Age-related differences in the distribution and occurrence of atherosclerotic aortic lesions in the hyperlipemic rabbit

The contribution of resident vascular stem cells to arterial pathology

Intimal accumulation of smooth muscle cells contributes to the development and progression of atherosclerotic lesions and restenosis following endovascular procedures. Arterial smooth muscle cells display heterogeneous phenotypes in both physiological and pathological conditions. In response to injury, dedifferentiated or synthetic smooth muscle cells proliferate and migrate from the tunica media into the intima. As a consequence, smooth muscle cells in vascular lesions show a prevalent dedifferentiated phenotype compared to the contractile appearance of normal media smooth muscle cells. The discovery of abundant stem antigen-expressing cells in vascular lesions also rarely detected in the tunica media of normal adult vessels stimulated a great scientific debate concerning the possibility that proliferating vascular wall-resident stem cells accumulate into the neointima and contribute to the progression of lesions. Although several experimental studies support this hypothesis, others researchers suggest a positive effect of stem cells on plaque stabilization. So, the real contribute of vascular wall-resident stem cells to pathological vascular remodelling needs further investigation. This review will examine the evidence and the contribution of vascular wall-resident stem cells to arterial pathobiology, in order to address future investigations as potential therapeutic target to prevent the progression of vascular diseases.

Contrasting patterns of spontaneous aortic disease in young and old rabbits

The pattern of spontaneous lipid deposition around aortic branch ostia was mapped in New Zealand White rabbits aged 1 month or 2 to 6 years. The young rabbits were studied within 1 day of weaning, and the older rabbits had been maintained on a low-protein, low-fat diet. Plasma concentrations of total cholesterol for the two groups averaged 75 and 18 mg/dL, respectively. Aortas were fixed in situ at a pressure of 90 to 100 cm H2O, stained with oil red O, and photographed en face under an epifluorescence microscope. Areas of staining contrasted in color with the fixative-stimulated autofluorescence of nondiseased tissue and were mapped by placing grids over the photomicrographs. Disease around intercostal ostia was rare, but two distributions were established by combining data from many branches. In weanlings, staining was seen within a triangular area downstream of the branch. In old animals, this area had the lowest frequency of disease; lesions tended to occur downstream of the spared region, along axes lying to either side of it, and at the lateral and upstream margins of the ostium. Disease was less rare at celiac branches. It occurred mainly downstream of the ostium in weanlings, whereas upstream sites were most affected in old animals, although significant disease remained at the juvenile locations. Earlier reports have described similar age-related distributions of disease in human aortas, consistent with a common underlying mechanism. The distributions also correlate with the spatial variations in arterial transport properties established in previous studies, and may be determined by them.

Hypercholesterolemia in the rabbit induced by feeding graded amounts of low-level cholesterol. Methodological considerations regarding individual variability in response to dietary cholesterol and development of lesion type

While a number of studies have presented detailed examinations of lesion development in the cholesterol-fed rabbit, individual variability in response to cholesterol feeding and type of lesion produced relative to the degree of cholesterol exposure is not well defined. This study analyzed such critical parameters in an attempt to further characterize the model and establish a baseline for future testing of treatments targeted at limiting atherosclerosis. For these experiments, male New Zealand White rabbits were fed atherogenic diets consisting of 0.05%, 0.10%, 0.15%, 0.20%, or 0.25% cholesterol dissolved in 6% peanut oil for 31 to 32 weeks. Raising dietary cholesterol from 0.05% to 0.15% resulted in a less than twofold stepwise increase in total plasma cholesterol (TPC) exposure (area under plasma cholesterol versus time curve), whereas further increases in cholesterol intake resulted in an exponential four- to fivefold increase in TPC exposure. Regression analysis of TPC exposure with aortic sudanophilia demonstrated a threshold of approximately 5000 cholesterol weeks; below this limit lesions were minimal, and above this value the degree of plaque correlated with TPC exposure. Furthermore, a wide biological variability occurred among rabbits with respect to individual responsiveness to dietary cholesterol. In the aorta, various types of plaques, from fatty streaks to atheromatous lesions, were observed, depending on the degree of cholesterol intake. Diets consisting of < 0.15% cholesterol resulted in the development of fatty streak lesions, while transitional lesions and atheromatous plaques were mostly found with higher cholesterol feeding. Coronary artery atherosclerosis was present in > 50% of animals fed diets > or = 0.15% cholesterol. Despite the level of TPC exposure, coronary lesions in epicardial vessels were generally the fibrous type, whereas intramyocardial arteries demonstrated predominantly intimal foam cells. In conclusion, by adjusting dietary cholesterol intake and selecting rabbits with a similar responsiveness to cholesterol, the overall cholesterol exposure can be more closely controlled to minimize the inherent individual variability among animals in this model. The nature of the target lesion must also be carefully considered, because the efficacy of some treatments may depend on the type of atherosclerotic plaque.

