The Effects of Hypergravity and Adrenalectomy on Bone Mineral Content, Urine Calcium and Body Mass in Rats

The effects of 14 days of increased gravitational load, and the absence of adrenal stress hormones on total body bone mineral content (BMC) were examined in male Sprague-Dawley rats. Centrifugation at 2 Gs (2G) was used to increase the gravitational load, and bilateral adrenalectomy (ADX) was used to eliminate the production of adrenal stress hormones. Stationary groups at 1 G (1G) and sham operated (SHAM) animals served as controls. Thirty rats (n=6 or 8) made up the four experimental groups (1G SHAM, 1G ADX, 2G SHAM and 2G ADX). BMC was assessed by dual energy x-ray absorptiometry (DXA) which was performed to determine the total body bone mineral content, and also through bone ashing of the left femur and the left humerus. Activity was determined through biotelemetry, also body mass and food intake were measured. Multi-factorial analysis of variance (MANCOVA) and Newman Keuls post hoc tests were used to analyze significant effects (p is less than 0.05) for the primary variables. Results from both DXA and the ashed femur indicated that BMC decreased significantly with increased G for both the SHAM and ADX groups. The BMC determined by DXA for the 1G ADX group was also significantly lower than the 1G SHAM group, however the 2G SHAM and 2G ADX groups were not significantly different. However, the bone ashing results showed the femur differed significantly only between the rates of centrifugation and not between the ADX and SHAM. The humerus showed no significant difference between any of the groups. There was a significant decrease in body mass with increased G and there was no ADX effect on body mass. When DXA BMC was normalized for body mass changes, there were no significant group differences. However, with bone ashing, the femur BMC/BW still showed significant difference between rates of centrifugation, with the 2G group being lower. Activity level decreased with body mass, and food intake data showed there was significant hypophagia during the first few days of centrifugation. Urine calcium was measured and was found decrease at the start of centrifugation for the 2G groups and rise to a level higher than that of the stationary groups. Finally, the correlation between BW and BMC was determined to be highly correlated (r = .71). These results suggest that the decrease in total body BMC seen with hypergravity may be based to a large extent on the differences in body mass induced by the 2G load.