Somatomedin (SM) activity and SM-carrier protein release by rat liver in organ culture

SM activity of rat liver culture medium was studied by measuring 35S uptake in chick embryo cartilage. In the absence of hormones, an inhibiting effect was observed, reduced but not suppressed by heating. GH (1 mU/ml) produced a slight but significant effect on the release of SM activity. Insulin was stimulating in a dose related manner (0.1 − 1 mU/ml). Both hormones were synergistic. Cortisol (1 − 100 ng/ml) stimulated the release of inhibitors. All these effects were suppressed by cycloheximide. Gel filtration study (G 75) showed that fractions containing the large molecules inhibited cartilage sulfation, while, at acidic pH, a SM activity was observed in the 6 −10,000 MW material. Studies with SM-A and NSILA-S (gifts of Dr Fryklund and Dr Zapf) showed the presence in the culture medium of high affinity proteins (MW ˜ 65,000) which bind specifically these molecules (K ˜ 2.108 M−1). A competitive binding assay using the rat liver carrier protein was set up. The results attest the validity of liver organ culture to study the hormonal control of Somatomedins and their carrier proteins biosynthesis.