The effect of ruthenium red on Ca 2+ transport and respiration in rat liver mitochondria

Convulsions or flaccid paralysis induced by ruthenium red depending on route of administration

Ruthenium red was administered to mice and cats intracranially or intraperitoneally. In mice, intracisternal administration produced status epilepticus and tonic convulsions. In contrast, intraperitoneal administration induced total flaccid paralysis lasting several hours. These effects of Ruthenium red were partially blocked by the simultaneous administration of CaCl2. EDTA, at doses much greater than those of Ruthenium red, produced effects similar to those of the dye, which were also blocked by CaCl2 administration. In cats, intraventricular or intrahippocampal administration of Ruthenium red through a permanently implanted cannula produced after a few minutes subclinical paroxysmal activity in all brain regions recorded. After several hours the animals developed typical grand mal seizures. Intraperitoneal injection of Ruthenium red to cats did not affect the EEG but markedly depressed muscular activity. Administration of carbachol to the latter animals produced myoclonic responses. These results are discussed in relation to the inhibitory effect of Ruthenium red on Ca2+ transport and binding to membranes, and to the role of this cation on neurotransmitter release.

Evidence for more than one Ca2+ transport mechanism in mitochondria

The active transport and internal binding of the Ca2+ analogue Mn2+ by rat liver mitochondria were monitored with electron paramagnetic resonance. The binding of transported Mn2+ depended strongly on internal pH over the range 7.7-8.9. Gradients of free Mn2+ were compared with K+ gradients measured on valinomycin-treated samples. In the steady state, the electrochemical Mn2+ activity was larger outside than inside the mitochondria. The observed gradients of free Mn2+ and of H+ could not be explained by a single "passive" uniport or antiport mechanism of divalent cation transport. This conclusion was further substantiated by observed changes in steady-state Ca2+ and Mn2+ distributions induced by La3+ and ruthenium red. Ruthenium red reduced total Ca2+ or Mn2+ uptake, and both inhibitors caused release of divalent cation from preloaded mitochondria. A model is proposed in which divalent cations are transported by at least two mechanisms: (1) a passive uniport and (2) and active pump, cation antiport or anion symport. The former is more sensitive to La3+ and ruthenium red. Under energized steady-state conditions, the net flux of Ca2+ or Mn2+ is inward over (1) and outward over (2). The need for more than one transport system inregulating cytoplasmic Ca2+ is discussed.

Calcium regulation in heart cells. The interaction of mitochondrial and sarcoplasmic reticulum with troponin-bound calcium

A study has been carried out of the interaction of dog heart mitochondria and sarcoplasmic reticulum with Ca2+ bound to troponin or to its Ca2+-binding component (troponin-C). Both organelles are able to release more than 80% of the Ca2+ bound to the two proteins. However, sarcoplasmic reticulum is only able to do so in the presence of oxalate, whereas mitochondria are active also in the absence of permeant Ca-complexing anions. On the other hand, calcium uptake in heart mitochondria is severely inhibited by Mg2+. The results are discussed with reference to the problem of the control of Ca2+ in heart sarcoplasm.

Glucose metabolism in perfused skeletal muscle. Pyruvate dehydrogenase activity in starvation, diabetes and exercise

1. The interconversion of pyruvate dehydrogenase between its inactive phosphorylated and active dephosphorylated forms was studied in skeletal muscle. 2. Exercise, induced by electrical stimulation of the sciatic nerve (5/s), increased the measured activity of (active) pyruvate dehydrogenase threefold in intact anaesthetized rated within 2 min. No further increase was seen after 15 min of stimulation. 3. In the perfused rat hindquarter, (active) pyruvate dehydrogenase activity was decreased by 50% in muscle of starved and diabetic rats. Exercise produced a twofold increase in its activity in all groups; however, the relative differences between fed, starved and diabetic groups persisted. 4. Perfusion of muslce with acetoacetate (2 mM) decreased (active) pyruvate dehydrogenase activity by 50% at rest but not during exercise. 5. Whole-tissue concentrations of pyruvate and citrate, inhibitors of (active) pyruvate dehydrogenase kinase and (inactive) pyruvate dehydrogenase phosphate phosphatase respectively, were not altered by excerise. A decrease in the ATP/ADP ratio was observed, but did not appear to be sufficient to account for the increase in (active) pyruvate dehydrogenase activity. 6. The results suggest that interconversion of the phosphorylated and dephosphorylated forms of pyruvate dehydrogenase plays a major role in the regulation of pyruvate oxidation by eomparison of enzyme activity with measurements of lactate oxidation in the perfused hindquarter [see the preceding paper, Berger et al. (1976)] suggest that pyruvate oxidation is also modulated by the concentrations of substrates, cofactors and inhibitors of (active) pyruvate dehydrogenase activity.

