Energy dissipation by calcium recycling and the efficiency of calcium transport in rat-liver mitochondria

Properties of Ca(2+) transport in mitochondria of Drosophila melanogaster

We have studied the pathways for Ca(2+) transport in mitochondria of the fruit fly Drosophila melanogaster. We demonstrate the presence of ruthenium red (RR)-sensitive Ca(2+) uptake, of RR-insensitive Ca(2+) release, and of Na(+)-stimulated Ca(2+) release in energized mitochondria, which match well characterized Ca(2+) transport pathways of mammalian mitochondria. Following larger matrix Ca(2+) loading Drosophila mitochondria underwent spontaneous RR-insensitive Ca(2+) release, an event that in mammals is due to opening of the permeability transition pore (PTP). Like the PTP of mammals, Drosophila Ca(2+)-induced Ca(2+) release could be triggered by uncoupler, diamide, and N-ethylmaleimide, indicating the existence of regulatory voltage- and redox-sensitive sites and was inhibited by tetracaine. Unlike PTP-mediated Ca(2+) release in mammals, however, it was (i) insensitive to cyclosporin A, ubiquinone 0, and ADP; (ii) inhibited by P(i), as is the PTP of yeast mitochondria; and (iii) not accompanied by matrix swelling and cytochrome c release even in KCl-based medium. We conclude that Drosophila mitochondria possess a selective Ca(2+) release channel with features intermediate between the PTP of yeast and mammals.

Control of oxidative phosphorylation in rat liver mitochondria: effect of ionic media

The aim of this work was to compare oxidative phosphorylation activity and its kinetic control on isolated rat liver mitochondria in either various ionic or sucrose isoosmotic media. Whatever the ionic medium, state 3 and uncoupled state respiratory rates were higher in ionic than in sucrose media, although state 4 respiration rate remained constant. Moreover, under isoosmotic conditions, the salt concentration necessary for half state 3 stimulation depends on the cation involved: for inorganic cations, these K0.5 values increased, as did the absolute value of hydratation enthalpy. The ATP/O ratio did not vary in any medium and matrix volume was about 20% increased in ionic media. JO2 versus delta p relationships were left-shifted in ionic media compared to sucrose medium: for the same respiratory rate, the protonmotive force maintained was lesser in ionic media. However, the relationship between JO2 and delta p is unique whatever the ionic medium under study. In ionic media compared to the sucrose medium, kinetic control was increased on one of the protonmotive force generating systems (cytochrome c oxidase) and decreased on one of the protonmotive force dissipating systems (adenine nucleotide translocator), even if the fluxes increased.

Effects of palmitoyl CoA and palmitoyl carnitine on the membrane potential and Mg2+ content of rat heart mitochondria

Palmitoyl CoA and palmitoyl carnitine added to rat heart mitochondria in amounts above 20 and 50 nmoles/mg protein, respectively, induced a fall in transmembrane potential and loss of endogenous Mg2+. The dissipation of membrane potential by low concentrations of palmitoyl CoA in the presence of Ca2+, but not that of high concentrations of palmitoyl CoA alone, was prevented by either ruthenium red, Cyclosporin A or Mg2+, but reversed only by Mg2+. The fall of membrane potential induced by palmitoyl carnitine was not prevented by any of these factors. It is suggested that the action of both palmitoyl CoA and palmitoyl carnitine at high concentrations is due to a non specific disruption of membrane architecture, while that of low concentrations of palmitoyl CoA in the presence of Ca2+ is associated specifically with energy dissipation due to Ca2+ cycling.

Experimental isobaric subarachnoid hemorrhage: regional mitochondrial function during the acute and late phase

