Membrane permeability transition promoted by phosphate enhances 1-anilino-8-naphthalene sulfonate fluorescence in calcium-loaded liver mitochondria

The mitochondrial permeability transition pore and its role in anaesthesia-triggered cellular protection during ischaemia-reperfusion injury

This review summarises the most recent data in support of the role of the mitochondrial permeability transition pore (mPTP) in ischaemia-reperfusion injury, how anaesthetic agents interact with this molecular channel, and the relevance this holds for current anaesthetic practice. Ischaemia results in damage to the electron transport chain of enzymes and sets into play the assembly of a non-specific mega-channel (the mPTP) that transgresses the inner mitochondrial membrane. During reperfusion, uncontrolled opening of the mPTP causes widespread depolarisation of the inner mitochondrial membrane, hydrolysis of ATP, mitochondrial rupture and eventual necrotic cell death. Similarly, transient opening of the mPTP during less substantial ischaemia leads to differential swelling of the intermembrane space compared to the mitochondrial matrix, rupture of the outer mitochondrial membrane and release of pro-apoptotic factors into the cytosol. Recent data suggests that cellular protection from volatile anaesthetic agents follows specific downstream interactions with this molecular channel that are initiated early during anaesthesia. Intravenous anaesthetic agents also prevent the opening of the mPTP during reperfusion. Although by dissimilar mechanisms, both volatiles and propofol promote cell survival by preventing uncontrolled opening of the mPTP after ischaemia. It is now considered that anaesthetic-induced closure of the mPTP is the underlying effector mechanism that is responsible for the cytoprotection previously demonstrated in clinical studies investigating anaesthetic-mediated cardiac and neuroprotection. Manipulation of mPTP function offers a novel means of preventing ischaemic cell injury. Anaesthetic agents occupy a unique niche in the pharmacological armamentarium available for use in preventing cell death following ischaemia-reperfusion injury.

A large Ca2+-dependent channel formed by recombinant ADP/ATP carrier from Neurospora crassa resembles the mitochondrial permeability transition pore

Strong support for the central role of the ADP/ATP carrier (AAC) in the mitochondrial permeability transition (mPT) is provided by the single-channel current measurements in patch-clamp experiments with the isolated reconstituted AAC. In previous work [Brustovetsky, N., and Klingenberg, M. (1996) Biochemistry 35, 8483-8488], this technique was applied to the AAC isolated from bovine heart mitochondria. Here we used recombinant AAC (rAAC) from Neurospora crassa expressed in E. coli, since AAC from mammalian sources cannot be expresssed in E. coli. The rAAC is free from residual mitochondrial components which might associate with the AAC in preparation from bovine heart. Ca(2+)-dependent channels with up to 600 pS are obtained, which are gated at >150 mV. The channel corresponds to a preferential matrix-outside orientation of rAAC in the patch membrane as shown with carboxyatractylate and a polar gating asymmetry. The channel is inhibited by ADP and bongkrekate, not by carboxyatractylate. Cyclophilin, isolated from Neurospora crassa, suppresses the gating, thus increasing conductivity at high positive voltage. Cyclosporin A abolishes the cyclophilin effect. ADP does not eliminate the cyclophilin effect but produces fast large-amplitude flickering of the channel without a stable decrease of the channel conductance. Also the pro-oxidant tert-butyl hydroperoxide reversibly suppresses voltage gating of the channel. The results show that the AAC can be a conducting component of the mPT pore, exhibiting similar characteristics as the mPT pore (response to Ca(2+), BKA, ADP), with a cyclophilin and pro-oxidant-sensitive gating at high voltage.

The irreversibility of inner mitochondrial membrane permeabilization by Ca2+ plus prooxidants is determined by the extent of membrane protein thiol cross-linking

We have previously shown that mitochondrial membrane potential (delta psi) drop promoted by prooxidants and Ca2+ can be reversed but not sustained by ethylene glycol-bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) unless dithiothreitol (DTT), a disulfide reductant, is also added [Valle, V. G. R., Fagian, M. M., Parentoni, L. S., Meinicke, A. R., and Vercesi, A. E. (1993). Arch. Biochem. Biophys. 307, 1-7]. In this study we show that catalase or ADP are also able to potentiate this EGTA effect. When EGTA is added long after (12 min) the completion of swelling or delta psi elimination, no membrane resealing occurs unless the EGTA addition was preceded by the inclusion of DTT, ADP, or catalase soon after delta psi was collapsed. Total delta psi recovery by EGTA is obtained only in the presence of ADP. The sensitivity of the ADP effect to carboxyatractyloside strongly supports the involvement of the ADP/ATP carrier in this mechanism. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of solubilized membrane proteins shows that protein aggregation due to thiol cross-linkage formed during delta psi drop continues even after delta psi is already eliminated. Titration with 5,5'-dithio-bis(2-nitrobenzoic acid) supports the data indicating that the formation of protein aggregates is paralleled by a decrease in the content of membrane protein thiols. Since the presence of ADP and EGTA prevents the progress of protein aggregation, we conclude that this process is responsible for both increased permeability to larger molecules and the irreversibility of delta psi drop. The protective effect of catalase suggests that the continuous production of protein thiol cross-linking is mediated by mitochondrial generated reactive oxygen species.