Dissociation of Bone Mineral Density From Age-related Decreases in Insulin-like Growth Factor-I and Its Binding Proteins in the Male Rat

Abstract

We evaluated the possibility that age-related decreases in circulating and/or bone-associated insulin-like growth factor-I (IGF-I) and its binding proteins (BPs) were associated with the development of osteopenia in 8-, 16-, and 24-month-old specific pathogen-free Brown Norway/Fischer 344 male rats. We measured bone mineral densities (BMD) of femurs by dual-energy x-ray absorptiometry. IGFs and IGFBPs were extracted from bone and separated by molecular exclusion HPLC before quantitation by specific radioligand assays. BMD did not change significantly between 8 and 24 months of age. IGF-I levels decreased by about 30% between 8 and 24 months in both serum and bone. Similarly, both circulating and bone-derived IGFBPs also declined (30% and 60%, respectively) with age. Thus, maintenance of femoral BMD throughout most of the adult rat life span was dissociated from the age-related decline in circulating and bone-associated IGF-I and IGFBPs.