Characterization of a Specific Somatomedin-C Receptor on Isolated Bovine Growth Plate Chondrocytes*

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Abstract
The interaction of somatomedin (Sm) with growth plate chondrocytes (GPCs) is believed to be the primary stimulus of skeletal growth. Using techniques designed to disrupt as little as possible the phenotypic characteristics of GPCs, we have been able to obtain 3–4 × 108 viable cells from the major physes of one newborn calf. The availability of these cells plus essentially pure Sm-C/insulin-like growth factor I, the most GH-dependent Sm, has now made possible detailed studies of the interaction of this radiolabeled peptide with the GPC receptor and of the subsequent processing of this hormone by these cells. The enzymatic methods required to free GPCs from their matrix led to loss of receptors, followed by rapid receptor regeneration by de novo synthesis in suspension cultures. Binding of [125I]iodo-Sm-C to GPCs was time dependent and saturable, with optima at 15 C and pH 7.8. At 37 C, binding peaked at 90 min and declined thereafter. Multiplication-stimulating activity, insulin, and nerve growth factor were less potent than unlabeled Sm-C in competition with [126I]iodo-Sm-C for its receptor. Human GH, epidermal growth factor, and fibroblast growth factor failed to show competition even at 10-6 M. Analysis of the fate of [126I]iodo-Sm-C bound to GPCs at 37 C provided evidencethat this hormone is internalized and extruded from the cell in a partially degraded form. Scatchard analysis of [125I]iodo-Sm-C binding to GPCs and to chondrocytes isolated from articular cartilage revealed similar Ka values, but reproducibly 2–6 times more receptors on growth plate than on articular chondrocytes.