Mitogen‐independent activation of Na⁺/H⁺ exchange in human epidermoid carcinoma A431 cells: Regulation by medium osmolarity

Abstract

Previous studies have documented the activation of Na+/H+ exchange in A431 cells by the addition of epidermal growth factor or serum (Rothenberg et al., 1983b). Here we show that exposure of A4 31 cells to medium of increased osmolarity also leads to activation of Na+/H+ exchange and to an increase in intracellular pH (pHi), which under a variety of conditions displays similar kinetics to that observed upon addition of mitogens to the cells. Measurements of cell volume using the 3-0-methylglucose equilibration technique clearly show that mitogens do not activate Na+/H+ exchange by an osmotic mechanism (i.e., a decrease in cell volume). In fact, mitogens can induce further intracellular alkalinization if added to cells which have been shrunken in hypertonic medium. Activation of the Na+/H+ antiport does not lead to an obligatory change in pHi. Addition of epidermal growth factor of hypertonic solution to A431 cells in bicarbonate buffer activates Na+/H+ exchange without a concomitant increase in pHi. Under these conditions the increased proton efflux via Na+/H+ exchange must therefore be compensated by other mechanisms that control cytoplasmic pH.

Effect of Elicitation and Changes in Extracellular pH on the Cytoplasmic and Vacuolar pH of Suspension-Cultured Soybean Cells

“…In the case of cultured animal cells, the simple inclusion of low concentrations of HC03-in the growth medium causes the disappearance of growth factorstimulated intracellular pH transitions (6,9,10,30). Based on these observations, the hormone-stimulated pH changes in animal cells now are believed to be more metabolically related than involved in signal transduction pathways (6,9,10,30). Third, it is conceivable that minor pH transitions were, in fact, elicited in our cell suspension and that they were simply too small to detect above the normal random pH fluctuations of the suspension.…”

“…Although the occurrence of such pH transitions under physiological conditions has been questioned recently in animal cells (6,9,30), there is still substantial evidence for their occurrence during the defense response in plants (4,13,20,21,27). This defense response, as it is currently understood, involves a rather global change in cell biochemistry encompassing transitions in pathways as diverse as phytoalexin synthesis (7), callose formation (5,14), biosynthesis of lignin-like material (5,22), hydroxyproline-rich protein synthesis (5), release of H202 (2), production of hydrolytic enzymes (5,27), synthesis of protease inhibitors (5), and hypersensitive cell death (4).…”

Hyperosmotic activation of the Na(+)‐H+ exchanger in a rat bone cell line: temperature dependence and activation pathways.

“…A hyperosmotic challenge leads to an initial cell shrinkage resulting in a coupled regulation of cellular volume and a pH increase due to activation of the exchanger (Hoffman & Simonsen, 1989). The intracellular pH response to a hyperosmotic load consists either of an initial acidification followed by an alkalinization or of an alkalinization alone, and is preceded in both cases by a brief lag period (Cassel, Whiteley, Zhuang & Glaser, 1985;Grinstein, Rothstein & Cohen, 1985). The mechanism underlying the activation of the Na+-H+ exchanger by the hyperosmotic stimulus, is yet unknown, though a phosphorylation step was suggested to occur along the activation pathway (Grinstein, Cohen, Goetz & Rothstein, 1985).…”

Tumorigenic 3T3 cells maintain an alkaline intracellular pH under physiological conditions.

“…Although preventing the increase in pHin generally inhibits proliferation (3,4,9), many cells do not increase their pHin at high pHex (e.g., pH 7.4) in the presence of bicarbonate (5,6,10,11), and artificially raising pHin with weak bases or high pHex is not sufficient to stimulate proliferation (12,13 tTo whom reprint requests should be addressed.…”