Relation between cytosolic free Ca2+ concentration and the control of pyruvate dehydrogenase in isolated cardiac myocytes

Supralinear Dependence of the IP3 Receptor-to-Mitochondria Local Ca2+ Transfer on the Endoplasmic Reticulum Ca2+ Loading

Calcium signal propagation from endoplasmic reticulum (ER) to mitochondria regulates a multitude of mitochondrial and cell functions, including oxidative ATP production and cell fate decisions. Ca2+ transfer is optimal at the ER-mitochondrial contacts, where inositol 1,4,5-trisphosphate (IP3) receptors (IP3R) can locally expose the mitochondrial Ca2+ uniporter (mtCU) to high [Ca2+] nanodomains. The Ca2+ loading state of the ER (Ca2 + ER) can vary broadly in physiological and pathological scenarios, however, the correlation between Ca2 + ER and the local Ca2+ transfer is unclear. Here, we studied IP3-induced Ca2+ transfer to mitochondria at different Ca2 + ER in intact and permeabilized RBL-2H3 cells via fluorescence measurements of cytoplasmic [Ca2+] ([Ca2+]c) and mitochondrial matrix [Ca2+] ([Ca2+]m). Preincubation of intact cells in high versus low extracellular [Ca2+] caused disproportionally greater increase in [Ca2+]m than [Ca2+]c responses to IP3-mobilizing agonist. Increasing Ca2 + ER by small Ca2+ boluses in suspensions of permeabilized cells supralinearly enhanced the mitochondrial Ca2+ uptake from IP3-induced Ca2+ release. The IP3-induced local [Ca2+] spikes exposing the mitochondrial surface measured using a genetically targeted sensor appeared to linearly correlate with Ca2 + ER, indicating that amplification happened in the mitochondria. Indeed, overexpression of an EF-hand deficient mutant of the mtCU gatekeeper MICU1 reduced the cooperativity of mitochondrial Ca2+ uptake. Interestingly, the IP3-induced [Ca2+]m signal plateaued at high Ca2 + ER, indicating activation of a matrix Ca2+ binding/chelating species. Mitochondria thus seem to maintain a "working [Ca2+]m range" via a low-affinity and high-capacity buffer species, and the ER loading steeply enhances the IP3R-linked [Ca2+]m signals in this working range.

Testosterone deficiency impairs cardiac interfibrillar mitochondrial function and myocardial contractility while inducing oxidative stress

Introduction

Clinical studies have shown that low levels of endogenous testosterone are associated with cardiovascular diseases. Considering the intimate connection between oxidative metabolism and myocardial contractility, we determined the effects of testosterone deficiency on the two spatially distinct subpopulations of cardiac mitochondria, subsarcolemmal (SSM) and interfibrillar (IFM).

Methods

We assessed cardiac function and cardiac mitochondria structure of SSM and IFM after 12 weeks of testosterone deficiency in male Wistar rats.

Results and Discussion

Results show that low testosterone reduced myocardial contractility. Orchidectomy increased total left ventricular mitochondrial protein in the SSM, but not in IFM. The membrane potential, size and internal complexity in the IFM after orchidectomy were higher compared to the SHAM group. However, the rate of oxidative phosphorylation with all substrates in the IFM after orchidectomy was lower compared to the SHAM group. Testosterone replacement restored these changes. In the testosterone-deficient SSM group, oxidative phosphorylation was decreased with palmitoyl-L-carnitine as substrate; however, the mitochondrial calcium retention capacity in IFM was increased. There was no difference in swelling of the mitochondria in either group. These changes in IFM were followed by a reduction in phosphorylated form of AMP-activated protein kinase (p-AMPK-α), peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) translocation to mitochondria and decreased mitochondrial transcription factor A (TFAM). Testosterone deficiency increased NADPH oxidase (NOX), angiotensin converting enzyme (ACE) protein expression and reduced mitochondrial antioxidant proteins such as manganese superoxide dismutase (Mn-SOD) and catalase in the IFM. Treatment with apocynin (1.5 mM in drinking water) normalized myocardial contractility and interfibrillar mitochondrial function in the testosterone depleted animals. In conclusion, our findings demonstrate that testosterone deficiency leads to reduced myocardial contractility and impaired cardiac interfibrillar mitochondrial function. Our data suggest the involvement of reactive oxygen species, with a possibility of NOX as an enzymatic source.