Calcium uptake in preterminal central synapses: importance of mitochondria

Energy dependent 45Ca2+ uptake in the synaptosomal preparation from guinea pig cortex has been investigated. 45Ca2+ uptake was stimulated by ATP, the absolute value of uptake being dependent on the extent of synaptosomal disruption caused by osmotic shock. A quantitative comparison of microsomal and mitochondrial ATP-dependent 45Ca2+ uptake showed that only mitochondria had a large enough capacity to account for the Ca uptake levels observed in the synaptosomal preparation. ATP-stimulated 45Ca2+ uptake in mitochondria, 'intact' and 'shocked' synaptosomes was inhibited by atractyloside, DNP, oligomycin and ruthenium red but unaffected by antimycin A and rotenone. This was interpreted as evidence that mitochondria were responsible for ATP-dependent synaptosomal Ca2+ uptake, the increase in uptake seen on osmotic lysis being due to the deocclusion of intraterminal mitochondria. Synaptosomal and mitochondrial 45Ca2+ uptake was also stimulated by the mitochondrial respiratory substrate glutamate; this uptake was sensitive to antimycin A, DNP, rotenone and ruthenium red but insensitive to atractyloside or oligomycin thus indicating it was of mitochondrial origin. No change in glutamate-dependent 45Ca2+ uptake was seen on osmotic lysis of the synaptosomes as the expected increase due to the release of occluded mitochondria was counterbalanced by the damaging effect of hypo-osmotic shock on the glutamate-stimulated 45Ca2+ uptake process.

The calcium conductance of the inner membrane of rat liver mitochondria and the determination of the calcium electrochemical gradient

1. A method is described for establishing steady-state conditions of calcium transport across the inner membrane of rat liver mitochondria and for determining the current of Ca2+ flowing across the membrane, together with the Ca2+ electrochemical gradient across the native Ca2+ carrier. These parameters were used to quantify the apparent Ca2+ conductance of the native carrier. 2. At 23 degrees C and pH7.0, the apparent Ca2+ conductance of the carrier is close to 1 nmol of Ca2+-min-1-mg of protein-1 mV-1. Proton extrusion by the respiratory chain, rather than the Ca2+ carrier itself, may often be rate-limiting in studies of initial rates of Ca2+ uptake. 3. Under parallel conditions, the endogenous H+ conductance of the membrane is 0.3 nmol of H+-min-1-mg of protein-1-mV-1. 4. Ruthenium Red and La3+ both strongly inhibit the Ca2+ conductance of the carrier, but are without effect on the H+ conductance of the membrane. 5. The apparent Ca2+ conductance of the carrier shows a sigmoidal dependence on the activity of Ca2+ in the medium. At 23 degrees C and pH7.2, half-maximum conductance is obtained at a Ca2+ activity of 4.7 muM. 6. The apparent Ca2+ conductance and the H+ conductance of the inner membrane increase fourfold from 23 degrees to 38 degrees C. The apparent Arrhenius activation energy for Ca2+ transport is 69kJ/mol. The H+ electrochemical gradient maintained in the absence of Ca2+ transport does not vary significantly with temperature. 7. The apparent Ca2+ conductance increases fivefold on increasing the pH of the medium from 6.8 to 8.0. The H+ conductance of the membrane does not vary significantly with pH over this range. 8. Mg2+ has no effect on the apparent Ca2+ conductance when added at concentration up to 1 mM. 9. Results are compared with classical methods of studying Ca2+ transport across the mitochondrial inner membrane.

