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 cartilage responded differently to liver somatomedin; 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 the processes of hypertrophy and new bone formation, did not directly affect [3H]thymidine uptake by proliferative chondrocytes, but inhibited stimulation of their activity by liver somatomedin.
Measurement 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.