This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by LEVY, J. Articles by OLEFSKY, J. M. Search for Related Content PubMed PubMed Citation Articles by LEVY, J. Articles by OLEFSKY, J. M.

The Effect of Insulin Concentration on Retroendocytosis in Isolated Rat Adipocytes^*

Division of Endocrinology and Metabolism, Departments of Medicine, University of California San Diego, La Jolla, California 92093
the Veterans Administration Medical Center, Medical Research Service San Diego, California 92161

Address correspondence and requests for reprints to: James Levy, M.D., Veterans Administration Medical Center, Medical Research Service (V-151), 3350 La Jolla Village Drive, San Diego, California 92161.

Abstract

After internalization by isolated rat adipocytes, insulin can be degraded or released intact from the cell, a process termed retroendocytosis. To determine whether the amount of ligand entering the cell could modulate its ultimate intracellular disposition, adipocytes were incubated with 0.4–25 ng/ml radiolabeled insulin for 20 min at 37 C to reach steady state binding and internalization. After this, surface bound insulin was removed by acid extraction and the cells were reincubated in insulin-free 37 C buffer. The fractional rate of release of internalized cell associated radioactivity was similar at all insulin concentrations. However, insulin enhanced the appearance of trichloroacetic acid (TCA)-precipitable material in a dose-dependent manner reaching a maximum at an insulin concentration of 10 ng/ml. At 0.4 ng/ml insulin, 18 ± 2% of the released radioactivity was TCA precipitable whereas 36 ± 3% was precipitable at 25 ng/ml. Sephadex G-50 gel chromatography and reverse phase HPLC analysis of the reincubation medium confirmed TCA-precipitable material was intact insulin. To further investigate the dual pathways of intracellular insulin processing, adipocytes were incubated with 0.4 and 25 ng/ml insulin for 20 min, acid extracted to remove surface receptor insulin, and solubilized. Sephadex G-50 and HPLC analysis revealed that proportionately less insulin intermediates and low molecular weight degradation products are found in cells incubated at the higher insulin concentrations. In conclusion, as adipocytes internalize more insulin, less is converted into insulin intermediates and low molecular weight degradation products and more is diverted to retroendocytosis. (Endocrinology 120: 450–456; 1987)

Footnotes

^* This work was supported by Grants AM-33650 and AM-33651 from NIH and grants form the Veterans Administration Medical Research Service, San Diego, California.

Received June 26, 1986.

This article has been cited by other articles:

				M. E. Valera Mora, A. Scarfone, M. Calvani, A. V. Greco, and G. Mingrone Insulin Clearance in Obesity J. Am. Coll. Nutr., December 1, 2003; 22(6): 487 - 493. [Abstract] [Full Text] [PDF]

				W. C. Duckworth, R. G. Bennett, and F. G. Hamel Insulin Degradation: Progress and Potential Endocr. Rev., October 1, 1998; 19(5): 608 - 624. [Abstract] [Full Text]