Receptor-mediated endocytosis of insulin: role of microvilli, coated pits, and coated vesicles.

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Abstract
When 125I-insulin [porcine] is incubated with 3T3-L1 [mouse] adipocytes and cells processed for EM autoradiography, the ligand initially localizes preferentially to microvilli and coated pits. As a function of time and temperature, this initial preferential localization to microvilli is lost and the ligand is internalized by the cell. Serial sections of apparent coated vesicles near the cell surface indicate that about half of these structures are true vesicels and, therefore, intermediates in this receptor-mediated endocytotic process. With time, 125I-insulin localizes to larger intracellular membrane-bounded structures. When cells are incubated with another ligand, cationic ferritin, that is taken up by adsorptive endocytosis, essentially the same structures are involved as for the endocytosis of 125I-insulin. Apparently, specificity for receptor-mediated endocytosis is conferred by the specific ligand receptor and possibly by ligand-induced receptor mobility in the plane of the plasma membrane. Other structures such as coated pits, coated vesicles, larger vesicles and secondary lysosomes are common for different ligands.