Oscillatory systems in mitochondria

Influence of cadmium on oxidative stress and NADH oscillations in mussel mitochondria

Biological organisms evolved to take advantage of recurring environmental factors which enabled them to assimilate and process metabolic energy for survival. Mitochondria display non-linear oscillations in NADH levels (i.e. wave behavior) that result from the balance between NADH production (aerobic glycolysis) and oxidation for ATP synthesis. The purpose of this study was to examine the effects of cadmium (Cd) on mitochondrial NADH oscillations in quagga mussels Dreissena bugensis exposed to 50 and 100 μg/L CdCl2 for 7 days at 15 °C. Metallothionein (MT) levels, thioredoxin reductase (TrxR) activity and NADH oxidation rate were also determined, as were oscillations in NADH and the formation of dissipative structures (turbidity), in isolated mitochondria suspensions. The results show that exposure to Cd readily induced MT levels at both concentrations tested and that TrxR and NADH oxidase activity was induced at 100 μg/L Cd only. In control mussels, NADH levels oscillated in mitochondria suspensions with a natural period of 2 to 2.5 min for up to 40 min. Exposure to Cd increased the complexity of the frequency profile of NADH oscillations and reduced the amplitudes of the natural signal with a period of 2 to 2.5 min. The formation of dissipative structures decreased in response to a Cd concentration of 100 μg/L but increased at a level of 50 μg/L. The amplitudes at the natural frequency were significantly correlated with NADH oxidase activity (r = -0.91) and with the formation of dissipative structures (r = -0.59). We conclude that Cd could alter the natural frequency in oscillations of NADH in mitochondria, thereby contributing to an increase in NADH oxidation rate and disruption of the spatial organization of mitochondria in suspension. In conclusion, changes in the wave behavior of NADH in mitochondria are proposed as a novel biomarker of toxicity in aquatic organisms.

Biological Timekeeping: The Business of a Blind Watchmaker

Fundamental understanding of life depends on both structural and functional details at the molecular level. Continually improving means of measurement of spatial and dynamic properties of biochemical constituents and cellular components complement studies of whole organisms. Integration of the interaction of components to provide coherent behaviour depends on highly elaborate orchestration in space and time. Whereas spatial information on a nanometre resolution is available, and fast dynamic analyses provide biochemical reaction rates measured in nanoseconds, functional coordination of the system requires integrated time dependence. While we are well aware of the special complexity of living organisms, appreciation of temporal scales and their organisation in time is still fragmentary. This article summarises current developments in research on biological time on scales from nanoseconds to years, the networks that connect different time domains and the oscillations, rhythms and biological clocks that coordinate and synchronise the complexity of the living state.

“It is the pattern maintained by this homeostasis, which is the touchstone of our personal identity. Our tissues change as we live: the food we eat and the air we breathe become flesh of our flesh, and bone of our bone, and the momentary elements of our flesh and bone pass out of our body every day with our excreta. We are but whirlpools in a river of ever-flowing water. We are not the stuff that abides, but patterns that perpetuate themselves”60.

Wiener, 1954

“What are called structures are slow processes of long duration, functions are quick processes of short duration”61.

Von Bertalanffy, 1952

Hexachlorobenzene treatment on hepatic mitochondrial function parameters and intracellular coproporphyrinogen oxidase location

