Normal Somatomedin and Somatomedin Receptors in Achondroplastic Dwarfism

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Abstract
The basic abnormality in achondroplasia may be a quantitative defect in endochondral new bone formation secondary to decreased synthesis of somatomedin (SM) or abnormal binding of SM to specific receptors. Plasma SM levels and SM receptors on circulating mononuclear cells obtained from 5 achondroplastic dwarfs and 5 age-matched controls were measured. Plasma SM levels were 0.82 .+-. 0.14 U[units]/ml (mean .+-. SE) for the achondroplastic dwarfs and 0.90 .+-. 0.12 U/ml for the controls. The specific binding of 125I-SM to 50 .times. 106 mononuclear cells was 7.66 .+-. 1.11% for the dwarf group and 7.66 .+-. 1.16% for the controls. Achondroplastic and control cells possessed equal numbers of receptor sites and identical receptor affinity for SM. Plasma SM levels and SM binding to circulating mononuclear cells are normal in achondroplastic dwarfs, so a primary intracellular defect may exist.