Effect of chronic centrifugation on the structural development of the musculoskeletal system of the rat

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Abstract
25 female Sprague-Dawley rats were placed on a 3.66 m radius centrifuge and subsequently exposed almost continuously for 810 days to 2.76 G. Compared to normal gravity controls, the most noticeable effect of hypergravity was, the inhibition of growth of the centrifuged animals. The rats exposed to hypergravity showed on average a smaller femur length (-6.5%), a smaller cross-sectional area (-7.7%), when expressed linearly, i.e. (area/π)1/2), and smaller outer and inner cross-sectional radii (linearly-9.3% and-12.3%) at the mid-shaft of the femoral bones. The growth inhibition of 3 hind-leg muscles was on average significantly less ranging from (-3.5% to-4.1%), compared to the growth inhibition of the linear dimension of the femur. Statistically there was no difference in the slope and elevation of the regression of the square-root of the cross-sectional area divided by π on the length of the femur between centrifuged animals and their 16 age matched controls. In the weight control group of 24 animals, comprised of 34, 74, and 102 day old rats, the corresponding regression line was parallel and lower in elevation by-12.8%, compared to the line for the centrifuged and age control groups. But, compared to the regression derived from all control animals ranging from 34 to 840 days of age, the cross-sectional area at the mid-shaft of the femur was 8.4% greater in the rats exposed to 2.76 G for 810 days. The slopes of the regression of the outer radius at mid-shaft on the length of the femur were the same in the centrifuged group and in the weight and age control groups of animals. But, the regression lines differed in elevation by-4.4% on average between the centrifuged and age control animals. The line for the regression of the inner radius at the mid-shaft on the length of the femur was parallel and lower in elevation by-7.6% in the centrifuged animals compared to the line for the age controls. But, compared to all control animals living at normal gravity, the outer radius was increased by 3.0% and the inner radius was decreased by 5.7% in the animals exposed to 2.76 G for 810 days. Since the centrifuged animals were all 840 days old, while the controls were from 34 to 840 days old, only further experiments comparing centrifuged and control animals of the same age at various growth stages will be able to furnish evidence for an unambiguous bone hypertrophy. The regressions of the cube-root of body weight on length of the femur deviate significantly in the 3 groups of animals. The heavier rats of the age control group have relatively shorter femurs than the lighter animals. The opposite applies to the centrifuged and the weight control groups of rats. Although the rats on the centrifuge are markedly smaller in overall body size than the controls, they exhibit on average the same absolute muscle weights as the animals at earth gravity, if rats of the same overall body size are compared. As shown by partial correlation analysis, the cross-sectional area along with both the inner and outer radii at the midshaft of the femur are significantly smaller in the age controls, compared to the centrifuged group, while the dry weight of the gluteus medius muscle is greater. In the centrifuged group, however, the cross-sectional area of the femoral shaft increases exclusively by a decrease of the inner radius, while the dry weight of the gluteus medius increases. It is further shown that centrifugation has a significant effect on the relationships between cross-sectional dimensions at the mid-length of the femoral shaft and the dry weights of the triceps and of the semimembranosus muscles.