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To determine whether growth hormone has a direct effect on skeletal tissues not mediated by somatomedins, and to better define the role of somatomedin-C/ insulin-like growth factor I (Sm-C/IGF-I) in skeletal development, bovine growth plate, and rabbit articular and growth plate chondrocytes in primary culture were evaluated under a variety of experimental conditions designed to elicit growth hormone and Sm-C/IGF-I stimulation. Under none of these conditions did bovine growth plate chondrocytes respond to either homologous bovine growth hormone or heterologous hGH. Under the same conditions, these cells were highly responsive to human Sm-C/IGF-I with respect to both 3Hthymidine and 35Ssulfate incorporation, indices of mitotic and differentiated cell functions, respectively. Similarly, both rabbit articular and growth plate chondrocytes showed enhanced incorporation of 3H thymidine and 35SJsulfate in the presence of Sm-C/IGF-I, but did not respond to either native or recombinant hGH. Cells at different stages of maturation within the bovine growth plate differed in their reaction to Sm-C/IGF-I with proliferative zone cells manifesting a greater response to the peptide than cells of the reserve zone. These results suggest that the action of Sm-C/IGF-I on growth plate and articular chondrocytes is direct and that the effect of GH on these cells is indirect. The data further suggest that within the growth plate, the transition from reserve to proliferative status is associated with an increased Sm-C/ IGF-I responsiveness, a change which may contribute to the functional differences in these cells

ABSTRACT: To determine whether growth hormone has a direct effect on skeletal tissues not mediated by somatomedins, and to better define the role of somatomedin-C/ insulin-like growth factor I (Sm-C/IGF-I) in skeletal development, bovine growth plate, and rabbit articular and growth plate chondrocytes in primary culture were evaluated under a variety of experimental conditions designed to elicit growth hormone and Sm-C/IGF-I stimulation. Under none of these conditions did bovine growth plate chondrocytes respond to either homologous bovine growth hormone or heterologous hGH. Under the same conditions, these cells were highly responsive to human Sm-C/IGF-I with respect to both [3H]thymidine and [35S]sulfate incorporation, indices of mitotic and differentiated cell functions, respectively. Similarly, both rabbit articular and growth plate chondrocytes showed enhanced incorporation of [3H] thymidine and [35SJsulfate in the presence of Sm-C/IGF-I, but did not respond to either native or recombinant hGH. Cells at different stages of maturation within the bovine growth plate differed in their reaction to Sm-C/IGF-I with proliferative zone cells manifesting a greater response to the peptide than cells of the reserve zone. These results suggest that the action of Sm-C/IGF-I on growth plate and articular chondrocytes is direct and that the effect of GH on these cells is indirect. The data further suggest that within the growth plate, the transition from reserve to proliferative status is associated with an increased Sm-C/ IGF-I responsiveness, a change which may contribute to the functional differences in these cells.

Abstract:: This prospective longitudinal study was undertaken to examine the short-term effects (6 months) of estrogen withdrawal on the circulating IGF system. A series of 40 patients suffering from endometriosis was studied before and after a 6-month treatment period with gonadotrophin releasing hormone (GnRH) agonist and calcium, with or without nasal salmon calcitonin. The plasma concentrations of insulin-like growth factor I (IGF-I9 and insulin-like growth factor II (IGF-II) were measured by radioimmunoassay and radioreceptor assay respectively. Plasma IGF binding proteins (IGFBPs) were quantified and characterized by ligand blot and immunoblot. In all patients, a secondary hypoestrogenism was observed, including a 4% decrease in lumbar bone mineral density (L-BMD). The plasma IGF-I and IGF-II concentrations increased after treatment (24%, p < 0.0005 and 40%, p < 0.004 respectively), with no significant difference between the treatment groups. There was a positive correlation between plasma IGF-I (but not IGF-II) changes and changes in urinary deoxypyridinoline (r = 0.32, p < 0.05), urinary C telopeptide of type I collagen (r = 0.33, p < 0.04) and total plasma alkaline phosphatases (r = 0.33, p < 0.04). No correlation was found between IGF-I and L-BMD changes, while there was a positive correlation between the changes in plasma IGF-II and L-BMD (r = 0.32, p < 0.05). Ligand blot analysis revealed a significant increase in IGF-II binding to a 29–31 kilodalton region where positive staining with specific antibodies to IGFBP-3 or IGFBP-1 was observed. In conclusion, IGF-I and IGF-II plasma concentrations are both increased following a short period of treatment with a GnRH agonist. The changes in individual IGF peptides are differently correlated with changes in markers of bone remodelling and L-BMD.

Conversion of 25-(OH)D3to 24,25-(OH)2D3and stimulation of proteoglycans synthesis in cultured chondrocytes has been previously demonstrated (Corvol et al., 1978). To investigate subcellular localization, labelled vitamin D metabolites, 26,27-3H, were incubated with cultured chondrocytes from prepubertal rabbits for various time periods. Medium and cells were extracted separately with methanol-chloroform (v : v) and analyzed by high pressure liquid chromatography. When labelled 25-(OH)D3was incubated, 8 × 10-9M, most radioactivity was recovered as 24,25-(OH)2D3in the mitochondrial fraction and reached a plateau after 60 min. Same labelled 24,25-(OH)2D3was also found in the nuclei. When labelled 24,25-(OH)2D3was incubated, the radioactivity was mainly retained in the nuclei. Nuclear uptake of this label was inhibited by preincubating the cells with excess non labelled 24,25-(OH)2D3. There was no specific nuclear binding of 25-(OH)D3. In conclusion, in chondrocytes the 24-hydroxylation of vitamin D is within mitochondria as already shown for the kidney. Specific nuclear binding of 24,25-(OH)2D3suggests that this is the active vitamin D metabolite in chondrocytes.