The effect of hormones on proton compartmentation in hepatocytes

An intracellular pH gradient in the anammox bacterium Kuenenia stuttgartiensis as evaluated by 31P NMR

The cytoplasm of anaerobic ammonium oxidizing (anammox) bacteria consists of three compartments separated by membranes. It has been suggested that a proton motive force may be generated over the membrane of the innermost compartment, the “anammoxosome”. ³¹P nuclear magnetic resonance (NMR) spectroscopy was employed to investigate intracellular pH differences in the anammox bacterium Kuenenia stuttgartiensis. With in vivo NMR, spectra were recorded of active, highly concentrated suspensions of K. stuttgartiensis in a wide-bore NMR tube. At different external pH values, two stable and distinct phosphate peaks were apparent in the recorded spectra. These peaks were equivalent with pH values of 7.3 and 6.3 and suggested the presence of a proton motive force over an intracytoplasmic membrane in K. stuttgartiensis. This study provides for the second time—after discovery of acidocalcisome-like compartments in Agrobacterium tumefaciens—evidence for an intracytoplasmic pH gradient in a chemotrophic prokaryotic cell.

Theoretical studies on the regulation of oxidative phosphorylation in intact tissues

The theoretical studies on the regulation of oxidative phosphorylation that were performed with the aid of kinetic models of this process are overviewed. A definition of the regulation of the flux through a metabolic pathway is proposed and opposed to the control exerted by particular enzymes over this flux. Different kinetic models of oxidative phosphorylation proposed in the literature are presented, of which only the model proposed by myself and co-workers was extensively used in theoretical studies on the regulation and compensation in the oxidative phosphorylation system. These theoretical studies have led to the following conclusions: (1) in isolated mitochondria, an increase in the activity of an artificial ATP-using system stimulates mitochondria mainly via changes in [ADP], while changes in [ATP] and [P(i)] play only a minor role; (2) in non-excitable tissues (e.g. liver), hormones (acting via some cytosolic factor(s)) activate directly both ATP usage and at least some enzymes of the ATP-producing block; (3) in excitable tissues (e.g. skeletal muscle), neural signals stimulate (via some cytosolic factor(s)) in parallel all the steps of oxidative phosphorylation together with ATP usage and substrate dehydrogenation; (4) the decrease in the flux through cytochrome oxidase caused by a decrease in oxygen concentration is, at least partially, compensated by a decrease in Delta p and increase in the reduction level of cytochrome c. A theoretical prediction is formulated that there should exist and be observable a universal cytosolic factor/regulatory mechanism which directly activates (at least in excitable tissues) all complexes of oxidative phosphorylation during an increased energy demand.

Influence of calcium and iron on cell death and mitochondrial function in oxidatively stressed astrocytes

Astrocytes protect neurons and oligodendrocytes by buffering ions, neurotransmitters, and providing metabolic support. However, astrocytes are also vulnerable to oxidative stress, which may affect their protective and supportive functions. This paper examines the influence of calcium and iron on astrocytes and determines if cell death could be mediated by mitochondrial dysfunction. We provide evidence that the events associated with peroxide-induced death of astrocytes involves generation of superoxide at the site of mitochondria, loss of mitochondrial membrane potential, and depletion of ATP. These events are iron-mediated, with iron loading exacerbating and iron chelation reducing oxidative stress. Iron chelation maintained the mitochondrial membrane potential, prevented peroxide-induced elevations in superoxide levels, and preserved ATP levels. Although increased intracellular calcium occurred after oxidative stress to astrocytes, the calcium increase was not necessary for collapse of mitochondrial membrane potential. Indeed, when astrocytes were oxidatively stressed in the absence of extracellular calcium, cell death was enhanced, mitochondrial membrane potential collapsed at an earlier time point, and superoxide levels increased. Additionally, our data do not support opening of the mitochondrial permeability transition pore as part of the mechanism of peroxide-induced oxidative stress of astrocytes. We conclude that the increase in intracellular calcium following peroxide exposure does not mediate astrocytic death and may even provide a protective function. Finally, the vulnerability of astrocytes and their mitochondria to oxidative stress correlates more closely with iron availability than with increased intracellular calcium.