These studies try to elucidate why isocoproporphyrin appears in hexachlorobenzene-poisoned rats' feces. Chronic exposure of hexachlorobenzene to rats produces an experimental model for human porphyria cutanea tarda. After 8 weeks of treatment, rats showed high porphyrin excreta and 50% inhibition of liver uroporphyrinogen decarboxylase activity. Uroporphyrin plus heptacarboxylic porphyrin exceeded coproporphyrin in urine, whereas in feces, isocoproporphyrin, from abnormal pentacarboxylic porphyrinogen III oxidative decarboxylation by liver coproporphyrinogen oxidase, became the main porphyrin. Trypsin-treated mitochondria showed that the outer and inner membrane permeability barrier was highly conserved after hexachlorobenzene intoxication. In digitonin-treated hexachlorobenzene mitochondria, coproporphyrinogen oxidase was free in the mitochondrial intermembrane space, whereas in normal mitochondria, 30% to 50% remained anchored to the inner membrane. Hexachlorobenzene led to a decrease in respiratory control and ADP/O ratios (uncoupled mitochondria). Albumin restored oxidative phosphorylation, indicating no irreversible inner membrane damage. Normal and hexachlorobenzene mitochondria oscillatory studies exhibited similar damping factor values, showing that hexachlorobenzene had no significant effect on membrane fluidity and elasticity. Mitochondrial uncoupling could explain the free state of the enzyme within the intermembrane space. The free state of the enzyme makes it more flexible and would allow pentacarboxylic porphyrinogen III, whose levels are increased, to compete with coproporphyrinogen III and being transformed into dehydroisocoproporphyrinogen, the liver forerunner of fecal isocoproporphyrin.

Respiratory oscillations in yeasts

Respiratory oscillations in yeasts have been studied in three time domains with periods of (a) about a minute, (b) about 40 min, and (c) about a day. Reactive responses (damped oscillations), rhythms and temperature-compensated clocks have been described for (b) and (c), but a timekeeping clock has not yet been shown for (a). Synchronous populations reveal the time-structure that can only otherwise be studied in single organisms; this is because time-averaging through an asynchronous population conceals its fine structure. Early studies with synchronous cultures made by size selection methods indicated ultradian-clock driven oscillations in respiration, pools of adenylates, total protein, RNA synthesis and many enzyme activities (tau = 40 min in Schizosaccharomyces pombe, 30 min in Candida utilis), and more recently in self-synchronised continuous cultures of Saccharomyces cerevisiae (tau = 48 min). Most detailed understanding comes from the latter system, where continuous, noninvasive real-time monitoring (of 02 uptake, CO2 production, and NAD(P)H redox state) is combined with frequent discrete time samples (for other redox components, including H2S, GSH and cytochromes, metabolites, and mRNA levels). A redox switch lies at the heart of this ultradian clock and a plethora of outputs is optimized to a time-base that is genetically-determined and differs in different organisms. It is suggested that the entire temporal landscape of all eukaryotic organisms and the cells of higher plants and animals is constructed on this basis. A time frame for the coordination and coherence of all intracellular processes and the construction and assembly of cellular structures is provided by the ultradian clock The circadian clock matches these functions to the daily cycle of the external environment.

Single and cell population respiratory oscillations in yeast: a 2-photon scanning laser microscopy study

Two-photon scanning laser and confocal microscopies were used to image metabolic dynamics of single or cell populations of Saccharomyces cerevisiae strain 28033. Autofluorescence of reduced nicotinamide nucleotides, and mitochondrial membrane potential (DeltaPsim), were simultaneously monitored. Spontaneous, large-scale synchronized oscillations of NAD(P)H and DeltaPsim throughout the entire population of yeasts occurred under perfusion with aerated buffer in a continuous single-layered film of organisms. These oscillations stopped in the absence of perfusion and the intracellular NAD(P)H pool became reduced. Individual mitochondria within a single yeast also showed in-phase synchronous responses with the cell population, in both tetramethylrhodamine ethyl ester (or tetramethylrhodamine methyl ester) and autofluorescence. A single, localized, laser flash also triggered mitochondrial oscillations in single cells suggesting that the mitochondrion may behave as an autonomous oscillator. We conclude that spontaneous oscillations of S. cerevisiae mitochondrial redox states and DeltaPsim occur within individual yeasts, and synchrony of populations of organisms indicates the operation of an efficient system of cell-cell interaction to produce concerted metabolic multicellular behaviour on the minute time scale in both cases.