Pro-inflammatory polarization of macrophages is associated with reduced endoplasmic reticulum-mitochondria interaction

Macrophages play a role in host defense, tissue remodeling and inflammation. Different inflammatory stimuli drive macrophage phenotypes and responses. In this study we investigated the relationship between macrophages immune phenotype and mitochondrial bioenergetics, cell redox state and endoplasmic reticulum (ER)-mitochondria interaction. Bacterial lipopolysaccharide (LPS) and interferon-γ (IFNγ) pro-inflammatory stimuli decreased oxidative metabolism (basal, phosphorylating and maximal conditions) and increased baseline glycolysis (117%) and glycolytic capacity (43%) in THP-1 macrophages. In contrast, interleukin-4 (IL4) and interleukin-13 (IL13) anti-inflammatory stimuli increased the oxygen consumption rates in baseline conditions (21%) and associated with ATP production (19%). LPS + IFNγ stimuli reduced superoxide anion levels by accelerating its conversion into hydrogen peroxide (H₂O₂) while IL4+IL13 decreased H₂O₂ release rates. The source of these oxidants was extra-mitochondrial and associated with increased NOX2 and SOD1 gene expression. LPS + IFNγ stimuli decreased ER-mitochondria contact sites as measured by IP3R1-VDAC1 interaction (34%) and markedly upregulated genes involved in mitochondrial fusion (9-10 fold, MFN1 and 2) and fission (∼7 fold, DRP1 and FIS1). Conversely, IL4+IL13 stimuli did not altered ER-mitochondria interactions nor MFN1 and 2 expression. Together, these results unveil ER-mitochondria interaction pattern as a novel feature of macrophage immunological, metabolic and redox profiles.

Improper Remodeling of Organelles Deputed to Ca2+ Handling and Aerobic ATP Production Underlies Muscle Dysfunction in Ageing

Proper skeletal muscle function is controlled by intracellular Ca2+ concentration and by efficient production of energy (ATP), which, in turn, depend on: (a) the release and re-uptake of Ca2+ from sarcoplasmic-reticulum (SR) during excitation-contraction (EC) coupling, which controls the contraction and relaxation of sarcomeres; (b) the uptake of Ca2+ into the mitochondrial matrix, which stimulates aerobic ATP production; and finally (c) the entry of Ca2+ from the extracellular space via store-operated Ca2+ entry (SOCE), a mechanism that is important to limit/delay muscle fatigue. Abnormalities in Ca2+ handling underlie many physio-pathological conditions, including dysfunction in ageing. The specific focus of this review is to discuss the importance of the proper architecture of organelles and membrane systems involved in the mechanisms introduced above for the correct skeletal muscle function. We reviewed the existing literature about EC coupling, mitochondrial Ca2+ uptake, SOCE and about the structural membranes and organelles deputed to those functions and finally, we summarized the data collected in different, but complementary, projects studying changes caused by denervation and ageing to the structure and positioning of those organelles: a. denervation of muscle fibers-an event that contributes, to some degree, to muscle loss in ageing (known as sarcopenia)-causes misplacement and damage: (i) of membrane structures involved in EC coupling (calcium release units, CRUs) and (ii) of the mitochondrial network; b. sedentary ageing causes partial disarray/damage of CRUs and of calcium entry units (CEUs, structures involved in SOCE) and loss/misplacement of mitochondria; c. functional electrical stimulation (FES) and regular exercise promote the rescue/maintenance of the proper architecture of CRUs, CEUs, and of mitochondria in both denervation and ageing. All these structural changes were accompanied by related functional changes, i.e., loss/decay in function caused by denervation and ageing, and improved function following FES or exercise. These data suggest that the integrity and proper disposition of intracellular organelles deputed to Ca2+ handling and aerobic generation of ATP is challenged by inactivity (or reduced activity); modifications in the architecture of these intracellular membrane systems may contribute to muscle dysfunction in ageing and sarcopenia.

