Intracellular pH during 'chemical hypoxia' in cultured rat hepatocytes. Protection by intracellular acidosis against the onset of cell death

The relationship between extracellular pH (pH(o)), intracellular pH (pH(i)), and loss of cell viability were evaluated in cultured rat hepatocytes after ATP depletion by metabolic inhibition with KCN and iodoacetate (chemical hypoxia). pH(i) was measured in single cells by ratio imaging of 2',7'-biscarboxyethyl-5,6-carboxyfluorescein (BCECF) fluorescence using multiparameter digitized video microscopy. During chemical hypoxia at pH(o) of 7.4, pH(i) decreased from 7.36 to 6.33 within 10 min. pH(i) remained at 6.1-6.5 for 30-40 min (plateau phase). Thereafter, pH(i) began to rise and cell death ensued within minutes, as evidenced by nuclear staining with propidium iodide and coincident leakage of BCECF from the cytoplasm. An acidic pH(o) produced a slightly greater drop in pH(i), prolonged the plateau phase of intracellular acidosis, and delayed the onset of cell death. Inhibition of Na⁺/H⁺ exchange also prolonged the plateau phase and delayed cell death. In contrast, monensin or substitution of gluconate for Cl^- in buffer containing HCO₃^- abolished the pH gradient across the plasma membrane and shortened cell survival. The results indicate that intracellular acidosis after ATP depletion delays the onset of cell death, whereas reduction of the degree of acidosis accelerates cell killing. We conclude that intracellular acidosis protects against hepatocellular death from ATP depletion, a phenomenon that may represent a protective adaptation against hypoxic and ischemic stress.