Subcellular distribution of phosphagens in isolated perfused rat heart

Integration of Eukaryotic Energy Metabolism: The Intramitochondrial and Cytosolic Energy States ([ATP]f/[ADP]f[Pi])

Maintaining a robust, stable source of energy for doing chemical and physical work is essential to all living organisms. In eukaryotes, metabolic energy (ATP) production and consumption occurs in two separate compartments, the mitochondrial matrix and the cytosol. As a result, understanding eukaryotic metabolism requires knowledge of energy metabolism in each compartment and how metabolism in the two compartments is coordinated. Central to energy metabolism is the adenylate energy state ([ATP]/[ADP][Pi]). ATP is synthesized by oxidative phosphorylation (mitochondrial matrix) and glycolysis (cytosol) and each compartment provides the energy to do physical work and to drive energetically unfavorable chemical syntheses. The energy state in the cytoplasmic compartment has been established by analysis of near equilibrium metabolic reactions localized in that compartment. In the present paper, analysis is presented for energy-dependent reactions localized in the mitochondrial matrix using data obtained from both isolated mitochondria and intact tissues. It is concluded that the energy state ([ATP]_f/[ADP]_f[Pi]) in the mitochondrial matrix, calculated from the free (unbound) concentrations, is not different from the energy state in the cytoplasm. Corollaries are: (1) ADP in both the cytosol and matrix is selectively bound and the free concentrations are much lower than the total measured concentrations; and (2) under physiological conditions, the adenylate energy states in the mitochondrial matrix and cytoplasm are not substantially different.

A 20/20 view of ANT function in mitochondrial biology and necrotic cell death

The adenosine nucleotide translocase (ANT) family of proteins are inner mitochondrial membrane proteins involved in energy homeostasis and cell death. The primary function of ANT proteins is to exchange cytosolic ADP with matrix ATP, facilitating the export of newly synthesized ATP to the cell while providing new ADP substrate to the mitochondria. As such, the ANT proteins are central to maintaining energy homeostasis in all eukaryotic cells. Evidence also suggests that the ANTs constitute a pore-forming component of the mitochondrial permeability transition pore (MPTP), a structure that forms in the inner mitochondrial membrane that is thought to underlie regulated necrotic cell death. Additionally, emerging studies suggest that ANT proteins are also critical for mitochondrial uncoupling and for promoting mitophagy. Thus, the ANTs are multifunctional proteins that are poised to participate in several aspects of mitochondrial biology and the greater regulation of cell death, which will be discussed here.

Cardiovascular applications of hyperpolarized contrast media and metabolic tracers

Modern hyperpolarization technology enhances the recordable magnetic resonance signal four to five orders of magnitude, making in vivo assessments of tracer pathways and metabolic compartments feasible. Existing hyperpolarization instrumentation and previous tracer studies using hydroxyethylpropionate (HEP) as an extracellular marker and 14-carbon label pyruvate as examples are described and reviewed as applicable to the working heart. Future metabolic imaging based on the use of hyperpolarized pyruvate needs to consider extra- and intra-cellular label dilution due to glycolysis, lactate oxidation and protein degradation. This dilution can substantially decrease the recordable signals from PDH flux (oxidative decarboxylation of pyruvate) and other pyruvate pathways. The review of previous literature and data suggests that the (13)C-alanine signal is a better index of mitochondrially oxidized pyruvate than L-lactate. These facts and considerations will help in the interpretation of the in vivo recorded hyperpolarization signals of metabolic tracers and contrast media.

The ADP and ATP transport in mitochondria and its carrier

Different from some more specialised short reviews, here a general although not encyclopaedic survey of the function, metabolic role, structure and mechanism of the ADP/ATP transport in mitochondria is presented. The obvious need for an "old fashioned" review comes from the gateway role in metabolism of the ATP transfer to the cytosol from mitochondria. Amidst the labours, 40 or more years ago, of unravelling the role of mitochondrial compartments and of the two membranes, the sequence of steps of how ATP arrives in the cytosol became a major issue. When the dust settled, a picture emerged where ATP is exported across the inner membrane in a 1:1 exchange against ADP and where the selection of ATP versus ADP is controlled by the high membrane potential at the inner membrane, thus uplifting the free energy of ATP in the cytosol over the mitochondrial matrix. Thus the disparate energy and redox states of the two major compartments are bridged by two membrane potential responsive carriers to enable their symbiosis in the eukaryotic cell. The advance to the molecular level by studying the binding of nucleotides and inhibitors was facilitated by the high level of carrier (AAC) binding sites in the mitochondrial membrane. A striking flexibility of nucleotide binding uncovered the reorientation of carrier sites between outer and inner face, assisted by the side specific high affinity inhibitors. The evidence of a single carrier site versus separate sites for substrate and inhibitors was expounded. In an ideal setting principles of transport catalysis were elucidated. The isolation of intact AAC as a first for any transporter enabled the reconstitution of transport for unravelling, independently of mitochondrial complications, the factors controlling the ADP/ATP exchange. Electrical currents measured with the reconstituted AAC demonstrated electrogenic translocation and charge shift of reorienting carrier sites. Aberrant or vital para-functions of AAC in basal uncoupling and in the mitochondrial pore transition were demonstrated in mitochondria and by patch clamp with reconstituted AAC. The first amino acid sequence of AAC and of any eukaryotic carrier furnished a 6-transmembrane helix folding model, and was the basis for mapping the structure by access studies with various probes, and for demonstrating the strong conformation changes demanded by the reorientation mechanism. Mutations served to elucidate the function of residues, including the particular sensitivity of ATP versus ADP transport to deletion of critical positive charge in AAC. After resisting for decades, at last the atomic crystal structure of the stabilised CAT-AAC complex emerged supporting the predicted principle fold of the AAC but showing unexpected features relevant to mechanism. Being a snapshot of an extreme abortive "c-state" the actual mechanism still remains a conjecture.

Oxygen reperfusion is limited in the postischemic hypertrophic myocardium

Studies have shown that hypertrophied hearts are unusually vulnerable to ischemia. Compromised O2supply has been postulated as a possible explanation for this phenomenon on the basis of elongated O2diffusion distance and altered coronary vasculature found in hypertrophied myocardium. To examine the postulate, perfused heart experiments followed the metabolic and functional responses of hypertrophic myocardium to ischemia.1H/31P NMR was used to measure cellular oxygenation and energy level during ischemia-reperfusion. The left ventricles from spontaneously hypertensive rats (SHR) were enlarged by 48%. With this moderate degree of hypertrophy, cellular O2and energy levels were normal during baseline perfusion. After an ischemic episode, however, cellular O2was severely deprived in the SHR hearts compared with the normal hearts. Depressed postischemic O2reperfusion correlated well with depressed energetic and functional recovery. The results from the current study thus demonstrate a critical relationship between reperfused O2level and functional recovery in hypertrophic myocardium. The role of reperfused O2, however, is time dependent. During early reperfusion, factor(s) other than O2appear to limit functional recovery. It is when the mechanical function of the heart approaches a new steady state that O2becomes a dominant factor. Meanwhile, the finding of a normal O2level in preischemic SHR hearts defies the notion of preexisting hypoxia as a primer of ischemic damage.

