Changes in abdominal aorta wall area in rats after 14 days 45° hind limb unweighting

Changes in the areas of the abdominal aorta walls in rats were measured after exposure to 14 days of 45° hind limb unweighting (45HU), a simulated microgravity model. Non-anesthetized rats were placed either in the 45° suspension group or allowed unrestrained movement in cages (control, C). Upon completion of the 14 day exposure, abdominal aortas were harvested from both rat groups at approximately 2 cm anterior to the bifurcation at the terminal segment of the aorta. Samples collected were stained with Toluene blue and cut in thick sections for light microscopy. Vessel area measurements of the most superficial layer of the tunica media and the lumen were made using the Bioquant OS/2 system (R & M Biometrics, Nashville, TN). The mean area of control (n=5) vessels was 1.40 × 10⁵ μm². The 45HU rat group showed two contradictory sets of results, labeled non-Responder and Responder. The non-Responder (NR) mean (x= 1.83 × 10⁵ μm², n=6) was not statistically higher than control values but was 30.6% larger than control values. The Responders (R) mean (x=0.59 × 10⁵ μm², n=6) was statistically lower than the control group with a 57.6% decrease in overall aorta wall area size. We speculate that the 45HU responder vs. non-responder phenomenon may correspond to astronauts who have orthostatic intolerance during spaceflight.