Uptake of insulin by plasmalemma and Golgi subcellular fractions of rat liver.

Abstract

Following the portal vein injection of '261-insulin, radioactivity was taken up by rat liver and concentrated in microsomes (30 to 50% of homogenate).Subfractionation of microsomes revealed that 30 to 60% of the label accumulated in Golgi elements.Uptake was inhibited by co-injected unlabeled insulin in a dose-dependent manner, by desalanine and desoctapeptide insulins, and by proinsulin in parallel with their capacities to inhibit insulin binding to its receptors.Structurally unrelated hormones did not inhibit uptake.Maximum labeling of the plasmalemma and Golgi heavy, intermediate, and light fractions occurred at 30 s, 2, 5, and 10 to 15 min, respectively.Redistribution of radiolabel during homogenization had no significant influence on the in vivo data.Electron microscope radioautography revealed grains intimately associated with Golgi elements with 50 to 60% of grains overlying the membranes of vesicles in the Golgi light and intermediate fractions.In Golgi heavy more grains (49%) were associated with small vesicles than flattened saccular elements (35%).'ZsII-insulin eluted from the Golgi fractions had an integrity varying from 75 to 90% at 2 min to 44 to 55% at 20 min as judged by Sephadex chromatography and rebinding studies.These studies indicate that *251-insulin was internalized by a receptor-mediated process and concentrated in Golgi elements of the cell in a substantially intact form and, thus, raise the possibility of an intracellular site of action.In previous studies, using a direct binding approach and electron microscope radioautography, we have identified insulin binding sites in morphologically defined Golgi elements (I).The binding sites of the Golgi elements from both rat and mouse liver were found to be of high affinity and specificity for insulin and to have other properties similar to those of the plasmalemma (2, 3).The Golgi apparatus is involved in concentrating and modifying secretory material prior to exocytosis (4).On the basis of cytochemical evidence that 5"nucleotidase activity can be localized to Golgi membranes (5,6) and that both insulin and lactogen receptors can be identified