[en] The acute somatomedin (SM) response to GH therapy has been examined in 21 GH-deficient children using a placental membrane radioreceptor assay (RRA) which measures a variety of SMs and a RIA specific for SM-C and insulin-like growth factor I (IGF-I). Plasma for determination of SM peptide content was obtained before initiation of therapy and 13 h after each of four daily injections of GH (0.1 U/kg). An additional SM determination was performed after 6 weeks of GH therapy (0.1 U/kg, three times per week) in seven of the subjects. RRA and RIA SM determiations were performed on the same acid-chromatographed sample and were compared to an acid chromatographed pooled plasma standard. The 4 days of GH therapy resulted in an increase in SM levels from 0.39+/- 0.24 to 1.18 +/- 0.62 (+/- SD) U/ml, determined by RIA. A single injection of GH resulted in a significant rise in plasma SM levels, measured by either RRA or RIA (P less than 0.001). Subjects who responded poorly to two injections of GH also had low SM levels after 4 days and even after 6 weeks of GH therapy. The RRA resulted in consistently higher value than the RIA. This difference was even greater when results were compared to a pure IGF-I/SM-C standard. The SM peptide contents determined by RRA and RIA were strongly correlated, not only for the group, but also among the determinations for each individual subject. However, the consistently higher values observed when the SM peptide content was measured by RRA compared to that measured by RIA and the variability in the RRA to RIA ratio among individual subjects suggest that the IGFI/SM-C RIA measures only one of a number of GH-dependent SM peptides

[en] Somatomedin C (SM) stimulates the incorporation of ³⁵SO₄ into glycosaminoglycans in chick cartilage cells. Medium from cell cultures contain increased amounts of ³⁵SO₄-labeled proteoglycan when the cells are stimulated by (SM). When these cells are incubated with ³²P[PO₄], ³²P is incorporated into material that is precipitate from the medium by cetylpyridinum chloride (CPC). The ³²P incorporation is also stimulated by SM. Following digestion by DNase and RNase, 6 to 14% of the ³²P remains precipitable by CPC and chromatographs at the void volume of Sephadex G-25 columns. After digestion with chondroitinase ABC approximately 70% of the ³²P chromatographs at the column volume, indicating a significant reduction in the molecular weight of the material containing ³²P. Based on these data, the phosphate is in proteoglycan. Medium from control cells not stimulated by somatomedin also contains material that is qualitatively the same. These data would suggest that like many other biological processes, phosphorylation may be an important regulatory event in proteoglycan biosynthesis

[en] Cell cultures obtained from chick embryo pelvic rudiments which respond to somatomedin C (SM) by increased incorporation of ³⁵[SO₄] into proteoglycans were stimulated with SM in the presence of ³²P[PO₄]. Cells were harvested at zero time, 50 min, and 7 hrs following addition of the ³²P and SM, homogenized, and dialyzed against phosphate buffered saline. After centrifugation at 100,000 xg, the pellet and the supernatant were incubated with DNase and RNase in the presence of benzamidine HCl, phenylmethylsulfonyl fluoride, 6-aminohexanoic acid and EDTA, and subjected to electrophoresis in a 5 to 15% polyacrylamide gel gradient in sodium dodecyl sulfate. In samples from cells stimulated for 50 min with SM, radioautography revealed two labeled bands at molecular weights of approximately 400K and 500K in the 100,000 xg pellet; no labeled bands were observed in the corresponding control cells. After 7 hr incubation extensive labeling was observed in both control and stimulated cells without discernible differences between control and SM-stimulated samples. These results suggest that an early event in SM stimulation of proteoglycan synthesis may be phosphorylation of proteins distinct from the SM receptor. It is interesting that one of the labeled bands corresponds approximately to the molecular weight of the proteoglycan core protein precursor