INSULIN-RESISTANCE IN A CASE OF COEXISTING INSULINOMA AND TYPE-2 DIABETES

Hyperinsulinaemia due to pancreatic beta-cell tumours has been reported to lead to insulin resistance. A possible contribution of dysregulated insulin receptors to the impaired insulin action of insulinoma has not been explored. Therefore, we studied insulin receptor function in a patient with insulin-producing adenoma. This patient was rather unusual in that she was found to have a very large tumour and strikingly high circulating levels of insulin. In addition, her previous history included type 2 (non-insulin-dependent) diabetes mellitus. We confirmed decreased glucose utilization and metabolic clearance rate for glucose in presence of marked endogenous hyperinsulinaemia (approximately 2000 pM). I-125-labelled insulin binding capacity and receptor affinity for insulin were normal in her intact blood monocytes and erythrocytes. Insulin receptors were purified from the patient's tumour as well as from the pancreas, omental fat, liver and erythrocytes. All parameters of insulin binding to these receptors were normal. Thus, no evidence of receptor downregulation due to the marked hyperinsulinaemia was found. As expected, addition of insulin in vitro stimulated receptor autophosphorylation and tyrosine kinase activity of the receptors isolated from the liver, fat and erythrocytes. However, the basal tyrosine kinase activities of the tumour and pancreatic receptors were very high when isolated and further addition of insulin in vitro increased the protein kinase activity only slightly. These results demonstrate that: (1) insulin receptor downregulation does not necessarily occur in the face of chronic endogenous hyperinsulinaemia; and (2) insulin resistance of insulinoma is not due to any structural or functional defect of the insulin receptors and is thus probably related to defect(s) at a level distal to the receptor function.