Mathematical model of mitochondrial ionic homeostasis: three modes of Ca2+ transport

Mitochondria play an important role in regulation of Ca2+ homeostasis in a cell. Here we present a mathematical model of mitochondrial ion transport and use this model to analyse different modes of Ca2+ uptake by mitochondria. The model includes transport of H+, Ca2+, K+, inorganic phosphate and oxidative substrates across the inner mitochondrial membrane harboring permeability transition pore (PTP). The detailed description of ion fluxes is based on the experimental ion kinetics in isolated mitochondria. Using the model we show that the kinetics of Ca2+ uptake by mitochondria is regulated by the total amount of Ca2+ in the system and the rate of Ca2+ infusion. Varying these parameters we find three different modes of ion transport. When the total amount of Ca2+ is below 140 nmol Ca2+/mg protein, all available Ca2+ is accumulated in the matrix without activation of the PTP. Between 140 and 160 nmol Ca2+/mg protein, accumulation of Ca2+ generates periodic opening and closure of the PTP and oscillations of ion fluxes. Higher levels of Ca2+ (> 160 nmol Ca2+/mg protein) result in a permanently open PTP, membrane depolarization and loss of small ions from the matrix. We show that in the intermediate range of Ca2+ concentrations the rate of Ca2+ infusion regulates the PTP state, so that slow Ca2+ infusion does not lead to PTP opening, while fast Ca2+ infusion results in an oscillatory state.

A mitochondrial oscillator dependent on reactive oxygen species

We describe a unique mitochondrial oscillator that depends on oxidative phosphorylation, reactive oxygen species (ROS), and mitochondrial inner membrane ion channels. Cell-wide synchronized oscillations in mitochondrial membrane potential (Delta Psi(m)), NADH, and ROS production have been recently described in isolated cardiomyocytes, and we have hypothesized that the balance between superoxide anion efflux through inner membrane anion channels and the intracellular ROS scavenging capacity play a key role in the oscillatory mechanism. Here, we formally test the hypothesis using a computational model of mitochondrial energetics and Ca(2+) handling including mitochondrial ROS production, cytoplasmic ROS scavenging, and ROS activation of inner membrane anion flux. The mathematical model reproduces the period and phase of the observed oscillations in Delta Psi(m), NADH, and ROS. Moreover, we experimentally verify model predictions that the period of the oscillator can be modulated by altering the concentration of ROS scavengers or the rate of oxidative phosphorylation, and that the redox state of the glutathione pool oscillates. In addition to its role in cellular dysfunction during metabolic stress, the period of the oscillator can be shown to span a wide range, from milliseconds to hours, suggesting that it may also be a mechanism for physiological timekeeping and/or redox signaling.

Repetitive transient depolarizations of the inner mitochondrial membrane induced by proton pumping

Single mitochondria show the spontaneous fluctuations of DeltaPsim. In this study, to examine the mechanism of the fluctuations, we observed DeltaPsim in single isolated heart mitochondria using time-resolved fluorescence microscopy. Addition of malate, succinate, or ascorbate plus TMPD to mitochondria induced polarization of the inner membrane followed by repeated cycles of rapid depolarizations and immediate repolarizations. ADP significantly decreased the frequency of the rapid depolarizations, but the ADP effect was counteracted by oligomycin. On the other hand, the rapid depolarizations did not occur when mitochondria were polarized by the efflux of K(+) from the matrix. The rapid depolarizations became frequent with the increase in the substrate concentration or pH of the buffer. These results suggest that the rapid depolarizations depend on the net translocation of protons from the matrix. The frequency of the rapid depolarizations was not affected by ROS scavengers, Ca(2+), CsA, or BA. In addition, the obvious increase in the permeability of the inner membrane to calcein (MW 623) that was entrapped in the matrix was not observed upon the transient depolarization. The mechanisms of the spontaneous oscillations of DeltaPsim are discussed in relation to the matrix pH and the permeability transitions.

