Observations on low amplitude Ca2+-induced swelling in mitochondria

Magic-angle phosphorus NMR of functional mitochondria: in situ monitoring of lipid response under apoptotic-like stress

Using a noninvasive, solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) approach, we track ex vivo the behavior of individual membrane components in isolated, active mitochondria (model system: potato tubers) during physiological processes. The individual phosphatidylcholine (PC), phosphatidylethanolamine (PE), and cardiolipin (CL) membrane constituents were identified as distinct lines by applying MAS (31)P NMR on extracted lipid membranes. However, the CL NMR signal appeared to be very broad in functional mitochondria, indicating a tight complex formation with membrane protein. Calcium stress induced severe membrane degradation without recovery of a single CL NMR resonance. This suggests that calcium overload destroys the outer mitochondrial membrane and does not modify strongly the CL protein complexes in the inner membrane; a conclusion confirmed by respiratory controls. Conversely, mitochondrial membrane disruption on time degradation or mechanical stress generates clearly visible identical CL NMR signals, similar to those observed in rehydrated lipid extracts. Similarly, noninvasive based NMR tracking of lipids in response to diverse physiological stimuli can easily be used for other organelles and whole living cells.

The transport of cations in mitochondria

The present paper has reviewed several factors related to ion transport and examined the properties of cation transport in mitochondria. The analysis suggests that: (1) The concept that a metabolically dependent electrical potential across the mitochondrial membrane plays a role in determining ion fluxes and steady-state concentrations is not justified and the data indicate that such exchanges are generally electroneutral. (2) Generally, the influx and efflux of an ion proceed by the same mechanism with at least one exception. (3) There are indications that some of the steps in transport are common to several cations. (4) The idea that carrier or ionophoric molecules are involved in cation transport has been examined in some detail together with the possible involvement of some known mitochondrial components. In particular, a model has been introduced in which local charge imbalances produced by H+ fluxes serve as the driving force of transport. The molecules of the complex are arranged in series in a tripartite arrangement including a filter or gate, a nonselective channel and an H+-transferring portion linked to either electron transport or the ATPase. Parts of this model have been introduced by other investigators. Models in which different portions of channels have differing functions have been proposed previously for other transport systems.

Ion transport underlying metabolically controlled volume changes of isolated mitochondria

An earlier study has indicated that the swelling of isolated mitochondria induced by inorganic phosphate (P(i)) can be accounted for largely or completely by the accumulation of ions. In the present study, a similar uptake was shown to be induced by a wider range of P(i) concentrations. Addition of ADP, KCN, or 2,4-dinitrophenol initiates a shrinkage which can be accounted for by the efflux of ions. The results are consistent with the explanations that (a) P(i) induces a transport of ions and a concomitant osmotic swelling, and (b) the addition of substances which compete or interfere with the energy available for transport results in ion efflux and a corresponding mitochondrial shrinkage. The results are not consistent with proposals that the changes in the light scattered by mitochondrial suspensions with alterations in metabolic states reflect a mechanochemical coupling phenomenon.