| Summary: | 碩士 === 高雄醫學大學 === 醫學研究所 === 92 === Disuse osteoporosis is defined as bone loss resulting from reduction of mechanical stress on bone. Previous report indicated that the unloading inhibited osteoblastogenesis, proliferation and differentiation of osteoblasts, however, the molecular mechanism remains uncertain. Since mechanical loading down-regulates the glutamate transporter, GLAST-1, and subsequent increase of extracellular glutamate may lead to increase of number and activity of osteoblasts, it is possible that alteration of glutamate and/or its receptor affects the physiological functions of bone cells under disuse condiction. Therefore, the present study was designed to investigate the role of NMDA receptor (NMDAR), a major glutamate receptor in bones, in regulating the bone remodeling in disused bones. The animal model of disused osteoporosis was established by tail-suspension for three weeks and osteoporosis was confirmed by bone mineral density and osteocalcin (OC) expression. The expression of subunit protein of NMDAR, NR1 and NR2D, in disused rats was compared to control rats to know whether NMDAR is involved in osteoporosis caused by disuse. Different stage of bone formation and bone resorption were evaluated by testing the level of mRNA and protein content of bone markers in femurs. Collagen type I, ALP, OC were estimate as bone formation markers while TRAP was evaluated as bone resorption markers. Furthermore, the NMDAR mediation on bone formation was elucidated by blocking NMDAR with MK-801 (a non-competitive NMDAR antagonist) using osteoblast cultures derived from fetal rat calvarias. The results showed that: (1) The RT-PCR product of TRAP was unchanged, while three bone formation markers (collagen type I. ALP and OC) were significantly decreased after disuse. (2) The expression of subunit protein of NMDAR, NR1 and NR2D, in bone tissue was significantly decreased in tail-suspension group compare to the control group, especially in osteoblasts. The proliferation and differentiation of cultured osteoblasts were significantly inhibited by MK-801 in a dose-dependent manner. These results suggested that disuse inhibits bone formation at least partially via suppressing the expression of the subunit protein of NMDA receptor.
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