Atherosclerotic Changes and Rheological Properties of Thoracic Aortas in WHHL Rabbits

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By means of a tensile testing instrument (MINEBEA, TOM 30J), tension-strain curves, stress-strain curves and stress relaxation curves were measured of circumferential strips excised from the proximal portion of the thoracic aorta in normal and WHHL rabbits aged from 8 to 30 months. Areas of atheromatous plaques on the intimal surface of the aorta in WHHL rabbits were measured using a area computing equipment (Planimex-25, Nihon Regulator) .<BR>Tension-strain curves were concave toward the tension-axis, indicating a decrease in wall distensibility with increasing strain. The wall distensibility of atherosclerotic aortas was lower than that of normal aortas and progressively decreased with advancing age. A positive relationship was obtained between wall thickness and age with the correlation coefficient of 0.66 and also between wall thickness and area ratio of the atherosclerotic lesion with the correlation coefficient of 0.77. In the old age group of WHHL rabbits, aortic walls showed fairly high elastic moduli calculated from the stress-strain curves. In the young age group, however, elastic moduli of the aortic walls decreased with increasing wall thickness. These results led us to the following speculations. The decreased distensibility of the atherosclerotic wall may be attributed mainly to an increase in wall thickness caused by an intimal thickening. In advanced atherosclerotic aortas, wall stiffness explained by an increase in elastic modulus may be an additional factor of the increased distensibility.<BR>Normal and atherosclerotic aortas exhibited some amount of stress relaxation curing the experimental period of five minutes, the degree of stress relaxation being smaller in the former than in the latter. There was a positive relationship between the degree of stress relaxation and wall thickness with the correlation coefficient of 0.58 for the aortas in WHHL rabbits aged 8 months. This may be mainly due to the increased viscoelasticity of the thickened intima associated with the proliferation of foam cells and the deposition of lipids.
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