AN ASSAY FOR PLASMA SOMATOMEDIN: [3H]THYMIDINE INCORPORATION BY ISOLATED RABBIT CHONDROCYTES

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Abstract
The incorporation of [3H]thymidine by rabbit chondrocytes in vitro has been developed as a sensitive assay for plasma somatomedin. A concentration of normal plasma of 2·5% enhanced [3H]thymidine incorporation by 5- to 20-fold compared with basal levels in the absence of plasma. The mean potency of plasma from normal adult men was 0·96 ± 0·1 u./ml (mean ± s.d.) and from acromegalic patients 1·9 ± 0·4 u./ml. The apparent potency of hypopituitary plasma alone increased on heating which suggested the presence of heatlabile inhibitors of somatomedin activity. The potency of heated hypopituitary plasma (0·6 ± 0·09 u./ml) remained significantly lower (P < 0·01) than normal plasma. Human growth hormone (0·1–20 μu./ml), bovine growth hormone (0·5–20 μu./ml), insulin (0·5–5 μu./ml) and glucose (0·3–2 mmol/l) had no direct effect on the incorporation of [3H]thymidine. Chondrocytes which had been previously stored frozen also showed a response to plasma somatomedin.