Change of insulin action with aging in conscious rats determined by euglycemic clamp.

To clarify the mechanism responsible for age-related changes in insulin action, the euglycemic clamp technique was performed with graded doses of insulin in conscious rats aged 2, 4, 10, and 20 mo. Insulin binding (IB) to muscle membranes was also studied. Maximal response of insulin-induced glucose disappearance rate (Rd) was decreased significantly between 2 and 4 mo of age. Dose-response curves shifted to the right progressively up to 20 mo of age. However, IB to the muscle membrane diminished between 1 and 4 mo of age without a decrease thereafter. When Rd was plotted against insulin bound to the membranes, the resulting curves shifted to the right with aging, suggesting a coupling defect between the binding and effector unit. In conclusion, insulin action alters in rats between 2 and 20 mo of age. The most pronounced impairment in IB and maximal response of insulin-induced Rd occurs during early life stage (through maturation) and then a coupling defect seems to be superimposed with further aging. However, we cannot exclude the possibility that these changes may be secondary to obesity or reduced physical activity, rather than aging per se.