Propionyl-L-carnitine prevents the progression of atherosclerotic lesions in aged hyperlipemic rabbits

We have characterized the extent and the phenotype of total and proliferating cell population of aortic plaques in aged rabbits receiving a long-term low-dose cholesterol hyperlipemic diet, which represents an experimental model of atherosclerosis. For nine months, rabbits received the hypercholesterolemic diet alone or in addition to a treatment with propionyl-L-carnitine (PLC), a derivative of carnitine, an intramitochondrial carrier of fatty acids present in most cell types. We observed that, in both PLC-treated and control hyperlipemic rabbits, the ratio between proliferating macrophage-derived and smooth muscle cells was 2:1. PLC in addition to the hypercholesterolemic diet induced a marked lowering of plasma triglycerides, very low density lipoprotein (VLDL) and intermediate density lipoprotein (IDL) triglycerides, while plasma cholesterol was slightly and transiently reduced. Moreover, PLC-treated hyperlipemic rabbits exhibited a reduction of plaque thickness and extent, a slight but significant reduction of the percentage of macrophage-derived cells as compared to control hyperlipemic animals and a reduction of the number of both proliferating macrophage- and smooth muscle cell-derived foam cells. Finally, both proliferating and non-proliferating plaque cells expressed large amounts of macrophage colony-stimulating factor protein, in particular macrophage-derived foam cells. These results indicate that a modification of plasma lipemic pattern obtained by a long-term oral administration of PLC was associated with a decrease of plaque cell proliferation and severity of aortic atherosclerotic lesions.

Cholesterol-fed and casein-fed rabbit models of atherosclerosis. Part 2: Differing morphological severity of atherogenesis despite matched plasma cholesterol levels

One-month-old male New Zealand White rabbits were fed either a cholesterol-free casein diet (n = 10) or low-level cholesterol-supplemented chow (n = 10) for 24 weeks, during which total plasma cholesterol levels were matched. After perfusion fixation, aortic tissue samples were taken from six predetermined locations and embedded in epoxy resin for examination by light and electron microscopy. Frozen sections were also obtained for histochemical demonstration of collagen and elastin. Lesion morphology was classified in toluidine blue-stained, semithin epoxy sections as early fatty streaks (round foam cells with little intervening extracellular matrix); advanced fatty streaks (foam cells with extracellular lipid); fibrous plaques (spindle-shaped cells within extracellular matrix); or atheromatous lesions (presence of an atheromatous core). In representative specimens, electron microscopy showed that the ultrastructure of round foam cells was consistent with macrophage derivation, whereas most spindle-shaped cells were clearly smooth muscle cells. Fibrous plaques were more common in the distal than the proximal aorta. Lesions in the casein-fed animals were essentially equally distributed among the four morphological categories, whereas lesions in the cholesterol-fed rabbits were predominantly of the atheromatous type. Thus, cholesterol-fed rabbits had, in general, more advanced lesions than casein-fed rabbits with matched total plasma cholesterol levels. Moreover, the feeding of a low-level cholesterol diet (0.125% to 0.5% by weight) to rabbits for a relatively short time (6 months) led to the development of advanced lesions similar to those seen in humans.

Age-related modifications of aorta and coronaries in the rabbit: a morphological and morphometrical assessment

Aging seems to be related to various vascular diseases, such as dissecting aneurysm and atherosclerosis. The nature of the relationship between aging and these vascular diseases has not been completely clarified. The goal of this study was to investigate, using morphological and morphometrical methods, the age related modifications of the arterial wall in rabbits of three different ages, evaluating separately two different vascular districts and the various aortic segments. Our results confirm that the most relevant age-related structural aortic changes were the increase of thickness, length, volume and diameter of the vessels, together with the development of an intimal thickening. The latter was diffuse in the aorta and focal in coronary vessels and it appears earlier in the aorta than in the coronaries, being absent in the coronaries of young rabbits. In addition, morphological and ultrastructural studies revealed the presence in aged rabbits of some marked intimal storage of a ground substance into intimal thickening of proximal aortic segments. Morphometric studies demonstrated an age-related decrease of aortic cellularity of tunica media and a parallel increase of the content of collagen and glycosaminoglycans, whereas elastin did not vary. The different relationships between cells and interstitial tissue occurring with aging are most probably a phenomenon of adaptation to the changing forces acting on the arterial wall and they might constitute the structural background of the increased arterial susceptibility to various noxae. Finally, the intimal storage of the ground substance, probably related to a functional disturbance of endothelium and or smooth muscle cells, may play an initiating role in atherogenesis.