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HU group showed reductions in the wt of wet bone by 20.9%, water content by 35.8%, collagen matrix by 12.2%, total inorganic content (ash wt) by 14.6% and calcium content by 33.4% of tibia, when these parameters were expressed as mg/ 100 gm BW. The bone calcium in dry bone was found to be reduced by 22.4% in HU group. These findings are in agreement with the reports of other workers (8, 12, 13). 4 HRWS group showed no significant difference in the water content and organic matrix content of the bone when compared with CON. Calcium content (mg/100 mg BW) of tibia in 4 HRWS remained 15.2% less as compared to CON. However, calcium in dry bone was found to be similar to CON. Student's unpaired t test was used to compare various bone
parameters of HU and 4 HRWS groups with CON group. In all cases,
the level of significance was set as P<0.05. DISCUSSIONBone is a modified connective tissue consisting of living cells (viz. osteoblast, osteoclast etc), organic intercellular matrix (viz. collagen, mucopolysaccharides and lipids etc) and inorganic (minerals viz. calcium, phosphorus etc) component. Results of CON group show that wet tibia consists of 1/3 water while 2/3 of it is dry bone consisting of organic matrix (collagen, lipids etc) and mineral content (calcium, phosphorus etc). Analysis of dry bone revealed that 450/c of dry bone is organic matrix and 55% of it is mineral component. These findings are in agreement with the data available in literature (13, 16, 17). S-W by tail suspension in rats for 15 d resulted in reduction of wet bone wt. This reduction in wet bone weight may be the result of reduction in water/organic matrix/ mineral content of bone. On comparing water content in tibia of HU group with CON group it was found 35.8% reduced in HU group. Reduction in the water content of bone may be due to reduction in collagen matrix leading to less osmotic binding of water molecule or as a part of reflex reduction in blood volume and body water due to cephalad fluid shift as induced during S-W by HU (8, 17). The wt of organic matrix (collagen and lipids etc) was found to be reduced by 12.2% in HU group. As majority of the organic matrix in a bone is made up of collagen fibres (16, 17), reduction in organic matrix can be interpreted as reduction in collagen fibres of the bone. Reduction in collagen matrix of bone in space flight has also been reported by others (12). It is also possible that some of the collagen fibres of bone was replaced by lipid material (1). HU group also showed reduction in total mineral content of bone as evident by the reduction in ash bone wt of HU group by 14.6%. Reduction in mineral content of bone during weightlessness has also been reported by others (1). The reduction in mineral content of bone may be due to reductions in calcium, phosphorus or any other mineral of the bone. Calcium content of tibia was found 33.4% reduced in HU group. Calcium in relation to dry bone and ash bone were also reduced in HU group. Reduction in calcium in dry bone may be the result of either less mineralisation of collagen fibres of bone and/or replacement of some collagen fibres by lipid materials in the bone (1). Lipid material of bone are normally not calcified in contrast to collagen fibres which are calcified as soon as these are formed by osteoblast (16, 17). A 33.4% decrease in calcium of tibia as compared to 12.2% decrease in organic matrix further indicate replacement of some of the collagen fibres of bone by lipid materials during SW. Reduction in the calcium content of the bone associated with increased calcium, phosphorus and other mineral loss in urine during actual and simulated weightlessness has been found by various other workers (1, 2, 18). Work done in the field of bone deconditioning due to weightlessness in human and animals has been extensively reviewed by Russell T Turner during the year 2000 (19). He observed that, it is still not clear whether the bone loss is associated with increased bone remodeling, reduced bone remodeling, or an uncoupling between bone formation and resorption, associated with decreased mRNA levels for bone matrix proteins. The molecular mechanism for bone deconditioning due to weightlessness is still unknown, but there is evidence for changes in selected cytokines (e.g., transforming growth factor-b and insulin-like growth factor I), that have been implicated in the regulation of bone formation (19). Irrespective of the mechanism involved in bone deconditioning, it is clear through our study that S-W by tail suspension in rats resulted in reduction of water, collagen and calcium content of the wt bearing bone i.e. tibia. 