We investigated the role of intracellular pH (pH_i) and Na/H exchange in cell death in human pulmonary artery endothelial cells (HPAEC) following a metabolic insult (inhibition-oxidative phosphorylation, glycolysis). Metabolic inhibition in medium at pH 7.4 decreased viability (0-15% live cells) over 6 h. Cell death was attenuated by maneuvers that decreased pH_i and inhibited Na/H exchange (acidosis, Na/H antiport inhibitors). In contrast, cell death was potentiated by maneuvers that elevated pH_i or increased Na/H exchange (monensin, phorbol ester treatment) before the insult. HPAEC demonstrated a biphasic pH_i response following a metabolic insult. An initial decrease in pH_i was followed by a return to baseline over 60 min. Maneuvers that protected HPAEC and inhibited Na/H exchange (acidosis, Na⁺-free medium, antiport inhibitors) altered this pattern. pH_i decreased, but no recovery was observed, suggesting that the return of pH_i to normal was mediated by antiport activation. Although Na/H antiport activity was reduced (55-60% of control) following a metabolic insult, the cells still demonstrated active Na/H exchange despite significant ATP depletion. Phorbol ester pretreatment, which potentiated cell death, increased Na/H antiport activity above the level observed in monolayers subjected to a metabolic insult alone. These results demonstrate that HPAEC undergo a pH-dependent loss of viability linked to active Na/H exchange following a metabolic insult. Potentiation of cell death with phorbol ester treatment suggests that this cell death pathway involves protein kinase C-mediated phosphorylation events.