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  • Title: Epidermal growth factor stimulates amiloride-sensitive 22Na+ uptake in A431 cells. Evidence for Na+/H+ exchange.
    Author: Rothenberg P, Glaser L, Schlesinger P, Cassel D.
    Journal: J Biol Chem; 1983 Apr 25; 258(8):4883-9. PubMed ID: 6300103.
    Abstract:
    Epidermal growth factor (EGF) increases Na+ uptake in several cell types through an electroneutral, amiloride-sensitive pathway putatively identified as Na+/H+ countertransport. The inferred cytosolic alkalinization resulting from this process has been proposed to be an important component of mitogenic stimulation. We studied the effect of EGF on the Na+/H+ exchange system of A431 cells, a cell line having a very high EGF receptor density but which is not mitogenically stimulated by EGF. We demonstrate that EGF rapidly activates net Na+ influx in A431 cells. Amiloride inhibits the EGF-dependent Na+ uptake (65% inhibition at 3 mM, ID50 approximately 0.3 mM) and inhibits much less the EGF-independent uptake. EGF is known to enhance 45Ca+ accumulation in A431 cells (Sawyer, S. T., and Cohen, S. (1981) Biochemistry 20, 6280-6286). The following findings indicate that EGF-dependent 22Na+ and 45Ca2+ uptake are two independent processes. 1) EGF effectively stimulates an amiloride-sensitive 22Na+ uptake in the absence of external Ca2+. 2) EGF-dependent 45Ca2+ uptake is not inhibited by amiloride. A new fluorescence technique is described for intracellular pH determination based on the introduction of fluorescein-labeled dextran into the cell cytoplasm. Using this method, the presence of amiloride-sensitive Na+/H+ exchange in A431 cells is documented. Although the lack of pH sensitivity of fluorescein fluorescence above pH 7.3-7.4 prevents a direct assessment of an EGF-induced increase of intracellular pH, the combined results of 22Na+ flux and intracellular pH measurements suggest that EGF activates Na+/H+ exchange in A431 cells. We conclude that enhanced Na+/H+ exchange may not necessarily be coupled to mitogenic triggering in different cell types, although the stimulation of Na+/H+ exchange may constitute a primary event in the mechanism of EGF action.
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