Demonstration of type I and type II somatomedin receptors on bovine growth plate chondrocytes

The chondrocytes of the epiphyseal growth plate are the presumed target cells for hormones regulating skeletal growth. The somatomedins, a family of low molecular weight peptides, are thought to play a stimulatory role in this regulation. The cellular actions of the somatomedins are themselves determined by binding to specific receptors on target cells. Previous studies have characterized a specific receptor for somatomedin‐C (Sm‐C) or insulin‐like growth factor I (IGF‐I) on bovine growth plate chondrocytes (GPCs). We now report the characterization of a second type of somatomedin receptor on these cells that is more specific for another class of somatomedin represented by multiplication‐stimulating activity (MSA) or rat insulin‐like growth factor II (rIGF‐II). Binding of [125I]MSA/rIGF‐II to isolated GPCs was time dependent and saturable. Unlabeled Mr 7.100 MSA/rIGF‐II and Sm‐C/IGF‐I were approximately equipotent in competing with [125I] MSA/rIGF‐II for binding. while Mr 8,600 MSA/rIGF‐II was an order of magnitude less potent. Low levels of competition by insulin appeared in some studies at concentrations of 10−7 M and higher. suggesting displacement of [125I]MSA/rIGF‐II binding. to the Sm‐C/IGF‐I receptor. In affinity‐labeling studies. [125I]Sm‐C/IGF‐I labeled a complex of Mr >300.000 (unreduced) and of Mr 140.000 (reduced). consistent with a type I somatomedin receptor composed of disulfide‐linked subunits. [125I]MSA/rIGF‐II labeled a Mr 240.000 moiety (unreduced) and Mr 260.000 (reduced). consistent with a type II somatomedin receptor. Both affinity‐labeling and kinetic data revealed cross‐binding of MSA/rIGF‐II and insulin with the type I receptor and of Sm‐C/IGF‐I with the type II receptor. In contrast. the type II receptor did not recognize insulin. These data suggest a complex pattern of graded specificity of these receptors for their ligands. These data are consistent with the hypothesis that IGF‐II as well as Sm‐C/IGF‐I participate in the stimulation of skeletal growth.