Effects of 2 Weeks Hypergravity on the Composition of the Intervertebral Disc

Background: The effects of loading on connective tissues similar to the intervertebral disk have primarily been studied using articular cartilage organ culture in vitro techniques. The effects of in vivo loading on the intervertebral disk are less well documented.

Methods: The lumbar annuli of rats centrifuged at 2G for 2 wk were compared with those of similar rats kept under analogous conditions at 1G. The studies involved the water, hydroxyproline (collagen), and uronic acid (proteoglycan, PG) content of the L4-5 and L5-6 annuli. Comparisons were also made between the 2G animals and the previous Cosmos 2044 spaceflight animals at 0G.

Results: The water content of annuli did not differ between the control and 2G annuli; however, the PG content of the tissue was decreased in the 2G animals and the collagen content slightly increased, resulting in an increased collagen-PG ratio in the 2G animals. When the annuli were immersed in water for 2 h, more PG leached from the annuli of the 2G animals than the control animals, suggesting an alteration in the nature of the PG population comprising the annuli. Interestingly, the direction of changes in the 2G annuli was identical to that of the previous Cosmos 2044 0G animals when they were compared with their respective controls. Therefore, directly opposite experimental conditions (2G and 0G) produced similar results.

Conclusion: Previous studies regarding PG synthesis and content in cartilaginous tissues suggest alterations in intratissue pH and hydration to be among the factors involved in controlling the PG population. If the observed changes are permanent, they may affect the integrity of the intervertebral disk although it is likely they are totally reversible on return to 1G after an interim period, providing injuries are avoided.