The mono-exponential pattern of phosphocreatine recovery after muscle exercise is a particular case of a more complex behaviour

A mathematical model is proposed showing that the mono-exponential recovery of phosphocreatine (PCr) after exercise is an approximation of a more complex pattern, which is identified by a second-order differential equation. The model predicts the possibility of three different patterns of PCr recovery: bi-exponential, oscillatory damped, and critically damped; the mono-exponential pattern being a particular case of the functions which are solutions of the differential equation. The model was tested on a sample of recovery data from 50 volunteers, checking whether the recovery patterns predicted by the model lead to a significant improvement of fit (IF) compared with the mono-exponential pattern. Results show that the IF is linked to pH. Bi-exponential solutions showed an IF in the pH range 6.65-6.85, and the oscillatory solutions at pH>6.9. Critically damped solutions displayed a poor IF. Oscillation frequencies found in the oscillatory recoveries increase at increasing pH. These results show that pH has a pivotal role on the pattern of PCr recovery and implications on the regulation of oxidative phosphorylation are discussed.

Oscillating Ca2+-induced channel activity obtained in BLM with a mitochondrial membrane component

Oscillations in ion fluxes and membrane potential may be observed in cells and in mitochondria as well. We obtained Ca2+-induced oscillations in channel activity in black-lipid membranes reconstituted with hydrophobic components extracted from mitochondria. Mitoplasts prepared from purified rat liver mitochondria were extracted with ethanol followed by Folch extraction and further partial purification by silicic acid chromatography. Channel activity was measured in lipid bilayers formed from bovine brain lipids and 10% cardiolipin with addition of the purified fractions. The conductance with 10 mM Ca2+ was 100 pS or its multiples. Ca2+ gradients of 4: 1 induced oscillating channel activity for several hours, with initial open states of 40 s and closed states of 56 s; the open times gradually decreasing to 8.6 s. No channel activity was seen without added fractions. The channel activity was associated with a Ca2+-binding lipid, nonpolar, low-molecular-weight fraction that in gel electrophoresis was not stained with Coomassie Blue and did not contain carbohydrate-staining material. 1H-Nuclear magnetic resonance spectra of the substance showed the presence of aliphatic chains and carbonyls, but the detailed structure remains to be elucidated.

Oscillations of membrane current and excitability driven by metabolic oscillations in heart cells

Periodic changes in membrane ionic current linked to intrinsic oscillations of energy metabolism were identified in guinea pig cardiomyocytes. Metabolic stress initiated cyclical activation of adenosine triphosphate-sensitive potassium current and concomitant suppression of depolarization-evoked intracellular calcium transients. The oscillations in membrane current and excitation-contraction coupling were linked to oscillations in the oxidation state of pyridine nucleotides but were not driven by pacemaker currents or alterations in the concentration of cytosolic calcium. Interventions that altered the rate of glucose metabolism modulated the oscillations, suggesting that the rhythms originated at the level of glycolysis. The energy-driven oscillations in potassium currents produced cyclical changes in the cardiac action potential and thus may contribute to the genesis of arrhythmias during metabolic compromise.

The Ca(2+)-induced permeability transition pore is involved in Ca(2+)-induced mitochondrial oscillations. A study on permeabilised Ehrlich ascites tumour cells

The Ca(2+)-induced permeability transition of the mitochondrial inner membrane was studied in digitonin-permeabilized Ehrlich ascites tumour cells respiring on succinate in an isotonic medium. Addition of a sufficient amount of Ca2+ to induce an efflux of accumulated Ca2+ from mitochondria produced an oscillatory state with periodically changing rates of respiration, transmembrane potential, delta pH and direction of Ca2+ fluxes. This contrasts with liver mitochondria in which only a Ca2+ efflux is induced under these conditions. Addition of traces of cyclosporin A (approximately 0.1 nM) damped the oscillations by inhibiting the phase in which Ca2+ efflux occurs and promoting the reestablishment of a higher transmembrane potential. Efflux was also prevented by addition of ATP or ADP, ATP being more potent. Efflux was also inhibited by low concentrations of spermine. It is concluded that Ca(2+)-induced oscillations involve the cyclosporin A-sensitive pore and that the Ehrlich ascites tumour cell mitochondria differ from liver mitochondria in being far more sensitive to cyclosporin A and ATP. The possible physiological role of the oscillatory state is discussed.