Coupling between cytosolic and mitochondrial calcium oscillations: role in the regulation of hepatic metabolism

Mitochondria are strategically localized at sites of Ca2+ release, such that increases in cytosolic free Ca2+ ([Ca2+]c) from either internal Ca2+ stores or Ca2+ influx across the plasma membrane can be rapidly transported into the mitochondrial matrix. The consequent elevation in mitochondrial Ca2+ ([Ca2+]m) stimulates the Ca2+-sensitive intramitochondrial dehydrogenases, resulting in elevation of NAD(P)H. The preferential coupling between increases in [Ca2+]c and [Ca2+]m is one proposed mechanism to coordinate mitochondrial ATP production with cellular energy demand. In liver cells, hormones that act through the second messenger inositol 1,4, 5-trisphosphate (IP3) generate oscillatory [Ca2+]c signals, which result from a periodic Ca2+- and IP3-mediated activation/deactivation of intracellular Ca2+ release channels. The [Ca2+]c spiking frequency increases with agonist dose, whereas the amplitude of each [Ca2+]c spike is constant. This frequency modulation of [Ca2+]c spiking encodes the signal from the extracellular agonist, which is then decoded by the internal Ca2+-sensitive proteins such as the Ca2+-sensitive intramitochondrial dehydrogenases. Our studies have investigated the relationship between IP3-dependent [Ca2+]c signals and [Ca2+]m in primary cultured hepatocytes. In addition, the changes in cellular [Ca2+] levels have been correlated with the regulation of intramitochondrial NAD(P)H levels, pyruvate dehydrogenase activity and the magnitude of the mitochondrial proton motive force.

Proportional activation coefficients during stimulation of oxidative phosphorylation by lactate and pyruvate or by vasopressin

A 'proportional activation' approach designed to deal with the influence of external effectors within biochemical systems is described. The proportional activation coefficient, which enables the quantitative determination of the relative stimulation of different parts of a system by a given effector, is defined. The proportional activation approach was used to calculate the relative activation of delta p-producing and delta p-consuming subsystems during stimulation of the respiration rate of cells by a variety of different effectors. Oxidative phosphorylation was stimulated by the addition of either lactate and pyruvate (10 mM and 1 mM) or vasopressin. The addition of lactate and pyruvate to suspensions of resting hepatocytes increased the respiration rate by about 50%. The delta p-consuming subsystem was stimulated about 60% as much as the delta p-producing subsystem. Quinolinic acid, commonly considered to be a specific inhibitor of gluconeogenesis, was found to block the delta p-producing oxidative subsystem as well as the delta p-consuming subsystem, indicating some nonspecific effects of this inhibitor. Addition of vasopressin to hepatocytes that were incubated in the presence of lactate and pyruvate resulted in an increase of the respiratory rate by up to 35%. The relative stimulation of the delta p-producing and delta p-consuming subsystems was essentially equal. Using the 'proportional activation approach' to analyse these and previously published data, it is shown that substrates (lactate/pyruvate and fatty acids), Ca(2+)-acting hormones (vasopressin and others) and calcium in muscles (heart muscle and skeletal muscle) activate both subsystems to a similar extent (it concerns especially Ca(2+)-dependent systems).

Theoretical studies on the control of the oxidative phosphorylation system

The dynamic model developed in our previous publications [1,2] was used to calculate the flux control coefficients of oxidation, phosphorylation and proton leak fluxes for isolated mitochondria and for three modes of work of intact cells (hepatocytes). The results obtained were compared with experimental data, especially those measured in the frame of the 'top-down approach' of the metabolic control theory. A good agreement for mitochondria and for intact cells was found. The control of the oxygen consumption flux is shared between the ATP utilization (main controlling factor), substrate dehydrogenation, proton leak and, in some conditions, the ATP/ADP carrier. The phosphorylation subsystem seemed to be controlled mainly by itself, while the proton leak was influenced by all three subsystems. It was also shown that the large relative change in the enzyme activity during inhibitor titration of mitochondria or cells could lead to the overestimation of some flux control coefficient values in experimental measurements. An influence of some hormones (glucagon, vasopressin, adrenaline and others) on the mitochondrial respiration was also simulated. Our results suggest that these hormones stimulate the substrate dehydrogenation as well as the phosphorylation system (ATP usage and, possibly, the ATP/ADP carrier).