Patients treated for aneurysmal subarachnoid hemorrhage show, in the long-term follow up, an elevated rate of cognitive disturbances that are mainly related to the impact of the initial bleeding: the neurotoxic effects of blood deposition in subarachnoidal spaces may result in a diffuse encephalopathy, but the intrinsic mechanism and the biochemical correlates are not known. In the present study we have evaluated mitochondrial function after experimental induction of subarachnoid hemorrhage. Mitochondrial function was evaluated in four different rat brain areas (frontal cortex, occipital cortex, hippocampus, and brain stem) after experimental isobaric subarachnoid hemorrhage in rats. Subarachnoid hemorrhage was induced by injecting 0.07 mL of arterial autologous blood into the cisterna magna. Intracranial pressure did not significantly increase. The nonsynaptic mitochondrial fraction was isolated from different rat brain areas, and the maximal rate of enzymatic reactions of some key enzymatic activities related to the Krebs cycle [nicotinamide adenine dinucleotide (oxidized form) (NAD+)-isocitrate dehydrogenase, citrate synthase, and succinate dehydrogenase] and of the electron transfer chain (cytochrome oxidase) were evaluated. The nonsynaptic mitochondrial fraction was utilized also to check parameters related to the mitochondrial respiration: state 3, state 4, uncoupled state, respiratory control ratio, and adenosine 5'-diphosphate/oxygen ratio. The biochemical parameters were measured at 1 and 72 hours after the subarachnoidal injection of blood. Subarachnoid hemorrhage did not affect the mitochondrial enzymatic activities both at 1 and 72 hours, while the mitochondrial enzymatic activities parameters were significantly affected: in particular, a significant decrease of respiratory control ratio in all tested brain areas was demonstrated. The increased mitochondrial vulnerability in the delayed phases could be one of the biochemical correlates of post-hemorrhagic encephalopathy.

High affinity insulin binding and insulin receptor-effector coupling: modulation by Ca2+

Insulin binding and insulin stimulated amino acid and glucose uptake were determined in cultured HTC hepatoma cells in the presence of Ca2+ and ruthenium red (RR) in order to further characterise the putative calcium binding site on the receptor. These ions increased insulin receptor high affinity binding and the sensitivity of these responses to insulin. The insulin concentration required to half-maximally stimulate amino acid uptake decreased significantly from 26.9 +/- 5.8 ng/ml to 6.0 +/- 1.3 ng/ml in the presence of 10 mM Ca2+ and to 1.3 +/- 0.5 ng/ml in the presence of RR. The effect of Ca2+ and RR was more pronounced on insulin stimulated glucose uptake. These agents also increased receptor-effector coupling, reducing the percentage of occupied receptors required for maximal insulin stimulation of amino acid uptake from 10.8% in control cells to 3.4 and 1.4% in the presence of Ca2+ and RR respectively. The receptor occupancy required to produce maximal insulin responses on glucose uptake decreased from 20% (control) to 3.8% (Ca2+ and RR). We hypothesize that since Ca2+ and RR have similar effects, that occupation of Ca2+ binding sites on the receptor produces a conformational change in the insulin receptor which increases insulin receptor affinity, insulin sensitivity and acts on an early post-receptor event responsible for coupling binding to insulin action.

Resting state respiration of mitochondria: reappraisal of the role of passive ion fluxes

Rat liver mitochondria respiring under resting state conditions in the presence of oligomycin were rapidly blocked with cyanide and the dissipation of the membrane potential, measured with a tetraphenylphosphonium-sensitive electrode, was followed over time. The plot of the rate of membrane potential dissipation versus the actual value of the membrane potential was nonlinear and identical to the plot of resting state respiration (titrated with small amounts of a respiratory inhibitor) versus the membrane potential. The relationship between the respiratory chain activity and the proton-motive force in mitochondria oxidizing succinate with either oxygen or ferricyanide as electron acceptors was also found to be identical. These results are interpreted as an indication that the passive permeability of the inner mitochondrial membrane toward ions is far more significant in maintaining resting state respiration than is the molecular slippage of the pumps in the respiratory chain. These results also confirm the non-ohmic characteristics of the inner mitochondrial membrane.

Slip and leak in mitochondrial oxidative phosphorylation

During oxidative phosphorylation by mammalian mitochondria part of the free energy stored in reduced substrates is dissipated and energy is released as heat. Here I review the mechanisms and the physiological significance of this phenomenon.