Imaging in five dimensions: time-dependent membrane potentials in individual mitochondria

Because of its importance in the chemiosmotic theory, mitochondrial membrane potential has been the object of many investigations. Significantly, however, quantitative data on how energy transduction might be regulated or perturbed by the physiological state of the cell has only been gathered via indirect studies on isolated mitochondrial suspensions; quantitative studies on individual mitochondria in situ have not been possible because of their small size, their intrinsic motility, and the absence of appropriate analytical reagents. In this article, we combine techniques for rapid, high resolution, quantitative three-dimensional imaging microscopy and mathematical modeling to determine accurate distributions of a potentiometric fluorescent probe between the cytosol and individual mitochondria inside a living cell. Analysis of this distribution via the Nernst equation permits assignment of potentials to each of the imaged mitochondrial membranes. The mitochondrial membrane potentials are distributed over a narrow range centered at -150 mV within the neurites of differentiated neuroblastoma cells. We find that the membrane potential of a single mitochondrion is generally remarkably stable over times of 40-80 s, but significant fluctuations can occasionally be seen. The motility of individual mitochondria is not directly correlated to membrane potential, but mitochondria do become immobile after prolonged treatment with respiratory inhibitors or uncouplers. Thus, three spatial dimensions, a key physiological parameter, and their changes over time are all quantitated for objects at the resolution limit of light microscopy. The methods described may be readily extended to permit investigations of how mitochondrial function is integrated with other processes in the intact cell.

Effect of ovarian hormones upon liver mitochondrial function in diabetic rats

In the present study it is shown that streptozotocin (SZ)-induced chronic diabetes of female albino rats produced significant alterations in liver mitochondrial function after 30-35 days of diabetes. The disturbances were as follows: (1) a significant fall of the mean values of the respiratory control ratio and of state 3 of respiration using three substrates, 3-hydroxybutyrate, malate-glutamate and succinate, and (2) a significant increase of the mean damping factor of the oscillatory osmotic variations (with valinomycin as K+ ionophore and succinate as substrate). The same mitochondrial function parameters were analyzed for comparison in control non-diabetic rats (group N) and in the following groups of female rats with chronic diabetes: intact (group I), oophorectomized (6 days after the injection of SZ) (group O), and oophorectomized with restitution therapy of 17 beta-estradiol (from the operation until the day before killing) (group O + Eol). The O group showed significantly higher values of the respiratory control ratio and of state 3 of respiration and significantly lower damping factors than group I. The restitution treatment in the O + Eol group restored the mitochondrial functions assayed to values similar to those of group I. These data provide strong evidence that estrogens exert a negative effect at the molecular level upon impaired liver mitochondrial functions in SZ-induced diabetes.

K+/H+ antiport in mitochondria

Mitochondria contain a latent K+/H+ antiporter that is activated by Mg2+-depletion and shows optimal activity in alkaline, hypotonic suspending media. This K+/H+ antiport activity appears responsible for a respiration-dependent extrusion of endogenous K+, for passive swelling in K+ acetate and other media, for a passive exchange of matrix 42K+ against external K+, Na+, or Li+, and for the respiration-dependent ion extrusion and osmotic contraction of mitochondria swollen passively in K+ nitrate. K+/H+ antiport is inhibited by quinine and by dicyclohexylcarbodiimide when this reagent is reacted with Mg2+-depleted mitochondria. There is good suggestive evidence that the K+/H+ antiport may serve as the endogenous K+-extruding device of the mitochondrion. There is also considerable experimental support for the concept that the K+/H+ antiport is regulated to prevent futile influx-efflux cycling of K+. However, it is not yet clear whether such regulation depends on matrix free Mg2+, on membrane conformational changes, or other as yet unknown factors.

Oscillating dissipative structures in mitochondrial suspensions

The occurrence of spatial structures in the unstirred layer of an oscillating mitochondrial suspension is reported. The structures are detected by photo camera by light scattering in the unstirred layer of suspension. The spatial structures observed are shown to oscillate with the same period as that of mitochondrial oscillations in the bulk phase. Patterning is not affected by the layer depth within the range 0.3-3.0 mm. Various types of oscillatory states of mitochondria are characterized by the corresponding patterns. Patterning is effectively suppressed by the inhibitors of the respiratory chain (antimycin A or CN-).

