Effects of NH4Cl-induced systemic metabolic acidosis on kidney mitochondrial coupling and calcium transport in rats

BACKGROUND

We have previously shown that chronic metabolic acidosis, induced in rats by NH(4)Cl feeding, leads to nephron hypertrophy and to a decreased water-salt reabsorption by the kidneys. Since mitochondria are the main source of metabolic energy that drives ion transport in kidney tubules, we examined energy-linked functions (respiration, electrochemical membrane potential and coupling between respiration and ADP phosphorylation) in mitochondria isolated from rat kidney and liver at 48 h after metabolic acidosis induced by NH(4)Cl.

METHODS

Mitochondria isolated from the kidneys and liver of metabolic acidotic rats, induced by NH(4)Cl, was used to study of the oxygen consumption by Clark-type electrode, mitochondrial electrical transmembrane potential estimated by the safranine O method and the variations in free medium Ca(2+) concentrations examined by absorbance spectrum of Arsenazo III set at the 675-685 nm wavelength pair.

RESULTS

Whole kidney and liver mitochondria isolated from 48 h acidotic rats presented higher resting respiration, lower respiratory control and a lower ADP/O ratio than controls. These differences in mitochondrial coupling, between respiration and oxidative phosphorylation (ATP synthesis), were totally corrected when experiments were carried out in the presence of carboxyatractyloside, GDP and BSA, indicating that mitochondrial uncoupling proteins are more active in acidotic rat kidneys. Interestingly, determination of Ca(2+) transport demonstrated a faster rate of initial Ca(2+) uptake by acidotic kidney mitochondria, which resulted in a lower concentration of extra-mitochondrial Ca(2+) under steady-state conditions (Ca(2+) set point) when compared with control mitochondria. In contrast, there were no significant differences in the rates of Na(+) or ruthenium red induced Ca(2+) efflux.

CONCLUSIONS

We suggest that the mild uncoupling and higher Ca(2+) accumulation represents an adaptation of the mitochondria to cope with conditions of oxidative stress and high cytosolic Ca(2+), which are associated with a decreased efficiency of oxidative phosphorylation that may explain, at least in part, the striking natriuresis observed under chronic acidosis. Finally, there were no changes in Ca(2+) transport or coupling in liver mitochondria isolated from the acidotic rats.

Suramin inhibits respiration and induces membrane permeability transition in isolated rat liver mitochondria

Suramin, a polysulfonated naphthylamine, caused a dose dependent inhibition of carbonyl cyanide p-(tri-fluoromethoxy)phenylhydrazone-stimulated respiration supported either by succinate or a cocktail of alphaketoglutarate, malate and isocitrate in isolated rat liver mitochondria. The half-maximum effect was obtained at 40 and 140 microM suramin for NADH- or FADH(2)-linked substrates, respectively. The respiration supported by N,N,N'N'-tetramethyl-p-phenylenediamine oxidation was unaffected by suramin (<or=500 microM). The latter respiratory substrate was used in the studies at examining the effects of suramin on the permeability properties of the inner mitochondrial membrane. These experiments provided evidence that suramin causes Ca(2+)-dependent mitochondrial swelling sensitive to the mitochondrial permeability transition (MPT) inhibitors cyclosporin A, ADP and Mg(2+). In addition, suramin decreased the content of reduced mitochondrial membrane protein thiols suggesting that thiol oxidation is the mechanism underlying suramin-induced MPT.

Low temperature and aging-promoted expression of PUMP in potato tuber mitochondria

In this communication, we show that the plant uncoupling mitochondrial protein (PUMP) present in potato tuber mitochondria is induced by aging at 28 degrees C and that this induction is strongly stimulated when the potato tubers are stored at low temperature (4 degrees C). PUMP activity was detected by the degree of linoleic acid (LA)-induced ATP-sensitive mitochondrial uncoupling measured as a function of the decrease in membrane potential (delta psi). The PUMP content was evaluated by immunoblot analysis using polyclonal antibodies raised against potato PUMP that specifically detected a 32 kDa band. In agreement with the effect of LA on delta psi, the content of the 32 kDa band increased during storage and was stimulated by low temperature. These results support the proposed role of PUMP in plant thermogenesis and possibly in fruit ripening and senescence.

