The uptake and extrusion of monovalent cations by isolated heart mitochondria

Summary The factors involved in the movement of monovalent cations across the inner membrane of the isolated heart mitochondrion are reviewed. The evidence suggests that the energy-dependent uptake of K⁺ and Na⁺ which results in swelling of the matrix is an electrophoretic response to a negative internal potential. There are no clear cut indications that this electrophoretic cation movement is carrier-mediated and possible modes of entry which do not require a carrier are examined. The evidence also suggests that the monovalent cation for proton exchanger (Na⁺ > K⁺) present in the membrane may participate in the energy-dependent extrusion of accumulated ions. The two processes, electrophoretic cation uptake (swelling) and exchange-dependent cation extrusion (contraction) may represent a means of controlling the volume of the mitochondrion within the functioning cell. A number of indications point to the possibility that the volume control process may be mediated by the divalent cations Ca⁺² and Mg⁺². Studies with mercurial reagents also implicate certain membrane thiol groups in the postulated volume control process.An invited article.     

Matrix free Mg2+ and the regulation of mitochondrial volume

Mitochondria must maintain volume homeostasis inorder to carry out oxidative phosphorylation. It has been postulatedthat the concentration of freeMg²⁺([Mg²⁺]) serves as thesensor of matrix volume and regulates aK⁺-extrudingK⁺/H⁺antiport (K. D. Garlid. J. Biol. Chem.255: 11273-11279, 1980). To test this hypothesis, the fluorescentprobe furaptra was used to monitor[Mg²⁺] and freeCa²⁺ concentration ([Ca²⁺]) in the matrix ofisolated beef heart mitochondria, andK⁺/H⁺antiport activity was measured by passive swelling in potassium acetate. Concentrations that result in 50% inhibition of maximum activity of 92 µM matrix [Mg²⁺] and 2.2 µM[Ca²⁺] were determined for theK⁺/H⁺ antiport. Untreated mitochondria average670 µM matrix [Mg²⁺], a value that would permit <1%of maximumK⁺/H⁺antiport activity. Hypotonic swelling results in large decreases inmatrix [Mg²⁺], butswelling due to accumulation of acetate salts does not alter[Mg²⁺]. Swelling inphosphate salts decreases matrix[Mg²⁺], but not tolevels that permit appreciable antiport activity. We conclude that1) it is unlikely that matrix[Mg²⁺] serves as themitochondrial volume sensor, 2) ifK⁺/H⁺antiport functions as a volume control transporter, it is probably regulated by factors other than[Mg²⁺], and3) alternative mechanisms formitochondrial volume control should be considered.

     

Ion transport by heart mitochondria. Retention and loss of energy coupling in aged heart mitochondria

The retention and loss of energy-coupling reactions in isolated beef heart mitochondria have been examined under anaerobic conditions using suspending media chosen to mimic the intracellular milieu. In long-term incubations at 37 °C, a loose coupling develops which can be controlled by adding serum albumin. This lesion closely resembles that produced by addition of free fatty acids which has been described in previous studies. Shorter incubation times produce an increased susceptibility to hydrogen peroxide which is characterized by elevated ATPase activity, increased permeability to monovalent cations, and increased proton ejection on transition from the anaerobic to the aerobic state. This peroxide sensitivity is prevented by chelators such as EGTA and appears to involve a time-dependent release of metal ions. Of the metabolites which are known to increase in concentration in the ischemic heart cell, Na⁺, P₁, lactate, and H⁺ all promote swelling of isolated heart mitochondria and contribute to a decline in energy coupling. The relationship of these results to the pathological deterioration of mitochondria in ischemic heart tissue is discussed.     

The nonspecific inner membrane pore of liver mitochondria: modulation of cyclosporin sensitivity by ADP at carboxyatractyloside-sensitive and insensitive sites

Cyclosporin A prevents the opening of a nonspecific pore in the inner membrane of liver mitochondria when added prior to Ca2+. In the presence of 10 microM Ca2+ cyclosporin is unable to close the pore and restore the original permeability unless ADP is also added. ADP acts at a high-affinity site (Km 5 microM), corresponding to the adenine nucleotide transporter. This effect of ADP is prevented and reversed by carboxyatractyloside. In the presence of carboxyatractyloside, cyclosporin added with higher concentrations of ADP (Km 70 microM) also can close the pore. This suggests that a lower-affinity ADP-binding component as well as cyclophilin and the adenine nucleotide transporter can modulate the sensitivity of the pore to cyclosporin.     

Mitochondrial enzyme retention by irreversibly damaged rectangular isolated adult rat heart myocytes

A one hour hypoxic incubation causes the release of a small but significant amount of cytosolic lactic dehydrogenase from glucose-deprived isolated adult rat heart myocytes. However, enzymes associated with the mitochondria are not liberated, and there is no increase in the number of hypercontracted cells. These observations led Piper et al. (Life Sciences 35, 127-134 [1984]) to conclude that reversibly injured myocytes can release cytosolic proteins. This conclusion was based on the supposition that irreversibly hypoxic injury must cause mitochondrial enzyme efflux and hypercontracture. The present study establishes that this supposition is invalid.     

Diurnal changes in near-surface ammonium concentration--interplay between zooplankton and phytoplankton

Data from a cruise in 1996 in the Southern Ocean near the islandof South Georgia indicate that ammonium concentrations in near-surfacewaters (top 30 m of the water column) varied diurnally by     

Microbiological leaching of a chalcopyrite concentrate and the influence of hydrostatic pressure on the activity of Thiobacillus ferrooxidans

Summary The effect of hydrostatic pressure on the activity of Thiobacillus ferrooxidans grown on chalcopyrite concentrate has been investigated. It was found that bacterial activity, measured by conventional respirometry, was little affected by subjecting these microorganisms to a pressure of 100 lbs/in² (690 kPa). The total cooper concentration was as high as 18 g/l in 28 days of leaching at atmospheric pressure.