Does insulin removal rate from plasma decline with age?

The effect of age on the rate of insulin removal from plasma was studied in both rat and man. The experimental approach was based on measurement of the steady-state plasma insulin concentration achieved during a period in which endogenous insulin secretion was suppressed and exogenous insulin infused. Rats, 1 1/2 and 12 mo of age, were infused with 2.5, 5.0, and 10.0 mU/kg of insulin during a 180-min period in which endogenous insulin secretion was suppressed by epinephrine and propranolol. Steady-state plasma insulin concentrations were approximately twice as high in the older rats at every insulin infusion rate. Similar results were seen in man; significant correlations were observed between height of steady-state plasma insulin concentration and advancing age during infusion of exogenous insulin and suppression of endogenous insulin with either exogenous insulin (r = 0.66, P less than 0.001) or epinephrine and propranolol (r = 0.47, P less than 0.01). Since infusion rates of exogenous insulin were identical in all studies, these results suggest that there is an age-related decrease in insulin catabolism.