1. The effects of clonidine on the aortic pressure-diameter relationship were studied in anaesthetized rabbits by means of an ultrasonic dimension technique. Midwall aortic stress and aortic elastic modulus were calculated.

2. In spite of a significant decrease in mean aortic blood pressure, aortic diameter was slightly but not significantly increased after intravenous clonidine injection (25 μg/kg body wt., intravenously over 30 s).

3. Midwall aortic stress was decreased when compared at the same aortic radius and aortic elastic modulus was increased when compared at a common blood pressure level. However, when aortic elastic modulus was plotted as a linear function of the stress, no difference was observed before and after clonidine.

4. The results suggest that the pressure-diameter changes induced by clonidine were primarily reflecting the mechanical changes in the aortic wall due to changes in smooth muscle tension. Furthermore it seems that the elastic modulus of the aorta tends to remain constant by adaptive changes in the aortic wall which over-ride the concurrent geometrical changes to maintain a level of elasticity consistent with the degree of aortic stress.

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