IN-VIVO AORTIC-WALL CHARACTERISTICS AT THE EARLY-STAGE OF ATHEROSCLEROSIS IN RABBITS

To assess whether vascular responsiveness to or-receptor agonist is al tered at the early stage of atherosclerosis, in vivo aortic pressure-d iameter relationship of the aorta over a wide range of pressures was a nalyzed before and after the acute administration of alpha-receptor ag onist (phenylephrine) in nine hypercholesterolemic fat-fed (7-wk-old) rabbits and eight normal diet-fed (7-wk-old) rabbits. In hypercholeste rolemic fat-fed rabbits, there was no major structural change in the a ortic 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, diastol ic stress-strain relationship was computed after applying several assu mptions to the actual aortic pressure-diameter relationship. After the intravenous administration of phenylephrine at a rate of 5 mu g . kg( -1) . min(-1), the stress (ordinate)strain (abscissa) relationship cur ves were shifted to the left, indicating the activation of aortic smoo th muscle by phenylephrine. The difference between the stress before a nd after phenylephrine showed a single peak at a cel tain 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 th e aorta.