Polyunsaturated fatty acid deficiency reverses effects of alcohol on mitochondrial energy metabolism

BACKGROUND/AIMS

Polyunsaturated fatty acids (PUFA) deficiency is common in patients with alcoholic liver disease. The suitability of reversing such deficiency remains controversial. The aim was to investigate the role played by PUFA deficiency in the occurrence of alcohol-related mitochondrial dysfunction.

METHODS

Wistar rats were fed either a control diet with or without alcohol (control and ethanol groups) or a PUFA deficient diet with or without alcohol (PUFA deficient and PUFA deficient+ethanol groups). After 6 weeks, liver mitochondria were isolated for energetic studies and fatty acid analysis.

RESULTS

Mitochondria from ethanol fed rats showed a dramatic decrease in oxygen consumption rates and in cytochrome oxidase activity. PUFA deficiency showed an opposite picture. PUFA deficient+ethanol group roughly reach control values, regarding cytochrome oxidase activity and respiratory rates. The relationship between ATP synthesis and respiratory rate was shifted to the left in ethanol group and to the right in PUFA-deficient group. The plots of control and PUFA deficient+ethanol groups were overlapping. Phospholipid arachidonic over linoleic ratio closely correlated to cytochrome oxidase and oxygen uptake.

CONCLUSIONS

PUFA deficiency reverses alcohol-related mitochondrial dysfunction via an increase in phospholipid arachidonic over linoleic ratio, which raises cytochrome oxidase activity. Such deficiency may be an adaptive mechanism.

Effect of chronic renal failure with metabolic acidosis on alanine metabolism in isolated liver cells

BACKGROUND & AIMS

Decreased ureagenesis and gluconeogenesis from alanine have been reported during chronic renal failure in rat. This study addressed the respective roles of plasma-membrane transport and intracellular metabolism in these abnormalities of alanine pathways.

METHODS

In hepatocytes isolated from uremic and control rats, we investigated: (1) the influence of uremia on gluconeogenesis and ureagenesis during incubations with alanine; (2) the kinetics of alanine plasma-membrane transport; (3) the relationships between intracellular alanine concentrations and its metabolism. Plasma-membrane alanine transport was assessed after addition of alanine (2 mM) by measuring its intracellular accumulation from 0 to 10 min, in the presence of a transaminase inhibitor. Alanine metabolism was studied in perifused hepatocytes by measuring intracellular alanine concentration together with urea, glucose and lactate production in the presence of increasing concentrations of alanine (0-8 mM).

RESULTS

Uremic rats showed decreased plasma bicarbonate. Uremia induced (P<0.05) a decrease in both gluconeogenesis (36%) and ureagenesis (22%). Alanine plasma-membrane transport decreased by 20% during uremia. During perifusions, uremia induced a 30-40% decrease in urea, glucose, and lactate production without modifying intracellular alanine concentration.

CONCLUSIONS

In uremic rats with acidosis, hepatocyte alanine utilization was impaired at both plasma-membrane transport and intracellular transamination steps.

Mitochondrial metabolism and type-2 diabetes: a specific target of metformin

Several links relate mitochondrial metabolism and type 2 diabetes or chronic hyperglycaemia. Among them, ATP synthesis by oxidative phosphorylation and cellular energy metabolism (ATP/ADP ratio), redox status and reactive oxygen species (ROS) production, membrane potential and substrate transport across the mitochondrial membrane are involved at various steps of the very complex network of glucose metabolism. Recently, the following findings (1) mitochondrial ROS production is central in the signalling pathway of harmful effects of hyperglycaemia, (2) AMPK activation is a major regulator of both glucose and lipid metabolism connected with cellular energy status, (3) hyperglycaemia by inhibiting glucose-6-phosphate dehydrogenase (G6PDH) by a cAMP mechanism plays a crucial role in NADPH/NADP ratio and thus in the pro-oxidant/anti-oxidant cellular status, have deeply changed our view of diabetes and related complications. It has been reported that metformin has many different cellular effects according to the experimental models and/or conditions. However, recent important findings may explain its unique efficacy in the treatment of hyperglycaemia- or insulin-resistance related complications. Metformin is a mild inhibitor of respiratory chain complex 1; it activates AMPK in several models, apparently independently of changes in the AMP-to-ATP ratio; it activates G6PDH in a model of high-fat related insulin resistance; and it has antioxidant properties by a mechanism (s), which is (are) not completely elucidated as yet. Although it is clear that metformin has non-mitochondrial effects, since it affects erythrocyte metabolism, the mitochondrial effects of metformin are probably crucial in explaining the various properties of this drug.

