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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 heat-labile inhibitors of somatomedin activity. The potency of heated hypopituitary plasma (0-6+/-0-09 u./ml) remained significantly lower (P less than 0-01) than normal plasma. Human growth hormone (0-1-20 muu./ml), bovine growth hormone (0-5 20 muu./ml), insulin (0-5-5 muu./ml) and glucose (0-3-2 mmol/1) had no direct effect on the incorporation of [3H]thymidine. Chondrocytes which had been previously stored frozen also showed a response to plasma somatomedin.

Plasma somatomedin activity enhanced the incorporation of [3H]thymidine into chondrocytes isolated from human foetal cartilage during weeks 13-21 of gestation. Human growth hormone (0.1-20 muu./ml), human placental lactogen (0.1-5 microgram/ml) and insulin (0.25-10 muu./ml) had no direct effects on the synthesis of DNA in these chondrocytes, although insulin at concentrations of 2.5-100 mu./ml increased [3H]thymidine incorporation by up to 400%.

Rat prolactin at a concentration of 50 ng/ml perfusion medium stimulated the production of somatomedin-like activity (SLA) from the perfused liver of normal rats. The effect was demonstrable in perfusions performed at 11.00 h in which rat prolactin caused a mean (+/- S.E.M.) increase in the uptake of [35S]sulphate into rat costal cartilage in vitro of 64 +/- 14% in comparison with controls, but at 15.00 h no effect was observed. No effect or rat prolactin on hypophysectomized rat liver was detectable at 11.00 h. Hypophysectomized and sham-operated rats were given five intravenous injections of 50 microgram rat prolactin or a similar volume of hormone solvent at 12 h intervals. Plasma somatomedin activity (SMA) and cartilage metabolism, measured by the uptake of radioactive sulphate and thymidine by costal cartilage in vitro, were similar in hypophysectomized animals given rat prolactin or hormone solvent. Sham-operated rats given rat prolactin showed a significant increase of plasma SMA and cartilage metabolism compared with control animals. The production of SLA by rat liver in response to rat prolactin may be related to the density of specific hepatic lactogenic receptors, since these are absent or present only in low numbers in hypophysectomized animals.

Isolated rat liver, when perfused with medium containing bovine growth homone produced somatomedin-like activity (liver somatomedin). Liver somatomedin is useful in studies of the hormonal control of the cartilage plate in vitro, since unlike serum it is not contaminated with other hormones or growth factors (apart from growth hormone). Chondrocytes isolated from various regions of the growth cartiage responded differently to liver s-omatomedin; proliferative chondrocytes, like those isolated from the articular cartilage, showed increased [3H]thymidine uptake in response to liver somatomedin, whereas hypertrophic chondrocytes did not respond. It is suggested that there is a reduction in the response to somatomedin by growth plate chondrocytes as they pass from the proliferative to the hypertrophic state. Thyroxine, thought to be involved in thr processes of hypertrophy and new bone formation, did not directly affect [3H]thymidine uptake by proliferative chondrocytes, but inhibited stimulation of both their activity by liver somatomedin. Mesurement of [3H]thymidine uptake by isolated articular chondrocytes may provide a useful assay for both liver and serum somatomedin. The graded response of chondrocytes to increasing concentrations of liver somatomedin paralleled the response to increasing levels of serum somatomedin.