Ammonia induces the mitochondrial permeability transition in primary cultures of rat astrocytes

Ammonia is a toxin that has been strongly implicated in the pathogenesis of hepatic encephalopathy (HE), and the astrocyte appears to be the principal target of ammonia toxicity. The specific neurochemical mechanisms underlying HE, however, remain elusive. One of the suggested mechanisms for ammonia toxicity is impaired cellular bioenergetics. Because there is evidence that the mitochondrial permeability transition (MPT) is associated with mitochondrial dysfunction, we determined whether the MPT might be involved in the bioenergetic alterations related to ammonia toxicity. Accordingly, we examined the mitochondrial membrane potential (Δψ_m) in cultured astrocytes and neurons using laser-scanning confocal microscopy after loading the cells with the voltage-sensitive dye JC-1. We found that ammonia induced a dissipation of the Δψ_m in a time- and concentration-dependent manner. These findings were supported by flow cytometry using the voltage-sensitive dye tetramethylrhodamine ethyl ester (TMRE). Cyclosporin A, a specific inhibitor of the MPT, completely blocked the ammonia-induced dissipation of the Δψ_m. We also found an increase in the mitochondrial permeability to 2-deoxyglucose in astrocytes that had been exposed to 5 mM NH₄Cl, further supporting the concept that ammonia induces the MPT in these cells. Pretreatment with methionine sulfoximine, an inhibitor of glutamine synthetase, blocked the ammonia-induced collapse of Δψ_m, suggesting a role of glutamine in this process. Over a 24-hr period, ammonia had no effect on the Δψ_m in cultured neurons. Collectively, our data indicate that ammonia induces the MPT in cultured astrocytes, which may be a factor in the mitochondrial dysfunction associated with HE and other hyperammonemic states.