Changes in calcium uptake rate by rat cardiac mitochondria during postnatal development

Ca2+ uptake, transmembrane electrical potential (Deltapsim) and oxygen consumption were measured in isolated ventricular mitochondria of rats from 3 days to 5 months of age. Estimated values of ruthenium red-sensitive, succinate-supported maximal rate of Ca2+ uptake (Vmax, expressed as nmol Ca2+/min/mg protein) were higher in neonates and gradually fell during postnatal development (from 435+/-24 at 3-6 days, to 156+/-10 in adults,P<0.001), whereas K0.5 values (approximately 10 microM were not significantly affected by age. Under similar conditions, mitochondria from adults (5 months old) and neonates (4-6 days old) showed comparable state 4 (succinate and alpha-ketoglutarate as substrates) and state 3ADP (alpha-ketoglutarate-supported) respiration rates, as well as Deltapsim values (approximately-150 mV). Respiration-independent Deltapsim and Ca2+ uptake, supported by valinomycin-induced K+ efflux were also investigated at these ages. A transient Deltapsim (approximately -30 mV) was evoked by valinomycin in both neonatal and adult mitochondria. Respiration-independent Ca2+ uptake was also transient, but its initial rate was significantly higher in neonates than in adults (49. 4+/-10.0v 28.0+/-5.7 mmol Ca2+/min/mg protein,P<0.01). These results indicate that Ca2+ uptake capacity of rat cardiac mitochondria is remarkably high just after birth and declines over the first weeks of postnatal life, without change in apparent affinity of the transporter. Increased mitochondrial Ca2+ uptake rate in neonates appears to be related to the uniporter itself, rather than to modification of the driving force of the transport.

Ca2+-independent permeabilization of the inner mitochondrial membrane by peroxynitrite is mediated by membrane protein thiol cross-linking and lipid peroxidation

Peroxynitrite anion, the reaction product of superoxide and nitric oxide, is a potent biological oxidant, which inactivates mammalian heart mitochondrial NADH-coenzyme Q reductase (complex I), succinate dehydrogenase (complex II), and ATPase, without affecting cytochrome c oxidase (complex IV). In this paper, we evaluated the effect of peroxynitrite on mitochondrial membrane integrity and permeability under low calcium concentration. Phosphate buffer was used in most of our experiments since Hepes, Tris, mannitol, and sucrose were found to inhibit the oxidative chemistry of peroxynitrite. Peroxynitrite (0.1-1.0 mM) caused a dose-dependent decrease in the ability of mitochondria to build up a membrane potential when N,N,N',N'-tetramethyl-p-phenylenediamine/ascorbate were used as substrate. Elimination of the membrane potential was accompanied by penetration of the osmotic support (KCl/NaCl) into the matrix as judged by the parallel occurrence of mitochondrial swelling. This swelling was partially inhibited by dithiothreitol (DTT) or butylated hydroxytoluene (BHT) and was insensitive to ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, ADP, and cyclosporin A. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of solubilized membrane proteins indicated that alterations in membrane permeability were associated with the production of protein aggregates due to membrane protein thiol cross-linking. The protective effect of DTT on both mitochondrial swelling and protein polymerization suggests the involvement of disulfide bonds in the membrane permeabilization process. In addition, the increase in thiobarbituric acid-reactive substances and the partial inhibitory effect of BHT indicate the occurrence of lipid peroxidation. These results support the idea that under our experimental conditions peroxynitrite causes mitochondrial structural and functional alterations by Ca2+-independent mechanisms through lipid peroxidation and protein sulfhydryl oxidation.

