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

Abstract

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 protondependent regulatory mechanism.An Na+/H+ antiporter is a constituent of the plasma membrane in a variety of animal cells and functions to extrude protons from the cytoplasm using the inwardly directed Na+ gradient.The antiporter, therefore, is thought to be involved in pH: homeostasis.Polypeptide growth factors stimulate Na+/H+ exchange activity in target cells (reviewed in Ref. l), and the resulting increase in pHi may play a permissive role in the induction of cellular proliferation by the growth factors (2,3).Na+/H+ exchange also becomes effectively activated in cells that have been loaded with relatively high concentrations of Na+ or Li+ (3-5), although the significance of this activation is unclear.