Regulation of oxidative phosphorylation in intact mammalian heart in vivo

A dynamic computer model of oxidative phosphorylation in intact heart was developed by modifying the model of oxidative phosphorylation in intact skeletal muscle published previously. Next, this model was used for theoretical studies on the regulation of oxidative phosphorylation in intact heart in vivo during transition between different work intensities. It is shown that neither a direct activation of ATP usage alone nor a direct activation of both ATP usage and substrate dehydrogenation, including the calcium-activated tricarboxylate acid cycle dehydrogenases, can account for the constancy of [ADP], [PCr], [P(i)] and [NADH] during a significant increase in oxygen consumption and ATP turnover encountered in intact heart in vivo. Only a direct activation of all oxidative phosphorylation complexes in parallel with a stimulation of ATP usage and substrate dehydrogenation enabled to reproduce the experimental data concerning the constancy of metabolite concentrations. The molecular background of the differences between heart and skeletal muscle in the kinetic behaviour of the oxidative phosphorylation system is also discussed.

Monitoring cytotoxic tumour treatment response by diffusion magnetic resonance imaging and proton spectroscopy

Exposure of tumours to anti-cancer drugs, gene or radiation therapy consistently leads to an increase in water diffusion in the cases expressing favourable treatment response. The diffusion change coincides cytotoxic cell eradication and precedes volume reduction in drug or gene therapy-treated experimental tumours. Interestingly, the recent studies from human brain tumour patients undergoing chemotherapy show similar behaviour of diffusion, suggesting important application for MRI in patient management. In this review observations from diffusion MRI and MRS in the tumours during cytotoxic treatment are summarized and the cellular mechanisms affecting molecular mobility are discussed in the light of tissue microenvironmental and microdynamic changes.

Macrocompartmentation of total creatine in cardiomyocytes revisited

Distribution of total creatine (free creatine + phosphocreatine) between two subcellular macrocompartments--mitochondrial matrix space and cytoplasm--in heart and skeletal muscle cells was reinvestigated by using a permeabilized cell technique. Isolated cardiomyocytes were treated with saponin (50 microg/ml for 30 min or 600 microg/ml for 1 min) to open the outer cellular membrane and release the metabolites from cytoplasm (cytoplasmic fraction, CF). All mitochondrial population in permeabilized cells remained intact: the outer membrane was impermeable for exogenous cytochrome c, the acceptor control index of respiration exceeded 10, the mitochondrial creatine kinase reaction was fully coupled to the adenine nucleotide translocator. Metabolites were released from mitochondrial fraction (MF) by 2-5% Triton X100. Total cellular pool of free creatine + phosphocreatine (69.6 +/- 2.1 nmoles per mg of protein) was found exclusively in CF and was practically absent in MF. When fibers were prepared from perfused rat hearts, cellular distribution of creatine was not dependent on functional state of the heart and only slightly modified by ischemia. It is concluded that there is no stable pool of creatine or phosphocreatine in the mitochondrial matrix in the intact muscle cells, and the total creatine pool is localized in only one macrocompartment--cytoplasm.

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.

Cardiac creatine kinase metabolite compartments revealed by NMR magnetization transfer spectroscopy and subcellular fractionation

In the perfused rat heart NMR inversion transfer revealed the existence of a compartment of ATP not exchanging through creatine kinase (CK), as demonstrated by an apparent discrepancy between the forward (F(f)) and reverse (F(r)) CK flux if this compartment was neglected in the analysis [Joubert et al. (2000) Biophys. J. 79, 1-13]. To localize this compartment, CK fluxes were measured by inversion of PCr (inv-PCr) or gamma ATP (inv-ATP), and the distribution of metabolites between mitochondria and cytosol was studied by subcellular fractionation. Physiological conditions were designed to modify the concentration and distribution of CK metabolites (control, adenylate depletion, inhibition of respiration, KCl arrest). Depending on cardiac activity, mitochondrial ATP (mito-ATP) assessed by fractionation varied from 11% to 30% of total ATP. In addition, the apparent flux discrepancy increased together with mito-ATP (F(f)/F(r) ranged from 0.85 to 0.50 in inv-PCr and from 1.13 to 1.88 in inv-ATP). Under conditions masking the influence of the ATP-P(i) exchange on CK flux, the ATP compartment could be directly quantified by the apparent flux discrepancy; its size was similar to that of mito-ATP measured by fractionation. Thus NMR inversion technique is a potential tool to assess metabolite compartmentation in the whole organ.

Differences in nucleotide compartmentation and energy state in isolated and in situ rat heart: assessment by 31P-NMR spectroscopy

Free cytosolic concentrations of ATP, PCr, ADP and 5'-AMP, and the cytosolic [ATP]/[ADP].[Pi] ratio, were determined in isolated and in situ rat hearts using 31P-NMR spectroscopy. Total tissue metabolite concentrations were determined by HPLC analysis of freeze-clamped, perchloric acid-extracted tissue. In in situ myocardium the PCr/ATP ratio was 2.7 +/- 0.2 determined from 31P-NMR data (using either PCr/beta-NTP or PCr/gamma-NTP), and 1.9 +/- 0.1 (P < 0.01) determined from total tissue concentrations. 31P-NMR-determined and total tissue [PCr] were in excellent agreement (49.6 +/- 8.4 and 49.5 +/- 1.0 mumol.g-1 dry wt, respectively), whereas 31P-NMR-determined [ATP] (18.6 +/- 3.2 mumol.g-1 dry wt) was only 71% of the total tissue concentration (26.1 +/- 1.7 mumol.g-1 dry wt, P < 0.01). Isolation and Langendorff perfusion of rat hearts with glucose as substrate reduced total tissue [ATP] and [PCr] and the 31P-NMR-determined PCr/ATP ratio fell to 1.5 +/- 0.1. This value agreed well with the total tissue ratio of 1.4 +/- 0.1, and there was excellent agreement between 31P-NMR-determined and total tissue [PCr] and [ATP] values in the perfused heart. Addition of pyruvate to perfusate increased the 31P-NMR-determined PCr/ATP ratio to 1.7 +/- 0.1 due to elevated [PCr], and there remained excellent agreement between NMR-determined and total tissue [PCr] and [ATP] values. Free cytosolic [ADP] (from the creatine kinase equilibrium) was 5% of total tissue ADP, and free cytosolic [5'-AMP] (from the adenylate kinase equilibrium) ranged from 0.2-0.3% of total tissue 5'-AMP. Bioenergetic state, indexed by [ATP]/[ADP].[Pi], was much lower in isolated perfused hearts (30 mM-1) vs. in situ myocardium (approximately 150 mM-1). In summary, we observe a substantial disproportionality between total tissue PCr/ATP and 31P-NMR-determined PCr/ATP in highly energised in situ myocardium but not in isolated perfused hearts. This appears due to an NMR invisible ATP compartment approximating 29% of total tissue ATP in situ. Additionally, more than 95% of ADP and more than 99% of 5'-AMP exist in bound forms in perfused and in situ myocardium. The physiological significance of these observations is unclear. However, substantial differences between 31P-NMR visible and total tissue [ATP] introduces significant errors in conventional estimation of free cytosolic [ADP], [5'-AMP] and [ATP]/[ADP].[Pi] from in vivo 31P-NMR data.

