Lactacidosis modulates glutathione metabolism and oxidative glutamate toxicity

In cerebral ischemia, reduced blood flow results in lactate accumulation and subsequent tissue acidosis. Lactate and acidosis have been demonstrated to increase infarct size in humans and in animal models of cerebral ischemia.

Oxidative glutamate toxicity is a form of glutamate-induced nerve cell death, where glutamate inhibits cystine uptake via the cystine/glutamate antiporter system xc-. Cystine is an essential building block of glutathione, the major small molecule antioxidant in the brain. Consequently, glutathione depletion leads to accumulation of reactive oxygen species and, ultimately, programmed cell death.

Using the hippocampal cell line, HT22, we show that lactate and acidosis exacerbate oxidative glutamate toxicity and decrease glutathione levels. The EC₅₀ for glutamate toxicity of 0.85 mM at pH 7.4 in the absence of lactate was reduced to 0.69 mM by 20 mM lactate, to 0.41 mM by pH 6.2 and to 0.32 mM by 20 mM lactate at pH 6.2. Lactate at pH 7.4 decreases glutathione by 7%, at pH 6.2 by 30% and the combination of both by 48%. Cystine uptake was reduced by 70% at pH 6.2 compared to pH 7.4, whereas the presence of 20 mM lactate did not influence system xc- activity. Furthermore, the enzymatic steps of glutathione synthesis downstream of cystine uptake were inhibited by lactate and acidosis. Interestingly acidosis also inhibited glutathione consumption. Thus, when glutathione synthesis was completely inhibited by cystine-free medium, acidosis partially protected against glutathione depletion and subsequent cell death. Both effects of acidosis were also present in primary neuronal and astrocyte cultures.

Furthermore, we asked whether compounds known to increase neuronal glutathione levels and to protect against oxidative glutamate toxicity are also active in the presence of lactate and acidosis. Whereas some compounds were still active at pH 6.2 in the presence of lactate, others lost protective activity.

Our finding show that lactacidosis modulates glutathione metabolism and neuronal cell death. Furthermore, lactacidosis may interfere with the action of some neuroprotective drugs rendering these less likely to be therapeutically effective in cerebral ischemia.