Bioenergetics of different brain areas after experimental subarachnoid hemorrhage in rats

We studied energy metabolism after experimental subarachnoid hemorrhage in rats. Four different cerebral areas were tested: frontal cortex, occipital cortex, hippocampus, and brainstem. Vmax of the following enzymatic activities was evaluated: in the homogenate: hexokinase, phosphofructokinase, and lactate dehydrogenase for the glycolytic pathway, and glucose-6-phosphate dehydrogenase for the hexose monophosphate shunt; in the purified nonsynaptic mitochondria: NAD+-isocitrate dehydrogenase, citrate synthase, and succinate dehydrogenase for the Krebs cycle, and cytochrome oxidase for the electron transfer chain. We also evaluated some parameters related to the respiration of nonsynaptic mitochondria (State 3, State 4, uncoupled state, respiratory control ratio, and ADP:O ratio). Subarachnoid hemorrhage did not significantly affect Vmax of the enzymatic activities related to anaerobic and aerobic metabolism; however, mitochondrial respiration was affected, particularly in the presence of NADH-producing substrates (glutamate + malate).

Pathways for Ca2+ efflux in heart and liver mitochondria

1. Two processes of Ruthenium Red-insensitive Ca2+ efflux exist in liver and in heart mitochondria: one Na+-independent, and another Na+-dependent. The processes attain maximal rates of 1.4 and 3.0 nmol of Ca2+.min-1.mg-1 for the Na+-dependent and 1.2 and 2.0 nmol of Ca2+.min-1.mg-1 for the Na+-independent, in liver and heart mitochondria, respectively. 2. The Na+-dependent pathway is inhibited, both in heart and in liver mitochondria, by the Ca2+ antagonist diltiazem with a Ki of 4 microM. The Na+-independent pathway is inhibited by diltiazem with a Ki of 250 microM in liver mitochondria, while it behaves as almost insensitive to diltiazem in heart mitochondria. 3. Stretching of the mitochondrial inner membrane in hypo-osmotic media results in activation of the Na+-independent pathway both in liver and in heart mitochondria. 4. Both in heart and liver mitochondria the Na+-independent pathway is insensitive to variations of medium pH around physiological values, while the Na+-dependent pathway is markedly stimulated parallel with acidification of the medium. The pH-activated, Na+-dependent pathway maintains the diltiazem sensitivity. 5. In heart mitochondria, the Na+-dependent pathway is non-competitively inhibited by Mg2+ with a Ki of 0.27 mM, while the Na+-independent pathway is less affected; similarly, in liver mitochondria Mg2+ inhibits the Na+-dependent pathway more than it does the Na+-independent pathway. In the presence of physiological concentrations of Na+, Ca2+ and Mg2+, the Na+-independent and the Na+-dependent pathways operate at rates, respectively, of 0.5 and 1.0 nmol of Ca2+.min-1.mg-1 in heart mitochondria and 0.9 and 0.2 nmol of Ca2+.min-1.mg-1 in liver mitochondria. It is concluded that both heart and liver mitochondria possess two independent pathways for Ca2+ efflux operating at comparable rates.

Tissue-specific modulation of the mitochondrial calcium uniporter by magnesium ions

This paper analyzes the kinetics of the Ca2+ uniporter of mitochondria from rat heart, kidney and liver operating in a range of Ca2+ concentrations near the steady-state value (1-4 microM). Heart mitochondria exhibit the lowest activity, and physiological Mg2+ concentrations inhibit the mitochondrial Ca2+ uniporter by approx. 50% in heart and kidney, and by 20% in liver. At physiological Ca2+ and Mg2+ concentrations the external free Ca2+ maintained by respiring mitochondria in vitro is higher in heart and kidney with respect to liver mitochondria. This behaviour could represent an adaptation of different mitochondria to their specific intracellular environment.

Steady-state kinetics of the overall oxidative phosphorylation reaction in heart mitochondria. Evidence for linkage of the energy-yielding and energy-consuming steps by freely diffusible intermediates and for an allosteric mechanism of respiratory control at coupling site 2

