Effect of 15 days hindlimb unloading on baroreflex sensitivity, Hear rate variability and Carotid artery structure in rats

Introduction In space, cephalad shift of body fluid equalizes the blood pressure throughout the body leading to various cardiovascular disturbances. Effects of microgravity have been perfectly simulated in the rat hindlimb unloading (HU) model. This study was planned to assess the structure-function relationship of the carotid artery before and after HU of 14 days Material and methods This study was done in Department of Physiology, AIIMS, New Delhi. 18 male Wistar rats weighing 250-300 gm were randomly divided into control and hindlimb unloaded groups and housed in separate cages for 14 days. Body weight, food and water intake was measured every day. The pelvic suspension method was used to simulate microgravity. Blood pressure was recorded directly from the carotid artery.ECG was recorded in lead II configuration. A graph was plotted of MAP v RRI and the slope was taken. Linear regression of mean arterial pressure and RR interval used to derive the Baroreflex sensitivity.Vessel wall thickness, intimal thickness, smooth muscle cell layer thickness & luminal diameter were observed after H&E staining. Ultrathin sectioning and counter-staining was done for electron microscopy and changes in endothelial thickness, elastic lamina, and deeper artery structures were observed. Statistical analysis was done using Graphpad Prism (version 9.0). Unpaired t-test was applied. Results We found significant differences were observed Food intake(p <0.001) Body weight(p <0.001), Heart rate (p=0.031), BRS (p= 0.026), HRV:R-R interval (p=0.0246), SDSD (p=0.0001),RMSD (p=0.0001),VLF(p=0.001),HF (p=0.0054), LF/HF Ratio (p= 0.0001).we did not found significant changes inWater intake, MAP, SBP, DBP, LFT, RFT, Vessel wall thickness(μm), Luminal Diameter (μm), Intimal thickness(μm), Smooth muscle thickness(μm), in H&E staining and in electron microscopy. Conclusion Chronic microgravity simulation using pelvic suspension showed a decrease in BRS, however 15 days are not insufficient to produce any structural changes in the carotid artery. BRS changes are accompanied by change vascular vasoconstriction and vasodilation properties which are due to alteration in the sympathetic and parasympathetic outflow. ### Competing Interest Statement The authors have declared no competing interest.