequate reduction in plasma cholesterol level hypothesis of this disease, would imply reduction to those levels found in populations where atherosclerosis

does not commot~ly reach an advanced stage and where its major To achieve such a goal, it is probable expressions are rare. that the dietary habits of developed communities may need to be altered more drastically than its members would be willing to adopt,

clinical

544

An aspect of this problem which has received little attention iS the factors which could influence the physical stat8 of ChOl8St8rOl trapped in the arterial wall. It has been inferred from in vitro studies that glucose,as well as sorbitol and fructose, could promote the crystallisation of cholesterol in the arterial wall. once ChOlest8rOl has crystallised from supersaturated solution it seems Unlikely that it If this is valid, then transient could be brought back into solution. elevations in blood glucose level, even in non-diabetics, could bring about such a change and thereby enhance the atherosclerotic process. An alternative attack on the problem may lie in identifying substances which have the capacity to prevent the crystallisation of chol8st8m~ from supersaturated solution (401. If such substances are transported together with cholesterol, it is conceivable that they might favourably affect its physical state in the arterial wall and modify the tissue response to this lipid, REFERENCES 1.

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