The three coupling segments of the respiratory chain of bovine heart mitochondria were examined individually by steady-state kinetic methods to determine whether or not freely diffusible intermediates occur between the energy-yielding and energy-consuming steps involved in the oxidative phosphorylation of extramitochondrial ADP. The principal method employed was the dual inhibitor technique, for which an appropriate model is provided. The results indicate that in accordance with the chemiosmotic theory the intermediate reactants that link the energy-yielding rotenone-sensitive (Site 1), cytochrome bc1 (Site 2), and cytochrome aa3 (Site 3) reactions of the respiratory chain to the energy-consuming ATP synthetase, AdN transport, and Pi transport reactions are freely diffusible (delocalized). Site 2 was found to differ from the others in regard to the mechanism by which the energy-linked respiratory chain reaction is controlled by the energy-consuming steps. Whereas the Site 1 and Site 3 respiratory chain reactions are controlled primarily by the thermodynamic mechanism of reaction reversal, the Site 2 respiratory reaction is controlled primarily by a kinetic mechanism in which an intermediate that links it to the energy-consuming steps inhibits it allosterically. From the effects of nigericin and valinomycin the allosteric intermediate appears to be the electrical component of the protonmotive force.

The apparent non-linearity of the relationship between the rate of respiration and the protonmotive force of mitochondria can be explained by heterogeneity of mitochondrial preparations

The apparent non-linear relationship between the rate of respiration and the protonmotive force in mitochondria under resting state conditions is an observation which has led to concepts such as non-chute characteristics of the proton leak through the mitochondrial membrane, or a slippage of proton pumps. We propose that this non-linearity may be a consequence of a heterogeneity of isolated mitochondria concerning the degree of coupling, since a small proportion of totally uncoupled, or loosely coupled, organelles may considerably contribute to the total respiration but not, or only slightly, to the protonmotive force. This hypothesis is supported by a fairly good fitting of computed relationships with those determined experimentally.

Role of the Ca2+ cycle in uncoupling of oxidative phosphorylation in liver mitochondria of cold-acclimated rats

Cold acclimation of Wistar rats for 2-4 weeks at about 3 degrees C resulted in an increased respiration rate and a reduced ADP/O ratio in liver mitochondria. With increasing duration of acclimation up to 10-12 weeks, these parameters returned to a normal level. The increase in the respiration rate and the decline of the mitochondrial ADP/O ratio were associated with a significant activation of the electroneutral release of Ca2+. When the animals were acclimated for 10-12 weeks the rate of Ca2+ release reduced to control values. The addition of 1 microM ruthenium red resulted in a decrease in the rates of mitochondrial respiration in control and cold-acclimated rats to approximately equal values and in a partial restoration of the ADP/O ratio in liver mitochondria of rats kept in the cold for 2-4 weeks. The respiratory activity of mitochondria isolated in the presence of 1 mM EGTA unaffected by ruthenium red.

An investigation on the effect of oligomycin on state-4 respiration in isolated rat-liver mitochondria

The inhibitory action of oligomycin on State-4 respiration in rat-liver mitochondria has been investigated in detail with regard to the extent, mode and characteristics of the inhibition. The possibility that this effect may be due either to some damage of the mitochondrial preparation used or to the presence of heavy contaminations by microsomes has been excluded. It has been found that the concentration of specific binding sites is the same in State 4 as in State 3. The extent of the inhibition appears to be related to the ADP concentration, rather than to ATP/ADP ratios. The inhibition of this antibiotic on State-4 respiration does not depend on the experimental conditions used (i.e., choice of substrates or composition of the reaction medium). In agreement with these observations, it has been found that the membrane potential of State 4 is significantly increased when oligomycin is added. All these results provide further evidence to the conclusion that a large portion of State-4 respiration is linked to phosphorylation.

Properties of a new calcium ion antagonist on cellular uptake and mitochondrial efflux of calcium ions

Compound YS 035 [NN-bis-(3,4-dimethoxyphenethyl)-N-methylamine] is a new synthetic compound capable of inhibiting Ca2+ uptake by different cells. The inhibition of Ca2+ uptake by muscle cells isolated from chicken embryo is dose-dependent in the compound YS 035 concentration range 10-30 microM. The new compound also inhibits Ca2+ entry into rat brain synaptosomes and less effectively into baby-hamster kidney cells. Compound YS 035 partially inhibits the slow Ca2+ release induced by Ruthenium Red and the rapid Na+-dependent efflux from heart mitochondria. The inhibition of the Na+/Ca2+ exchange appears to be of a non-competitive type with an apparent Ki of 28 microM. The new Ca2+ antagonist totally inhibits the Ca2+ efflux from liver mitochondria induced by Ruthenium Red, but it does not affect the release induced by uncoupler, respiratory inhibitor or chelator, nor the mitochondrial ATP synthesis and membrane potential. The properties shown by the new compound indicate it to be a Ca2+ antagonist and a useful tool for studies on the mitochondrial Ca2+ transport.

