title: Epidermal growth factor stimulates amiloride-sensitive 22Na+ uptake in A431 cells. Evidence for Na+/H+ exchange.
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.
mesh_terms: D000584:Amiloride / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism; D002294:Carcinoma, Squamous Cell / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D004815:Epidermal Growth Factor / Q000494:pharmacology*; D006801:Humans; D010042:Ouabain / Q000494:pharmacology; D011719:Pyrazines / Q000494:pharmacology*; D012964:Sodium / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers
chemical_list: D002352:Carrier Proteins; D011719:Pyrazines; D017923:Sodium-Hydrogen Exchangers; D010042:Ouabain; D004815:Epidermal Growth Factor; D000584:Amiloride; D012964:Sodium; D002118:Calcium