Glutamate-supported calcium movements in rat liver mitochondria effects of anions and pH

Temperature dependence of the atractyloside-induced mitochondrial Ca2+ release

1. Mitochondrial Ca2+, accumulated by succinate oxidation was released by addition of 50 microM atractyloside. Beside this Ca2+ efflux, a large oxidation of pyridine nucleotides and sustained membrane depolarization occurs. An absolute requirement for acetate to support Ca2+ release is demonstrated. 2. Membrane de-energization, NAD(P)H oxidation, and Ca2+ efflux as induced by atractyloside were temperature-dependent, since it occurs when mitochondria are incubated at 22 degrees C and was abolished at 4 degrees C. 3. Taking into account this latter, the effects of atractyloside on mitochondrial Ca2+ release appears not to be a simple result of the binding of the inhibitor to adenine nucleotide translocase. 4. It is proposed that the mechanism involved in atractyloside-driven membrane permeability to Ca2+ must be related with the transference of the conformational change of the carrier, to another membrane structure responsible for the maintenance permeability to ions.

Control of mitochondrial Ca2+ retention by ADP-stimulated glutamic dehydrogenase

The protective effect of ADP on unspecific Ca2+ release and collapse of the transmembrane potential was analyzed in mitochondria from kidneys of rats. The presence of ADP in the incubation mixture prevents Ca2+ leakage and collapse of delta psi in sucrose-containing medium, but fails to do so in KCl medium. The effect of the adenine nucleotide in sucrose media correlates with an increase in the level of reduced pyridine nucleotides; the increase was due to a stimulatory effect on the activity of glutamic dehydrogenase. It also was observed that in KCl media, in the presence and in the absence of ADP the rate of NADH oxidation through the respiratory chain was higher than in sucrose; in this latter medium a high level of reduced pyridine nucleotides was found, in comparison to KCl media. It is proposed that the role of ADP is to increase glutamic dehydrogenase activity and in consequence to provoke a higher rate of formation of NADH which in turn controls Ca2+ release.

Ion permeability induction by the SH cross-linking reagents in rat liver mitochondria is inhibited by the free radical scavenger, butylhydroxytoluene

The hydrophobic, potentially SH cross-linking reagent, phenylarsine oxide (PhAsO), was found to induce K+ and Ca2+ effluxes from mitochondria and to accelerate the respiration rate in state 4. The hydrophobic monofunctional electrophilic agent, N-ethylmaleimide, does not exhibit this effect but prevents the action of PhAsO. The polar potentially SH cross-linking regents (arsenite, diamide) induce ion fluxes only in the presence of Pi. Ion fluxes induced by the SH reagents are inhibited by butylhydroxytoluene (an inhibitor of free radical reactions), and N,N'-dicyclohexylcarbodiimide, not by oligomycin. It is inferred that the induction of ion fluxes in mitochondria caused by cross-linking of two juxtaposed SH groups is related to the development of free radical reactions.

Ca2+ transport in isolated mouse liver mitochondria; role of reductive carboxylation and citrate?

The uptake of Ca2+ in isolated mouse liver mitochondria respiring on succinate in the presence of rotenone and added Pi, was inhibited by dibucaine, fluorocitrate, p-hydroxymercuribenzoate (PMB), malonate, palmitoyl-CoA, succinyl-CoA and trifluoroperazine. The release of accumulated Ca2+ was stimulated by arsenite, malonate, PMB, palmitoyl-CoA and succinyl-CoA, whereas the release was inhibited by dibucaine, fluorocitrate, trifluoroperazine, and by oligomycin, especially in the presence of ADP. The pyridine nucleotides were oxidized in mitochondria incubated with PMB. The observations suggest a possible contributory role of reductive carboxylation for the uptake of Ca2+, and a possible role of citrate for the retention of Ca2+ in isolated mouse liver mitochondria.