Crucial role of intracellular effectors on glycogenolysis in the isolated rat heart: potential consequences on the myocardial tolerance to ischemia

The role played by glycogenolysis in the ischemic heart has been recently put into question because it is suspected that a slowing down of this process could be beneficial for the tolerance of the myocardium to ischemia. The role of the intracellular effectors that control the rate of glycogenolysis has therefore regained interest. We aimed to understand the role played by those intracellular effectors which are directly related to the energy balance of the heart. To this end, we review some of the previously published data on this subject and we present new data obtained from P-31 and C-13 NMR spectroscopic measurement on isolated rat heart. Two conditions of ischemia were studied: 15 min global no-flow and 25 min low-flow ischemia. The hearts were isolated either from control animals or from rats pre-treated with isoproterenol (5 mg.kg-1 b.w. i.p.) 1 h before the perfusion in order to C-13 label glycogen stores. Our main results are as follows: (1) the biochemically determined glycogenolysis rate during the early phase of ischemia (up to 10-15 min) was larger in no-flow ischemia than in low-flow conditions for both groups, (2) direct measurement of the glycogenolysis rate, as determined by C-13 NMR, after labelling of the glycogen pool in the hearts from isoproterenol-treated rats, confirms the estimations from the biochemical data, (3) glycogenolysis was slower in the hearts from pre-treated animals than in control hearts for both conditions of ischemia, (4) the total activity of glycogen phosphorylase (a + b) increased, by 50%, after 5 min no-flow ischemia, whereas it decreased by 42% after the same time of low-flow ischemia. However, the ratio phosphorylase a/a + b was not altered, whatever the conditions, (5) the concentration of inorganic phosphate (Pi) increased sharply during the first minutes of ischemia, to values above 8-10 mM, under all conditions studied. The rate of increase was larger during no-flow ischemia than during low-flow ischemia. The concentration of Pi was thereafter higher in controls than in the hearts from isoproterenol-treated animals. The calculated cytosolic concentration of free 5'AMP increased sharply at the onset on ischemia, reaching in a few minutes values above 30 microM in controls and significantly lower values around 15 microM, in the hearts from isoproterenol-treated rats. (6) The hearts from isoproterenol-treated rats displayed a reduced intracellular acidosis, when compared to controls, under both conditions of ischemia. We conclude that the intracellular effectors, mainly free AMP, play an essential role in the control of glycogenolysis via allosteric control of phosphorylase b activity. The alteration in the concentration of free Pi, the substrate of both forms of phosphorylase, can be considered as determinant in the control of the rate of glycogenolysis. The attenuation of ischemia-induced intracellular acidosis in the hearts from isoproterenol-treated rats could be a consequence of a reduced glycogenolytic rate and is likely to be related to a better resumption of the mechanical function on reperfusion.

Increase of blood NAD+ and attenuation of lactacidemia during nicotinamide treatment of a patient with the MELAS syndrome

Decreased activity of complex I (NAD:ubiquinone oxidoreductase) is the most frequent biochemical finding associated with mutation at the base pair 3243 of the mitochondrial DNA. The mutation has been previously shown to lead to a defective translation. We hypothesized that due to an imperfect assembly of complex I subunits the substrate affinity of this enzyme may be lowered and this may be counteracted by increasing the mitochondrial NAD+NADH concentration. Therefore, we studied the effect and mechanism of action of nicotinamide treatment in a MELAS patient with the base pair 3243 mutation. Nicotinamide treatment was initiated after his first stroke-like episode. The blood NAD concentration (representing the intracellular concentration in erythrocytes) increased linearly being 24-fold at 6 weeks of treatment. Blood lactate and pyruvate concentration decreased by 50% within three days and 24 h urine lactate content within 2 weeks and we observed a clinical improvement together with a decrease in the lesion volume in magnetic resonance imaging within the first month. The cellular NAD increase upon nicotinamide administration was probably universal, because it occurred in a time and dose-dependent manner in cultured fibroblasts from both the patient and the controls. Alleviation of the lactate accumulation during the nicotinamide treatment suggests that an increase in the cellular NAD+NADH concentration leads to enhancement of the oxidation of reducing equivalents. However, the Km of complex I for NADH in skeletal muscle from the patient was similar to that of controls. This may indicate that physiologically mitochondrial complex I operates at non-saturating substrate concentration, and this may explain the effect of nicotinamide treatment.

Regulation of energy metabolism in liver

Energy metabolism in liver has to cope with the special tasks of this organ in intermediary metabolism. Main ATP-generating processes in the liver cell are the respiratory chain and glycolysis, whereas main ATP-consuming processes are gluconeogenesis, urea synthesis, protein synthesis, ATPases and mitochondrial proton leak. Mitochondrial respiratory chain in the intact liver cell is subject to control mainly by substrate (hydrogen donors, ADP, oxygen) transport and supply and proton leak/slip. Whereas hormonal control is mainly on substrate supply to mitochondria, proton leak/slip is supposed to play an important role in the modulation of the efficiency of oxidative phosphorylation.

The content of glutathione and glutathione S-transferases and the glutathione peroxidase activity in rat liver nuclei determined by a non-aqueous technique of cell fractionation

Hepatocellular nuclei require glutathione, glutathione S-transferases (GSTs) and Se-dependent glutathione peroxidase (GPx) for intranuclear protection against damage from electrophiles or products of active oxygen. Data so far available from the literature on nuclei isolated in aqueous systems range from glutathione, GSTs and GPx either being absent altogether to being present in quantities in excess of those in the cytoplasm. This paper describes a small-scale preparation of a nuclear fraction from rat liver by a non-aqueous technique, designed to retain nuclear water-soluble molecules in situ, since low-molecular-mass compounds can diffuse freely into other compartments during aqueous separation. This non-aqueous procedure shows the nucleus to contain glutathione at 8.4 mM and soluble GSTs at 38 micrograms/mg of protein, the enrichment over the homogenate being 1.2-1.4-fold. Se-dependent GPx activity was also present in the nucleus (56 m-units/mg), although with slightly lower activity than in the homogenate (0.7-fold).

Antibody-mediated imbalance of myocardial energy metabolism. A causal factor of cardiac failure?

The ADP-ATP carrier of the inner mitochondrial membrane is an autoantigen in myocarditis and dilated cardiomyopathy. Sera of patients with these diseases contain carrier-specific autoantibodies that inhibit the transmembrane nucleotide transport on isolated mitochondria. Guinea pigs immunized with the isolated ADP-ATP carrier protein also generate specific carrier-inactivating antibodies. In this study, we measured the cardiac function of guinea pigs immunized with the ADP-ATP carrier by determining the external heart work (EHW) of their isolated perfused spontaneously beating hearts stimulated by 4.0 mmol/L calcium and aortic ligature. Further, the electrogenic transport activity of the ADP-ATP carrier was estimated by calculating the cytosolic-mitochondrial difference of the phosphorylation potential of ATP [delta G(cyt-mit)] in the freeze-clamped isolated hearts by nonaqueous fractionation. The EHW of immunized guinea pigs was seen to be reduced by 54% (P < .005) compared with nonimmunized control guinea pigs, and delta G(cyt-mit) declined from 4.9 kJ/mol ATP in nonimmunized control hearts to 2.3 kJ/mol ATP in the hearts of the immunized guinea pigs (P < .005). The decisive result of this study, however, is the close relation observed between the magnitude of reduction of delta G(cyt-mit) and the size of the decrease in EHW (r = .87). Therefore, it seems plausible that antibody-mediated carrier dysfunction (creating the observed imbalance in myocardial energy metabolism) is responsible for the impairment of cardiac function. Our data support the hypothesis that immunopathic mechanisms in myocarditis and dilated cardiomyopathy can trigger subsequent heart failure.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic compartmentation and substrate channelling in muscle cells. Role of coupled creatine kinases in in vivo regulation of cellular respiration--a synthesis

The published experimental data and existing concepts of cellular regulation of respiration are analyzed. Conventional, simplified considerations of regulatory mechanism by cytoplasmic ADP according to Michaelis-Menten kinetics or by derived parameters such as phosphate potential etc. do not explain relationships between oxygen consumption, workload and metabolic state of the cell. On the other hand, there are abundant data in literature showing microheterogeneity of cytoplasmic space in muscle cells, in particular with respect to ATP (and ADP) due to the structural organization of cell interior, existence of multienzyme complexes and structured water phase. Also very recent experimental data show that the intracellular diffusion of ADP is retarded in cardiomyocytes because of very low permeability of the mitochondrial outer membrane for adenine nucleotides in vivo. Most probably, permeability of the outer mitochondrial membrane porin channels is controlled in the cells in vivo by some intracellular factors which may be connected to cytoskeleton and lost during mitochondrial isolation. All these numerous data show convincingly that cellular metabolism cannot be understood if cell interior is considered as homogenous solution, and it is necessary to use the theories of organized metabolic systems and substrate-product channelling in multienzyme systems to understand metabolic regulation of respiration. One of these systems is the creatine kinase system, which channels high energy phosphates from mitochondria to sites of energy utilization. It is proposed that in muscle cells feed-back signal between contraction and mitochondrial respiration may be conducted by metabolic wave (propagation of oscillations of local concentration of ADP and creatine) through cytoplasmic equilibrium creatine and adenylate kinases and is amplified by coupled creatine kinase reaction in mitochondria. Mitochondrial creatine kinase has experimentally been shown to be a powerful amplifier of regulatory action of weak ADP fluxes due to its coupling to adenine nucleotide translocase. This phenomenon is also carefully analyzed.