4 h S during 15 d of S-W resulted in partial improvement of wet tibia wt. On further analysis water content of tibia was found to be improved. Although water Content in 4 HRWS was still 14% less than CON but difference was not found significant. Organic matrix content was also found to be improved and it was not found to be significantly different in 4 HRWS when compared to CON. Total mineral content of the tibia (ash wt) did not improve at all and it was still 13.2% less in 4 HRWS as compared to CON. It was still possible that organic matrix in bone of 4 HRWS group had less of collagen fibres and more of lipid material (1) while ash bone of 4 HRWS group had more of calcium content and less of other mineral. Improvement in the calcium content with out any improvement in the total mineral content suggests that bone calcium in 4 HRWS group improved at the cost of other minerals of the bone. Improvement in the bone calcium was not complete as it was found still 15.21% less than CON group. As calcium in bone gets deposited only on collagen fibres (16, 17), it is concluded that collagen content of bone was also improved at the cost of lipid material of bone. Calcium content in relation to dry bone in 4 HRWS was not significantly different from CON group. It is further suggestive of proportionate improvement in calcium and collagen fibres of the bone in 4 HRWS group (16, 17). 4 h WS during S-W was not found sufficient in total prevention of calcium loss of bone as calcium content of tibia were found still 15.2% less than CON. A 10.8% reduction in the wt of wet tibia, inspite of complete improvement in tibia water, in 4HRWS group as compared to CON group, further suggests above view point. In one of our earlier study conducted at IAM, effect of 2 h daily weight support in preventing the atrophic changes in tibia due to simulated weightlessness was found ineffective in preventing reductions in organic matrix, bone minerals and calcium (14). 4 h WS during S-W is found effective in complete prevention of water loss and partial prevention of bone demineralisation of tibia as induced by HU. HU in rats simulates deconditioning effects of weightlessness
on weight bearing bone tibia resulting in reduction of water
content, organic matrix and calcium content of bone. 4 h WS
during S-W resulted in complete prevention of water loss and
partial prevention of demineralisation of' tibia as induced
by HU. REFERENCES1. Te WSS, Wronski TJ, Morey ER et al. Effects of spaceflight on trabecular bone in rats. Am J Physiol 1983; 244: R310-R314. 2. Whedon GD, Lutwak L, Rambaut PC et al. Mineral and nitrogen balance study observation : The second manned Skylab mission. Aviat Space Eituiron Med 1976; 47(4): 391-396. 3. Stupakov GP. Microgravity induced changes in human bone strength. The Physiologist 1989; 32(l) Suppl: S41-S44. 4. Morey ER, Baylink D,T. Inhibition of bone formation during spaceflight. Science 1978; 201: 1138-1141. 5. Cann CE, Adachi RR. Bone resorption and mineral exceretion in rats during spaceflight. Am J Physiol, 1983; 244: R327-R331. 6. Fielder PJ, Morey ER, Robert WE. Osteoblast histogenesis in periodontal liganient and tibial metaphysis during simulated weightlessness. Aviat Space Environ Med 1986; 57: 1125-1130. 7. Bloomfield S. Modification of bone atrophy seen with hind limb suspension by exercise and dobutamine. The Physiologist 1989; 31(1)Supl)l: S27-S28. 8. Mishra SS, Banerjee PK, Jaiii PK. Studies on skeletomuscular deconditioning and hematological changes in rats following antiorthostatic hypokinesis induced by tail suspension. AFMRC Project NO., 1711/88. 9. Russel RB. Periodic icceleratioti stimulation in a weightlessness environment (PAS-WE): A new scheme. The Physiologist t989; 32(1)Suppl: S5-S7. 10. Burton RR. A human use centrifuge for space stations : Proposed ground based studies. Aviat Space Environ Med 1988; 59: 579-582. 11. Jain PK, Banerjee PK, Baboo NS et al. Physiological properties of rat hind limb muscles after 15 days of simulated weightless environment. Indian J Physiol Pharmacol 1997; 41(l): 23-28. 12. Roer Robert D, Dillaman RM. Bone growth and calcium balance during simulated weightlessness in the rat. J Appl Physiol 1990; 68(l): 13-20. 13. Jain PK, lyer EM, Banerjee PK et al. Bone changes during simulated weightlessness in rats. Indian J Physiol Pharmacol 2000; 44(3): 359-362. 14. Jain PK, lyer EM, Baneijee PK et al. Modification of bone atrophy by daily 2 hour weight support during simulated weightlessness in rats. Ind J Aerospace Med Winter 97; 41(2): 22-25. 15. Nath RL, Nath RK. Practical biochemistry in clinical medicine. 2nd edn, Calcutta: Academic Publisher, 1990; 385-386. 16. Keele CA, E Neil, N Joels. Samson Wriglit's Applied Physiology. 13th edn, New York Toronto: Oxford University Press, 1984; 546-555. 17. Guyton AC. Textbook of Medical Physiology. 8th edn, Philadelphia: WB Saunders Company, 1991; 469 and 872-874. 18. Portugalov VV, Savina EA, Kaplansky AS et al. Effect of space flight factors on the mammal : Experimental-morphological study. Aviat Space Environ Med 1976; 47(8): 813-816. 19. Russell T. Turner. Physiology of a Microgravity Environment; Invited Review: What do we know about the effects of spaceflight on bone ? J Appl Physiol 2000; 89(2): 840-847. |
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