Characterization of the effect of insulin on collagen synthesis in fetal rat bone.

We characterized the effect of insulin on collagen synthesis in 21-day-old fetal rat calvaria maintained in organ culture. All experiments were done in the presence of 100 mg/dl glucose and 3 mM phosphate, which were found to be optimal concentrations for insulin responsiveness. All concentrations of insulin tested (1 nM to 1 microM) increased the percentage of collagen being synthesized in the central bone, whereas only high concentrations of hormone (100 nM to 1 microM) increased the percentage of collagen being synthesized in periosteum. Insulin at 3 nM increased the labeling of type I collagen in the central bone, but did not alter the labeling of type I or III collagen in the periosteum. Proinsulin was approximately 10-100 times less effective than insulin in stimulating collagen synthesis, whereas porcine relaxin and C-peptide were ineffective. Insulin did not enhance the deposition of newly synthesized collagen in the bone by a mechanism that involved decreasing the degradation of collagen. To determine whether insulin enhanced collagen synthesis by increasing the replication of collagen-synthesizing cells, we tested the effect of insulin in the presence of hydroxyurea, a DNA synthesis inhibitor. Hydroxyurea at 1 mM had little effect on collagen synthesis in control cultures or those treated with 1 or 10 nM insulin. However, hydroxyurea blunted the stimulation of collagen synthesis that occurred at higher concentrations of insulin. These experiments suggest that insulin at physiological levels appears to increase bone collagen synthesis by a direct effect on the osteoblast, whereas insulin at high concentrations has an additional action to increase the replication of collagen-synthesizing cells.