Effect of cardiopulmonary bypass on lactate metabolism

OBJECTIVE

We have investigated the role of cardiopulmonary bypass on lactate metabolism in patients undergoing uncomplicated surgery for elective coronary artery bypass grafting (CABG).

DESIGN

Prospective non-randomized observational study.

SETTINGS

National Cardiovascular Center. PATIENTS. Three independent groups were studied: preoperative ( n=20), postoperative with bypass (CPB, n=20) and postoperative without bypass (NO-CPB, n=20).

INTERVENTIONS

Lactate metabolism was investigated with the use of an exogenous lactate challenge test (2.5 mmol Na-lactate/kg body weight in 15 min). Blood lactate was sequentially determined after the end of infusion. Lactate clearance and endogenous production were estimated from the area under the curve, and a bi-exponential fitting permitted modeling the lactate-decay into two compartments.

MEASUREMENTS AND MAIN RESULTS

Lactate metabolism parameters (basal lactate, clearance, endogenous production and half-lives [HL] I and II) were not different between the NO-CPB and preoperative groups. In the CPB group, as compared to the other two groups, basal lactate and endogenous production were not significantly affected while lactate clearance (CPB: 6.02+/-0.97 versus preoperative: 9.41+/-0.93 and NO-CPB: 9.6+/-0.8 ml/kg per min) and HL-I (CPB: 10.6+/-1.4 versus preoperative: 17.2+/-2.3 and NO-CPB: 18.8+/-2.5 min) were decreased ( p<0.001) and HL-II was increased (CPB: 171+/-41 versus preoperative: 73+/-12 and NO-CPB: 48+/-2.9 min, p<0.01).

CONCLUSION

While surgery and anesthesia per se do not seem to alter lactate metabolism, CPB significantly decreased lactate clearance, this effect being possibly related to a mild liver dysfunction even in uncomplicated elective surgery.

Metabolic and hemodynamic effects of hypertonic solutions: sodium-lactate versus sodium chloride infusion in postoperative patients

Although hypertonic saline has been proposed as an intravenous resuscitation fluid, the beneficial effects of the sodium load are associated with potentially deleterious effects of chloride. Since the physiological lactate anion is well metabolized, hypertonic lactate solution could represent an interesting alternative. The aim of this study was to compare metabolic and hemodynamic effects of hypertonic infusion of sodium lactate versus sodium chloride in three groups of surgical patients who underwent elective coronary artery bypass grafting (CABG). Hypertonic lactate solution was infused to patients 14 to 16 h after surgery either involving a cardiopulmonary bypass (CPB-Lac, n = 20) or on-off pump (OPCAB-Lac, n = 20), whereas the third group consisted of patients undergoing cardiopulmonary bypass but receiving hypertonic saline solution (CPB-NaCl, n = 20). An equal fluid and sodium load (2.5 mL/2.5 mmol x kg(-1)) was infused in all patients over 15 min. Plasma glucose and sodium increased after infusion in the three groups, but the changes, although significant, were small. As expected, lactate rose only in CPB-Lac and OPCAB-Lac groups, the changes being more marked in CPB-Lac, indicating a slower lactate metabolism in this group compared with OPCAB-Lac. Although both solutions produced significant increases in cardiac index and oxygen delivery, there was a significant decrease in oxygen extraction only in groups receiving sodium lactate (CPB-Lac and OPCAB-Lac) and not in CPB-NaCl. Finally, hypertonic NaCl infusion induced a modest, although significant, decrease in arterial pH and bicarbonate, whereas hypertonic lactate infusion increased these two parameters in both CPB-Lac and OPCAB-Lac. This study demonstrates that hypertonic lactate infusion is safe and well tolerated in patients undergoing elective cardiac surgery.

Malnutrition in hemodialysis diabetic patients: evaluation and prognostic influence

BACKGROUND

This work aimed to evaluate the role of malnutrition in the increased mortality rate of hemodialysis diabetic patients from a French cooperative series.

