Reduction of glycogen stores in a rat model of chronic hyperinsulinism

To assess a suitable model for the study of the mechanisms of development of insulin resistance in vivo, liver and muscle glycogen levels (as a metabolic index of tissue insulin sensitivity) were investigated in rats with functioning islet cell adenomas induced by streptozotocin and nicotinamide. These rats have basal moderate hyperinsulinemia and hypoglycemia and show a remarkable increase in insulin secretion after glucose administration. Plasma glucagon concentrations are normal. Nevertheless, in tumor-bearing rats, a reduction of tissue glycogen stores occurs, related to plasma glucose concentrations, and liver glycogen fails to increase even after a glucose load. The lack of excess fat in tumor-bearing rats also suggests a certain insulin insensitivity of the adipose tissue and distinguishes this model of chronic hyperinsulinism from other reported models, such as genetically obese animals.