A 31P NMR study of mitochondrial inorganic phosphate visibility: effects of Ca2+, Mn2+, and the pH gradient

The effects of external pH, temperature, and Ca2+ and Mn2+ concentrations on the compartmentation and NMR visibility of inorganic phosphate (Pi) were studied in isolated rat liver mitochondria respiring on succinate and glutamate. Mitochondrial matrix Pi is totally visible by NMR at 8 degrees C and at low external concentrations of Pi. However, when the external Pi concentration is increased above 7 mM, the pH gradient decreases, the amount of matrix Pi increases, and the fraction not observed by NMR increases. Raising the temperature to 25 degrees C also decreases the pH gradient and the Pi fraction observed by NMR. At physiologically relevant concentrations, Ca2+ and Mn2+ do not seem to play a major role in matrix Pi NMR invisibility. For Ca2+ concentrations above 30 nmol/mg of protein, formation of insoluble complexes will cause loss of Pi signal intensity. For Mn2+ concentrations above 2 nmol/mg of protein, the Pi peak can be broadened sufficiently to preclude detection of a high-resolution signal. The results indicate that mitochondrial matrix Pi should be mostly observable up to 25 degrees C by high-resolution NMR. While the exact nature of the NMR-invisible phosphate in perfused or in vivo liver is yet to be determined, better success at detecting and resolving both Pi pools by NMR is indicated at high field, low temperature, and optimized pulsing conditions.

Continuous measurement of mitochondrial pH gradients in isolated hepatocytes by difference ratio spectroscopy

The pH gradient, delta pH, present across the inner mitochondrial membrane in isolated rat hepatocytes was continuously monitored with a novel spectroscopic technique that utilizes the weak acid fluorescein. Unlike most cytosolic pH indicators, such as 2',7'-bis(carboxyethyl)-5,(6)-carboxyfluorescein (BCECF), fluorescein freely distributes between the cytosolic and mitochondrial compartments. As is typical for weak acids, the distribution between these two compartments is governed by the magnitude of the pH gradient. Since fluorescein has two ionizable groups, the fluorescein dianion is concentrated in the mitochondrial compartment 100-fold per delta pH unit. In this compartment, fluorescein absorbance (or excitation) spectra are red-shifted about 6-8 nm in the matrix environment, as compared to the cytosolic dye at equivalent pH values. The combination of favorable mitochondrial accumulation and red-shifted spectra enables mitochondrial pH to be continuously monitored qualitatively in whole cells by dual wavelength spectroscopy (510 minus 540 nm). When the cytosolic pH is determined by independent means, the mitochondrial pH can be quantitated, based on the theoretical dependence of the fluorescein distribution ratio on delta pH, the ratio of cytosolic to mitochondrial volumes, and the known extinction coefficients for the dye in the cytosolic and mitochondrial compartments. The sensitivity of the method for following kinetic responses in mitochondrial pH is especially noteworthy; a 0.1-unit change in delta pH is easily distinguished, with a time resolution of less than a second.