Use of cytosolic metabolite patterns to estimate free magnesium in normoxic myocardium

Cytosolic free magnesium (Mgf) is considered relatively constant. To test this concept, Mgf was estimated during hyperkalemic ventricular akinesis, normal and maximum adrenergic stimulation, and sulfate loading of the normoxic perfused guinea-pig heart. The Mgf estimates utilized a new sliding scale derived from the Mg(2+)-dependence of glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase (GAPDH/PGK). The pseudo constant K'GAPDH.K'PGK was measured as ([creatine phosphate][3-phosphoglycerate][lactate]KLDH)/([creatine][Pi] [glyceraldehyde 3-phosphate][pyruvate]KCK), which varied with magnesium due to KCK (CK, LDH = creatine kinase, lactate dehydrogenase). However, the correct magnesium dependencies of the true constants KGAPDH.KPGK and KCK were taken from the literature. The [Mg2+] at which pseudo K'GAPDH.K'PGK equalled true KGAPDH.KPGK was the best estimate of Mgf.Mgf fell to approximately 0.13 mM in hyperkalemic arrest from a control of approximately 0.6 mM, rising to approximately 0.85 mM only during maximum adrenergic stress. Mgf increased further to approximately 1.3 mM during sulfate loading which induced ATP catabolism. Mgf and ATP were reciprocally related. Thus; (1) myocardial free [Mg2+] judged from GADPH/PGK mass-action relations changed appreciably only under extreme physiological states; (2) ATP was a major chelator of Mg2+ in perfused myocardium, i.e., acute ATP pool size reduction may be associated with increments in Mgf.

The role of the mitochondrial creatine kinase system for myocardial function during ischemia and reperfusion

The subcellular distribution of ATP, ADP, creatine phosphate and creatine was studied in normoxic control, isoprenaline-stimulated and potassium-arrested guinea-pig hearts as well as during ischemia and after reperfusion. The mitochondrial creatine phosphate/creatine ratio was closely correlated to the oxidative activity of the hearts. This was interpreted as an indication of a close coupling of mitochondrial creatine kinase to oxidative phosphorylation. To further investigate the functional coupling of mitochondrial creatine kinase to oxidative phosphorylation, rat or guinea-pig heart mitochondria were isolated and the mass action ratio of creatine kinase determined at active or inhibited oxidative phosphorylation or in the presence of high phosphate, conditions which are known to change the functional state of the mitochondrial enzyme. At active oxidative phosphorylation the mass action ratio was one-third of the equilibrium value whereas at inhibited oxidative phosphorylation (N2, oligomycin, carboxyatractyloside) or in the presence of high phosphate, the mass action ratio reached equilibrium values. These findings show that oxidative phosphorylation is essential for the regulation of the functional state of mitochondrial creatine kinase. The functional coupling of the mitochondrial creatine kinase and oxidative phosphorylation indicated from the correlation of mitochondrial creatine phosphate/creatine ratios with the oxidative activity of the heart in situ as well as from the deviation of the mass action ratio of the mitochondrial enzyme from creatine kinase equilibrium at active oxidative phosphorylation in isolated mitochondria is in accordance with the proposed operation of a creatine shuttle in heart tissue.

5'-Nucleotidase activity and adenosine production in rat liver mitochondria

The controversial subject of mitochondrial 5'-nucleotidase in the liver was studied employing density gradient fractionation combined with a method for analyzing the distribution profiles of marker enzymes based on multiple regression analysis. Triton WR-1339 was used to improve the separation of mitochondria from lysosomes by the gradient centrifugation technique. Adenosine production was examined further using acetate to increase intramitochondrial AMP, and thus adenosine production, in incubations with gradient centrifugation-purified mitochondria. Distribution analysis of the crude homogenate showed that 5'-nucleotidase activity exists in the mitochondrial fraction. To increase the resolution of this approach with respect to mitochondria, a crude mitochondrial fraction was also studied. In this case the relative mitochondrial activity decreased but 5'-nucleotidase activity was still clearly detectable. The mitochondrial 5'-nucleotidase exhibited a Km of 94 microM and a Vmax of 31 nmol/min per mg protein for AMP. The kinetic data for the Mg2+, ATP, ADP and AOPCP sensitivity of the enzyme showed that it differs from the plasma membrane, lysosome and cytosol 5'-nucleotidases. AOPCP was only a moderate inhibitor, and ATP was a more potent inhibitor than ADP at a 1 mM concentration. The enzyme also showed a requirement of Mg2+. Acetate caused the conversion of intramitochondrial adenylates to AMP and the formation of adenosine. Adenosine concentration increased in the extramitochondrial space in a time-dependent manner, but only trace amounts of nucleotides were detected. The data show that 5'-nucleotidase activity producing adenosine exists in rat liver mitochondria and a concentration-dependent adenosine output from mitochondria by diffusion or facilitated diffusion is also suggested.

Coenzyme A sequestration in rat hearts oxidizing ketone bodies

Previous studies have indicated that ketone body-mediated contractile failure in rat hearts is due to inhibition of 2-oxoglutarate dehydrogenase, and it has been speculated that this inhibition is due to the sequestration of intramitochondrial CoA as acetoacetyl-CoA and acetyl-CoA. These studies were performed to determine whether oxidation of acetoacetate by isolated rat heart mitochondria results in a fall in intramitochondrial nonesterified CoA [CoASH] and whether increasing the available CoA improves contractile performance in hearts oxidizing acetoacetate. The oxidation of acetoacetate by isolated rat heart mitochondria resulted in depressed state 3 respiration as well as in a decrease in [CoASH]. Increasing the tissue content of CoASH in perfused hearts by providing the precursors for CoA relieved inhibition of 2-oxoglutarate dehydrogenase and improved the contractile performance of isolated working hearts. In contrast, the addition of carnitine increased the tissue content of CoASH but did not improve function. These findings suggest the presence of two different pools of CoASH. We conclude that ketone body-mediated inhibition of 2-oxoglutarate dehydrogenase is due to decreased intramitochondrial CoASH and that this inhibition of the citric acid cycle is a plausible mechanism for concomitant contractile failure.

