Depletion of cellular ATP inhibits Na+/H+ antiport in cultured human cells. Modulation of the regulatory effect of intracellular protons on the antiporter activity.

The metabolic energy dependency of Na+/H+ exchange activity was investigated in cultured human A431 carcinoma cells and foreskin fibroblasts. Following the activation of Na+/H+ exchange which results from loading cells with Na+ or Li+, depletion of cellular ATP by incubation with metabolic inhibitors causes an inhibition of Na+/H+ exchange activity. This inhibition is reversible and correlates with the extent of the reduction in the ATP pool. On the other hand, reduction of the intracellular pH (pHi) to approximately 6.0-6.2 results in a similar effective activation of Na+/H+ exchange in both control and ATP-depleted cells. Na+/H+ exchange activity in ATP-depleted cells that either have or have not been loaded with Na+ shows a steep dependency on pHi, being essentially abolished above pHi of 6.4-6.5, whereas control cells show a considerable activity also at more alkaline pHi values. Thus, Na+/H+ exchange activity in ATP-depleted cells shows an increased dependency on intracellular protons. These findings suggest that although metabolic energy is not required as a driving force for Na+/H+ antiport, depletion of metabolic energy substrates inhibits the antiporter activity due to a modulation of an intracellular proton-dependent regulatory mechanism.