The mechanism of Ca2(+)-related control of gluconeogenesis in perfused liver

A kinetic expression for rat-liver mitochondrial aspartate formation in situ was developed in order to determine whether hormonally induced decreases in 2-oxoglutarate levels can regulate hepatic gluconeogenesis from lactate via control of aspartate formation. Previous studies from this laboratory showed that 2-oxoglutarate can inhibit aspartate production by isolated mitochondria. These present studies were designed to probe the physiological significance of the decrease in 2-oxoglutarate levels observed when Ca2(+)-mobilizing gluconeogenic hormones are administered to isolate perfused rat livers. First, estimates were made of the kinetic constants which determine the rate of aspartate formation in isolated mitochondria. The concentrations of the substrates and products of this process were then measured in perfused livers. From these values, it was possible to estimate aspartate efflux from mitochondria in situ. The calculated rates of aspartate production were increased by decreases in 2-oxoglutarate levels which occurred when glucagon or phenylephrine was added to the perfused livers. Glucagon also effected an inhibition of pyruvate kinase, evidenced by the fact that the calculated rate of aspartate efflux equalled the rate of gluconeogenesis (the difference between the two is equivalent to the pyruvate-kinase flux). By contrast, in control livers and with phenylephrine stimulation, aspartate formation was higher than gluconeogenesis suggesting significant pyruvate-kinase flux in this condition. The calculations also show a correlating increase in flux through pyruvate carboxylase (30% with phenylephrine, 15% with glucagon, compared with approximately 50% increases in gluconeogenic flux). The mechanism of this increase is discussed.

Activation by spermine of citrate synthase from porcine heart

Spermine activated citrate synthase from porcine heart by decreasing the Km value for the substrate oxaloacetate without affecting the maximal velocity. Spermine markedly increased the maximal velocity of the saturation function with respect to acetyl-CoA as the substrate under conditions of intracellular concentrations of oxaloacetate, but the enzyme was not activated by spermine under conditions of higher concentrations of oxaloacetate. The concentration of spermine required for 50% activation of the enzyme was about 50 microM. Spermidine showed only a little activation, while putrescine caused no activation. Spermine, which contributes to an activation of Ca2(+)-sensitive dehydrogenases of the citric acid cycle by enhancing Ca2+ uptake into mitochondria, can activate citrate synthase directly, and is responsible for the stimulation of oxidative metabolism in mitochondria.

Stimulation of the electron transport chain in mitochondria isolated from rats treated with mannoheptulose or glucagon

We investigated the kinetics of the mitochondrial respiratory chain, proton leak, and phosphorylating subsystems of liver mitochondria from mannoheptulose-treated and control rats. Mannoheptulose treatment raises glucagon and lowers insulin; it had no effect on the kinetics of the mitochondrial proton leak or phosphorylating subsystems, but the respiratory chain from succinate to oxygen was stimulated. Previous attempts to detect any stimulation of cytochrome c oxidase by glucagon are shown by flux control analysis to have used inappropriate assay conditions. To investigate the site of stimulation of the respiratory chain we measured the relationship between the thermodynamic driving force and respiration rate for the span succinate to coenzyme Q, the cytochrome bc1 complex and cytochrome c oxidase. Hormone treatment of rats altered the kinetics of electron transport from succinate to coenzyme Q in subsequently isolated mitochondria and activated succinate dehydrogenase. The kinetics of electron transport through the cytochrome bc1 complex were not affected. Effects on cytochrome c oxidase were small or nonexistent.

A synthetic peptide of the pseudosubstrate domain of protein kinase C blocks cytoplasmic alkalinization during activation of the sea urchin egg

Multiple second messenger pathways have been proposed for transduction of the sperm-egg fusion event during fertilization of sea urchin eggs. Cytoplasmic alkalinization due to increased Na(+)-H+ antiport has been causally linked to many of the metabolic events during fertilization. Two possible second messenger pathways coupling sperm-egg fusion and antiporter activity are activation of protein kinase C (PKC) and Ca2(+)-calmodulin kinase. A selective inhibitor of PKC is PKC(19-36), a synthetic peptide of the pseudosubstrate domain of the kinase. Injection of PKC(19-36) into unfertilized sea urchin eggs blocked cytoplasmic alkalinization during activation by phorbol 12-myristate 13-acetate, a PKC agonist. The rise in pH during fertilization was partially blocked by PKC(19-36), which suggested that multiple pathways regulate the antiporter during fertilization. The use of fluorescein chromophores to measure intracellular pH in sea urchin eggs is also discussed.