Chlortetracycline-mediated continuous Ca2+ oscillations in mitochondria of digitonin-treated Tetrahymena pyriformis

Ca2+ transport in mitochondria was studied in situ using digitonin-permeabilized cells of the ciliate protozoan Tetrahymena pyriformis GL. In the presence of oxidizable substrates and inorganic phosphate, mitochondria were able to accumulate a large amount of the added Ca2+ without subsequent uncoupling and mitochondrial damage. However, the maximal Ca2+ uptake dramatically decreased in the presence of micromolar concentrations of the fluorescent calcium indicator, chlortetracycline, which in aerobic conditions caused an uncoupling of the respiration in Ca2+-loaded mitochondria. Moreover, on reaching hypoxia, when the rate of oxygen diffusion from the air to the stirred incubation medium became a limiting factor, continuous Ca2+ oscillations were observed. Ca2+ fluxes were synchronous with the cyclic changes of the membrane potential and were followed with a significant delay by the changes of the membrane-associated fluorescence of Ca-chlortetracycline complexes. Both the chlortetracycline-induced uncoupling of the respiration and the oscillations were prevented by either EGTA or ruthenium red. It is suggested that in conditions of the limited rate of respiration the oscillations are generated as a result of the functioning of the two Ca2+-transport pathways: a Ca2+ uniport and a chlortetracycline-mediated electroneutral Ca2+ efflux.

Effects of Stevia rebaudiana natural products on rat liver mitochondria

The effects of several natural products extracted from the leaves of Stevia rebaudiana on rat liver mitochondria were investigated. The compounds used were stevioside (a non-caloric sweetener), steviolbioside, isosteviol and steviol. Total aqueous extracts of the leaves were also investigated. S. rebaudiana natural products inhibited oxidative phosphorylation, ATPase activity NADH-oxidase activity, succinate-oxidase activity, succinate dehydrogenase, and L-glutamate dehydrogenase. The ADP/O ratio was decreased. Substrate respiration (state II respiration) was increased at low concentrations (up to 0.5 mM) and inhibited at higher concentrations (1 mM or more). In uncoupled mitochondria, inhibition of substrate respiration was the only effect observed. Net proton ejection induced by succinate and swelling induced by several substrates were inhibited. Of the compounds investigated, the sweet principle stevioside was less active. It was concluded that, in addition to the inhibitory effects, S. rebaudiana natural products may also act as uncouplers of oxidative phosphorylation. The possible physiologic consequences of the ingestion of stevioside and S. rebaudiana aqueous extracts are discussed.

Experimental evidence and theoretical discussion for long-term oscillations of phosphofructokinase in a compartmentalized system

The activity of rabbit muscle phosphofructokinase (EC 2.7.1.11) has been followed as a function of time under conditions where the enzyme is separated from the bulk solution by an inert membrane. An enzymatic coupling assay allows continuous measurement of the variations in NADH concentration, which is directly related to the enzyme catalytic activity. For given concentrations of substrates (ATP and Fru6P) in the outside reservoir and a given ratio between diffusion coefficients of both substrates, the activity of phosphofructokinase exhibits an oscillatory behavior during a period of about 5 h. The phenomenon is explained in terms of coupling between diffusion of metabolites and non-linear enzyme reaction.