Combined glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase in catecholamine-stimulated guinea-pig cardiac muscle. Comparison with mass-action ratio of creatine kinase

The steady-state reactant levels of triose-phosphate isomerase and the glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase system were examined in guinea-pig cardiac muscle. Key glycolytic intermediates, including glyceraldehyde 3-phosphate were directly measured and compared with those of creatine kinase. Non-working Langendorff hearts as well as isolated working hearts were perfused with 5 mM glucose (plus insulin) under normoxia conditions to maintain lactate dehydrogenase near-equilibrium. The cytosolic phosphorylation potential ([ATP]/([ADP].[Pi])) was derived from creatine kinase and the free [NAD+]/([NADH].[H+]) ratio from lactate dehydrogenase. In Langendorff hearts glycolysis was varied from near-zero flux (hyperkalemic cardiac arrest) to higher than normal flux (normal and maximum catecholamine stimulation). The triose-phosphate isomerase was near-equilibrium only in control or potassium-arrested Langendorff hearts as well as in postischemic 'stunned' hearts. However, when glycolytic flux increased due to norepinephrine or due to physiological pressure-volume work the enzyme was displaced from equilibrium. The alternative phosphorylation ratio [ATP]'/([ADP]).[Pi]) was derived from the magnesium-dependent glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase system assigning free magnesium different values in the physiological range (0.1-2.0 mM). As predicted, [ATP]/([ADP].[Pi]) and [ATP]'/([ADP]'.[Pi]') were in excellent agreement when glycolysis was virtually halted by hyperkalemic arrest (flux approximately 0.2 mumol C3.min-1.g dry mass-1). However, the equality between the two phosphorylation ratios was not abolished upon resumption of spontaneous beating and also not during adrenergic stimulation (flux approximately 5-14 mumol C3.min-1.g dry mass-1). In contrast, when flux increased due to transition from no-work to physiological pressure-volume work (rate increase from approximately 3 to 11 mumol C3.min-1.g dry mass-1), the two ratios were markedly different indicating disequilibrium of the glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase. Only during adrenergic stimulation or postischemic myocardial 'stunning', not due to hydraulic work load per se, glyceraldehyde-3-phosphate levels increased from about 4 microM to greater than or equal to 16 microM. Thus the guinea-pig cardiac glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase system can realize the potential for near-equilibrium catalysis at significant flux provided glyceraldehyde-3-phosphate levels rise, e.g., due to 'stunning' or adrenergic hormones.

Differential extractability of creatine phosphate and ATP from cardiac muscle with ethanol and perchloric acid solution

To compare the extractability of creatine phosphate with that of ATP by alcohol extraction, both compounds were extracted from normal perfused rat heart tissues by using various stepwise concentrations of ethanol and 0.4 M HClO4. Powdered samples (6-15 mg wet wt) from the freeze-clamped tissues were homogenized in 2 ml of the ethanol solutions. After centrifugation, the supernatant was removed; each centrifuged sediment was rehomogenized with 2 ml of 0.4 M HClO4 and centrifuged. The supernatant was neutralized with 0.4 m KHCO3. The same powdered samples were directly homogenized with 2 ml of 0.4 M HClO4 and treated in the same manner. Only a small amount of ATP in the tissues was extracted by an 85% or higher concentration of ethanol. Further, about 13% of the tissue ATP was not extractable by the subsequent perchloric acid extraction. In contrast to ATP, creatine phosphate in the tissues was partially extracted by 95% ethanol and nearly all of the tissue creatine phosphate was extracted by 70% ethanol. The total creatine phosphate obtained by 70% ethanol and by subsequent perchloric acid extraction was significantly higher than that obtained by direct perchloric acid extraction. From these results, it was concluded that the extractability of creatine phosphate in the tissue by alcohol extraction is clearly different from that of ATP. Additionally, the stepwise extraction is recommended as a useful method for the extraction of energy metabolites in perfused rat heart tissue.

Subcellular distribution of energy metabolites in the pre-ischaemic and post-ischaemic perfused working rat heart

Isolated working rat hearts were subjected to 20 min of global ischaemia and either 5 min or 15 min of reperfusion. The subcellular distribution of ATP, ADP, AMP, phosphocreatine and Pi were determined before and after ischaemia by the method of non-aqueous tissue fractionation. Ventricular function and the cytosolic, mitochondrial and ATPase-associated compartmentation of metabolites were measured. After 5 min of reperfusion, only 13 +/- 9% of the pre-ischaemic contractile function was restored compared to 67 +/- 8% after 15 min reperfusion. ATP was reduced in all cellular compartments after 5 min of reperfusion but was only decreased from pre-ischaemic values in the cytosolic compartment after 15 min of reperfusion (17.1 +/- 3.9 nmol/mg vs. 4.3 +/- 1.5 nmol/mg total protein; P less than 0.05). The mitochondrial [ATP]/[ADP] was reduced from a normal value of 4.36 to 1.79 after 5 min but recovered to 4.62 after 15 min of reperfusion. Most of the Pi was located in the mitochondria or with the ATPase fraction of the cell, with only 16% of the total Pi free in the cytosol. This study indicates that the capacity of the heart to recover function may be compromised during early reperfusion by a 59% increase in mitochondrial phosphate content and during late reperfusion by a reduced cytosolic/mitochondrial concentration ratio of both ATP (from 0.85 to 0.19) and phosphocreatine (from 3.9 to 1.24).

31P NMR measurements of the ADP concentration in yeast cells genetically modified to express creatine kinase

Rabbit muscle creatine kinase has been introduced into the yeast Saccharomyces cerevisiae by transforming cells with a multicopy plasmid containing the coding sequence for the enzyme under the control of the yeast phosphoglycerate kinase promoter. The transformed cells showed creatine kinase activities similar to those found in mammalian heart muscle. 31P NMR measurements of the near-equilibrium concentrations of phosphocreatine and cellular pH together with measurements of the total extractable concentrations of phosphocreatine and creatine allowed calculation of the free ADP/ATP ratio in the cell. The calculated ratio of approximately 2 was considerably higher than the ratio of between 0.06 and 0.1 measured directly in cell extracts.

Adenine nucleotide transport and adenosine production in isolated rat heart mitochondria during acetate metabolism

In view of its vasodilatory effect on the coronary circulation (probably mediated by adenosine) and its metabolic compartmentalization (intramitochondrial activation to form acetyl-CoA), the metabolic effects of acetate were studied in isolated rat heart mitochondria. Acetate caused conversion of adenylates to AMP and the formation of adenosine. Adenylate efflux was inhibited by carboxyatractyloside but not by N-ethylmaleimide. The intramitochondrial accumulation of AMP was enhanced by carboxyatractyloside during acetate metabolism and the formation of extramitochondrial adenosine inhibited. A carboxyatractyloside-sensitive unidirectional AMP influx with a Km of 50 microM and Vmax of 11 nmol/min per mg mitochondrial protein was also observed. The mitochondrial adenosine content was high and constant during the experiments. The steep apparent concentration gradient of adenosine indicates that most of the mitochondrial adenosine is tightly bound to protein. Adenosine formation was proportional to the extramitochondrial AMP concentration, showing that the 5'-nucleotidase activity of cardiac mitochondrial preparations is extramitochondrial in origin. The data suggest that the mitochondrial ATP/ADP carrier is capable of transporting AMP and that intramitochondrial AMP is recycled during acetate metabolism in the myocardium partially by means of the ATP/ADP translocator, leading to an increase in extramitochondrial AMP and adenosine formation.

