ABSTRACT
Both hypoxia and a sharp decline of pH occur during ischemia. Upon reperfusion, reoxygenation and a return to physiologic pH occur together. The role of reoxygenation and oxygen-free radical formation in reperfusion injury has received much attention, but the possible importance of pH changes to lethal cell injury has received little attention. In recent experiments, we evaluated the effect of pH in several models of lethal anoxic, toxic, and reperfusion injury to hepatocytes, cardiac myocytes, and hepatic sinusoidal endothelial cells, both in intact organs and in isolated cell populations. Acidosis (pH ≤ 7.0) protected against lethal cell injury in virtually every model tested, including ATP depletion by metabolic inhibition (“chemical hypoxia”), anoxia/reoxygenation, ischemia/reperfusion, and exposure to toxic chemicals. During ATP depletion, intracellular pH fell. Protection against cell killing was mediated by this intracellular acidification. Treatments that decreased intracellular pH (e.g., amiloride, extracellular acidosis) delayed onset of cell death, whereas measures that blocked intracellular acidification (e.g., monensin) accelerated lethal cell injury. Moreover, in models of ischemia and reperfusion, the rapid return to normal pH rather than reoxygenation precipitated lethal reperfusion injury. Our working hypothesis is that ATP depletion or ion deregulation during cell injury leads to activation of hydrolytic enzymes, whose activity is inhibited at acidotic pH. In support of this hypothesis, phospholipase and protease inhibitors delayed lethal cell injury in these models. Phospholipase A2 activity increased during chemical hypoxia, an effect inhibited by acidotic pH. Understanding these phenomena led to the design of Carolina rinse solution (pH 6.5) to prevent reperfusion injury to organs stored for transplantation surgery. In conclusion, acidotic pH greatly reduced loss of cell viability after ischemia/reperfusion injury. Cells and organisms may exploit this phenomenon to survive hypoxic environments, a strategy physicians might one day adopt to reduce tissue damage after heart attacks, strokes, and organ storage for transplantation surgery.