Calcium-binding of synaptosomes isolated from rat brain cortex. IV. Effects of ruthenium red on the Co-operative nature of calcium-binding

Ruthenium red combines with isolated synaptosomes, resulting in strong inhibition of their Ca2+-binding. In isotonic saline media, however, the dye-induced inhibition of Ca2+-binding is significantly greater than that expected for the amount of bound dye and Hill's exponent of the Ca2+-binding decreases to 1 with an increase in the amount of the dye bound. On the other hand in isotonic mannitol-sucrose solution, inhibition of synaptosmal Ca2+-binding brought about by the dye is proportional to the amount of dye bound. Based on these results, the effects of the dye on the co-operative nature of synaptosomal Ca2+-binding is discussed.

Respiration-dependent efflux of magnesium ions from heart mitochondria

Energy-linked respiration causes a net movement of Mg2+ between rat heart mitochondria and the ambient medium. When the extramitochondrial concontration of Mg2+ is less that about 2.5 mM the net movement of Mg2+ constitutes an efflux, whereas a net influx of Mg2+ occurs when the external concentration of Mg2+ is greater than this. Both the efflux and the influx are induced to only a very small degree by externally added ATP. Evidence suggests that Pi may be required for the respiration-induced efflux of Mg2+.

Effects of La+++, Mn++ and ruthenium red on Mg-Ca-ATPase activity and ATP-dependent Ca-binding of the synaptic plasma membrane

The effects of La+++, Mn++ and ruthenium red (R.R.) on Ca-uptake of synaptic plasma membranes (S.P.M.) were investigated. La+++ (0.1 mM), Mn++ (0.2 mM) and R.R. (0.1 mM) selectively inhibited Mg-Ca-ATPase but did not significantly affect Mg-ATPase activity. The apparent Ki values of La+++, Mn++ and R.R. for Mg-Ca-ATPase were 0.05, 0.06 and 0.03 mM, respectively. La+++, Mn++ and R.R. did not affect Ca-uptake at concentrations which strongly inhibited Mg-Ca-ATPase activity. These results indicate that Ca-uptake by S.P.M. differ from that by sarcoplasmic reticulum.

The uptake and extrusion of monovalent cations by isolated heart mitochondria

The factors involved in the movement of monovalent cations across the inner membrane of the isolate 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+ greater than K+) present in the membrane may participate in the energy-dependent extrusion of accumulated ions. The two processes, electrophoreti c 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.

Oxalate, calcium uptake and ATPase activity of sarcoplasmic reticulum vesicles

Ca++-uptake and Mg++-Ca++-dependent ATPase activity of skeletal muscle sarcoplasmic reticulum vesicles were reciprocally affected by increasing the oxalate concentration from 0 to 4 mM. At 0-0.1 mM oxalate approximately 17% of the calcium was removed by the vesicles from the medium while the ATPase activity was maximal (approximately 0.66 mumoles Pi mg-1 protein min-1). Between 0.1 to 0.2 mM oxalate the ATPase activity was reduced to one-fifth but the uptake rose sharply and 100% of the 45Ca++ was removed from the medium. The uptake was maintained at this level at oxalate concentrations greater than 0.4 mM but the ATPase activity remained inhibited. The kinetics of Ca++-uptake and ATPase activity were also differentially affected by oxalate. In the presence of oxalate, ruthenium red had only a very slight inhibitory effect on the calcium uptake. Addition of 0.1 mM EGTA removed 80% of the Ca++ from preloaded vesicles within 10 min. The formation of insoluble Ca-oxalate salt on the surface of the vesicle is suggested by these results. Calculations based on the Ksp of the calcium oxalate salt are presented to show its formation and the possible speciation of a Ca-oxalate complex which may affect the Ca++-uptake and ATPase activity.