METHODS

Body mass index (BMI), serum albumin, prealbumin, cholesterol, and pre-dialysis creatinine, normalized protein catabolic rate and lean body mass (LBM) were measured in 734 diabetic and 6389 non-diabetic patients (aged 63.4 +/- 12.2 and 62.0 +/- 15.9 years; 1.01 male to 1.40 female ratio). The outcome of 1610 of these patients, including 170 diabetics, was assessed during a 30-month follow-up.

RESULTS

Diabetic as compared to non-diabetic patients showed a significant (P < 10-4) increased BMI (25.9 +/- 5.2 vs. 23.1 +/- 4.3) and cholesterol (5.5 +/- 1.6 vs. 5.3 +/- 1.5 mmol/L), and decreased albumin (37.8 +/- 5.4 vs. 38.9 +/- 5.3 g/L), prealbumin (317 +/- 91 vs. 340 +/- 94 mg/L), creatinine (711 +/- 184 vs. 816 +/- 217 micromol/L) and LBM (76 +/- 18 vs. 87 +/- 21%). Normalized protein catabolic rate was similar in the two groups (1.11 +/- 0.31 vs. 1.13 +/- 0.32 g/kg/L). One and two-year survival was 83.7 +/- 2.9% and 65.5 +/- 3.8% in diabetic patients versus 90.3 +/- 0.8% and 79.9 +/- 1.1% in non-diabetics (relative risk 1.26, P < 0.01). Independent predictors of survival were age, albumin and prealbumin in non-diabetics and only age in diabetics.

CONCLUSION

Diabetic patients compared to non-diabetics were characterized by an increased incidence of protein malnutrition and decreased survival. However, the higher death risk associated with diabetes was not related to malnutrition.

Determination of mitochondrial reactive oxygen species: methodological aspects

The generation of Reactive Oxygen Species (ROS) as by-products in mitochondria Electron Transport Chain (ETC) has long been admitted as the cost of aerobic energy metabolism with oxidative damages as consequence. The purpose of this methodological review is to present some of the most widespread methods of ROS generation and to underline the limitations as well as some problems, identified with some experiments as examples, in the interpretation of such results. There is now no doubt that besides their pejorative role, ROS are involved in a variety of cellular processes for the continuous adaptation of the cell to its environment. Because ROS metabolism is a complex area (low production, instability of species, efficient antioxidant defense system, several places of production...) bias, variances and limitations in ROS measurements must be recognized in order to avoid artefactual conclusions, and especially to improve our understanding of physiological and pathophysiological mechanisms of such phenomenon.

Thyroid status is a key regulator of both flux and efficiency of oxidative phosphorylation in rat hepatocytes

Thyroid status is crucial in energy homeostasis, but despite extensive studies the actual mechanism by which it regulates mitochondrial respiration and ATP synthesis is still unclear. We studied oxidative phosphorylation in both intact liver cells and isolated mitochondria from in vivo models of severe not life threatening hyper- and hypothyroidism. Thyroid status correlated with cellular and mitochondrial oxygen consumption rates as well as with maximal mitochondrial ATP production. Addition of a protonophoric uncoupler, 2,4-dinitrophenol, to hepatocytes did not mimic the cellular energetic change linked to hyperthyroidism. Mitochondrial content of cytochrome oxidase, ATP synthase, phosphate and adenine nucleotide carriers were increased in hyperthyroidism and decreased in hypothyroidism as compared to controls. As a result of these complex changes, the maximal rate of ATP synthesis increased in hyperthyroidism despite a decrease in ATP/O ratio, while in hypothyroidism ATP/O ratio increased but did not compensate for the flux limitation of oxidative phosphorylation. We conclude that energy homeostasis depends on a compromise between rate and efficiency, which is mainly regulated by thyroid hormones.

Lactate: A key metabolite in the intercellular metabolic interplay

Most physicians involved in intensive care consider lactate solely as a deleterious metabolite, responsible for high morbidity and bad prognosis in severe patients. For the physiologist, however, lactate is a key metabolite, alternatively produced or consumed. Many studies in the literature have infused animals or humans with exogenous lactate, demonstrating its safety and usefulness, but the bad reputation of lactate is still widespread. The metabolic meaning of glucose–lactate cycling exceeds its initial role described by Cori and Cori. According to recent works concerning lactate, it can be predicted that a new role as a therapeutic agent will arise for this metabolite.

