Changes in serum somatomedin A and its binding to kidney membranes in growing rats

Changes in serum somatomedin A levels and [125I]somatomedin A binding to membrane fractions from kidney were studied in rats 1-80 days of age. The mean level of serum somatomedin A was 0.80 U/ml at birth and increased with age; at 80 days the mean level was 7.41 +/- 0.67 U/ml. There was a close correlation between serum levels of somatomedin A and body weight. Labelled somatomedin A binding to membrane fractions from kidney was highest at birth and decreased with age up to 50 days. In Scatchard analysis of the data the affinity constant did not show a clear change with age, but the binding capacity decreased with age up to 30 days. An inverse correlation was observed between serum somatomedin A levels and labelled somatomedin A binding to membrane fractions from kidney. Compared to changes in circulating somatomedin A, the change in tissue binding was modest. The observation suggests that other circulating growth factors not measured by this radioreceptor assay or altered post-receptor sensitivity to somatomedins may be involved in growth.