Platelet-derived growth factor stimulates Na+/H+ exchange and induces cytoplasmic alkalinization in NR6 cells

Stimulation of Na+/H+ exchange by growth factors has been implicated as a mechanism allowing quiescent cells to resume growth because of a predicted elevation of intracellular pH(pHi). We tested this prediction in NR6 cells by using a further development of our technique for pHi measurement, based on introduction of the fluorescent pH indicator 4',5'-dimethylfluorescein (pKa = 6.75) coupled to dextran into the cytoplasm. Addition of the potent mitogens platelet-derived growth factor (PDGF) or serum to NR6 cells stimulates an amiloride-sensitive 22Na+ uptake and causes an elevation of pHi. The PDGF-dependent pHi increase follows a lag period of approximately equal to 2 min, reaches a maximal level within 10 min (delta pHi approximately equal to 0.1 at an external pH of 7.18), and remains at this level for at least 1 hr. Serum addition initially produces a large elevation of pHi, which later declines to a level similar to that obtained with PDGF. The effects of PDGF and serum are partially additive (delta pHi approximately equal to 0.14). The magnitude of pHi elevation by PDGF decreases with increasing extracellular pH. Serum- and PDGF-dependent elevations of pHi are inhibited by amiloride and by eliminating Na+ from the medium. Under conditions in which Na+/H+ exchange is inhibited, PDGF and serum induce an initial cytoplasmic acidification that does not show a lag period. The results show that a single purified growth factor, as well as serum, can promote a sustained elevation of pHi by stimulating Na+/H+ exchange. The extent of pHi elevation may be modulated by the concomitant stimulation by the growth factor of a process generating H+ within the cell.