Electron Microscope Autoradiographic Analysis of [¹²⁵I]Iodoinsulin Entry into Adult Rat Hepatocytesin Vivo:Evidence for Multiple Sites of Hormone Localization*

The binding and electron microscope autoradiographic analysis of [125I]insulin in livers of fed and fasted rats were examined. One minute after the injection of 0.75 μg [125I]iodoinsulin into the portal vein of fed rats, 36.2% of the label had bound to liver. Extraction and gel filtration of the bound radioactivity revealed that it was intact hormone. Ten minutes after injection, 19.2% of the radioactivity was still bound to liver, but extraction and gel filtration data revealed that only 70% of the radioactivity represented intact insulin. This degraded insulin, however, was lost from the tissue during processing for the electron microscope autoradiographs. Grain density histograms of electron microscopic autoradiographs of livers from fed and fasted animals demonstrated that 1 min post injection, grains of [125I]iodoinsulin were primarily located on the plasma membrane of hepatocytes. At this early time point, 10% of the insulin had an intracellular location. Ten minutes after injection, over 80–90% of the grains were inside the cell. Organelle distribution and volume density analysis revealed that 1 min after injection, insulin was primarily associated with plasma membranes, but was also seen inside of the cell over the smooth endoplasmic reticulum and small vesicles (<0.1 μm). After 10 min, insulin was associated with small vesicles, multivesicular bodies, and smooth and rough endoplasmic reticula. These electron microscope autoradiograph studies demonstrated, therefore, that insulin translocates into the hepatocyte interior in a time-dependent manner and then becomes associated with intracellular organelles. This entry of insulin into the cell interior may reflect pathways of insulin degradation, pathways of insulin action, or both (Endocrinology108: 1821, 1981)