Evidence for abnormal regulation of insulin receptors in Friedreich's ataxia

Friedreich's ataxia is associated with a high incidence of diabetes mellitus. We have previously demonstrated that insulin resistance is present in nondiabetic patients with Friedreich's ataxia. This was associated with a reduction in the affinity of insulin receptors on freshly isolated monocytes. In this study we investigated the ability of the monocyte insulin receptor to acutely alter its affinity in response to oral glucose. Glucose and insulin concentrations were higher in the patients with Friedreich's ataxia after an oral glucose load, consistent with the presence of insulin resistance. The normal increase in the affinity of insulin receptors on monocytes 5 h after oral glucose was absent in the five patients with Friedreich's ataxia. Receptor affinity actually decreased in three of the five patients. These findings support the concept that a membrane abnormality that alters the binding function of the insulin receptor is present in these patients.