Antibodies to the ADP/ATP carrier, an autoantigen in myocarditis and dilated cardiomyopathy, penetrate into myocardial cells and disturb energy metabolism in vivo

We identified the ADP/ATP carrier, located within the inner mitochondrial membrane, to be an organ- and conformation-specific autoantigen in myocarditis and dilated cardiomyopathy. We also showed that autoantibodies to the ADP/ATP carrier inhibit the nucleotide transport in vitro. Specific binding of the autoantibodies to the carrier was demonstrated by radioimmunoassay and the immunoblot technique; the inhibition of the nucleotide transport was determined by the inhibitor stop method. To establish if these autoantibodies might also affect cardiac energy metabolism in vivo, we measured whether they are capable of penetrating into myocytes and whether subcellular ATP/ADP ratios and phosphorylation potentials of ATP change in hearts of guinea pigs that have been immunized with the isolated ADP/ATP carrier. An intracellular deposition of autoantibodies was observed by direct immunofluorescence and by immunoperoxidase staining on cryosections of the myocardial tissue of animals immunized with the ADP/ATP carrier. Furthermore, binding of autoantibodies to mitochondrial membrane structures was shown by immunoelectron-microscopic methods. The cytosolic and intramitochondrial distribution of adenine nucleotides in stimulated, isolated perfused hearts of guinea pigs immunized with the ADP/ATP carrier was measured by nonaqueous fractionation. Compared with controls performing equal external heart work, the cytosolic ATP decreased in the immunized animals, whereas the mitochondrial ATP increased strongly; ADP concentrations showed an opposite change. Thus, a resultant cytosolic decrease and a marked mitochondrial increase of the ATP/ADP ratio was established. As a consequence, the cytosolic-mitochondrial phosphorylation potential of ATP was diminished. These findings demonstrate that antibodies against intracellular antigens are able to penetrate into living cells, and that autoimmunity to the ADP/ATP carrier may contribute to the pathophysiology of myocarditis and dilated cardiomyopathy by causing an autoantibody-mediated imbalance between intracellular energy delivery and demand.

Careful consideration of the effects induced by glutathione depletion in rat liver and heart. The involvement of cytosolic and mitochondrial glutathione pools

One of the most widely used mechanisms by which the role of glutathione (GSH) in cellular functions has been withdrawn, is to deplete GSH intracellularly. The importance of the procedure and xenobiotic chosen to get it is discussed. Mitochondrial GSH plays certainly an important role in maintaining cellular homeostasis. This contribution varies depending on the tissue and the conclusions obtained about the functions of this GSH pool in one organ may not be applied to others. Original data on the subcellular distribution of GSH in myocardial tissue of the rat are presented, and the effect of phorone on both cardiac GSH pools is compared with the effect in liver. The mechanical failure of myocardium after ischemic or reperfusion damage might involve mitochondrial GSH, in view of the literature data referring to the role of thiol groups in energy transfer from mitochondria to cytosol.

Subcellular distribution of malate-aspartate cycle intermediates during normoxia and anoxia in the heart

The subcellular distribution of adenine nucleotides, phosphocreatine and intermediates of the malate-aspartate cycle was investigated in adult rat heart myocytes under normoxia and anoxia. Cytosolic and mitochondrial concentrations of metabolites were determined by a fractionation method using digitonin. Under normoxia, cytosolic/mitochondrial gradients were found for ATP (c/m = 4), AMP (c/m less than 0.01), citrate (c/m = 0.5), aspartate (c/m = 3), glutamate (c/m = 2), while phosphocreatine and glutamine were confined to the cytosolic space. No gradients were found for malate and 2-oxoglutarate. The results show that the transport of electrons from the cytosol into the mitochondria is supported by the glutamate gradient and by a high glutamate/aspartate ratio inside the mitochondria (Glu/Asp = 15) which is maintained by the energy-dependent Glu-Asp exchange across the mitochondrial membrane. Under anoxia, cytosolic glutamate is transaminated with pyruvate, yielding alanine and 2-oxoglutarate, which is oxidized to succinate inside the mitochondria and leaves the cell. The data indicate that stimulation of transamination is caused by a mass action effect following a decrease in cytosolic 2-oxoglutarate which may be due to succinate-2-oxoglutarate exchange across the mitochondrial membrane. Inhibition of the energy-dependent inward transport of glutamate may support this process.

Ca2+-dependent and Ca2+-independent glutamate release, energy status and cytosolic free Ca2+ concentration in isolated nerve terminals following metabolic inhibition: possible relevance to hypoglycaemia and anoxia

Hypoglycaemia and anoxia both cause massive release of glutamate from the brain in vivo, and the nature of this release was investigated using guinea-pig cerebral-cortical synaptosomes and iodoacetate and rotenone to simulate the energetic consequences of these conditions. Glutamate release (by continuous fluorimetry), cytoplasmic free Ca2+ (by fura-2), membrane potentials, ATP, ADP and creatine phosphate were determined in parallel, following the addition of iodoacetate or rotenone, alone or in combination. Ca2+-dependent glutamate release had a high energy requirement which could only be satisfied by aerobic glycolysis. Respiration using endogenous substrates, or anaerobic glycolysis following rotenone, caused a progressive inhibition of Ca2+-dependent release, correlating with a decline in the total ATP/ADP ratio and creatine phosphate. With rotenone, an increase in Ca2+-independent glutamate release was observed, correlating with a decline in plasma membrane potential. Only a slight increase in free Ca2+ was seen. Rotenone plus iodoacetate caused an almost immediate collapse of ATP/ADP ratio and a parallel loss of Ca2+-dependent glutamate release before free Ca2+ had risen to a level sufficient for exocytosis. In contrast, Ca2+-independent glutamate release increased. The Ca2+-dependent release of L-glutamate had the characteristics of an exocytotic transmitter release mechanism, being energy-dependent and triggered by elevated cytoplasmic free Ca2+ concentration. A distinct Ca2+-independent release of cytoplasmic glutamate occurred by reversal of the Na+-coupled uptake carrier, which was accelerated by a decline in the Na+ gradient. It is concluded that the Ca2+-independent release of cytoplasmic glutamate may make the major contribution to the excitotoxic release of glutamate in hypoglycaemic and anoxic conditions.

Role of NADP+ (corrected)-linked malic enzymes as regulators of the pool size of tricarboxylic acid-cycle intermediates in the perfused rat heart

Cytosolic and mitochondrial concentrations of malate, 2-oxoglutarate, isocitrate and pyruvate in the isolated perfused rat heart were measured by non-aqueous tissue fractionation, taking the NADP-linked isocitrate dehydrogenase as indicator reactions for the free [NADPH]/[NADP+] ratios. The mass-action ratios of NADP-linked malic enzymes (EC 1.1.1.40) were found to be on the side of pyruvate carboxylation by more than one order of magnitude in both the cytosolic and the mitochondrial spaces in hearts perfused with glucose, whereas during propionate perfusion this ratio approached the equilibrium constant (Keq.) of malic enzyme. The results consequently indicate that the NADP-linked malic enzymes cannot be responsible for the feed-out (cataplerotic) reactions from the tricarboxylic acid cycle which occur during glucose perfusion. Only when other anaplerotic fluxes into the cycle are high, as during propionate oxidation, which results in accumulation of tricarboxylic acid-cycle intermediates, is a steady state reached which allows efflux via the malic enzyme.

31P-NMR saturation transfer measurements of exchange between Pi and ATP in the reactions catalysed by glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase in vitro

Recent 31P-NMR saturation transfer measurements of flux between Pi and ATP in the perfused rat heart (Kingsley-Hickman, P., Sako, E.Y., Andreone, P.A., St. Cyr, J.A., Michurski, S., Foker, J.E., From, A.H.L., Petein, M. and Ugurbil, K. (1986) FEBS Lett. 198, 159-163) have given a P/O ratio (mols ATP synthesised/atoms oxygen consumed) which was close to 6. This anomalously high value was attributed to exchange in the reaction catalysed by the mitochondrial F1F0-ATP synthase. We show here that this exchange could also be catalysed by the glycolytic enzymes glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase. 31P-NMR saturation transfer measurements of the exchange catalysed by these enzymes in vitro, under conditions designed to mimic those present in the perfused rat heart, have shown that they could catalyse a quantitatively significant Pi-ATP exchange in vivo. A three-site exchange model is used to investigate the effects of Pi-ATP exchange on saturation transfer measurements of the reverse flux in the creatine kinase reaction. A discrepancy in the measured and forward and reverse fluxes in this reaction has been attributed previously to the participation of the gamma-phosphate of ATP in other exchange reactions.

