Bank A J
Department of Medicine, University of Minnesota, Minneapolis 55455, USA.
Vasc Med. 1997;2(1):44-50. doi: 10.1177/1358863X9700200107.
Vasoactive drugs alter smooth muscle tone not only in arterial resistance vessels, but also in large conduit arteries. The resultant changes in smooth muscle tone alter both conduit vessel size and stiffness and hence influence pulsatile components of left ventricular afterload. The effects of smooth muscle relaxation and contraction on arterial elastic properties are complex and have not been fully characterized. Several recent studies have utilized a new intravascular ultrasound technique to study the effects of changes in smooth muscle tone on brachial artery elastic mechanics in normal human subjects in vivo. Smooth muscle relaxation with nitroglycerin improves isobaric brachial artery compliance without significantly altering arterial wall stiffness as measured by incremental elastic modulus (Einc). The improvement in compliance with smooth muscle relaxation is the net result of factors that: (1) increase wall stiffness (increased tension in parallel elastin and collagen fibers); (2) decrease wall stiffness (decreased tension in the smooth muscle and its associated series elastic component); and (3) increase vessel lumen size. Using a modified Maxwell model for the arterial wall, smooth muscle relaxation is also shown to shift the predominant elements contributing to wall stress and Einc from smooth muscle and the collagen fibers in series with the smooth muscle to collagen fibers in parallel with the smooth muscle. A better understanding of the mechanisms contributing to changes in arterial elastic mechanics following alterations in smooth muscle tone will help in developing pharmacologic therapies aimed at reducing pulsatile components of left ventricular afterload.
血管活性药物不仅会改变动脉阻力血管的平滑肌张力，还会改变大的输送动脉的平滑肌张力。平滑肌张力的变化会改变输送血管的大小和硬度，从而影响左心室后负荷的搏动成分。平滑肌舒张和收缩对动脉弹性特性的影响很复杂，尚未完全明确。最近的几项研究利用一种新的血管内超声技术，在体内研究正常人体受试者平滑肌张力变化对肱动脉弹性力学的影响。硝酸甘油使平滑肌舒张可改善等压状态下肱动脉的顺应性，而用增量弹性模量（Einc）测量时，动脉壁硬度无明显改变。平滑肌舒张时顺应性的改善是以下因素的综合结果：（1）增加壁硬度（平行弹性蛋白和胶原纤维中的张力增加）；（2）降低壁硬度（平滑肌及其相关串联弹性成分中的张力降低）；（3）增加血管腔大小。使用改良的麦克斯韦动脉壁模型还表明，平滑肌舒张会使导致壁应力和Einc的主要成分从平滑肌以及与平滑肌串联的胶原纤维，转变为与平滑肌平行的胶原纤维。更好地理解平滑肌张力改变后动脉弹性力学变化的机制，将有助于开发旨在降低左心室后负荷搏动成分的药物治疗方法。
