In vivo regulation of cell surface and intracellular insulin binding sites by insulin.

Purified plasma membranes, smooth and rough endoplasmic reticulum, and nuclei were prepared from the livers of rats made either hyperinsulinemic with insulin injections or hypoinsulinemic with streptozotocin. Plasma membranes from insulin-treated animals bound 46% less lz51-insulin, and membranes from diabetic animals 33% more labeled hormone, than membranes from controls. Binding plots indicated that the decrease and increase in binding observed in these two states were due to changes in the number of insulin binding sites. Smooth endoplasmic reticulum from insulin-treated animals bound 35% less labeled insulin, and smooth endoplasmic reticulum from diabetic animals bound 26% more labeled insulin than did endoplasmic reticulum from controls. Unlike plasma membranes, the major changes in the binding of insulin to the smooth endoplasmic reticulum appeared to be due to changes in binding affinity. Similar results were seen with rough endoplasmic reticulum. Nuclei from insulintreated animals bound 36% less, and nuclei from diabetic animals 25% more, labeled insulin than controls. When the binding of ‘251-insulin to both plasma membranes and nuclei was plotted as a function of the plasma insulin concentration, a significant negative correlation was observed up to 60 microunits/ml. The present study indicates that extracellular insulin in uiuo regulates not only its cell surface binding sites, but also those within the cell. In addition, this study suggests the possibility that the sensitivity of target tissues to insulin may be a function of the ability of both the cell surface and intracellular insulin binding sites to bind insulin.