Receptors for insulin-like growth factors in rabbit articular and growth plate chondrocytes in culture

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Receptors for the insulin-like peptide ILAs have been identified in cultured rabbit chondrocytes. The cell-ILAs interaction is a time-dependent, reversible and saturable process. The cell-bound radioactive material appears as intact hormone. Insulin-like growth factor II (IGF II) is as potent as ILAs in competing for 125I-ILAs binding, whereas insulin-like growth factor I is somewhat less potent. Insulin does not affect ILAs binding. Our results suggest that the cultured chondrocytes possess a common receptor for the somatomedin peptides and that insulin, which is able to stimulate sulfate incorporation into proteoglycans, acts through an insulin receptor distinct from the somatomedin site. In growth plate chondrocytes the specific binding of 125I-ILAs is ten times lower than in articular cells in culture, whereas the sulfation activity of ILAs, at all concentrations studied, is 2.5 – 3 times lower in growth plate than in articular chondrocytes. The total binding of 125I-ILAs is higher to a particulate cell fraction than to intact cultured cells, by a factor of 10 for the growth plate cells, and by a factor of 2 for the articular cells. These findings suggest a poor accessibility of the hormone to the receptor sites of the growth plate chondrocytes in culture, which are known to be surrounded by a thick matrix.