The effect of hypercholesterolemia on the transverse wall properties of the upper descending thoracic aorta was studied in canines. Hypothyroid animals were fed a free diet supplemented with cholesterol, propylthiouracil, and saturated fat (lard) for 11 mo. The mechanical properties of the wall showed marked changes following experimental pathology. The failure stress, failure elongation, and energy to failure decreased following hypercholesterolemia. The tensile response was bilinear in both the pathological and control tissues. In the initial region, which is controlled by elastic fibers, the response was left unchanged by the experimental pathology. In the secondary region, which is controlled by stretching of the collagen fibers, the intercept with the strain axis and the slope were decreased by the pathology. This indicates that there are changes in the strength-bearing collagen and its interaction with the amorphous matrix. However, no significant parallel histological changes were observed in the structure of collagen and elastic fibers. Although this phenomenon precedes the development of atherosclerotic plaques which are charateristic of a diagnosed human disease, the aortic wall is already “hardened” because of its reduced compliance resulting from the shift in the onset of secondary (collagenous) response.

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