Ruthenium red-insensitive Ca2+ uptake and release by mitochondria

Calcium uptake by rat liver mitochondria driven by an artificial pH gradient is ruthenium red insensitive, electrically neutral, and inhibited by the local anesthetic, nupercaine. This pH-driven Ca2+ transport is also inhibited by NH3, Pi, and acetate. Direct measurements of Pi indicate it is not translocated with Ca2+ during pH-driven Ca2+ uptake. Calcium is therefore not transported by a Ca2+-Pi symport mechanism. Ruthenium red-insensitive Ca2+ efflux is similar in its inhibition by nupercaine and its kinetics, and is also electroneutral. This suggests that the Ca2+ uptake described here occurs via reversal of the principal pathway of mitochondrial Ca2+ release. From the available data, pH-driven Ca2+ uptake (and presumably Ca2+ efflux) is hypothesized to occur by Ca2+ symport with unidentified anions. Protons may move counter to Ca2+ or reversibly dissociate from cotransported anions, which therefore couples Ca2+ transport to the pH gradient.

Uncoupler-stimulated release of Ca2+ from Ehrlich ascites tumor cell mitochondria

Ruthenium red-insensitive, uncoupler-stimulated release of Ca2+ from Ehrlich ascites tumor cell mitochondria is much slower than from rat liver mitochondria under comparable conditions. In the presence of Pi and at moderate or high Ca2+ loads, ruthenium red-insensitive Ca2+ efflux elicited with uncoupler is approximately 20 times more rapid for rat liver than Ehrlich cell mitochondria. This is attributed to resistance of tumor mitochondria to damage by Ca2+ due to a high level of endogenous Mg2+ that also attenuates Ca2+ efflux. Calcium release from rat liver and tumor mitochondria is inhibited by exogenous Mg2+. This applies to ruthenium red-insensitive spontaneous Ca2+ efflux associated with Ca2+ uptake and uncoupling, and (b) ruthenium red-insensitive Ca2+ release stimulated by uncoupling agent. The endogenous Mg2+ level of Ehrlich tumor mitochondria is approximately three times that of rat liver mitochondria. Endogenous Ca2+ is also much greater (six fold) in Ehrlich tumor mitochondria compared to rat liver. Despite the quantitative difference in endogenous Mg2+, the properties of internal Mg2+ are much the same for rat liver and Ehrlich cell mitochondria. Ehrlich ascites tumor mitochondria exhibit slow, metabolically dependent Mg2+ release and rapid limited release of Mg2+ during Ca2+ uptake. Both have been observed with rat liver and other types of mitochondria. The proportions of apparently "bound" and "free" Mg2+ (inferred from release by the ionophore, A23187) do not differ significantly between tumor and liver mitochondria. Thus, the endogenous Mg2+ of tumor mitochondria has no unusual features but is simply elevated substantially. Ruthenium red-insensitive Ca2+ efflux, when expressed as a function of the intramitochondrial Ca2+/Mg2+ ratio, is quite similar for tumor and rat liver. It is proposed, therefore, that endogenous Mg2+ is a major regulatory factor responsible for differences in the sensitivity to damage by Ca2+ and Ca2+ release by Ehrlich ascites tumor mitochondria compared to mitochondria from normal tissues.

Protection of canine cardiac mitochondrial function by verapamil-cardioplegia during ischemic arrest

Hemodynamic and mitochondrial function recover following 60 minutes of ischemic arrest and reperfusion in hearts pretreated with verapamil. The present study was carried out to determine whether verapamil prevents the onset of mitochondrial oxidative impairment after 60 minutes of ischemic arrest without reperfusion. Two preparations of mitochondria isolated following Polytron homogenization and subsequent treatment of the myofibrillar pellet with Nagarse were examined for phosphorylating respiration. The Polytron mitochondria were more sensitive to ischemic arrest than were the Nagarse mitochondria with either glutamate-malate (57% vs. 22% inhibition), succinate (+ rotenone) (41% vs. 14% inhibition), or palmitoylcarnitine (57% vs. 27% inhibition) as respiratory substrates. Verapamil pretreatment significantly increased oxidation of all substrates by the subsequently isolated Polytron mitochondria, but only succinate-supported respiration returned to control levels. In contrast, the small amount of respiratory inhibition exhibited by the Nagarse mitochondria after ischemic arrest was insensitive to verapamil pretreatment. We conclude that the Polytron preparation of mitochondria is more susceptible to ischemia than the Nagarse mitochondria, and this susceptibility correlates with a striking sensitivity to verapamil protection. In general, oxidation of NADH-linked substrates, including palmitoylcarnitine, is more affected by ischemic arrest than succinate, and only oxidation of the latter substrate is totally protected by verapamil. The beneficial action of verapamil on mitochondrial function occurs prior to reperfusion. The data suggest that alterations in calcium homeostasis occur during the ischemic period, as well as in the subsequent reperfusion period.

