Kinetics of cell age-dependent decline of insulin receptors in human red cells

Insulin receptors are present in human erythrocytes and correlate negatively with cellular age. Little is known about the function of these receptors, about the precise kinetics of their decline during cell aging or about their fate after disappearance from the cells. To elucidate some of these questions, we have prepared red blood cell populations of widely varying cellular ages (ranging from the erythroblast stage to senescent mature erythrocytes) by isopycnic centrifugation on isosmolar density gradients. In addition, young red cells were cultured for 4 days in vitro to permit observation of short-term changes. In mature erythrocytes, insulin receptors decreased as an exponential function of cell age with an estimated half time of 40 days. A more rapid decline of insulin receptors occurred coincident with reticulocyte maturation. Loss of receptors from cultured cells was accompanied by appearance of a soluble insulin receptor in the medium. The effect of insulin on glucose utilization in erythroblast and reticulocyte preparations was negligible, as assessed by CO2 and lactate production. We conclude that 1) insulin receptors are progressively lost from the red blood cell after the erythroblast stage; 2) receptor loss is particularly rapid during reticulocyte maturation; 3) shedding of receptors into the extracellular environment is one reason for their depletion from cells; and 4) in basophilic erythroblasts and reticulocytes, insulin exhibits little metabolic action despite the relatively high receptor complement present in these cells.