Mitochondrial oscillation and activation of H+/cation exchange

Mitochondria incubated aerobically in the presence of tetrapropylammonium and weak acids and in the presence of trace amounts of tetraphenylboron undergo a series of damped oscillations reflecting cycles of osmotic swelling and shrinkage. The matrix volume changes are consequent to transport of tetrapropylammonium catalytically stimulated by tetraphenylboron. The amplitude and frequency of the oscillations increase with the concentration of tetrapropylammonium, as required for critical rates and extents of ion influx. Addition of bovine serum albumin abolishes both the uptake of tetrapropylammonium and the oscillations. Volume oscillations are paralleled by cyclic activation and depression of the respiratory rate. Two lines of evidence suggest that the train of damped oscillations depends on the cyclic activation of an electroneutral exchange of H+ with organic cations rather than on cyclic uncoupling. First, further increase of cation permeability due to a pulse of tetraphenylboron, after initiation of cation efflux, restores cation influx. Second, addition of Mg2+, which abolishes the oscillations, has a much more marked inhibitory effect on the process of cation efflux than on cation influx. Conversely, addition of A23187, which removes membrane-bound Mg2+, promotes cation efflux and thus the oscillations. It is suggested that, in the present system, stretching of the inner membrane and Mg2+ depletion result in activation of an electroneutral H+/organic cation exchange, and that cyclic activation of this reaction results in damped oscillations.

Involvement of periodic deacylation-acylation cycles of mitochondrial phospholipids during Sr2+-induced oscillatory ion transport in rat liver mitochondria

Lysophosphatidylcholine and lysophosphatidylethanolamine levels were determined during Sr2+-induced oscillating ion fluxes in mitochondria prelabelled in vivo with 32Pi. Periodic fluctuations of both lyso compounds were established with an increase at the stage of simultaneously monitored K+ influx and a decrease at K+ efflux. The periodic activations and inactivations of phospholipase were found to be associated with periodic changes in the incorporation rates of labelled polyunsaturated fatty acids with an apparent phase difference of 180 degrees. Periodic deacylation-acylation cycles of phospholipids accompanying the periodic cycles of reversible ion accumulation and release are suggested to be involved in the trigger mechanism generating the permeability changes during oscillatory ion transport.

Oscillations in vesicular compartments

The oscillatory phenomena which occur in metabolic processes are of great interest to chemists and biologists for a better understanding of far-from-equilibrium behavior that exists in biological systems. In this paper, we present a model for metabolic oscillations in a vesicular compartment and show how oscillations of the components involved in an energy transducing system may arise from our model under certain external conditions.

Vanadate-induced inhibition of sodium transport and of sodium-independent anion transport in turtle bladder

Vanadate in the serosal bathing fluid of turtle bladders inhibits the Na+ moiety of the short-circuiting current (Isc), the anion (Cl-, HCO3-) moiety of Isc, and net Cl- flux. Since the anion transport is Na+-independent and ouabain-insensitive, its inhibtion by vanadate is uniquely different from the well known vanadate-induced inhibition of (Na++K+)-ATPase and Na+ transport-dependent anion movement of some other epithelia. Vanadate also generates damped oscillations in the bladders' electrical parameters, an unusual effect by an ion in epithelial systems.

Small-angle X-ray scattering from mitochondria

X-ray (CuKalpha) scattering curves of rat liver mitochondria are characterized by continuously decreasing intensity from 0.5 to 5 mrad and a broad maximum centered near 20 mrad. The condensed-to-orthodox morphological transition of the inner membranes of intact mitochondria causes a dramatic decrease in scattering at very small angle and a marked shift of the 20 mrad maximum to smaller angle. A similar small-angle scattering maximum is observed with inner mitochondrial membrane fractions prepared by digitonin treatment and osmotic shock/step gradient centrifugation procedures. However, the small-angle X-ray scattering curves of mitochondria after acetone treatment and osmoticlysis/sonication are essentially continuous. These characteristics of mitochondrial X-ray scattering are discussed in terms of known structural features of the organelle.