Mitochondrial respiratory chain adjustment to cellular energy demand

Because adaptation to physiological changes in cellular energy demand is a crucial imperative for life, mitochondrial oxidative phosphorylation is tightly controlled by ATP consumption. Nevertheless, the mechanisms permitting such large variations in ATP synthesis capacity, as well as the consequence on the overall efficiency of oxidative phosphorylation, are not known. By investigating several physiological models in vivo in rats (hyper- and hypothyroidism, polyunsaturated fatty acid deficiency, and chronic ethanol intoxication) we found that the increase in hepatocyte respiration (from 9.8 to 22.7 nmol of O(2)/min/mg dry cells) was tightly correlated with total mitochondrial cytochrome content, expressed both per mg dry cells or per mg mitochondrial protein. Moreover, this increase in total cytochrome content was accompanied by an increase in the respective proportion of cytochrome oxidase; while total cytochrome content increased 2-fold (from 0.341 +/- 0.021 to 0.821 +/- 0.024 nmol/mg protein), cytochrome oxidase increased 10-fold (from 0.020 +/- 0.002 to 0.224 +/- 0.006 nmol/mg protein). This modification was associated with a decrease in the overall efficiency of the respiratory chain. Since cytochrome oxidase is well recognized for slippage between redox reactions and proton pumping, we suggest that this dramatic increase in cytochrome oxidase is responsible for the decrease in the overall efficiency of respiratory chain and, in turn, of ATP synthesis yield, linked to the adaptive increase in oxidative phosphorylation capacity.

Role of substrates in the regulation of mitochondrial function in situ

Investigations of mitochondrial oxidative phosphorylation have been mainly carried out in isolated mitochondria, where the experimental conditions can be precisely set. However, in intact living systems oxidative phosphorylation takes place in a complex environment, whose experimental dissection is a major challenge. It has long been recognized that the efficiency of oxidative phosphorylation depends on the nature of the respiratory substrates, which feed electrons to the respiratory chain at different levels. Yet, the role of substrates in determining mitochondrial function and their response to energetic stress has been largely overlooked. Here we review recent work showing that the nature of the energetic substrates profoundly affects the mitochondrial responses to manipulations of pathophysiological relevance, such as uncoupling and opening of the permeability transition pore (PTP). Uncoupling of intact hepatocytes caused very different metabolic effects depending on whether carbohydrates or lipids were the energy source. With dihydroxyacetone as the substrate dinitrophenol caused a collapse of the mitochondrial membrane potential and of the ATP/ADP ratio, while the respiratory rate was increased only transiently. With octanoate as the substrate, on the other hand, dinitrophenol caused a dramatic stimulation of the respiratory rate, while the mitochondrial membrane potential and ATP/ADP ratio were affected only marginally. We then review results indicating that the activity of complex I directly regulates the PTP, a finding that emphasizes the importance of the respiratory substrates in PTP regulation.

Glucose 6-phosphate hydrolysis is activated by glucagon in a low temperature-sensitive manner

Glucagon affects liver glucose metabolism mainly by activating glycogen breakdown and by inhibiting pyruvate kinase, whereas a possible effect on glucose-6-phosphatase has also been suggested. Although such a target is of physiological importance for liver glucose production it was never proven. By using a model of liver cells, perifused with dihydroxyacetone, we show here that the acute stimulation of gluconeogenesis by glucagon (10(-7) m) was not related to the significant inhibition of pyruvate kinase but to a dramatic activation of the hydrolysis of glucose 6-phosphate. We failed to find an acute change in glucose-6-phosphatase activity by glucagon, but the increase in glucose 6-phosphate hydrolysis was abolished at 21 degrees C; conversely the effect on pyruvate kinase was not affected by temperature. The activation of glucose 6-phosphate hydrolysis by glucagon was confirmed in vivo, in postabsorptive rats receiving a constant infusion of glucagon, by the combination of a 2-fold increase in hepatic glucose production and a 60% decrease in liver glucose 6-phosphate concentration. Besides the description of a novel effect of glucagon on glucose 6-phosphate hydrolysis by a temperature-sensitive mechanism, this finding could represent an important breakthrough in the understanding of type II diabetes, because glucose 6-phosphate is proposed to be a key molecule in the transcriptional effect of glucose.

