Effect of aging on the kinetic characteristics of the insulin receptor autophosphorylation in rat adipocytes.

The effect of aging on the insulin binding parameters and on the kinetic characteristics of the insulin receptor autophosphorylation in rat adipose tissue has been investigated. Using solubilized receptors from adipocyte plasma membranes, no significant differences were identified in either affinity or receptor number in adult vs old rats. Time courses for in vitro receptor phosphorylation revealed that both the initial rate of autophosphorylation and the maximal 32P incorporation were decreased by 40% in old (24-month) animals as compared to adult (3-month) control rats. The tyrosine phosphatase activity associated with the adipocyte plasma membranes does not account for the decreased kinase activity found in old rats. Insulin sensitivity (measured as the dose of insulin required for 50% maximal stimulation of kinase activity) was similar in both groups of rats. However, the kinase activity showed a decreased responsiveness to the hormone in the old rats. Double reciprocal plot analysis of receptor phosphorylation revealed that the Km for ATP was not modified. In contrast, the insulin-stimulated Vmax value was decreased by two-fold in 24-month-old rats. The decrease in Vmax does not appear to be related to an increased basal phosphorylation level on Ser/Thr residues of the C terminus of the receptor beta-subunit. Thus, we conclude that the reduced insulin receptor kinase activity in adipose tissue from old rats is due, at least in part, to a defect of the intrinsic kinase activity of the insulin receptor.