In vivo aortic wall characteristics at the early stage of atherosclerosis in rabbits

To assess whether vascular responsiveness to alpha-receptor agonist is altered at the early stage of atherosclerosis, in vivo aortic pressure-diameter relationship of the aorta over a wide range of pressures was analyzed before and after the acute administration of alpha-receptor agonist (phenylephrine) in nine hypercholesterolemic fat-fed (7-wk-old) rabbits and eight normal diet-fed (7-wk-old) rabbits. In hypercholesterolemic fat-fed rabbits, there was no major structural change in the aortic wall except fatty streak, despite a marked increase in the level of plasma cholesterol, indicating the early stage of atherosclerosis of the aorta. By using a modified three-element Maxwell model, diastolic stress-strain relationship was computed after applying several assumptions to the actual aortic pressure-diameter relationship. After the intravenous administration of phenylephrine at a rate of 5 micrograms.kg-1.min-1, the stress (ordinate)-strain (abscissa) relationship curves were shifted to the left, indicating the activation of aortic smooth muscle by phenylephrine. The difference between the stress before and after phenylephrine showed a single peak at a certain strain. The peak difference in the stress was smaller in hypercholesterolemic fat-fed rabbits than in normal diet-fed rabbits, indicating the reduction of vascular responsiveness at the early stage of atherosclerosis of the aorta.