Histomorphometric, physical, and mechanical effects of spaceflight and insulin-like growth factor-I on rat long bones

Previous experiments have shown that skeletal unloading resulting from expo sure to microgravity induces osteopenia in rats. In maturing rats, this is primarily a function of reduced formation, rather than increased resorption , Insulin-like growth factor-I (IGF-I) stimulates bone formation by increas ing collagen synthesis by osteoblasts. The ability of IGF-I to prevent oste openia otherwise caused by spaceflight was investigated in 12 rats flown fo r 10 days aboard the Space Shuttle, STS-77, The effect IGF-I had on cortica l bone metabolism was generally anabolic, For example, humerus periosteal b one formation increased a significant 37.6% for the spaceflight animals tre ated with IGF-I, whereas the ground controls increased 24.7%, This increase in humeral bone formation at the periosteum is a result of an increased. p ercent mineralizing perimeter (%Min.Pm), rather than mineral apposition rat e (MAR), for both spaceflight and ground control rats. However, IGF-I did i nhibit humerus endocortical bone formation in both the spaceflight and grou nd control rats (38.1% and 39.2%, respectively) by limiting MAR. This effec t was verified in a separate ground-based study. Similar histomorphometric results for spaceflight and ground control rats suggest that IGF-I effects occur during normal weight bearing and during spaceflight, Microhardness me asurements of the newly formed bone indicate that the quality of the bone f ormed during IGF-I treatment or spaceflight was not adversely altered. Spac eflight did not consistently change the structural (force-deflection) prope rties of the femur or humerus when tested in three-point bending. IGF-I sig nificantly increased femoral maximum and fracture strength. (Bone 23:527-63 5; 1998) (C) 1998 by Elsevier Science Inc. All rights reserved.