Compartmentation of high-energy phosphates in resting and beating heart cells

The subcellular distribution of ATP, ADP, creatine phosphate and creatine has been analyzed by fast detergent fractionation of isolated frog heart cells. Digitonin fractionation (0.5 mg/ml, 10 s at 2 degrees C in 20 mM 4-morpholinepropanesulfonic acid/3 mM EDTA/230 mM mannitol medium) was used to separate mitochondria and myofilaments from cytosol. To separate myofilaments from the other cellular compartments. Triton X-100 was used (2%, 15 s in the same medium as digitonin). For either resting or beating cells the total cellular contents of ATP, ADP, creatine phosphate and creatine was similar, nevertheless the O2 consumption was 6-times higher. The compartmentation of these metabolites was also identical. Myofilaments contain 1.1 nmol ADP per mg total cellular proteins. In the cytosolic compartment the metabolite concentrations, all measured in nmol per mg total cellular proteins, were: ATP, 13; ADP, 0.25-0.05; creatine phosphate, 18.5 and creatine, 14. This indicated that the reaction catalyzed by creatine kinase was in a state of (or near) equilibrium.

Cytosolic adenylates and adenosine release in perfused working heart. Comparison of whole tissue with cytosolic non-aqueous fractionation analyses

Free cytosolic adenylates were examined in relation to adenosine plus inosine released from perfused working guinea-pig hearts. Whole-tissue adenylate data from freeze-clamped hearts were quantitatively compared with corresponding values obtained by subcellular fractionation of homogenized myocardium in non-aqueous media. Adenosine and inosine in venous cardiac effluents were measured by high-performance liquid chromatography. Hearts, perfused at their natural flows, were subjected to various workloads, substrates and catecholamines to alter myocardial energy metabolism and respiration over a wide physiological range. Non-aqueous cytosolic ATP and creatine phosphate (CrP) accounted for more than 80% of the respective total myocardium content. The cytosolic CrP/Pi ratio was in near-quantitative agreement with the overall tissue CrP/Pi ratio when the latter parameter was corrected for extracellular Pi. This was conclusive evidence that ATP, CrP and Pi were predominantly located in the cytosol of the well-oxygenated cardiomyocyte. Measured myocardial oxygen uptake (MVO2) was reciprocally related to the phosphorylation state of CrP [( CrP]/[Cr] X [Pi]) and hence that of ATP [( ATP]/[ADP] X [Pi]) assuming the creatine kinase at near-equilibrium at a near-constant pH of 7.2. On the other hand, calculated mean free cytosolic ADP concentrations increased essentially linearly up to threefold with increasing MVO2 in the presence of virtually unchanged or only slightly decreased ATP levels; this was found both according to the whole tissue and the special subcellular fractionation data. Employing the myokinase mass-action ratio and substituting total cardiac ADP by the mean free cytosolic ADP concentrations, the mean free cytosolic AMP concentrations proved to be in the nanomolar range, i.e. up to three orders of magnitude lower than the overall tissue AMP content. We propose, therefore, that in the normoxic heart, AMP is located predominantly in the mitochondrial compartment. Nevertheless, both free cytosolic AMP concentration and release of adenosine plus inosine were apparently square or even higher-power functions of the rate of cardiac respiration. On the other hand, the mean purine nucleoside release seemed linearly correlated (r = 0.920) with the calculated free cytosolic AMP concentration. Our observations seem to suggest that the concentrations of free ADP and AMP in the cytosol are major determinants of the production of inosine and coronary vasodilator adenosine.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of palmitoylcarnitine oxidation in isolated rat liver mitochondria. Role of the redox state of NAD(H)

The redox-mediated regulation of palmitoylcarnitine oxidation was studied in isolated rat liver mitochondria in which the mitochondrial free NADH/NAD+ ratio was controlled by graded concentrations of acetoacetate and ketomalonate in a rotenone and malonate-inhibited system in the presence of ADP. The NADH/NAD+ ratio was buffered kinetically by adjusting the concentrations of the hydrogen acceptor substances and determined by calibrated NAD(P)H fluorometry of the mitochondrial suspension. A two-fold variation in the beta-oxidation rate and a five-fold variation in the free NADH/NAD+ ratio was obtained in the presence of rotenone. A non-linear negative correlation was found between the acetyl-CoA concentration and the beta-oxidation rate and a negative correlation between the long-chain acyl-CoA concentration and the beta-oxidation rate. The data indicate that the redox state is a partial controller of the beta-oxidation rate in liver mitochondria. The contribution of acetyl-CoA, a putative regulator of beta-oxidation at the acyl-CoA thiolase step is small under the conditions used.

Function dependent changes in the subcellular distribution of high energy phosphates in fast and slow rat skeletal muscles

Function dependent changes in the subcellular distribution of ATP, ADP, creatine phosphate (CrP) and creatine (Cr) in rat fast-twitch gastrocnemius and slow-twitch soleus muscles were studied by fractionation of freeze-clamped and freeze-dried tissue in non-aqueous solvents. During 5 min of isotonic contraction of gastrocnemius muscles the mitochondrial content of total creatine [sigma(CrP + Cr)] decreases by 9.5 nmol/mg total protein whereas there is an increase in extramitochondrial total creatine by 12.3 nmol/mg total protein, indicating a net transfer of approximately 10 nmol total creatine/mg total protein/5 min across the mitochondrial inner membrane. During short-term stimulation (6 s) of gastrocnemius muscles the socalled "additionally-bound ADP" correlates not only with force (Hebisch et al. 1984) but also with filament overlap. This confirms the previous suggestion that "additionally-bound ADP" represents actomyosin-ADP-complexes. Following long-term stimulation (10 s), the rate of decay of force is at least two orders of magnitude faster than that of "additionally bound ADP". This indicates a decrease of actomyosin-ADP complexes due to formation of myosin-ADP complexes. Short-term stimulation (6 s) of slow-twitch soleus muscles does not lead to any force-dependent change in the content of "additionally-bound ADP", similar to the finding in long-term contracting gastrocnemius muscles. Denervation of soleus muscles leads to a decrease in "additionally-bound ADP" to values comparable to those found in resting fast-twitch gastrocnemius muscles.

Mitochondrial respiratory control in the myocardium

The heart muscle has proved to be a practical model for studying respiratory control in intact tissues. It also demonstrates that control at the level of the respiratory chain is augmented by metabolic control at the substrate level as exemplified by the very narrow range of changes in the redox state of the mitochondrial NADH/NAD couple even during extensive changes in ATP and oxygen consumption. The behaviour of mitochondria when isolated can largely be duplicated in the intact myocardium. Moreover, the high intracellular concentrations of enzymes, coenzymes and adenine nucleotides create conditions of high reaction rates, enabling the formation of a near equilibrium network of certain main pathways. This equilibrium network in connection with metabolic regulation of the hydrogen pressure upon the matrix NADH/NAD pool is a prerequisite for the regulation of cellular respiration at a high efficiency of energy transfer. Experimentation on the intact myocardium also seems to be capable of resolving some of the uncertainties about prevailing mechanisms for the regulation of cellular respiration.

