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Abstract Using isolated segments of the abdominal aorta of normotensive rats, the dependence of the dynamic circumferential elastic modulus (E d), the loss modulus (ωηw), and and the coefficient of wall viscosity (ωηw) on the mean circumferential wall stress (σt) and the frequency of radius changes were studied under conditions of strong smooth muscle activation, induced by norepinephrine (NE), and during relaxation, induced by papaverine (PAP). The arterial segments were subjected to quasistatic and to small sinusoidal volume changes of 0.1–20 Hz at mean pressure levels of 1–23 kPa. The diameter changes were recorded by means of a photoelectric device with high spatial and temporal resolution.E d, ωηw, and ηw were calculated from the mean external and internal radii and from the dynamic pressure-radius changes determined at each pressure level. Results and Conclusions 1. The relative decrease in mean radius produced by NE-activation of the resting smooth muscle, is only of the order of 10–15%. The maximum active decrease in radius occurs at a pressure level of about 10 kPa. 2. The quotient of the dynamic to the quasistatic elastic modulus increases from 1.5–2.1 under NE, and from 1.2–1.5 under PAP when σt is increased from 1·102 to 15·102 kPa. 3. E d and ηw increase with increasing σt. At a given σt,E d is virtually independent of frequency, while ωηw slightly increases with increasing frequency. The values ofE d and ηw obtained under NE and PAP are virtually identical. From these findings it is concluded that the elastic behaviour of the vessel wall is determined chiefly by the stiffness of the passive elements. 4. At a given frequency, ηw increases with increasing σt, while ηw decreases markedly with increasing frequency when σt remains unchanged. This behaviour is called, in the terms of polymer rheology, thixotropy or pseudoplasticity. The values of ηw obtained under NE and under PAP are virtually identical. This leads to the conclusion that the viscous properties of the arterial wall, under pulsatile conditions, reflect the viscosity of the passive elements rather than the viscosity of the contractile element.
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