Mechanical properties of normotensive and hypertensive female rat carotid arteries

Abstract

In response to chronic hypertension, the heart and vasculature exhibit morphologic changes that reflect alterations in net gains and losses of their cellular components, i.e. they grow and remodel. Vascular remodeling is manifested mechanically as an altered response to applied loads. This remodeling may differ by sex and direction within the vascular wall. In an effort to better understand remodeling, this study examined female Sprague-Dawley rats 10-12 days post aortic coarctation or sham surgery. The objectives of this study were threefold: (1) to design and build a system to allow quantification of thromboxane A₂ produced from cylindrical segments of hypertensive and normotensive rat aortas, (2) to quantify the response of hypertensive and normotensive female rat carotid arteries to applied loads considering active and passive responses in the longitudinal and circumferential directions, and (3) to compare the female response in both groups to data available for males. The first objective provides the means to examine sex differences in thromboxane A₂ production, which may be important in remodeling. The second objective involved pressure-diameter tests (with specimen preconditioning) and opening angle tests in both active and passive states. There were no statistically significant differences between normotensive and hypertensive opening angle results or the slopes of passive mean circumferential stress and mean axial stress vs. circumferential stretch plots. Mean passive axial stress was greater in hypertensives than controls for a given axial stretch. Altered passive axial properties indicate a difference from controls in collagen, elastin, or the passive properties of smooth muscle cells. No generalizations were made from the active tests since only the first loading cycle showed evidence of contraction, and most data collected were post preconditioning. Comparable studies on male rats demonstrate statistically significant differences in both histological and mechanical properties by 8 days post-coarctation (Fridez et al, 2001). The lack of similar findings here may relate to gender differences in the time-course or extent of remodeling. Future studies should devote careful attention to sex differences in multi-axial properties and active vs. passive remodeling. The role of preconditioning in active mechanical tests and the time course of remodeling should be addressed.