GROWTH-HORMONE AND PARATHYROID-HORMONE STIMULATE IGFBP-3 IN RAT OSTEOBLASTS

Osteoblast-like cells prepared from calvaria of newborn rats produce i nsulin-like growth factor (IGF) I and several insulinlike growth facto r binding proteins (IGFBPs) in vitro. Among the IGFBPs found in condit ioned cell culture medium, IGFBP-3 is the most abundant. Intact IGFBP- 3, as assessed by I-125-labeled IGF-II ligand blot analysis, is more a bundant in culture media of cells exposed to growth hormone (GH) or to parathyroid hormone (PTH), both at 5 x 10(-9) mol/l, for 24 h. At the same time, concentrations of IGF-I are increased in media of cells ex posed to PTH but not to GH, compared with hormone-free control culture s. IGFBP-3 mRNA is increased in osteoblasts exposed to PTH or to GH bu t not in response to 5 x 10(-9) mol/l IGF-I. PTH exerts a rapid (withi n 2 h) stimulatory effect on IGF-I and IGFBP-3 production, both at the message and peptide levels, whereas GPI increases only IGFBP-3, both at the message and peptide levels (after 24 h). We conclude that IGF-I does not mediate increased IGFBP-3 production by rat osteoblasts in r esponse to GH and PTH.