The regulation of the matrix volume of mammalian mitochondria in vivo and in vitro and its role in the control of mitochondrial metabolism

The purpose of this article is to describe briefly the methods by which the intra-mitochondrial volume may be measured both in vitro and in situ, to summarise the mechanisms thought to regulate the mitochondrial volume and then to review in more detail the evidence that changes in the intra-mitochondrial volume play an important part in the regulation of liver mitochondrial metabolism by glucogenic hormones such as glucagon, adrenaline and vasopressin. It will be shown that these hormones cause an increase in matrix volume sufficient to produce significant activation of fatty acid oxidation, respiration and ATP production, pyruvate carboxylation, citrulline synthesis and glutamine hydrolysis. These are all processes activated by such hormones in vivo. I will go on to demonstrate that the increase in matrix volume is brought about by an increase in mitochondrial [PPi]. This is able to stimulate K+ entry into the matrix, perhaps through an interaction with the adenine nucleotide translocase. The rise in matrix [PPi] is a consequence of an increase in cytosolic and hence mitochondrial [Ca2+] which inhibits mitochondrial pyrophosphatase. In the final section of the review I provide evidence that changes in mitochondrial volume may be important in the responses of a variety of tissues to hormones and other stimuli. I write as a metabolist with a working knowledge of bioenergetics rather than the converse, and this will certainly be reflected in the approach taken. If I cause offence to any dedicated experts in the field of bioenergetic by my ignorance or lack of understanding of their studies I can only offer my apologies and ask to be corrected.

Intracellular mitochondrial membrane potential as an indicator of hepatocyte energy metabolism: further evidence for thermodynamic control of metabolism

The lipophilic triphenylmethylphosphonium cation (TPMP+) has been employed to measure delta psi m, the electrical potential across the inner membrane of the mitochondria of intact hepatocytes. The present studies have examined the validity of this technique in hepatocytes exposed to graded concentrations of inhibitors of mitochondrial energy transduction. Under these conditions, TPMP+ uptake allows a reliable measure of delta psi m in intracellular mitochondria, provided that the ratio [TPMP+]i/[TPMP+]e is greater than 50:1 and that at the end of the incubation more than 80% of the hepatocytes exclude Trypan blue. Hepatocytes, staining with Trypan blue, incubated in the presence of Ca2+, do not concentrate TPMP+. The relationships between delta psi m and two other indicators of cellular energy state, delta GPc and Eh, or between delta psi m and J0, were examined in hepatocytes from fasted rats by titration with graded concentrations of inhibitors of mitochondrial energy transduction. Linear relationships were generally observed between delta psi m and delta GPc, Eh or J0 over the delta psi m range of 120-160 mV, except in the presence of carboxyatractyloside or oligomycin, where delta psi m remained constant. Both the magnitude and the direction of the slope of the observed relationships depended upon the nature of the inhibitor. Hepatocytes from fasted rats synthesized glucose from lactate or fructose, and urea from ammonia, at rates which were generally linear functions of the magnitude of delta psi m, except in the presence of oligomycin or carboxyatractyloside. Linear relationships were also observed between delta psi m and the rate of formation of lactate in cells incubated with fructose and in hepatocytes from fed rats. The linear property of these force-flow relationships is taken as evidence for the operation of thermodynamic regulatory mechanisms within hepatocytes.

Sites of action of glucagon and other Ca2+ mobilizing hormones on the malate aspartate cycle

Data from a number of laboratories suggest that the exchange of glutamate for aspartate across the mitochondrial inner membrane is stimulated by glucagon and by Ca2+-mobilizing hormones. The purpose of this study was to determine the site of action of these hormones. Two possibilities were considered and tested. The first hypothesis is that the mitochondrial membrane electrical potential gradient (delta psi m) in the cells is increased by the hormones; and that the putative increase in delta psi m stimulates aspartate efflux. The second possibility is that Ca2+ mediates decreases in cellular levels of alpha-ketoglutarate, secondary to stimulation of alpha-ketoglutarate dehydrogenase, and that the decrease in alpha-ketoglutarate stimulates aspartate production by mitochondria. The effect of glucagon on delta psi m was estimated in intact hepatocytes using the lipophilic cation tetraphenyl phosphonium. No increase in delta psi m was observed due to hormone treatment. On the other hand, alpha-ketoglutarate was found to be an effective competitive inhibitor of aspartate formation via glutamate transamination by isolated liver mitochondria (Ki = 0.55 mM).