Regulation of mammalian pyruvate dehydrogenase

In mammalian tissues, two types of regulation of the pyruvate dehydrogenase complex have been described: end product inhibition by acetyl CoA and NADH: and the interconversion of an inactive phosphorylated form and an active nonphosphorylated form by an ATP requiring kinase and a specific phosphatase. This article is largely concerned with the latter type of regulation of the complex in adipose tissue by insulin (and other hormones) and in heart muscle by lipid fuels. Effectors of the two interconverting enzymes include pyruvate and ADP which inhibit the kinase, acetoin which activates the kinase and Ca2+ and Mg2+ which both activate the phosphatase and inhibit the kinase. Evidence is presented that all components of the pyruvate dehydrogenase complex including the phosphatase and kinase are located within the inner mitochondrial membrane. Direct measurements of the matrix concentration of substrates and effectors is not possible by techniques presently available. This is the key problem in the identification of the mechansims involved in the alterations in pyruvate dehydrogenase activity observed in adipose tissue and muscle. A number of indirect approaches have been used and these are reviewed. Most hopeful is the recent finding in this laboratory that in both adipose tissue and heart muscle, differences in activity of pyruvate dehydrogenase in the intact tissue persist during preparation and subsequent incubation of mitochondria.

The mechanism of the halothane-dependent efflux of calcium from rat-liver mitochondria

The halothane-dependent, calcium-induced loss of respiratory control in rat liver mitochondria [1, 2] is Mg2plus -dependent and is accompanied by an enhanced mitochondrial swelling. It is suggested that this swelling reflects an increase in calcium activity in the matrix space, due to a decrease in binding of the accumulated cation. This change in the partition of intramitochondrial calcium is correlated with an inhibition by halothane of energy-independent, calcium-induced swelling. The enhanced swelling associated with the active accumulation of calcium in the presence of halothane does not lead to a marked increase in permeability to other ions. Nevertheless, under conditions of energised calcium uptake, and in the presence of Mg2plus, a halothane-dependent, ruthenium red-insensitive efflux of calcium is observed. This is consistent with the proposed halothane-dependent increase in the matrix activity of accumulated Ca2plus. It is suggested that this mechanism accounts for the previously postulated [2] futile cycle of calcium uptake and release induced by halothane in rat liver mitochondria.

Evidence for a halothane-dependent cyclic flux of calcium in rat-liver mitochondria

The previously reported (Hall et al., Biochem. Soc. Trans. 1973) halothane-dependent, calcium-induced loss of respiratory control in rat liver mitochondria is relatively specific to calcium; the effect of strontium ions is much smaller, and comparable additions of potassium salts have no effect on mitochondrial respiration on succinate in the presence of halothane. The calcium-dependent loss of respiratory control can be prevented, or reversed, respectively, by the prior or subsequent addition of agents that either chelate extramitochondrial Ca2plus or inhibit calcium accumulation, or that inhibit the efflux of accumulatec calcium. These results suggest that the halothane-dependent, calcijm-induced loss of respiratory control is due to a cyclic flux of calcium uptake and release.

A kinetic study of mitochondrial calcium transport

This report describes a kinetic analysis of energy-linked Ca2+ transport in rat liver mitochondria, in which a ruthenium red/EGTA [ethanedioxy-bis(ethylamine)-tetraacetic acid] quenching technique has been used to measure rates of 45Ca2+ transport. Accurately known concentrations of free 45Ca2+ were generated with Ca2+/nitrilotriacetic acids buffers for the determination of substrate/velocity relationships. The results show that the initial velocity of transport is a sigmoidal function of Ca2+ concentration (Hill coefficient = 1.7), the Km being 4 muM Ca4 at 0 degrees C and pH 7.4. These values for the Hill coefficient and the Km remain constant in the presence of up to 2 mM phosphate, but with 10 mM acetate both parameters are increased slightly. Both permeant acids increase the maximum velocity to an extent dependent on their concentration. The Ca2+-binding site(s) of the carrier contains a group ionizing at pH approximately 7.5 at 0 degrees C, which is functional in the dissociated state. The stimulatory effect of permeant acids is ascribed to their facilitating the release of Ca2+ from the carrier to the internal phase, an interpretation which is strengthened by the lack of effect of the permeant anion SCN- on Ca2+ transport. Studies on the time-course of Ca2+ uptake and of EFTA-induced Ca2+ efflux from pre-loaded mitochondria demonstrate the reversibility of the carrier in respiring mitochondria and the extent to which this property is influenced by permeant acids. These data are accommodated in a carrier mechanism based on electrophoretic transport of Ca2+ bound to pairs of interacting acidic sites.

