t-Butylhydroperoxide and gliotoxin stimulate Ca2+ release from rat skeletal muscle mitochondria

Rat liver mitochondria have a specific Ca2+ release pathway which operates when NAD+ is hydrolysed to nicotinamide and ADPribose. NAD+ hydrolysis is Ca(2+)-dependent and inhibited by cyclosporine A (CSA). Mitochondrial Ca2+ release can be activated by the prooxidant t-butylhydroperoxide (tbh) or by gliotoxin (GT), a fungal metabolite of the epipolythiodioxopiperazine group. Tbh oxidizes NADH to NAD+ through an enzyme cascade consisting of glutathione peroxidase, glutathione reductase, and the energy linked transhydrogenase, whereas GT oxidizes some vicinal thiols to the disulfide form, a prerequisite for NAD+ hydrolysis. We report now that rat skeletal muscle mitochondria also contain a specific Ca2+ release pathway activated by both tbh and GT. Ca2+ release increases with the mitochondrial Ca2+ load, is completely inhibited in the presence of CSA, and is paralleled by pyridine nucleotide oxidation. In the presence of tbh and GT, mitochondria do not lose their membrane potential and do not swell, provided continuous release and re-uptake of Ca2+ ('Ca2+ cycling') is prevented. These data support the notion that both tbh- and GT-induced Ca2+ release are not the consequence of an unspecific increase of the inner membrane permeability ('pore' formation). Tbh induces Ca2+ release from rat skeletal muscle less efficiently than from liver mitochondria indicating that the coupling between tbh and NADH oxidation is much weaker in skeletal muscle mitochondria. This conclusion is corroborated by a much lower glutathione peroxidase activity in skeletal muscle than in liver mitochondria. The prooxidant-dependent pathway promotes, under drastic conditions (high mitochondrial Ca2+ loads and high tbh concentrations), Ca2+ release to about the same extent and rate as the Na+/Ca2+ exchanger. This renders the prooxidant-dependent pathway relevant in the pathophysiology of mitochondrial myopathies where its activation by an increased generation of reactive oxygen species probably results in excessive Ca2+ cycling and damage to mitochondria.