Physiologic Aspects of Drug Therapy and Large Artery Elastic Properties

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.