Abstract
The insulin receptor was evaluated at different disease stages in the sand rat (Psammomys obesus), a model for nutrition-induced diabetes. Nondiabetic sand rats showed markedly low receptor number in liver compared with albino rats. Their receptor had an intact tyrosine kinase activity but a higher Km for ATP in the phosphorylation reaction of exogenous substrates. The initial effects of overeating (i.e., development of hyperinsulinemia without hyperglycemia) were associated in the sand rat with a dramatic decrease in in vitro and in vivo insulin-induced receptor tyrosine kinase activity in both liver and muscle. In muscle, this coincided with a decrease in receptor number and an increase in basal tyrosine kinase activity. Similar changes were observed upon development of hyperinsulinemia with hyperglycemia. Upon recovery from the diabetic state by diet restriction, the impaired receptor kinase activation was corrected. Complete restoration occurred only in animals that fully recovered from the diabetic state and became normoinsulinemic. These observations indicate that loss and gain of receptor tyrosine kinase activity were dependent on insulin levels. Thus, overeating may lead to the development of hyperinsulinemia through ineffective extraction of excess insulin by the scarce liver receptors. Hyperinsulinemia, in turn, causes a reversible reduction in receptor kinase activity, leading to insulin resistance. This sequence of events may be relevant to diet-related changes in human non-insulin-dependent diabetes mellitus.