Butylated hydroxytoluene and inorganic phosphate plus Ca2+ increase mitochondrial permeability via mutually exclusive mechanisms

Mitochondria undergo a permeability transition (PT)(2), i.e., become nonselectively permeable to small solutes, in response to a wide range of conditions/compounds. In general, opening of the permeability transition pore (PTP) is Ca2+- and P-i-dependent and is blocked by cyclosporin A (CsA), trifluoperazine (TFP), ADP, and butylated hydroxytoluene (BHT). Gudz and coworkers have reported [7th European Bioenergetics Conference, EBEC Short Reports (1992) 7, 125], however, that, under some conditions, BHT increases mitochondrial permeability via a process that may not share all of these characteristics. Specifically, they determined that the BHT-induced permeability transition was independent of Ca2+ and was insensitive to CsA. We have used mitochondrial swelling to compare in greater detail the changes in permeability induced by BHT and by Ca2+ plus P-i with the following results. (1) The dependence of permeability on BHT concentration is triphasic: there is a threshold BHT concentration (ca. 60 nmol BHT/mg mitochondrial protein) below which no increase occurs; BHT enhances permeability in an intermediate concentration range; and at high BHT concentrations (>120 nmol/mg) permeability is again reduced. (2) The effects of BHT depend on the ratio of BHT to mitochondrial protein. (3) Concentrations of BHT too low to induce swelling block the PT induced by Ca2+ and P-i. (4) The dependence of the Ca2+-triggered PT on P-i concentration is biphasic. Below a threshold of 50-100 mu M, no swelling occurs. Above this threshold swelling increases rapidly. (5) P-i levels too low to support the Ca2+-induced PT inhibit BHT-induced swelling. (6) Swelling induced by BHT can be stimulated by agents and treatments that block the PT induced by Ca2+ plus P-i. These data suggest that BHT and Ca2+ plus P-i increase mitochondrial permeability via two mutually exclusive mechanisms.