IGF-I production by mouse osteoblasts.

Mouse osteoblasts contain and secrete insulinlike growth factor I (IGF-I), which can be measured by radioimmunoassay after separation from endogenous IGF-I binding activity. Our studies indicate that IGF-I is produced by all bone cell populations prepared by sequential digestion of mouse calvaria with collagenase and protease. Furthermore, relatively small amounts of IGF-I are cell associated, and IGF-I is recovered primarily in the cell medium after 24 h of culture. Basal IGF-I secretion is also density dependent, and secretion per cell is approximately 20-fold higher when cultures are inoculated at 0.125 versus 1.0 x 10(5) cells per cm2. Growth hormone increased the secretion of IGF-I only in cells released in the earlier stages of digestion. These growth hormone-responsive populations were previously shown to differ from late released cells in that they show a lower expression of the osteoblastic phenotype, harbor more EGF receptors per cell, and have a higher proliferative response to low doses of exogenous IGF-I and EGF. These data reaffirm the presence of different subclasses of bone cells in populations obtained by sequential digestion of bone and suggest that growth hormone stimulates IGF-I secretion by immature osteoblasts.