Nutrition and somatomedin. V. Action and measurement of somatomedin inhibitor(s) in serum from diabetic rats.

Serum from normal rats stimulates growth cartilage in vitro; this stimulation is attributed to somatomedin activity. In contrast, serum from diabetic rats may produce dose-response lines with negative slopes and, in combination studies, suppress the stimulatory activity of serum from normal rats; this is attributed to somatomedin inhibitory factor(s). Somatomedin inhibitory activity in serum from diabetic rats cannot be attributed to recognized catabolic factors, such as glucocorticoids or free fatty acids. The inhibitory activity is resistant to 100 C heat at neutral pH but is partially removed by 100 C heat at pH 5.5. Somatomedin inhibitory activity can be estimated as the ability to decrease the stimulation of rat cartilage incubated for 2 days with serum from normal rats. With this method, serum from diabetic rats provides linear inhibition of both sulfate and thymidine uptake. This procedure is simple, reproducible, and allows detection of inhibitory activity in as little as 5 microliter whole serum. It allows identification of individual samples enriched in inhibitory activity and should be useful in further studies of somatomedin inhibitory factor(s).