Smooth Muscle Relaxation

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Abstract
Compliance, distensibility, incremental elastic modulus (E(inc)), and pulse wave velocity are all terms used to describe the mechanical properties of arteries. Previous studies assessing the effects of smooth muscle relaxation on each of these parameters have produced conflicting results. Our laboratory has previously demonstrated that intrabrachial infusion of nitroglycerin in normal human subjects results in a large increase in brachial artery compliance without changing arterial wall stiffness as measured by E(inc). In the present study, the relationships among compliance, distensibility, E(inc), and pulse wave velocity under different levels of vascular tone are shown using data acquired by intravascular ultrasound as well as theoretical curves. We demonstrate that the effects of smooth muscle relaxation can be depicted as 2 separate steps: (1) a rightward shift to a new theoretical curve describing the relationship between 2 of the above elastic parameters that is solely due to changes in vessel geometry and (2) a shift along the new curve that is dependent on changes in wall stiffness.