Effects of ruthenium red and KCL on responses of guinea pig umbilical veins

5-Hydroxytryptamine (10(-5) M), norepinephrine (10(-5) M) and acetylcholine (10(-5) M) induced maximal contractions in isolated guinea pig umbilical vein strips. Pretreatment with ruthenium red (2.7--90 mug/ml) for ten minutes reduced 5-hydroxytryptamine and norepinephrine responses in a dose-related manner but did not affect acetylcholine-induced responses. Ruthenium red alone did not produce contractions. In the presence of KC1 (125 MM) the responses to norepinephrine and acetylcholine were enhanced significantly. The sensitivity of umbilical veins to ruthenium red may be useful in determining to what extent various vasoactive agents utilize extracellular calcium to induce contractions.

Evidence of a calcium-ion-transport system in mitochondria isolated from flight muscle of the developing sheep blowfly Lucilia cuprina

The EGTA (ethanedioxybis(ethylamine)tetra-acetic acid)-Ruthenium Red-quench technique (Reed & Bygrave, 1974a) was used to measure initial rates of Ca-2+ transport in mitochondria from flight muscle of the blowfly Lucilia cuprina. Evidence is provided for the existence in these mitochondria of a Ca-2+-transport system that has many features in common with that known to exist in rat liver mitochondria. These include requirement for energy, saturation at high concentrations of Ca-2+, a sigmoidal relation between initial rates of Ca-2+ transport and Ca-2+ concentration, a high affinity for free Ca-2+ (Km approx. 5 muM) and high affinity for the Ca-2+-transport inhibitoy, Ruthenium Red (approx. 0.03 nmol of carrier-specific binding-sites/mg of protein; Ki approx. 1.6 x 10- minus 8 M). Controlled respiration can be stimulated by Ca-2+ after a short lag-period provided the incubation medium contains KCl and not sucrose. The ability of Lucilia mitochondria to transport Ca-2+ critically depends on the stage of mitochondrial development; Ca-2+ transport is minimal in mitochondria from pharate adults, is maximal between 0 and 2h post-emergence and thereafter rapidly declines to reach less than 20% of the maximum value by about 2-3 days post-emergence. Respiration in mitochondria from newly emerged flies does not respond to added Ca-2+; that from 3-5-day-old flies is stimulated approx. 50%. Whereas very low concentrations of Ca-2+ inhibit ADP-stimulated respiration and oxidative phosphorylation in mitochondria from newly emerged flies (Ki approx. 60 ng-ions of Ca-2+/mg of protein); much higher concentrations (approx. 200 ng-ion/mg of protein) are needed to inhibit these processes in those from older flies. The potential of this system for studying the function and development of metabolite transport systems in mitochondria is discussed.

THE EFFECTS OF TEN PHENOLIC COMPOUNDS ON HYPOCOTYL GROWTH AND MITOCHONDRIAL METABOLISM OF MUNG BEAN

Ten phenolic compounds were examined for their effect on mung bean (Phaseolus aureus L.) hypocotyl growth and on respiration and coupling parameters of isolated mung bean hypocotyl mitochondria. Three compounds-tannic, gentisic, and p-coumaric acids-inhibited hypocotyl growth and when incubated with isolated hypocotyl mitochondria released respiratory control, inhibited respiration, and prevented substrate-supported Ca²⁺ and PO₄ transport. Vanillic acid also inhibited hypocotyl growth and reduced mitochondrial Ca²⁺ uptake but did not affect respiration or respiratory control of isolated mitochondria. This is the first compound reported to selectively inhibit Ca²⁺ uptake in plant mitochondria. Two other phenolic compounds-α, 3,5-resorcylic and protocatechuic acids-showed no significant effect on hypocotyl growth and did not affect mitochondrial oxidative phosphorylation either separately or in various combinations. Four phenolic compounds-ferulic, caffeic, p-hydroxybenzoic, and syringic acids-showed a significant reduction in mung bean hypocotyl growth but did not inhibit any of the mitochondrial processes examined. The results show that phenolic compounds which alter respiration or coupling responses in isolated mitochondria also inhibit hypocotyl growth and may reflect a mechanism of action for these natural growth inhibitors.

