Effects of Increased Muscle Mass on Bone in Male Mice Overexpressing IGF-I in Skeletal Muscles

It has been hypothesized that increase in muscle mass increases the strain on bone resulting in increase in bone mass. The aim of the present study was to determine the effects of increased muscle mass on bone. A colony of transgenic mice that overexpress hIGF-I in muscle, resulting in larger muscles, was established. Six-month-old heterozygous and wild type males were used in this study. The tibial diaphysis, femoral diaphysis and distal femoral metaphysis were analyzed using pQCT densitometry. Heterozygous animals had significantly higher body weight, muscle weight and muscle area when compared with wild type animals. Tibia and femur of the heterozygous mice had significantly higher weights and lengths. The tibial and femoral diaphyses of heterozygous animals had significantly higher cortical bone area, cortical bone mineral content, cortical bone mineral density, cortical thickness and periosteal perimeter when compared with wild type animals. In the distal femoral metaphysis, the total bone area and the cancellous bone area of heterozygous mice were significantly higher than those of wild type animals. In conclusion, increased muscle mass was associated with bigger bones in animals overexpressing IGF-I. Only pure cortical bone increased in both area and mineral content in these animals; cancellous bone, however, increased only in area and not in mineral content and density.