Direct evidence that insulin does not down-regulate its own receptors in circulating monocytes of human newborns

To determine the effect of insulin on its own receptor concentrations in circulating monocytes of normal human adults and full-term newborns, these cells were chronically exposed (2-18 h) to 10(-9) to 10(-7) mol/l insulin at 37 degrees C in vitro. The reduced 125I-insulin binding observed in monocytes from adult individuals was dependent on the concentration of unlabelled insulin and on the duration of exposure, while in the monocytes obtained from umbilical cord blood, insulin did not induce any reduction of insulin binding. The modifications observed in insulin binding were accounted for by changes in receptor concentrations rather than by any change in receptor affinity for the hormone. These findings show that insulin does not down-regulate its own receptors in monocytes of human newborns.