Characteristics of the calcium-triggered mitochondrial permeability transition in nonsynaptic brain mitochondria:: Effect of cyclosporin A and ubiquinone 0

The objective of the present study was to assess the capacity of nonsynaptic brain mitochondria to accumulate Ca(2+) when subjected to repeated Ca(2+) loads, and to explore under what conditions a mitochondrial permeability transition (MPT) pore is assembled. The effects of cyclosporin A (CsA) on Ca(2+) accumulation and MPT pore assembly were compared with those obtained with ubiquinone 0 (Ub(0)), a quinone that is a stronger MPT blocker than CsA, when tested on muscle and liver mitochondria. When suspended in a solution containing phosphate (2 mM) and Mg(2+) (1 mM), but no ATP or ADP, the brain mitochondria had a limited capacity to accumulate Ca(2+) (210 nmol/mg of mitochondrial protein), Furthermore, when repeated Ca(2+) pulses (40 nmol/mg of protein each) saturated the uptake system, the mitochondria failed to release the Ca(2+) accumulated. However, in each instance, the first Ca(2+) pulse was accompanied by a moderate release of Ca(2+), a release that was not observed during the subsequent pulses. The initial release was accompanied by a relatively marked depolarization, and by swelling, as assessed by light-scattering measurements. However, as the swelling was <50% of that observed following addition of alamethicin, it is concluded that the first Ca(2+) pulse gives rise to an MPT in a subfraction of the mitochondrial population. CsA, an avid blocker of the MPT pore, only marginally increased the Ca(2+)-sequestrating capacity of the mitochondria. However, CsA eliminated the Ca(2+) release accompanying the first Ca(2+) pulse. The effects of CsA were shared by Ub(0), but when the concentration of Ub(0) exceeded 20 mu M, it proved toxic. The results thus suggest that brain mitochondria are different from those derived from a variety of other sources. The major difference is that a fraction of the brain mitochondria, studied presently, depolarized and showed signs of an MPT. This fraction, but not the remaining ones, contributed to the chemically and electron microscopically verified mitochondrial swelling.