Changes in the microindentation properties of aortic intimal surface during cholesterol feeding of rabbits

A new microindentation technique using a small, loaded spherical probe (25 microns diameter) was developed to quantify early mechanical changes in the arterial intima during atherogenesis. Routine use of forces of 10 microN on normal intima for 10 S resulted in indentations of about 2.5 microns which gives an intimal compliance of 0.245 +/- 0.031 m/N. There was a decrease in intimal compliance in the proximal to distal direction in both the rabbit and the dog thoracic aorta. The region immediately distal to an intercostal orifice in the rabbit was often up to 1.6 times more compliant (softer) than surrounding areas while the corresponding region in the dog tended to be less compliant than elsewhere. Feeding a 1% cholesterol diet to rabbits for 8 weeks increased the intimal compliance to 0.368 +/- 0.041 m/N. The regions distal to the intercostal ostium were 32% more compliant after 4 weeks of cholesterol diet than non-orifice-related areas but after 8 weeks of the diet there was no significant difference between these regions. The time course of indentation (viscous component) was characterised by a rapid indentation phase which almost became non-exponentially asymptotic by 6 S and was 66 +/- 3% SD of this value by 1 S. Cholesterol feeding (8 weeks) altered this phase to 76 +/- 4% SD of the asymptotic value after 1 S of indentation, indicating a decrease in initial viscosity. A very slow indentation or creep was also found when the force continued for a period up to 25 min. Cholesterol feeding increased this rate of creep by approximately 15%. The results show that early atheromatous changes induced by cholesterol feeding are accompanied by softening and increased fluidity of the aortic intima.