A re-evaluation of energy-independent calcium-ion binding by rat liver mitochondria

The impermeability of the mitochondrial inner membrane to the chelator ethanedioxybis(ethylamine)tetra-acetic acid permits discrimination between Ca(2+) which has been transported to the internal (matrix) phase and Ca(2+) which binds to the external surfaces of the mitochondrion. With this technique, it is shown that ;energy-independent high-affinity' binding is a measure of carrier-mediated active Ca(2+) transport in respiration-inhibited mitochondria; the carrier also transports Ca(2+) to the internal phase after treatment with carbonyl cyanide m-chlorophenylhydrazone, but in this case the active-transport component is inhibited. The Ca(2+)-binding sites associated with the external membrane surfaces are similar in concentration and affinity for both inhibited and uncoupled mitochondria; it was not possible to measure external Ca(2+) binding which could be identified as carrier specific. The results are discussed in relation to the mechanism of mitochondrial Ca(2+) transport, and to previous studies of energy-independent Ca(2+) binding.

Calcium-binding properties and ATPase activities of rat liver plasma membranes

Plasma membranes from rat liver purified according to the procedure of Neville bind calcium ions by a concentration-dependent, saturable process with at least two classes of binding sites. The higher affinity sites bind 45 nmol calcium/mg membrane protein with a K(D) of 3 microM. Adrenalectomy increases the number of the higher affinity sites and the corresponding K(D). Plasma membranes exhibit a (Na(+)-K(+))-independent-Mg(2+)-ATPase activity which is not activated by calcium between 0.1 microM and 10 mM CaCl(2). Calcium can, with less efficiency, substitute for magnesium as a cofactor for the (Na(+)-K(+))-independent ATPase. Both Mg(2+)- and Ca(2+)-ATPase activities are identical with respect to pH dependence, nucleotide specificity and sensitivity to inhibitors. But when calcium is substituted for magnesium, there is no detectable membrane phosphorylation from [gamma-(32)P] ATP as it is found in the presence of magnesium. The existence of high affinity binding sites for calcium in liver plasma membranes is compatible with a regulatory role of this ion in membrane enzymic mechanisms or in hormone actions. Plasma membranes obtained by the procedure of Neville are devoid of any Ca(2+)-activated-Mg(2+)-ATPase activity indicating the absence of the classical energy-dependent calcium ion transport. These results would suggest that the overall calcium-extruding activity of the liver cell is mediated by a mechanism involving no direct ATP hydrolysis at the membrane level.

Calcium and magnesium ions as effectors of adipose-tissue pyruvate dehydrogenase phosphate phosphatase

The metal-ion requirement of extracted and partially purified pyruvate dehydrogenase phosphate phosphatase from rat epididymal fat-pads was investigated with pig heart pyruvate dehydrogenase [(32)P]phosphate as substrate. The enzyme required Mg(2+) (K(m) 0.5mm) and was activated additionally by Ca(2+) (K(m) 1mum) or Sr(2+) and inhibited by Ni(2+). Isolated fat-cell mitochondria, like liver mitochondria, possess a respiration- or ATP-linked Ca(2+)-uptake system which is inhibited by Ruthenium Red, by uncouplers when linked to respiration, and by oligomycin when linked to ATP. Depletion of fat-cell mitochondria of 75% of their total magnesium content and of 94% of their total calcium content by incubation with the bivalent-metal ionophore A23187 leads to complete loss of pyruvate dehydrogenase phosphate phosphatase activity. Restoration of full activity required addition of both MgCl(2) and CaCl(2). SrCl(2) could replace CaCl(2) (but not MgCl(2)) and NiCl(2) was inhibitory. The metal-ion requirement of the phosphatase within mitochondria was thus equivalent to that of the extracted enzyme. Insulin activation of pyruvate dehydrogenase in rat epididymal fat-pads was not accompanied by any measurable increase in the activity of the phosphatase in extracts of the tissue when either endogenous substrate or (32)P-labelled pig heart substrate was used for assay. The activation of pyruvate dehydrogenase in fat-pads by insulin was inhibited by Ruthenium Red (which may inhibit cell and mitochondrial uptake of Ca(2+)) and by MnCl(2) and NiCl(2) (which may inhibit cell uptake of Ca(2+)). It is concluded that Mg(2+) and Ca(2+) are cofactors for pyruvate dehydrogenase phosphate phosphatase and that an increased mitochondrial uptake of Ca(2+) might contribute to the activation of pyruvate dehydrogenase by insulin.