The participation of reactive oxygen species and protein thiols in the mechanism of mitochondrial inner membrane permeabilization by calcium plus prooxidants

We have recently shown that permeabilization of the inner mitochondrial membrane by calcium plus prooxidants is associated with oxidation of protein thiols forming cross-linked protein aggregates [Fagian, M. M., Pereira da Silva, L., Martins, I. S. and Vercesi, A. E. (1990) J. Biol. Chem. 265, 19955-19960]. In this study we show that mitochondria could regenerate and sustain a membrane potential (delta psi) comparable to the control experiment after the protein aggregates were cleaved by dithiothreitol. The addition of ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid, which removes Ca2+ but does not eliminate the protein aggregates, caused an incomplete and nonsustainable recovery of delta psi. Exogenous catalase prevented the disruption of membrane potential and decreased the production of membrane protein aggregates when mitochondria were incubated in the presence of Ca2+ alone or Ca2+ plus a prooxidant. This strongly indicates that H2O2 and possibly other H2O2-derived reactive oxygen species are involved in the mechanism of membrane protein aggregates production that may result in the process of membrane pore formation.

Membrane protein thiol cross-linking associated with the permeabilization of the inner mitochondrial membrane by Ca2+ plus prooxidants

In a previous report (Macedo, D.V., Ferraz, V. L., Pereira-da-Silva, L., and Vercesi, A. E. (1988) in Integration of Mitochondrial Functions (Lemasters, J. J., et al., eds) pp. 535-542, Plenum Publishing Corp., New York), we proposed that the alterations in the inner mitochondrial membrane permeability caused by Ca2+ plus prooxidants could be the consequence of membrane protein sulfhydryl-disulfide transitions. In this study, we show that Ca2+ plus diamide, a thiol oxidant, significantly decrease the ability of beef heart submitochondrial particles to build up and sustain a membrane potential generated by succinate oxidation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of solubilized membrane proteins indicates that these effects on the membrane potential are associated with the production of protein aggregates due to thiol cross-linking. Evidence is also presented that these protein aggregates can be produced in mitoplasts previously loaded with Ca2+ and that this is potentiated by the presence of either diamide or t-butylhydroperoxide. Furthermore, dithiothreitol, a disulfide reductant, was found to be much more effective than NAD(P)+ reductants in reversing Ca2+ efflux induced by prooxidants. It is concluded that the perturbation of the inner mitochondrial membrane caused by Ca2+ plus prooxidants is associated with protein polymerization due to thiol cross-linking, resulting in the production of high molecular mass protein aggregates.

Inhibition of oxidative phosphorylation by Ca2+ or Sr2+: a competition with Mg2+ for the formation of adenine nucleotide complexes

Intramitochondrial Sr2+, similar to Ca2+, inhibits oxidative phosphorylation in intact rat-liver mitochondria. Both Ca2+ and Sr2+ also inhibit the hydrolytic activity of the ATPase in submitochondrial particles. Half-maximal inhibition of ATPase activity was attained at a concentration of 2.5 mM Ca2+ or 5.0 mM Sr2+ when the concentration of Mg2+ in the medium was 1.0 mM. The inhibition of ATPase activity by both cations was strongly decreased by increasing the Mg2+ concentration in the reaction medium. In addition, kinetical data and the determination of the concentration of MgATP, the substrate of the ATPase, in the presence of different concentrations of Ca2+ or Sr2+ strongly indicate that these cations inhibit ATP hydrolysis by competing with Mg2+ for the formation of MgATP. On the basis of a good agreement between these results with submitochondrial particles and the results of titrations of oxidative phosphorylation with carboxyatractyloside or oligomycin in mitochondria loaded with Sr2+ it can be concluded that intramitochondrial Ca2+ or Sr2+ inhibits oxidative phosphorylation in intact mitochondria by decreasing the availability of adenine nucleotides to both the ADP/ATP carrier and the ATP synthase.