Toxicity of alpidem, a peripheral benzodiazepine receptor ligand, but not zolpidem, in rat hepatocytes: Role of mitochondrial permeability transitionand metabolic activation

Whereas alpidem is hepatotoxic, zolpidem is not. Despite closely related ch emical structures, alpidem, but not zolpidem, is a peripheral benzodiazepin e receptor (PBR) ligand, and is also more lipophilic than zolpidem. We comp ared their effects in isolated rat liver mitochondria and rat hepatocytes. Zolpidem did not affect calcium-induced mitochondrial permeability transiti on (MPT) in mitochondria, caused little glutathione depletion in hepatocyte s, and was not toxic, even at 500 muM. At 250 to 500 muM, alpidem prevented calcium-induced MPT in isolated mitochondria, but caused severe glutathion e depletion in hepatocytes that was increased by 3-methylcholanthrene, a cy tochrome P4501 A inducer, and decreased by cystine, a glutathione precursor . Although cell calcium increased, mitochondrial cytochrome c did not trans locate to the cytosol and cells died of necrosis. Cell death was prevented by cystine, but not cyclosporin A, an MPT inhibitor. At low concentrations (25-50 muM), in contrast, alpidem accelerated calcium-induced MPT in mitoch ondria. It did not deplete glutathione in hepatocytes, but nevertheless cau sed some cell death that was prevented by cyclosporin A, but not by cystine . Alpidem (10 muM) also increased the toxicity of tumor necrosis factor-alp ha (1 ng/ml) in hepatocytes. In conclusion, low concentrations of alpidem i ncrease both calcium-induced MPT in mitochondria, and TNF-alpha toxicity in cells, like other PBR ligands. Like other lipophilic protonatable amines, however, alpidem inhibits calcium-induced MPT at high concentrations. At th ese high concentrations, toxicity involves cytochrome P4501A-mediated metab olic activation, glutathione depletion, and increased cell calcium, without MPT involvement. In contrast, zolpidem has no mitochondrial effects, cause s little glutathione depletion, and is not toxic.