Transient kinetics of thermodynamic buffering

The transient response of mitochondrial ATP production towards perturbations was studied by analyzing the trajectories leading from arbitrary initial conditions of the adenine nucleotide pool to the final steady state. These trajectories were calculated from differential equations based on linear relations between flows and thermodynamic forces of the adenylate kinase system including oxidative phosphorylation. The motion of the system along the trajectories consists of two phases: (1) a rapid phase leading from initial states to a common relaxation curve; and (2) a slow phase leading along the relaxation curve to the final steady state. The first phase corresponds to a motion close to the loci of constant adenylic energy charge. In line with this observation is the finding that the energy charge is a constant of motion of the adenylate kinase reaction. The second phase corresponds to a motion along a relaxation curve characterized by minimal Lyapunov exponents in the concentration space of the adenine nucleotides. Thus, both phases of the transient kinetics can be approximated in terms of thermodynamic functions to a high degree of precision. Incubations with isolated rat liver mitochondria were in excellent agreement with the theoretical predictions. In summary, these studies show that the adenylate kinase system not only optimizes the efficiency of oxidative phosphorylation through thermodynamic buffering but, in addition, also deeply influences the transient response of the whole system.

Calcium efflux parallel to total phosphate retention in rat liver mitochondria

Phosphate efflux from uncoupled rat liver mitochondria was completely inhibited when mersalyl plus butylmalonate and ATP were added to a sucrose suspending medium. Despite the total retention of phosphate a calcium efflux was observed even in presence of ruthenium red. Under the above conditions no phosphate is transported in association with the ADP/ATP carrier. While mersalyl completely blocked the phosphate release induced by ruthenium red or EGTA from coupled mitochondria it only partially inhibited the CA2+-efflux. The inhibition of Ca2+ efflux was almost completely abolished in the presence of acetate. The existence of a co-transport of Ca2+ associated with phosphate is discussed.

Phosphorylating efficiency of isolated rat liver mitochondria respiring under the conditions of steady-State 4

A limited, but significant net formation of ATP was observed during the very first period of respiratory State 4. The synthesis appeared to depend on respiration, since it was completely inhibited by KCN or by 2,4-dinitrophenol. Accordingly, State 4 respiration was observed to be inhibited to a large extent by oligomycin. After the initial increase, the level of ATP remained unmodified under the conditions of steady-state 4. Also, the maintenance of the equilibrium level of ATP was very sensitive to KCN or 2,4-dinitrophenol. Under the very same conditions of State 4, the mitochondria exhibited a significant ATPase activity, which appeared to be competitively inhibited by ADP. Therefore, it might be concluded that the apparently constant level of ATP observed in State 4 results from a balanced equilibrium between a respiration-dependent synthesis and a continuous hydrolysis. A comparison between the amount of ATP hydrolysed in State 4 and the amount of oxygen consumed under the same conditions indicated that the phosphorylating efficiency of respiring mitochondria in State 4 is as high as in State 3.

The effect of oligomycin on rat liver mitochondria respiring in state 4

It has been found that oligomycin inhibits up to at least 50% state-4 mitochondrial respiration. A time dependence of oligomycin inhibition has been shown. A titration curve for state-4 respiration of sigmoidal profile has been presented. The possibility of misreading this oligomycin effect, so far never reported, has been excluded by evaluating the quality of mitochondrial preparations used in respect to their morphological, functional and electrochemical properties. The conclusion has therefore been put forward that the most part of respiration in steady-state-4 is driven by ATP synthesis.

Mitochondrial Ca2+ transport in lean and genetically obese (ob/ob) mice

Isolated mitochondria from liver or brown adipose tissue of obese ob/ob mice demonstrated increased rates of Ca2+ uptake and release compared with those of lean mice. This enhanced transport activity was not found in mitochondria from kidney or skeletal muscle. Respiration-induced membrane potential was the same in mitochondria from lean and ob/ob mice. It is therefore concluded that the increased Ca2+ uptake rates reflect an activation of the Ca2+ uniporter rather than a change in the electrophoretic driving force. As mitochondria from pre-obese ob/ob mice did not show elevated rates of Ca2+ transport, the activated transport in the obese animals was thus a consequence of the state of obesity rather than being a direct effect of the ob/ob genotype. It is suggested that the enhanced activity of the Ca2+-transport pathways in liver and brown adipose tissue may alter metabolic functions in these tissues by modifying cytoplasmic or intramitochondrial Ca2+ concentrations.