The inhibition of mitochondrial calcium transport by lanthanides and ruthenium red

An EGTA (ethanedioxybis(ethylamine)tetra-acetic acid)-quench technique was developed for measuring initial rates of (45)Ca(2+) transport by rat liver mitochondria. This method was used in conjunction with studies of Ca(2+)-stimulated respiration to examine the mechanisms of inhibition of Ca(2+) transport by the lanthanides and Ruthenium Red. Ruthenium Red inhibits Ca(2+) transport non-competitively with K(i) 3x10(-8)m; there are 0.08nmol of carrier-specific binding sites/mg of protein. The inhibition by La(3+) is competitive (K(i)=2x10(-8)m); the concentration of lanthanide-sensitive sites is less than 0.001nmol/mg of protein. A further difference between their modes of action is that lanthanide inhibition diminishes with time whereas that by Ruthenium Red does not. Binding studies showed that both classes of inhibitor bind to a relatively large number of external sites (probably identical with the ;low-affinity' Ca(2+)-binding sites). La(3+) competes with Ruthenium Red for most of these sites, but a small fraction of the bound Ruthenium Red (less than 2nmol/mg of protein) is not displaced by La(3+). The results are discussed briefly in relation to possible models for a Ca(2+) carrier.

Inhibitory action of Ruthenium red on neuromuscular transmission

The effect of Ruthenium Red on synaptic transmission was examined at isolated junctions of the frog, by conventional methods for stimulation and intracellular recording. Ruthenium Red (2.5-10.0 muM) reduces the synaptic potential to subthreshold levels. An analysis of this phenomenon shows that the main action of Ruthenium Red is on the presynaptic nerve terminal where it decreases the number of quanta of transmitter liberated by the nerve impulse. It has the following additional effects: a reduction in the amplitude of the spontaneous miniature end plate potentials; an increase in their frequency; and an increase in delayed release of transmitter after a nerve impulse. Some of these results are discussed in terms of the known inhibitory action of Ruthenium Red on calcium transport across mitochondrial membranes.

Mechanisms for the abnormal energetics of pressure-induced hypertrophy of cat myocardium

Depressed myocardial contractility with paradoxically increased oxygen consumption has been demonstrated in previous studies of pressure overload hypertrophy. To determine whether altered mitochondrial respiration participates in the abnormal energetics of this muscle, right ventricular hypertrophy (RVH) was produced in 12 cats by pulmonary artery banding. A polarographic muscle bath was used to study eight control and eight RVH papillary muscles, and the respiration of mitochondria isolated from these right ventricles was characterized. RVH muscles demonstrated depressed force-velocity and length-tension curves. The myocardial oxygen consumption per gram of peak active tension was increased from 0.65 ± 0.05 nliters/mg beat -1 (control) to 1.10±0.07 nliters/mg beat -1 (RVH) ( P <0.001). Abnormal mitochondrial respiration was shown by an increase in the rate of state 4 oxygen consumption from 12.5±0.8 natoms/mg min -1 (control) to 19.9±0.8 natoms/mg min -1 (RVH) ( P < 0.001). The altered oxygen cost of active isometric tension in the RVH muscles was linearly correlated with the altered rate of mitochondrial state 4 oxygen consumption ( r = 0.91). Ruthenium red, a compound that blocks mitochondrial calcium uptake, reduced the rate of RVH state 4 oxygen consumption to the control level. The present study suggests a mechanism for the abnormal myocardial oxygen consumption in pressure overload hypertrophy and relates it to nonphosphorylating mitochondrial respiration linked to calcium transport.