INSULIN-INDUCED RECEPTOR LOSS IN THE CULTURED HUMAN LYMPHOCYTE: QUANTITATIVE MORPHOLOGICAL PERTURBATIONS IN THE CELL AND PLASMA MEMBRANE

When cultured human lymphocytes (IM-9) are exposed to IO~6M porcine insulin for 6 h, washed, and incubated with 1I4I-insulin, the ability of the cell to bind the labelled hormone is reduced by a mean of 78 %. Under these experimental conditions that induce insulin-receptor loss in this cell there is a mean 95 % increase in microinvaginations in the plasma membrane revealed by electron microscopy on freeze-fractured replicas of the cell. At the same time, horseradish peroxidase uptake, a marker of endocytosis, is increased in the cells incubated with insulin. Coupled with our recent EM autoradiographic evidence that labelled insulin is acutely internalized by this cell, these studies are consistent with the possibility that endocytosis represents a mechanism by which receptor is removed from the cell surface