Exogenous Mg-ATP induces a large inhibition of pyruvate kinase in intact rat hepatocytes

Mg-ATP infusion in vivo has been reported to be beneficial both to organ function and survival rate in various models of shock. Moreover, a large variety of metabolic effects has been shown to occur in several tissues due to purinergic receptor activation. In the present work we studied the effects of exogenous Mg-ATP in rat liver cells perifused with dihydroxyacetone to investigate simultaneously gluconeogenetic and glycolytic pathways. We found a significant effect on oxidative phosphorylation as characterized by a decrease in oxygen consumption rate and in the cellular ATP-to-ADP ratio associated with an increase in lactate-to-pyruvate ratio. In addition, exogenous Mg-ATP induced rapid and reversible inhibition of both gluconeogenesis and glycolysis. The main effect on gluconeogenesis was located at the level of the fructose cycle, whereas the decrease in glycolysis was due to a strong inhibition of pyruvate kinase. Although pyruvate kinase inhibition induced by exogenous Mg-ATP was allosteric when assessed in vitro after enzyme extraction, we found a large decrease in the apparent maximal velocity when kinetics were assessed in vivo in intact perifused hepatocytes. This newly described short-term regulation of pyruvate kinase occurs only in the intact cell and may open new potentials for the pharmacological regulation of pyruvate kinase in vivo.

Inter-organ substrate exchanges in the critically ill

Metabolic inter-organ exchange is a major field of research for improving the treatment of the critically ill. Adapting regional blood flows is the first regulatory step, although the relationships between hypoperfusion and metabolic disorders are matter of controversy. Metabolic steady state results from a vast inter-organ interplay and several nutrients or metabolites are signalling molecules in the regulation of gene transcription. Inter- or intra-organ substrate recycling shares or delays the mandatory need for aerobic ATP synthesis in some conditions. Nitrogen metabolism is highly compartmentalised in an inter-organ co-operation and liver, muscle, kidney and gut are the most important organs. By remodelling the amino acid mixture delivered to peripheral cells after intestinal absorption, the liver plays a determinant role in whole body protein synthesis. Albumin turnover increases after brain injury. Since the location of synthesis is different to that of breakdown this turnover can be viewed as an inter-organ exchange. The metabolic side of pH homeostasis is also an inter-organ exchange mainly shared by liver, kidney and muscle.

Mitochondrial adaptation to in vivo polyunsaturated fatty acid deficiency: increase in phosphorylation efficiency

Polyunsaturated fatty acid (PUFA) deficiency affects respiratory rate both in isolated mitochondria and in hepatocytes, an effect that is normally ascribed to major changes in membrane composition causing, in turn, protonophoriclike effects. In this study, we have compared the properties of hepatocytes isolated from PUFA-deficient rats with those from control animals treated with concentrations of the protonophoric uncoupler 2,4-dinitrophenol (DNP). Despite identical respiratory rate and in situ mitochondrial membrane potential (delta psi), mitochondrial and cytosolic ATP/ADP-Pi ratios were significantly higher in PUFA-deficient cells than in control cells treated with DNP. We show that PUFA-deficient cells display an increase of phosphorylation efficiency, a higher mitochondrial ATP/ADP-Pi ratio being maintained despite the lower delta psi. This is achieved by (1) decreasing mitochondrial Pi accumulation, (2) increasing ATP synthase activity, and (3) by increasing the flux control coefficient of adenine nucleotide translocation. As a consequence, oxidative phosphorylation efficiency was only slightly affected in PUFA-deficient animals as compared to protonophoric uncoupling (DNP). Thus, the energy waste induced by PUFA deficiency on the processes that generate the proton motive force (pmf) is compensated in vivo by powerful adaptive mechanisms that act on the processes that use the pmf to synthesize ATP.

Chronic ethanol ingestion increases efficiency of oxidative phosphorylation in rat liver mitochondria

The efficiency of oxidative phosphorylation was compared between rats chronically fed with ethanol and controls. (i) Results showed that the liver mitochondria state 4 respiratory rate was strongly inhibited, while the corresponding proton-motive force was not affected; (ii) the cytochrome oxidase content and activity were decreased and (iii) the oxidative-phosphorylation yield was increased in the ethanol exposed group. Furthermore, oxidative phosphorylation at coupling site II was not affected by ethanol. Cytochrome oxidase inhibition by sodium-azide mimicked the effects of ethanol intoxication in control mitochondria. This indicates that the decrease in cytochrome oxidase activity induced by ethanol intoxication directly increases the efficiency of oxidative phosphorylation.

Lactic acidosis. A new insight?

