Regional specific adaptation of the endothelial glycocalyx dimension in tail-suspended rats

Previous animal studies by using tail-suspended hindlimb-unloaded rat model have shown that simulated microgravity-induced vessel structural and functional remodeling may be anatomic region dependent. However, little care has been taken to assess the structural adaptation of the endothelial glycocalyx, the apical surface of the endothelium, the key mechanosensor mediating nitric oxide (NO) production, and the natural protective barrier of the vasculature. Therefore, the present study extended simulated microgravity-induced vessel remodeling to the endothelial glycocalyx level. The percents of bone mineral density (BMD) change from both control and tail-suspended (TS) rats were measured by micro-computed tomography (Micro-CT). Structural parameters such as the luminal diameter (D), the thickness of each layer, the ratio of intima to media (IMR), the cross-sectional areas of the intima (CSAI) and media (CSAM) of vessels from three different regions (the common carotid artery, abdominal aorta, and femoral artery) were assessed by hematoxylin and eosin staining. Dimensions of the glycocalyx above, below, and away from the endothelial cell nucleus were examined by fluorescein isothiocyanate-labeled wheat germ agglutinin (WGA-FITC) binding to the cryosection of vessels. Our results show that 3-week tail suspension of rats increases the thickness and CSA of the abdominal aortic endothelium by 23.7 and 21.1 %, respectively, thickens the media layer of the common carotid artery by 34.0 %, and increases the luminal diameter, the CSA of the intima and media of the femoral artery by 75.7, 93, and 61.2 %, respectively. Correspondingly, the dimension of the glycocalyx away from the common carotid arterial and the abdominal aortic endothelial cell nucleus from tail-suspended rats shows a 1.66- and 1.64-fold increase respectively, while it shows a 0.79-fold reduction on the top of the femoral endothelial cells. These results suggest that simulated microgravity induces vascular endothelial glycocalyx remodeling in a regional-dependent manner. The perturbation of the endothelial glycocalyx at the lower body artery may be the first event of vascular remodeling initiating endothelial dysfunction, contributing to postspaceflight orthostatic intolerance.