Quantitative 31P nuclear magnetic resonance analysis of metabolite concentrations in Langendorff-perfused rabbit hearts

The quantitative analysis of the mobile high-energy phosphorus metabolites in isovolumic Langendorff-perfused rabbit hearts has been performed by 31P NMR utilizing rapid pulse repetition to optimize sensitivity. Absolute quantification required reference to an external standard, determination of differential magnetization saturation and resonance peak area integration by Lorentzian lineshape analysis. Traditionally accepted hemodynamic indices (LVDP, dp/dt) and biochemical indices (lactate, pyruvate) of myocardial function were measured concomitantly with all NMR determinations. Hemodynamically and biochemically competent Langendorff-perfused rabbit hearts were found to have intracellular PCr, ATP, GPC, and Pi concentrations of 14.95 +/- 0.25, 8.08 +/- 0.13, 5.20 +/- 0.58 and 2.61 +/- 0.47 mM respectively. Intracellular pH was 7.03 +/- 0.01. Cytosolic ADP concentration was derived from a creatine kinase equilibrium model and determined to be approximately 36 microM. Reduction of perfusate flow from 20 to 2.5 ml/min demonstrated statistically significant decreases in PCr, ATP, and pH as well as an increase in Pi that correlated closely with the independent hemodynamic and biochemical indices of myocardial function. The decrease in ATP and PCr concentrations precisely matched the increase in Pi during reduced flow. These results constitute the first quantitative determination of intracellular metabolite concentrations by 31P NMR in intact rabbit myocardium under physiologic and low flow conditions.

Compartmentation of ATP within renal proximal tubular cells

Studies on the relations between active solute transport and cell metabolism require not only knowledge of the total cellular ATP, but also of the separate mitochondrial and cytosolic ATP levels. For this purpose, mitochondrial and cytosolic fractions were separated from isolated proximal tubular suspensions by the digitonin technique and the amount of ATP analyzed separately for each compartment. In a parallel series of experiments, the absolute volumes of mitochondrial and extramitochondrial spaces were determined in rat renal cortical tubular suspension utilizing electron microscopic morphometry. When referring ATP measurements to the morphometrically determined absolute volumes, the ATP concentrations were calculated to be 4.33 mmol/l for the cytosolic and 2.62 mmol/l for the mitochondrial space. The cytosolic and mitochondrial ATP, thus, represent 70 and 30% of the total cellular ATP, respectively.

Adenine nucleotide metabolism and compartmentalization in isolated adult rat heart cells

The metabolism and intracellular compartmentalization of adenine nucleotides in a preparation of adult rat heart myocytes showing good morphology, viability, and tolerance to calcium ion has been examined by high performance liquid chromatography. These myocytes contain an average of 23 nmol adenine nucleotide per milligram protein which is about 60% of the adenine nucleotide content of intact rat heart tissue. The loss of adenine nucleotide occurs during the incubation and washing steps that increase the yield of viable cells, rather than during the collagenase perfusion. An analysis of cellular compartments shows that the adenine nucleotide of the cell consists of 17 nmol adenine nucleotide in the cytosol, 5 nmol in the mitochondria, and 1.3 nmol adenosine diphosphate bound to myofibrils per milligram cell protein. Myocytes lose both adenosine triphosphate and adenine nucleotide when incubated anaerobically in the absence of glucose, and the lost adenine nucleotide can be accounted for as increased inosine, adenosine, and inosine monophosphate. Myocytes that contain less than 0.1 nmol of cytosol adenosine triphosphate per milligram cell protein maintain an intact sarcolemma, but are unable to carry out anaerobic glycolysis. Reoxygenation of anaerobic cells results in restoration of energy charge and a net resynthesis of about 2 nmol adenine nucleotide per milligram protein. Adenosine and inosine monophosphate decrease on reaeration of anaerobic cells, whereas inosine levels increase. When iodoacetate is added to block glycolysis, the decline in adenine nucleotide and production of inosine monophosphate are accelerated and there is no resynthesis of adenine nucleotide when anaerobic cells are reoxygenated . Large accumulations of inosine monophosphate are also seen in myocytes treated with an uncoupler of oxidative phosphorylation.

Compartmentation of high-energy phosphates in resting and working rat skeletal muscle

The subcellular distribution of high-energy phosphates in various types of skeletal muscle of the rat was analysed by subfractionation of tissues in non-aqueous solvents. Different glycolytic and oxidative capacities were calculated from the ratio of phosphoglycerate kinase and citrate synthase activities, ranging from 25 in m. soleus to 130 in m. tensor fasciae latae. In the resting state, the subcellular contents of ATP, creatine phosphate and creatine were similar in m. soleus, m. vastus intermedius, m. gastrocnemius and m. tensor fasciae latae but, significantly, a higher extramitochondrial ADP-content was found in m. soleus. A similar observation was made in isometrically and isotonically working m. gastrocnemius. The extramitochondrial, bound ADP accounted fully for actin-binding sites in resting fast-twitch muscles, but an excess of bound ADP was found in m. soleus and working m. gastrocnemius. The amount of non-actin-bound ADP reached maximal values of approx. 1.2 nmol/mg total protein. It could not be enhanced further by prolonged isotonic stimulation or by increased isometric force development. It is suggested that non-actin-bound ADP is accounted for by actomyosin-ADP complexes generated during the contraction cycle. Binding of extramitochondrial ADP to actomyosin complexes in working muscles thus acts as a buffer for cytosolic ADP in addition to the creatine system, maintaining a high cytosolic phosphorylation potential also at increasing rates of ATP hydrolysis during muscle contraction.

Mitochondrial membrane potential, transmembrane difference in the NAD+ redox potential and the equilibrium of the glutamate-aspartate translocase in the isolated perfused rat heart

The distribution of glutamate and aspartate and the mitochondrial membrane potential (delta psi) were studied in isolated rat heart mitochondria and in the intact perfused rat heart. The diffusion potential imposed by the glutamate-aspartate exchange through mediation of the electrogenic glutamate-aspartate translocator attained a value close to the mitochondrial delta psi measured from the distribution of triphenylmethylphosphonium ion (TPMP+) both in isolated mitochondria and in intact myocardium. Distributions of the delta psi probe and metabolites were determined by subcellular fractionation of the heart muscle in a non-aqueous medium. The results indicate that the glutamate-aspartate translocator is in near equilibrium in the myocardium. The diffusion potential of the glutamate-aspartate exchange, and the mitochondrial/cytosolic difference in the redox potentials of the free NAD+/NADH pools are equal allowing for experimental error. These data obtained from intact tissue can therefore be interpreted as supporting the notion of the transmembrane uphill transport of reducing equivalent from the cytosolic free NAD+/NADH pool being driven by the malate-aspartate cycle energized by the mitochondrial delta psi.

Proton electrochemical potential of the inner mitochondrial membrane in isolated perfused rat hearts, as measured by exogenous probes

The membrane potential (delta psi) and delta pH of the inner mitochondrial membrane were studied in isolated perfused rat hearts using exogenous labelled probes and tissue fractionation in non-aqueous media. The mitochondrial delta psi, measured by means of the subcellular distribution of [3H]triphenylmethylphosphonium (TPMP+), was 125 +/- 7 mV (negative inside) in hearts beating at 5 Hz and 150 +/- 3 mV (negative inside) in hearts beating at 1.5 Hz. The mitochondrial membrane delta pH, measured by means of the subcellular distribution of low concentrations of [1-14C]propionate, was 0.63 +/- 0.06 pH units (alkaline inside) in hearts beating at 5 Hz and 0.53 +/- 0.12 pH units (alkaline inside) in hearts beating at 1.5 Hz. The implication of proton and electron gradients in the regulation of cellular respiration is discussed. In combination with previous evidence on adenylate distribution in the isolated perfused rat heart, the results indicate that the mitochondrial electrogenic adenylate translocator is in near equilibrium with delta psi.