Ammonium ion transport-a cause of cell death

Ammonium can be transported into the cell by ion pumps in the cytoplasmic membrane. Ammonia then diffuse out through the cell membrane. A futile cycle is created that results in cytoplasmic acidification and extracellular alkalinisation. Ammonium transport can be quantified by measuring the extracellular pH changes occurring in a cell suspension (in PBS) after addition of ammonium. By using this technique, in combination with specific inhibitors of various ion pumps, it was shown that ammonium ions are transported across the cytoplasmic membrane by the Na(+)K(+)2Cl(-)-cotransporter in both hybridoma and myeloma cells. Further, the Na(+)/H(+) exchanger, which regulates intracellular pH by pumping out protons, was shown to be active during ammonium exposure. The viability of hybridoma cells suspended in PBS and exposed to NH (inf4) (sup+) for only 90 min, was reduced by 11% (50% necrosis and 50% apoptosis). A control cell suspension did not loose viability during this time. Turning off the activity of the Na(+)/H(+) exchanger (by amiloride) during ammonium exposure decreased viability further, while inhibiting transport itself (by bumetanide) restored viability to the same level as for the control experiment with bumetanide alone. These results show that one effect of ammonia/ammonium on cell physiology is specifically related to the inward transport of ammonium ions by membrane bound ion pumps.