Interpretation of insulin radioreceptor assays

Evidence that the insulin resistance of pregnancy may not involve a post-receptor defect in human adipocytes

Insulin binding and the capacity of insulin to stimulate the conversion of glucose to carbon dioxide and lipid, and to activate the protein tyrosine kinase associated with the insulin receptor have been investigated in adipocytes isolated from pregnant and non-pregnant women. Insulin binding and the conversion of glucose to lipid were the same for both groups. However, conversion of glucose to CO2 was higher in the non-pregnant group due to an elevated basal activity, and the increase produced by insulin was similar in both groups. The tyrosine kinase activity of the isolated receptor preparations was higher in the pregnant group due to an increase in the basal non-insulin dependent activity, and the increase produced by insulin was similar in both groups. These findings show the in vitro insulin responsiveness of isolated adipocytes is similar for both groups, and suggests that the in vivo insulin resistance of late pregnancy, as far as adipose tissue is concerned, is not due to any inherent defect in insulin action at the receptor or post-receptor level. In vivo insulin resistance may result from an increased level of circulating insulin antagonists.

Regional differences in insulin receptor function in Werner's syndrome

Werner's syndrome is a genetic disease characterized by premature aging and is often associated with glucose intolerance due to insulin resistance. The clinical manifestations in this syndrome are preferentially expressed in the face and acral regions without apparent involvement of the trunk. We compared insulin receptor binding and amino acid uptake of fibroblasts derived from the forearm that had sclerodermoid features, and from the abdomen that was apparently normal in a patient with Werner's syndrome. In normal controls, specific insulin binding was not different in forearm and abdomen-derived fibroblasts (10.72 +/- 2.11%, 10.40 +/- 1.27%, respectively). In the patient, however, specific insulin binding was reduced in the fibroblasts derived from the forearm compared with those derived from the abdomen (3.55%, 8.16%, respectively). Scatchard analysis revealed that the reduction in insulin binding of the forearm fibroblasts from the patient was due to a reduction in receptor number with no change in receptor affinity. The dose-response curve for insulin of alpha-aminoisobutyric acid (AIB) uptake is shifted to the right in the fibroblasts derived from the acral area. The results show that in a patient with Werner's syndrome, regional differences occur in fibroblast insulin receptor binding and function. This suggests early phenotypic expression of the genetic abnormality of insulin receptor function in these patients.