A simple universal mechanism of use and conservation of energy: its application to movements of ions and other materials across cell, mitochondrial and other membranes and to oxidative phosphorylation

A single simple mechanism by which all cells might both use energy to drive active transport to all solutes and also conserve energy in the form of adenosinetriphosphate (ATP) is descirbed. The basic assumption is that injection of energy results in a conformational change of the membrane which both generates transient highly-ordered water structures on its inside surface and changes membrane permeability. Ordered water is propagated through the cell by means of cooperative interactions with proteins, so that during the ordered period intracellular water is incompatible with small cations which require strong primary hydration, but has enhanced affinity for water-structure-breaking solutes. In animal cells cytoplasmic water is ordered by the activity of the plasma-membrane-bound transport ATPases. In mitochondria and bacteria the state of ordered water is identified with the energised state, which can be generated either by passage of electrons down the electron chain, or by ATPase activity. The mechanism is shown to be consistent with the observed transport activities of mitochondria and bacteria, and also provides a simple direct explanation of oxidative phosphorylation.

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.

Mechanism of active shrinkage in mitochondria. II. Coupling between strong electrolyte fluxes

1. Addition of succinate to valinomycin-treated mitochondria incubated in KCL causes a large electrolyte penetration. The process depends on a steady supply of energy and involves a continuous net extrusion of protons. Rates of respiration and of electrolyte penetration proceed in a parallel manner. 2. A passive penetration of K+ salt of permeant anions occurs in respiratory-inhibited mitochondria after addition of valinomycin. Addition of succinate at the end of the passive swelling starts an active extrusion of anions and cations with restoration of the initial volume. The shrinkage is accompanied by a slow reuptake of protons. The initiation of the active shrinkage correlates with the degree of stretching of the inner membrane. The extrusion of electrolytes is inhibited by nigericin, while it is only slightly sensitive to variations of the valinomycin concentration larger than two orders of magnitude. 3. Passive swelling and active shrinkage occurs also when K+ is replaced by a large variety of organic cations. The rate of organic cation penetration is enhanced by tetraphenylboron, while the rate of electrolyte extrusion is insensitive to variation of the tetraphenylboron concentration. 4. Active shrinkage, either with K+ or organic cation salts, is inhibited by weak acids. The phosphate inhibition is removed by SH inhibitors. The active shrinkage is also inhibited by mersalyl to an extent of about 60%. 5. Three models of active shrinkage are discussed: (a) mechanoprotein, (b) electrogenic proton pump, and (c) proton-driven cation anion pump.

Oscillations of an electrogenic pump in the plasma membrane of Neurospora

The presence of the poky mutation in Neurospora crassa produces mitochondria which are defective in cytochromes b and aa3 but which compensate by means of an alternate, cyanide-insensitive oxidase. As previously reported (Slayman, Rees, Orchard & Slayman, J. Biol. Chem., 250:396, 1975) cyanide blockade of the poky strain carrying the partial suppressor f results in a metabolic downshift of only 56%, compared with a downshift of 98% in wild-type Neurospora; the downshift is accompanied by exponential decay of ATP in the wild type, but by an undershoot and monotonic recovery of ATP in poky f. Whereas the membrane potential declines with ATP in wild-type Neurospora, it oscillates near the resting level (ca. -- 185 mV) in poky f. Oscillations begin with a depolarizing swing of 30--100 mV, followed by slight hyperpolarization, then by 2--4 damped cycles having a frequency near 1/min. Similar oscillations arise with antimycin, salicyl hydroxamic acid, and several uncoupling agents, and depend on partial maintenance of respiration through either the defective cytochrome chain or the alternate oxidase. Small oscillations (maximally +/- 30% of the control value) in membrane conductance also occur, roughly in phase with the oscillations of membrane potential. The amplitude of these, in comparison with the nonlinearity of the normal current-voltage relationship for the membrane, strongly suggests that they arise as a secondary consequence of the voltage changes. Therefore, since it has previously been argued (Slayman, Long & Lu, J. Membrane Biol. 14:305, 1973) that most of the resting membrane potential in the organism arises from active extrusion of H+ ions, the simolest interpretation of the cyanide-induced voltage oscillations is that current through the H+ pump is modulated cyclically. The ultimate mechanism for this modulation is unresolved, but could plausible involve a metabolic feedback system, oscillations of intracellular pH, or both. In many respects the observed voltage oscillations resemble the well-known oscillations of mitochondrial H+ flux which are produced by sudden metabolic shifts.