From an intensivist point of view, lactic acid is (i) responsible for metabolic acidosis, (ii) related to anoxia or ischemia and (iii) associated with poor prognosis. Conversely, from a biochemist point of view lactate is a good cellular substrate which can be easily converted to pyruvate and used as gluconeogenic substrate, or oxidised or transaminated into alanine. Hence the main question is not anymore to assess the value of lactate concentration as a marker of severity (it is well established) but rather to understand the metabolic meaning of its increase: is it beneficial or deleterious? In fact several recent experimental works have shown that instead of being a negative consequence, lactate production and related metabolic acidosis due to the stimulation of anaerobic ATP-production pathway could be a protective adapted response.

Energy metabolism in critically ill patients: lactate is a major oxidizable substrate

The potential role of an energy defect in acute diseases is still in the centre of the pathophysiological understanding of such states and therefore of our attempts to limit or to reverse the possible deleterious consequences of such defect. In fact several recent experimental works have shown that instead of being a negative consequence, the lactate production and the related metabolic acidosis due to the stimulation of anaerobic ATP-production pathway is rather a protective adapted response.

In vivo hypoxic exposure impairs metabolic adaptations to a 48 hour fast in rats

Hypoxia is well known to affect carbohydrate metabolism through its action on liver function and thus on glucose homeostasis. The aim of this study was to examine the carbohydrate, lipid and protein metabolic responses to 48 h of hypoxia, as well as the hormonal adaptations using both normoxic controls and hypoxic animals in the fasted state to standardize for the marked hypophagia observed in response to hypoxia. Hypoxia exposure (inspiratory oxygen fraction (FI,O2) = 0.1) resulted in a greater weight loss (-23 +/- 3.6% versus -16 +/- 2% in controls, p<0.001). Hypoxia plus fasting led to a significant increase in plasma glucose, lactate, insulin and catecholamine concentrations, while the increase in free fatty acid and beta-hydroxybutyrate was abolished. Changes in plasma amino acid patterns were not affected by hypoxia. Liver glycogen depletion was significantly less pronounced in the hypoxic group, while phosphoenolpyruvate carboxykinase (a key enzyme of liver gluconeogenesis) activity and transcription enhancements were abolished by hypoxia. Overall, hypoxic exposure in rats fasted for 48 h resulted in a unique pattern that differed from responses to injury or fasting per se. Oxygen seems to play a central role in the metabolic adaptation to fasting, from gene expression to weight loss. Since hypoxaemia associated with fasting has detrimental effects on nutritional balance, the present observations may be clinically relevant in the setting of acute exacerbation with hypoxaemia for chronic respiratory disease.

Inhibition of glycerol metabolism in hepatocytes isolated from endotoxic rats

Sepsis or endotoxaemia inhibits gluconeogenesis from various substrates, the main effect being related to a change in the phosphoenolpyruvate carboxykinase transcription rate. In addition, sepsis has been reported to affect the oxidative phosphorylation pathway. We have studied glycerol metabolism in hepatocytes isolated from rats fasted and injected 16 h previously with lipopolysaccharide from Escherichia coli. Endotoxin inhibited glycerol metabolism and led to a very large accumulation of glycerol 3-phosphate; the cytosolic reducing state was increased. Furthermore glycerol kinase activity was increased by 33% (P<<0.01). The respiratory rate of intact cells was significantly decreased by sepsis, with glycerol or octanoate as exogenous substrates, whereas oxidative phosphorylation (ATP-to-O ratio or respirations in state 4, state 3 and the oligomycin-insensitive state as well as the uncoupled state) was unchanged in permeabilized hepatocytes. Hence the effect on energy metabolism seems to be present only in intact hepatocytes. An additional important feature was the observation of a significant increase in cellular volume in cells from endotoxic animals, which might account for the alterations induced by sepsis.

The yield of oxidative phosphorylation is controlled both by force and flux

Dissipation of energy during oxidative phosphorylation may be due to two distinct mechanisms: passive permeability to protons and/or cations (leak) or decrease in the efficiency of some proton pumps (slip). Whatever the mechanism involved, it is admitted that the wastage depends on the protonmotive force. However, the most relevant question in physiology is to determine whether other factors contribute or not to this efficiency. By comparing phosphorylating (high respiratory flux) or non phosphorylating (low respiratory flux) states at similar protonmotive force, we have shown that the wastage is higher in phosphorylating than in non-phosphorylating conditions. This strongly argues for the fact that the flux of oxidative phosphorylation is an important parameter in the control of the yield of this major energetic pathway.