Mechanisms of Insulin Action and Na⁺/H⁺ Exchanger in Adipocytes

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Characteristics of insulin binding to isolated rat adipocytes were studied by use of fluorescein isothiocyanate (FITC)-labeled insulin and flow cytometry, and were compared with those of insulin-induced uptake of ³H-2-deoxy-D-glucose (³H-2-DG). FITC-labeled insulin bound to the adipocytes during the initial incubation, but washing once with Hanks' buffer completely removed the bound insulin from the cells. Uptake of ³H-2-DG by the adipocytes was induced by insulin but ceased when the cells were washed. 5-(N-ethyl-N-isopropyl)amiloride (EIPA), a Na⁺/H⁺ exchanger inhibitor, suppressed both the insulin-induced ³H-2-DG uptake and insulin-induced translocation of glucose transporter type 4 (Glut-4) from the microsomal membranes to the plasma membrane of the cells. In the absence of insulin, when the Nay in the Hanks' buffer was replaced with Li⁺[Na⁺(-)Li⁺(+) Hanks' buffer], the level of ³H-2-DG uptake by the cells was significantly higher than in the presence of Na⁺. The increase in the uptake of ³H-2-DG induced by Li⁺ was similar to the increase induced by insulin, but phosphorylation of the insulin receptors was not detected by Western blotting. The increased ³H-2-DG uptake induced by the Na⁺(-)Li⁺(F) Hanks' buffer solution was abolished by adding EIPA. This result suggests that the Na⁺/H⁺ exchanger influences Li⁺-induced ³H-2-DG uptake. Based on these results, we conclude that the observed insulin-induced glucose uptake was tightly regulated by the Na⁺/H⁺ exchanger.