On the mechanisms of ATP-induced and succinate-induced redistribution of cations in isolated rat liver cells

1. The ability of external ATP to induce calcium uptake in isolated rat liver cells was further characterized. Stimulation of calcium uptake was specific for ATP, other nucleotides or ATP metabolites had no comparable effect. ATP was dephosphorylated while stimulating calcium uptake, but there was no stoichiometry between ATP hydrolysis and calcium uptake nor did dephosphorylation depend on calcium concentration. ATP acted from outside and was dephosphorylated by an ecto-ATPase of the cells. 2. In addition to its direct action, ATP enhanced succinate-dependent calcium uptake in a cooperative fashion. This is best explained by different sites of action. ATP increases cell membrane permeability while succinate stimulates uptake into mitochondria. 3. ATP was able to lower Na+ and K+ gradients and the pH gradient between cells and incubation medium. Increasing calcium concentration counteracted this effect though calcium uptake was then stimulated. 4. Succinate alone did not affect monovalent cation gradients but raised the pH gradient. It partially counteracted the ATP effects on these gradients. 5. Since catecholamine-like actions of ATP may be mediated by an increase in cytoplasmic calcium concentration, the action of extracellular ATP can be taken as a model to study the role of calcium as a transmitter of hormone actions. From interdependence between ATP-stimulated and succinate-stimulated calcium uptake, conclusions can be drawn on the resulting cytoplasmic calcium concentration and its effect on plasma membrane permeability.

Induction of 2H+/Me2+ exchange in rat-liver mitochondria

The time dependency of CA2+ efflux from Ca2+-loaded rat liver mitochondria has been investigated. The rate of ruthenium-red-insensitive Ca2+ efflux is continuously increased during the retention as a result of induction of an electroneutral H+ Ca2+ exchange system. The activation of the Ca2+ efflux pathway takes place under the constant value of the membrane potential and is accompanied by oxidation of mitochondrial pyridine nucleotides. It has also been found that the ruthenium-red-insensitive H+/Sr2+ exchange occurs in mitochondria during Sr2+-induced oscillation of ion fluxes. The rate of H+/Sr2+ exchange is variable and depends on the stage of the oscillatory cycle.

Calcium metabolism in vertebrate photoreceptors

So far all attempts to demonstrate a rapid, light-stimulated release of calcium from disks into the cytosol at a sufficiently high stoichiometry have failed. Either the release stoichiometry was too small or the velocity too slow to account for the amplification in visual transduction. The multitude of failures demonstrate that regulation of intracellular calcium is a very delicate process and the idea of a robust calcium channel in the disk membrane that is opened by rhodopsin itself is certainly an oversimplification. The strongest evidence in favour of the "calcium transmitter hypothesis" is the large calcium efflux from rods in a retina. However as long as the source of the calcium efflux inside the rod cells is unknown conclusions about the role of this calcium efflux are premature. Unfortunately, measurements of intracellular calcium, such as those by Brown and coworkers (93,94) in their pioneering work on photoreceptors in the ventral eye of Limulus, have not yet been feasible in vertebrates.

The transport of cations in mitochondria

The present paper has reviewed several factors related to ion transport and examined the properties of cation transport in mitochondria. The analysis suggests that: (1) The concept that a metabolically dependent electrical potential across the mitochondrial membrane plays a role in determining ion fluxes and steady-state concentrations is not justified and the data indicate that such exchanges are generally electroneutral. (2) Generally, the influx and efflux of an ion proceed by the same mechanism with at least one exception. (3) There are indications that some of the steps in transport are common to several cations. (4) The idea that carrier or ionophoric molecules are involved in cation transport has been examined in some detail together with the possible involvement of some known mitochondrial components. In particular, a model has been introduced in which local charge imbalances produced by H+ fluxes serve as the driving force of transport. The molecules of the complex are arranged in series in a tripartite arrangement including a filter or gate, a nonselective channel and an H+-transferring portion linked to either electron transport or the ATPase. Parts of this model have been introduced by other investigators. Models in which different portions of channels have differing functions have been proposed previously for other transport systems.