Mitochondrial transmembrane potential and pH gradient during anoxia

The effect of anoxia on the mitochondrial transmembrane potential and pH gradient was studied in a preparation of isolated hepatocytes. Transmembrane potential (delta psi) was calculated from the distribution of triphenylmethylphosphonium between the mitochondrial, cytosolic, and extracellular compartments, which were separated by digitonin fractionation and centrifugation. Mitochondrial and cytosolic pH values were calculated from the distribution of the weak acid, dimethadione, which was determined similarly. After 30 min anoxia, the magnitude of mitochondrial delta psi was decreased from -163 to -133 mV and the delta pH (mitochondria vs. cytoplasm) was essentially unchanged (aerobic, 0.78 +/- 0.08; anaerobic, 0.76 +/- 0.11). Thus the protonmotive force (delta p = delta psi-Z delta pH), is largely retained even in the absence of electron flow and ATP synthesis. Inhibitors of the ATP synthase (oligomycin), mitochondrial adenine nucleotide carrier (atractyloside), and glycolytic pathway (2-deoxy-D-glucose) do not affect the ability of the cell to maintain delta psi during anoxia. Therefore, the results indicate that retention of the protonmotive force is not due to utilization of ATP produced by glycolysis and suggest that mechanisms exist to preserve ion distribution during anoxia.

Hepatic metabolism during acute ethanol administration: a phosphorus-31 nuclear magnetic resonance study on the perfused rat liver under normoxic or hypoxic conditions

The effect of ethanol metabolism on the energetic parameters and intracellular pH of the isolated perfused rat liver from fed rats was studied by phosphorus-31 nuclear magnetic resonance spectroscopy. This technique allowed us to analyze nondestructively and in real time the role of low oxygen tension on the possible injurious effect of ethanol on the liver cells. A quantitative analysis of nuclear magnetic resonance data recorded on a perfused rat liver within a 30 mm diameter probe has been performed at 80.9 MHz. Under normoxic and normothermic conditions, the levels of phosphorylated metabolites detected by nuclear magnetic resonance were 2.8, 0.3 and 2 mumoles per gm liver wet weight for ATP, ADP and inorganic orthophosphate, respectively. The cytosolic pH was 7.25 +/- 0.05. During a period of 4 min of hypoxia induced by reducing the perfusion flow rate to 25% of its initial value (i.e., from 12 ml to 3 ml per min per 100 gm body weight), the level of ATP dropped to 2.2 mumoles per gm liver wet weight. Concomitantly, ADP and inorganic orthophosphate increased to 0.6 and 3.3 mumoles per gm liver wet weight. Cytosolic pH fell to 7.02 +/- 0.05. Perfusion of the liver with a Krebs medium containing 70 mM (0.4%) ethanol induced a sharp decrease in intracellular inorganic orthophosphate to reach 1.3 mumole per gm liver wet weight and after a lag time of 4 to 6 min, a decrease in ATP level (2.15 mumoles per gm liver wet weight). A large increase in phosphomonoesters (mainly sn-glycerol 3-phosphate) up to 6 mumoles per gm liver wet weight was also observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial function after acute alteration of the endogenous insulin-to-glucagon ratio

Mannoheptulose (2g/kg i.p.) increases serum glucagon and decreases serum insulin via its effect on pancreatic islet cells. These changes in endogenous hormone status had effects on rat liver mitochondria that were comparable to the effects of injecting porcine glucagon (0.5 mg/kg i.p.). Mitochondrial adenine nucleotide content was increased 38 or 39% by mannoheptulose or glucagon respectively, citrulline synthesis by 165 or 193%, pyruvate carboxylation by 113 or 135%, coupled respiration by 34 or 42%, and uncoupled respiration by 40 or 54%. We conclude that the reciprocal changes in endogenous insulin and glucagon brought about by mannoheptulose offer a useful and interesting alternative to glucagon injection for studying the effects of these pancreatic hormones on liver mitochondria.

