Smooth Muscle Tone and Arterial Wall Viscosity

Abstract

Abstract-The relationships between steady and pulsatile pressures, smooth muscle tone, and arterial viscoelastic behavior remain a matter of controversy. We previously showed that arterial wall viscosity (AWV) was 3-fold lower in vivo than in vitro and suggested that in vivo active mechanisms could minimize intrinsic AWV to improve the efficiency of heart-vessel coupling energy balance. The aim of the present study was to determine the role of smooth muscle tone on AWV, under various levels of steady and pulsatile pressures, both in vivo and in vitro. AWV of rat abdominal aorta was studied first in vivo after bolus injections of phenylephrine (PE) or sodium nitroprusside (SNP), then in vitro in response to PE or SNP. In vitro, arterial segments were submitted first to steady pressure (0 to 200 mm Hg) by increments of 20 mm Hg, then to increasing levels of pulse pressure (20 to 50 mm Hg) at various mean arterial pressures (75 to 150 mm Hg). AWV was quantified as the area of the pressure/diameter relationship hysteresis, issued from the simultaneous measurements of pressure (Millar micromanometer) and diameter (NIUS echotracking device). In vivo, AWV increased after PE and decreased after SNP, in parallel with pressure changes. In vitro, AWV was not significantly influenced by PE and SNP. After both PE and SNP, AWV increased with pulse pressure but was not influenced by mean arterial pressure. At any given pulse pressure, AWV was higher in vitro than in vivo. The relation between AWV and pulse pressure was significantly steeper in vitro than in vivo. These results show that AWV is strongly influenced by steady and pulsatile mechanical load but not by smooth muscle tone, both in vivo and in vitro. Factors other than sustained smooth muscle activation should be explored to explain the minimization of AWV in vivo compared with intrinsic in vitro values. (Hypertension. 1998;32:360-364.)Key Words: viscosity Ⅲ arteries Ⅲ muscle, smooth Ⅲ sodium nitroprusside Ⅲ phenylephrine Ⅲ aorta A lthough it is well known that biological tissues, including the arterial wall, respond to stress through both elastic and viscous behaviors, the viscous aspect has often been neglected. Indeed, authors acknowledged the theoretical difficulties raised by taking viscosity into account and the methodological difficulties for measuring it.1,2 In most studies, viscosity was considered a dampening phenomenon and expressed in term of phase delay. An alternative approach, developed by mechanical engineers, was to consider viscosity an energy-dissipating phenomenon during the mechanical transduction.3-5 Indeed, a major function of large arteries is to store mechanical energy generated by the heart during systole and to restore it during diastole to optimize the heart-vessel coupling. [6][7][8][9] We used this approach in a recent study 10 and showed that the viscosity measured in vivo in intact animals was 3-fold lower than viscosity measured in vitro at the same arterial site under similar pressure conditions. We hypothesized that active mechanisms...

Ex vivo biomechanical properties of the female urethra in a rat model of birth trauma

“…They concluded that urethral stress relaxation was significantly greater in SUI than for healthy females, with the greatest differences occurring at the bladder neck or proximal urethral segment. The viscoelasticity of muscular tissues is related to the basal tone of the tissue (2,8,32). Therefore, our observation that VD leads to a decrease in basal tone in the urethra is consistent with the finding by Thind and Lose (38) that urethral viscoelasticity is altered in SUI.…”

“…Due to the nonuniform nature of the urethra along its length (10), a thickness must be determined for each segment (i.e., proximal, middle, and distal segments) separately to calculate R i from the measured Ro for use in Eqs. [1][2][3][4][5]. Thus, after the biomechanical ex vivo testing was performed, specimens were fixed under zero transmural pressure in 4% paraformaldehyde overnight, followed by 30% sucrose solution.…”

Quantitative Analysis of the Relationship between Blood Vessel Wall Constituents and Viscoelastic Properties: Dynamic Biomechanical and Structural In Vitro Studies in Aorta and Carotid Arteries

“…Indeed, the structural bases for the static and dynamic mechanical properties of the vascular wall have been extensively studied both in in vitro experiments [2] and in in vivo experimental animals [3]. Furthermore, the influence of the specific constituents of the arterial wall on the mechanical properties of the vessel has been investigated, focusing the analyses on determining their role in the elasticity of arteries [4].…”

Assessment of the viscosity of the pulmonary artery wall

“…More recently, however, BAUER et al [28] reported that, in contrast with the static modulus, the dynamic elastic modulus and the coefficient of wall viscosity were virtually uninfluenced by smooth muscle tone, when various arterial segments were studied in vitro under the condition of activation induced by noradrenalin and relaxation induced by papaverine. BOUTOUYRIE et al [30] confirmed in vivo that wall viscosity was not influenced by smooth muscle tone. The above observations were made on systemic arteries, and it remains to be established whether pulmonary arteries behave in a similar fashion.…”

“…wall viscosity. Recently, however, it was shown that sustained changes in smooth muscle tone did not influence aortic wall viscosity [30].…”