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Title: GROWTH HORMONE MAINTAINS BONE DENSITY AND CIRCULATING OSTEOCALCIN LEVELS IN HYPOPHYSECTOMIZED RATS SUBJECTED TO SIMULATED WEIGHTLESSNESS

Author
item MATHIAS, E - IOWA STATE UNIVERSITY
item BOLT, D - 1265-10
item ANDERSON, L - IOWA STATE UNIVERSITY

Submitted to: Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/1995
Publication Date: N/A
Citation: N/A

Interpretive Summary: This study evaluated whether growth hormone can be used to prevent the decrease in bone mineral density in long bones noted under conditions of weightlessness, such as space flight, prolonged bed rest and long-term limb immobilization. Hypophysectomized and intact rats were used as test animals. Hypophysectomy was performed to prevent variation in endogenous secretion of growth hormone. Weightlessness was simulated in a cage that prevented the rats hind limbs from bearing weight. Simulated microgravity decreased bone density in both hypophysectomized and intact rats. Daily injections of growth hormone in both suspended and non-suspended hypophysectomized rats maintained osteocalcin levels at near normal levels. We conclude that growth hormone stimulates the synthesis of new non- calcified bone matrix, but may not play a role in calcifying newly formed bone matrix. Data were consistent with the view that growth hormone may potentiate trabecular bone formation and thus should receive further study for use in preventing bone loss under conditions of weightlessness.

Technical Abstract: Tail suspension was used to study the effects of simulating space flight (microgravity) on the skeletal system of the rat. We tested whether bovine growth hormone (bGH) increased bone mineral density in tail suspended (S) and nonsuspended (NS) hypophysectomized (Hypox) rats over their respective controls by measuring epiphyseal cartilage width, osteoblast cell population, histomorphometric analysis and bone mineral density of the tibia, and plasma osteocalcin concentrations. Daily doses of 150 ug bGH maintained active growth in nonsuspended hypophysectomized rats similar to that in intact controls. A significant decrease in osteoblast population was observed in suspended compared with nonsuspended animals within each treatment group, and this decrease was consistent when adjusted for body weight and tibia circumference. Simulated weightlessness significantly increased epiphyseal plate width in hypophysectomized rats given bGH (P < 0.01) or PBS (P < 0.05) compared with their respective nonsuspended controls, but plate with width remained unchanged in intact animals. High resolution dual energy X-ray obsorptiometric (SEXA) scans on the tibiae revealed significantly decreased bone mineral density caused by tail suspension in intact (P < 0.01) as well as in hypophysectomized rats given bGH (P < 0.01) or PBS (P < 0.05) compared with their respective nonsuspended controls. In contrast, bGH in both suspended and nonsuspended rats maintained plasma osteocalcin concentrations after hypophysectomy similar (P < 0.05) to those in the intact controls. Bone density and osteocalcin plasma concentration can be maintained in hypophysectomized rats given bGH. Simulated microgravity decreased bone density without a significant shift in circulating osteocalcin concentrations.