Inhibition of oxidative phosphorylation by a Ca2+-induced diminution of the adenine nucleotide translocator

The mechanism through which internal Ca2+ inhibits oxidative phosphorylation of rat heart mitochondria has been explored. In parallel to a Ca2+-induced diminution of the activity of the adenine nucleotide translocator, an efflux of internal adenine nucleotides is observed. The efflux of adenine nucleotides depends on the amount of Ca2+ accumulated by the mitochondria and on the time that Ca2+ remains in the mitochondria; this efflux is atractyloside insensitive. These results suggest that internal Ca2+, by inducing a lowering of the internal concentration of adenine nucleotides, diminishes the rate of exchange of adenine nucleotides via the translocase, and in consequence of oxidative phosphorylation. Under conditions in which the Ca2+-induced release of adenine nucleotides takes place, no gross changes of the permeability properties of the membrane are observed. As revealed by studies with arsenate, respiratory activity and the function of the ATPase in the direction of ATP synthesis are not affected by internal Ca2+.

Respiratory activities of subsarcolemmal and intermyofibrillar mitochondrial populations isolated from denervated and control rat soleus muscles

Ultraturrax and Nagarse released populations of mitochondria isolated from control and day 21 denervated rat soleus muscle were characterized with respect to their oxidative phosphorylation, ADP translocase and ATPase activities. Both Ultraturrax and Nagarse released mitochondrial populations displayed lower capacities for oxidative phosphorylation; lower ADP translocase activities and higher Mg2+ stimulated ATPase activities than their corresponding controls. For both the denervated and control states, the Nagarse-released mitochondrial populations displayed significantly higher respiratory activities than the Ultraturrax released fractions. The significance of these findings is discussed with regard to the process of mitochondrial respiratory control. In addition the role of mitochondrial dysfunction in denervation muscular atrophy is assessed.

Exercise-induced alterations of hepatic mitochondrial function

In order to examine the effect of a single bout of exercise on hepatic mitochondrial function, starved untrained male rats swam at 34-35 degrees C with a tail weight (5% of body wt.) for 100 min. The rates of ADP-stimulated and uncoupled respiration were higher in the mitochondria isolated from the exercised rats regardless of the substrate utilized. Succinate-linked Ca2+ uptake was 48% greater in the exercised group; however, Ca2+ efflux was markedly depressed. The inhibition of Ca2+ uptake by Mg2+ was higher in the control group, so that the difference in Ca2+ uptake between the two groups was greater in the presence of Mg2+ than in its absence. The response of phosphorylating respiration and Ca2+ fluxes to exogenous phosphate and the pH of the assay medium differed in the exercise group. These observations with the exercised group were not related to non-specific stress. The exercise-induced mitochondrial-functional alterations are reminiscent of those obtained from mitochondria isolated from glucagon- or catecholamine-treated sedentary rats. Thus, adrenergic stimulation as well as other factors may be operating during exercise, leading to an alteration of mitochondrial function in vitro.

H+-dependent efflux of Ca2+ from heart mitochondria

A rapid loss of accumulated Ca2+ is produced by addition of H+ to isolated heart mitochondria. The H+-dependent Ca+ efflux requires that either (a) the NAD(P)H pool of the mitochondrion be oxidized, or (b) the endogenous adenine nucleotides be depleted. The loss of Ca2+ is accompanied by swelling and loss of endogenous Mg2+. The rate of H+-dependent Ca2+ efflux depends on the amount of Ca2+ and Pi taken up and the extent of the pH drop imposed. In the absence of ruthenium red the H+-induced Ca2+-efflux is partially offset by a spontaneous re-accumulation of released Ca2+. The H+-induced Ca2+ efflux is inhibited when the Pi transporter is blocked with N-ethylmaleimide, is strongly opposed by oligomycin and exogenous adenine nucleotides (particularly ADP), and inhibited by nupercaine. The H+-dependent Ca2+ efflux is decreased markedly when Na+ replaces the K+ of the suspending medium or when the exogenous K+/H+ exchanger nigericin is present. These results suggest that the H+-dependent loss of accumulated Ca2+ results from relatively nonspecific changes in membrane permeability and is not a reflection of a Ca2+/H+ exchange reaction.