Investigating the impact of duration of oscillatory shear stress on endothelial function

Abstract

The endothelium is a single layer of cells lining the arteries and is critical for the regulation of vascular health and function. The endothelium stimulates dilation in response to increased blood-flow associated shear stress, a force excreted on the arterial wall as blood flow increases. Endothelial function is assessed in large arteries using a non-invasive technique called flow-mediated dilation (FMD). This protocol involves measuring the degree of FMD following the release of a temporary limb occlusion which creates a transient increase in blood flow resulting in increased shear stress. A larger FMD indicates greater endothelial function while a small FMD indicates endothelial dysfunction. Oscillatory shear stress (OSS), characterized by decreased mean shear stress and increased retrograde shear stress, occurs at arterial branch points and is associated with endothelial dysfunction and atherosclerosis. OSS in the large artery of the upper arm (brachial artery) induced with 30 minutes of mild forearm cuff inflation has been shown to invoke transient impairment in brachial artery FMD. No study has compared the degree of FMD impairment between two different durations of OSS exposure, therefore, the sensitivity of the endothelium to the duration of OSS is unknown. Furthermore, there is inconclusive evidence to suggest women may be protected against OSS induced endothelial dysfunction. We hypothesized that 15 minutes of OSS exposure would cause a more modest decline in FMD compared to 30 minutes of OSS exposure. Twenty-eight healthy men and women (21 ± 2 years [Mean ± SD]) participated in two experimental visits in which brachial artery endothelial function was assessed via FMD pre- and post- either 15 or 30 minutes of OSS exposure. The OSS intervention significantly decreased FMD in both the 15- and 30-minute condition, such that FMD was significantly smaller post- versus pre-intervention (p< 0.026). However, the decline of FMD was similar in the 15- and 30-minute conditions (p<0.977). We did not find any sex differences in FMD. These results demonstrate a similar endothelial sensitivity to 15 and 30 minutes of OSS exposure, suggesting that the magnitude of endothelial impairment present at 30 minutes occurs before or around 15 minutes of OSS exposure.