Acid-induced death in neurons and glia

Abstract

Lactic acidosis has been proposed to be one factor promoting cell death following cerebral ischemia. We have previously demonstrated that cultured neurons and glia are killed by relatively brief (10 min) exposure to acidic solutions of pH <5 (Goldman et al., 1989). In the present series of experiments, we investigated the relationship between changes in intracellular pH (pH(i)) and cellular viability. pH(i) was measured using fluorescent pH probes and was manipulated by changing extracellular pH (pH(e)). Homeostatic mechanisms regulating pH(i) in neurons and glia were quickly overwhelmed: neither neurons nor glial cells were able to maintain baseline pH(i) when incubated at pH(e) below 6.8. Neuronal and glial death was a function of both the degree and the duration of intracellular acidification, such that the LD50 following timed exposure to HCl increased from pH(i) 3.5 for 10-min acid incubations to pH(i) 5.9 for 2-hr exposures and pH(i) 6.5 for 6-hr exposures. Replacement of HCl with lactic acid raised the LD50 to pH(i) 4.5 for 10-min acid exposures, but did not change the LD50 for longer exposures: pH(i) measurements concurrent with extracellular acidification suggested that the greater cytotoxicity of lactic acid relative to that of HCl was caused by the more rapid intracellular acidification associated with lactic acid. The onset of death after exposure to moderately acidic solutions was delayed in some cells, such that death of the entire cell population became evident only 48 hr after acid exposure. During this latency period, cellular viability indices and ATP levels fell in parallel. The course of death could be experimentally manipulated: hypothermia initiated after the acid exposure attenuated the degree of cell death, indicating that processes occurring during this postinjury period were responsible for demise. Thus, a delayed process of cell death can be initiated in vitro by an initial insult other than glutamate exposure, and this process can occur in glia as well as in neurons.