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

IntroductionIn space, cephalad shift of body fluid equalizes the blood pressure throughout the body leading to various cardiovascular disturbances in space and upon return on earth. Effects of microgravity on humans can be simulated on earth by hindlimb unloading (HU) rat models l. This study was planned to examine the Effect of 15 days hindlimb unloading on baroreflex sensitivity, Herat rate variability (HRV) and carotid artery structure in rats. Material and methods18 male Wistar rats weighing 250-300 gm were randomly divided into control and hindlimb unloading groups and housed in separate cages for 15 days. Body weight, food and water intake was measured every day. The pelvic suspension method was used to simulate effects of microgravity. To record HRV, ECG was recorded in lead II configuration on the 15th day. Then blood pressure was recorded directly from the carotid artery. Linear regression of arterial pressure and RR interval used to derive the Baroreflex sensitivity. Vessel wall thickness, intimal thickness, smooth muscle cell layer thickness & luminal diameter were compared 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, Mann Whitney and 2 way ANOVA were used according to groups and distribution of parameters. ResultsWe found significant differences in Food intake (p {square}0.001), Body weight (p {square}0.001), Heart rate (p=0.031), BRS (p= 0.026), R-R interval (p=0.0246), heart rate variability parameters {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 in Water intake, MAP, SBP, DBP, Vessel wall thickness, Luminal Diameter, Intimal thickness, Smooth muscle thickness, in H&E staining and in electron microscopy. ConclusionChronic microgravity simulation using pelvic suspension showed a decrease in BRS after 15 days of HU; however 15 days of HU was not insufficient to produce any structural changes in the carotid artery. BRS changes were accompanied by change in vascular vasoconstriction and vasodilation properties which were due to alteration in the sympathetic and parasympathetic outflow.