ARTERIOSCLEROSIS IN MAN, OTHER MAMMALS AND BIRDS

Effect of hypoxia on cholesterol accumulation in cultured rabbit aortic smooth muscle cells

We studied the effect of hypoxia on cholesterol accumulation in cultured rabbit aortic smooth muscle cells, which were incubated in a medium with normolipemic rabbit serum (NRS) or hyperlipemic rabbit serum (HRS). The cells were incubated in a humidified atmosphere of either 20% O2, 75% N2 and 5% CO2 (control cells) or 2% O2, 93% N2 and 5% CO2 (hypoxic cells). In a medium containing 20% NRS, the free cholesterol level of hypoxic cells was only a little higher than that of control cells, and there was no significant difference in esterified cholesterol content. On the other hand, in a medium containing 20% HRS, the free cholesterol level was slightly higher and the esterified cholesterol level was markedly higher in hypoxic cells compared with control cells. These results show that hypoxia promotes the accumulation of cholesterol, especially as ester, in smooth muscle cells cultured with hyperlipemic serum. These in vitro experiments indicate that hypoxia in the arterial wall associated with hyperlipidemia may play an important role in atherogenesis, although the precise mechanism remains unclear.

Pathological principles involved in regression of atherosclerosis

The view is advanced that regression of atherosclerosis depends on normal pathological principles, namely the process of organization. On account of the hypoxic milieu of the arterial wall and lack of capillary ingrowths into it, the hypoxia-resistant arterial smooth muscle cell subserves the functions of both phagocyte and fibroblast in organization. The proliferation of smooth muscle in atherosclerosis is, thus, attributed to a variant of a basic pathological mechanism, and does not require more a complex explanation, such as the action of a somatotropin, mitogen or mutagen. Moderate dilatation of the arterial wall, caused by the basic pathological mechanisms of either atrophy or hypertrophy would, within definite limits, offset inward encroachment by atherosclerosis and, thus, could constitute another type of regression, or at least a failure of lumen calibre to get worse.

The antiocclusive effect of coronary dilatation with age

Human coronary arteries were perfusion-fixed; sectioned and their external and lumenal circumferences measured by microscopic planimetry. They were found to dilate with increasing age, and this change seems to be more a degenerative process than a response to increasing heart weight. It is inferred that a moderate degree of coronary dilatation compensates for the tendency of atherosclerosis to occlude the lumen. Absence of any coronary dilatation might be hazardous in that the stenosing effects of atherosclerosis would be enhanced. By contrast over-dilatation (ectasia) is dangerous in that it causes a reduced flow-rate and, hence, promotes thrombosis.

Is atheroma a reversible lesion?

In this review atherogenic factors are discussed in relation to the possibility of regression. Evidence for regression of human lesions comes mainly from postwar studies and observations on persons with chronic wasting diseases. The entry, exit and effects of lipids in the arterial wass are considered as important factors which might determine regression. A variety of experiments in different animals which have been done in order to study regression are described. Some involve cholesterol feeding and withdrawal, others are concerned with the effects of hyperoxia and drugs. It is concluded that certain forms of atheroma can be induced to regress.

Experimental arteriosclerosis. I. Fibrous plaque formation in primates, an electron microscope study

Arteriosclerotic lesions have been produced in monkeys (Macaca nemestrina) by selective removal of the vascular endothelium with an intra-arterial balloon catheter. Immediately after de-endothelialization a platelet layer covers the denuded area. This thrombus is gradually removed and by 7 days the vessel appears to be largely reendothelialized. Beginning at day 4, smooth muscle cells undergo modification and migrate through fenestrae in the internal elastic lamina into the intima where they proliferate. By 28 days, the intimal lesion consists of multiple layers of smooth muscle cells surrounded by collagen and elastic fibers and basement-like material. After 3 months the lesions are markedly hyperplastic and contain new extracellular connective tissue elements. In contrast, with no further injury after 6 months the lesion has decreased markedly in size suggesting that it may be reversible in the absence of continued endothelial injury. The importance of endothelial "injury" exposing medial smooth muscle to plasma constituents may be the principal factors associated with the migration and proliferation of the smooth muscle cells into the intima resulting in the lesion. The smooth muscle cells do not contain lipid. The similarities of this lesion to the fibromusculo-elastic lesion or preatherosclerotic intimal hyperplasia in man makes it a useful model for the further study of atherosclerosis.

Venous endothelium of experimental arteriovenous fistulas in rabbits

Since hemodynamic factors have been implicated in the localization and pathogenesis of atherosclerosis, the effect of hemodynamic stress on endothelium was investigated. The endothelium of the inferior vena cava of 24 rabbits with aortocaval fistulas was stained in situ and examined by the Hautchen technique at postoperative intervals ranging from 1 to 50 weeks. Cell counts revealed that, in the anastomosed vein of the fistula, an initially high frequency of abnormal cells decreased 3-12 weeks postoperatively but rose again within 20-50 weeks to levels significantly higher than those in either normal venous endothelium or sham-operated veins. An originally high mitotic index at 1-2 weeks decreased with time but still remained significantly larger than that observed in sham-operated rabbits. In most rabbits with arteriovenous shunts, a small area of the inferior vena cava near the fistula was devoid of endothelium, covered with a thin deposit of mural thrombi, and considered to be a jet lesion. The findings demonstrate that hemodynamic factors can cause endothelial injury, increased turnover of endothelial cells, and formation of multinucleated endothelial cells characteristic of regenerating endothelium.