Phosphorus-31 nuclear-magnetic-resonance study of phosphorylated metabolites compartmentation, intracellular pH and phosphorylation state during normoxia, hypoxia and ethanol perfusion, in the perfused rat liver

A quantitative analysis of the phosphorus-31 NMR spectra of excised perfused rat liver has been carried out at 80.9 MHz using a 30-mm sample cell. The results indicate that in liver from fed rats, all intracellular ATP is detected by NMR. In contrast, only the cytosolic fractions of Pi and ADP can be observed as indicated by careful analysis of spectra obtained from perchloric acid liver extracts and intact liver under valinomycin perfusion. In well-oxygenated perfused liver the ATP concentration is 7.4 mM. Values of 5.3 mM and 0.9 mM are found respectively for Pi and ADP concentrations in the cytosolic compartment. Cytosolic pH value (pHi) is 7.25 +/- 0.05 and free magnesium concentration 0.5 mM. Addition of 70 mM (0.4%) ethanol to the perfusate of a fed rat liver induces 25% and 38% reduction of ATP and Pi levels, respectively. A large amount of sn-glycerol 3-phosphate is synthesized (up to 11 mM) in the cytosol. After ethanol withdrawal, a large overshoot in cytosolic Pi is observed, which is indicative of a net uptake of Pi across the plasma membrane that occurred during ethanol oxidation. No significant pH variation is observed during ethanol infusion. In perfused liver of rats subjected to 48-h fasts, the concentrations of cytosolic phosphorylated metabolites are 5.3 mM, 0.8 mM and 11.5 mM for ATP, ADP and Pi, respectively. The perfusion of the liver with 70 mM ethanol does not change the adenine nucleotide levels, while the Pi content is decreased by 10%. During a 4-min hypoxia, induced by reducing the perfusion flow rate from 12 ml to 3 ml min-1 (100 g body weight)-1, ATP concentration decreases to 5.8 mM in the fed rat liver. Cytosolic Pi and ADP increase to 8.7 mM and 1.6 mM, respectively. The cytosolic pH evolves to more acidic values and reaches 7.02 +/- 0.05 at the end of the 4-min hypoxic period.

Stimulation of mitochondrial functions by glucagon treatment, starvation and by treatment of isolated mitochondria with glycogen-bound enzymes

Liver mitochondria isolated from rats starved overnight, or fed rats injected with glucagon, exhibited a similar increase of the respiration rate with succinate (by 30-40%) and glutamate plus malate (by 20-30%), as compared to mitochondria from control fed animals. The content of mitochondrial adenine nucleotides was elevated by 30-45% by glucagon treatment or starvation. Mitochondrial respiration and citrulline synthesis were stimulated by 30-40% when mitochondria isolated from fed rats were briefly preincubated with the extract from liver glycogen granules, ATP and MgCl2. This effect was abolished by heating the extract at 100 degrees C.

Increased adenine nucleotides in liver mitochondria after mannoheptulose injection in vivo

In adult rats, mannoheptulose injection causes a transient decrease in the serum insulin-to-glucagon ratio and a concomitant increase in serum glucose concentration. These effects attain a maximum 1 h after the injection and then decline toward normal. Correlated with the hormone changes is a dramatic increase in the adenine nucleotide content (ATP + ADP + AMP) of liver mitochondria, which peaks to over 50% of control values at 1 h. The increase in mitochondrial adenine nucleotides must occur by uptake from the cytosol, because the adenine nucleotide content of the whole tissue remains constant. The accumulation of adenine nucleotides by the mitochondria probably occurs over the recently characterized carboxyatractyloside-insensitive transport pathway that allows exchange of ATP-Mg for Pi. The actual mechanism by which net uptake is regulated after mannoheptulose injection has not yet been elucidated; however, changes in the Km or Vmax of the carrier and an increase in the tissue ATP/ADP ratio were eliminated as possibilities. The increase in matrix adenine nucleotide content in response to hormone changes brought about by mannoheptulose was much greater and more reproducible than what is achieved with glucagon injection. Mannoheptulose treatment may therefore be preferable as a model for further study of hormone effects on mitochondrial function.

