The role of membrane carbohydrate in the function of insulin receptors and glucose transporters was investigated with neuraminidase to release sialic acid from isolated rat adipocytes. Pretreatment of adipocytes with neuraminidase resulted in an increase in basal glucose transport. At the same time, insulin-stimulated glucose transport was reduced by an average of 30%. The enzyme action on glucose transport was not due to a nonspecific membrane perturbation because neuraminidase caused only a nonsignificant decrease in the uptake of the amino acid analog α-(methylamino)isobutyric acid and had no effect on basal or insulin-stimulated protein synthesis. Insulin binding was slightly increased in neuraminidase-treated cells, yet the shapes of the dose-response curves for insulin stimulation of glucose transport were similar (EC50 = 0.087 ± 0.010 and 0.082 ± 0.008 nM for control and treated cells, respectively). The neuraminidase-induced increase in basal transport was the result of an increase in the affinity of the glucose-transport system (Km = 7.3 ± 1.4 and 3.6 ± 0.7 mM before and after treatment, respectively) with no change in Vmax. Conversely, enzyme treatment decreased the Vmax of insulin-stimulated 3-O-methylglucose transport from 87.8 ± 13.2 to 41.3 ± 4.9 pmol · 2 × 105 cells−1 · S−1 while not altering the Km. These changes in glucose-transport activity resulting from enzyme treatment were not due to alterations in glucose-transporter concentration on the plasma membrane as measured by the D-glucose-inhibitable binding of [3H]cytochalasin B. Thus, sialic acid plays multiple roles in the control of glucose-transport activity. In the absence of insulin, it places constraints on the affinity of the transporter for substrate, and it may also be involved in maintaining the intrinsic activity of insulin-stimulated transporters, possibly acting through a regulatory protein on the cell surface.
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Original Articles|
August 01 1989
Neuraminidase Treatment of Isolated Rat Adipocytes and Differential Regulation of Basal and Insulin-Stimulated Glucose Transport
Theodore P Ciaraldi
Theodore P Ciaraldi
Veterans Administration Medical Center, Medical Research Service
San Diego
Division of Endocrinology/Metabolism, Department of Medicine, University of California at San Diego
La Jolla, California
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Address correspondence and reprint requests to Theodore P. Ciaraldi, PhD, University of California at San Diego, Department of Medicine, V-111G, La Jolla, CA 92093.
Diabetes 1989;38(8):951–958
Article history
Received:
December 01 1988
Revision Received:
March 28 1989
Accepted:
March 28 1989
PubMed:
2666202
Citation
Theodore P Ciaraldi; Neuraminidase Treatment of Isolated Rat Adipocytes and Differential Regulation of Basal and Insulin-Stimulated Glucose Transport. Diabetes 1 August 1989; 38 (8): 951–958. https://doi.org/10.2337/diab.38.8.951
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