Influence of mild cold on 24 h energy expenditure, resting metabolism and diet-induced thermogenesis

1. It has been suggested previously that people in developed countries do not expose themselves to cold severe enough to induce a metabolic response. The energy expenditure, at both heat production and total heat loss, of nine women was therefore measured continuously while each lived for 30 h in a whole-body calorimeter on two occasions, one at 28 degrees and the other at 22 degrees. All subjects followed a predetermined pattern of activity and food intake. The environmental conditions were judged by the subjects to be within those encountered in everyday life. In the standard clothing worn, 28 degrees was considered to be comfortably warm but not too hot, while 22 degrees was judged to be cool but not too cold. 2. Heat production for 24 h was significantly greater at the lower temperature, by (mean +/- SE) 7 . 0 +/- 1 . 1%. The range was between 2 and 12%. Total heat loss was also significantly greater, by 6%, and there was a large change in the partition of heat loss. At the lower temperature sensible heat loss increased by 29% while evaporative heat loss decreased by 39%. 3. Resting metabolism measured in the morning 12--13 h after the last meal was significantly greater at 22 degrees than at 28 degrees, whereas there was no difference when the resting measurement was made for 2 . 5 h following a meal. 4.

IN CONCLUSION

(a) environmental temperature may play a more important role than was previously recognized in the energy balance of those living in this country, and (b) there is an indication of at least a partial replacement of cold-induced by diet-induced thermogenesis in man.

The role of inorganic phosphate in the release of Ca2+ from rat-liver mitochondria

The effect of inorganic phosphate on Ca2+ retention has been investigated using phosphate-depleted liver mitchondria. Phosphate induces the release of Ca2+ through an efflux route insensitive to ruthenium red. This effect is not due to functional or structural damage, since mitochondria maintain their membrane potential during phosphate-induced Ca2+ efflux. Direct enzymatic measurement of mitochondria pyridine nucleotides has established that changes in their redox state (i.e. increased oxidation) do not play a role in the phosphate-effect. The phosphate-induced Ca2+ efflux requires transport of phosphate out of mitochondria. However, the fluxes of Ca2+ and phosphate do not coincide: the release of phosphate preceeds that of Ca2+.

The optimal efficiency and the economic degrees of coupling of oxidative phosphorylation

A phenomenological theory considering the output characteristics of oxidative phosphorylation has been worked out by adopting the formalism of linear nonequilibrium thermodynamics. The linearity of oxidative phosphorylation in the range of the output forces of practical interest has been experimentally verified. the efficiency of oxidative phosphorylation is zero if either a load with a zero conductance (open-circuited situation) or a load with an infinite conductance (short-circuited situation) is attached to oxidative phosphorylation. In between these extreme conductances there exists a finite load conductance permitting oxidative phosphorylation to operate with optimal efficiency. The necessary and sufficient condition for optimal efficiency was found to be L33/L11 = square root 1 - q2 where L11 is the phenomenological conductance of phosphorylation, L33 the phenomenological conductance of the load and q the degree of coupling of oxidative phosphorylation driven by respiration. This condition was called conductance matching. Under the condition of conductance matching, four output functions of oxidative phosphorylation of practical interest were optimized. A maximal net rate of oxidative phosphorylation occurs at a degree of coupling qf = 0.78. A maximal output power of oxidative phosphorylation, i.e. net rate times established phosphate potential, resuls at qp = 0.91. The maximization of the function net rate times efficiency yielded an economic degree of coupling qfec = 0.95 for maximal ATP flow. Finally, maximization of the function output power times efficiency led to a degree of coupling qpec = 0.97. This last function simultaneously maximized net rate of ATP production, developed phosphate potential and efficiency and reflects therefore the most economic solution to the output problem under the condition of conductance matching. In isolated rat livers perfused in a metabolic resting state, the condition of conductance matching is fulfilled. In addition, the degree of coupling oxidative phosphorylation under these conditions corresponds to the economic degree of coupling qpec.