Carbon-13 and phosphorus-31 NMR study of hepatic metabolism in the perfused rat liver

Phosphorus-31 nuclear magnetic resonance (NMR) has been used to determine non-invasively absolute concentrations of phosphorylated metabolites in the perfused rat liver. It has been shown that the NMR method does detect cytoplasmic ATP and ADP (ATP:ADP ratio of 15 +/- 3) with no contribution from mitochondrial adenine nucleotides. The concentration of ATP was 7.2 +/- 0.3 mM in the cytosol of well-oxygenated liver, after two hours of perfusion with a Krebs-Ringer buffer. Other phosphorylated metabolites were detected, mainly inorganic phosphate (1.1 mumol/g liver wet weight), phosphorylcholine (1.0 mumol/g wet weight), glycerophosphorylethanolamine (0.34 mumol/g wet weight) and glycerophosphorylcholine (0.30 mumol/g wet weight). The intracellular pH measured from the position of the Pi resonance has a value of 7.2 +/- 0.1. It is likely that the detectable Pi originates from the cytosolic compartment since a pH value of 7.4-7.6 would be expected for the mitochondrial matrix. Natural abundance carbon-13 NMR has also been used to follow the glycogen breakdown in situ by measuring the intensity of the glycogen C-1 resonance in the perfused liver spectrum as a function of the perfusion time. The glycogenolytic process has been studied as a function of the glucose content of the perfusate. Rate of glycogenolysis from 2.7 to 0.16 muEq glycosyl units g wet weight-1 min-1 were found when glucose concentration in the perfusate was varied from 0 to 50 mM. The fate of 90% enriched [2-13C] acetate has been studied in the perfused rat liver by 13C-NMR in order to investigate the mitochondrial metabolism and the interrelations between cytosolic and mitochondrial pools of metabolites. Some compounds of the intermediary metabolism where found to be extensively labelled, e.g. glutamate, glutamine, acetoacetate and beta-hydroxybutyrate. Under our experimental conditions, labelling of glutamate reached a steady-state within 30 min after the onset of perfusion of 20 mM acetate. In addition, the observed incorporation of carbon-13 isotope into glutamine can be linked to the operation of the glutamate-glutamine antiporter and to the high activity of the cytosolic glutamate synthetase. The finding of both active glutaminase and glutamine synthetase activity in the same liver cells is an evidence of the existence of an active glutamine-glutamate futile cycle.

Effects of metabolic acidosis on viability of cells exposed to anoxia

The effects of metabolic acidosis were examined in isolated rat hepatocytes under substrate-free oxygenated or anoxic conditions. Lowering extracellular pH to 6.6 under aerobic conditions had no deleterious effects on the cells as determined by trypan blue exclusion, lactate dehydrogenase (LDH) release, cellular K+ and Ca2+ content, and ability to increase ATP levels after nutrients and adenosine were added to media. Cytosolic pH was measured in aerobic cells at varying extracellular pH using 6-carboxyfluorescein. By using values for cytosolic pH obtained in this manner together with 5,5-dimethyl[2-14C]oxazolidine-2,4-dione (DMO) distribution data, a method was derived for determining intramitochondrial pH. The pH gradient across the mitochondrial membrane was found not to change with a decrease in extracellular pH from 7.4 to 6.9. At pH 6.9 hepatocytes were protected against anoxic injury as compared with cells incubated at pH 7.5 or 6.6. This protection was manifested by a decrease in vital dye uptake and LDH release, maintenance of higher cellular K+ content, less stimulation of respiration with succinate, improved recovery of ATP levels after return to an oxygenated nutrient environment, and maintenance of normal cellular Ca2+ content after reoxygenation. Recovery of cellular ATP content was independent of ATP levels, total adenine nucleotide pool, and energy charge ratio at the end of the anoxic period. Measurement of cytoplasmic pH in anaerobic cells by [14C]DMO distribution showed progressive cellular acidification with lowering of extracellular pH. The protective effects observed at pH 6.9 are not unique to hepatocytes since isolated renal cortical tubules exposed to anoxia have improved ATP levels on reoxygenation at this pH when compared with tubules incubated at pH 7.5.