Mitochondrial calcium transport and the redox nature of the calcium-induced membrane permeability transition

Mitochondria possess a Ca²⁺ transport system composed of separate Ca²⁺ influx and efflux pathways. Intramitochondrial Ca²⁺ concentrations regulate oxidative phosphorylation, required for cell function and survival, and mitochondrial redox balance, that participates in a myriad of signaling and damaging pathways. The interaction between Ca²⁺ accumulation and redox imbalance regulates opening and closing of a highly regulated inner membrane pore, the membrane permeability transition pore (PTP). In this review, we discuss the regulation of the PTP by mitochondrial oxidants, reactive nitrogen species, and the interactions between these species and other PTP inducers. In addition, we discuss the involvement of mitochondrial redox imbalance and PTP in metabolic conditions such as atherogenesis, diabetes, obesity and in mtDNA stability.

Regulation of cardiac cellular bioenergetics: mechanisms and consequences

The regulation of cardiac cellular bioenergetics is critical for maintaining normal cell function, yet the nature of this regulation is not fully understood. Different mechanisms have been proposed to explain how mitochondrial ATP production is regulated to match changing cellular energy demand while metabolite concentrations are maintained. We have developed an integrated mathematical model of cardiac cellular bioenergetics, electrophysiology, and mechanics to test whether stimulation of the dehydrogenase flux by Ca²⁺ or Pi, or stimulation of complex III by Pi can increase the rate of mitochondrial ATP production above that determined by substrate availability (ADP and Pi). Using the model, we show that, under physiological conditions the rate of mitochondrial ATP production can match varying demand through substrate availability alone; that ATP production rate is not limited by the supply of reducing equivalents in the form of NADH, as a result of Ca²⁺ or Pi activation of the dehydrogenases; and that ATP production rate is sensitive to feedback activation of complex III by Pi. We then investigate the mechanistic implications on cytosolic ion homeostasis and force production by simulating the concentrations of cytosolic Ca²⁺, Na⁺ and K⁺, and activity of the key ATPases, SERCA pump, Na⁺/K⁺ pump and actin-myosin ATPase, in response to increasing cellular energy demand. We find that feedback regulation of mitochondrial complex III by Pi improves the coupling between energy demand and mitochondrial ATP production and stabilizes cytosolic ADP and Pi concentrations. This subsequently leads to stabilized cytosolic ionic concentrations and consequentially reduced energetic cost from cellular ATPases.

A Novel Nicotinamide Adenine Dinucleotide Correction Method for Mitochondrial Ca(2+) Measurement with FURA-2-FF in Single Permeabilized Ventricular Myocytes of Rat

Fura-2 analogs are ratiometric fluoroprobes that are widely used for the quantitative measurement of [Ca(2+)]. However, the dye usage is intrinsically limited, as the dyes require ultraviolet (UV) excitation, which can also generate great interference, mainly from nicotinamide adenine dinucleotide (NADH) autofluorescence. Specifically, this limitation causes serious problems for the quantitative measurement of mitochondrial [Ca(2+)], as no available ratiometric dyes are excited in the visible range. Thus, NADH interference cannot be avoided during quantitative measurement of [Ca(2+)] because the majority of NADH is located in the mitochondria. The emission intensity ratio of two different excitation wavelengths must be constant when the fluorescent dye concentration is the same. In accordance with this principle, we developed a novel online method that corrected NADH and Fura-2-FF interference. We simultaneously measured multiple parameters, including NADH, [Ca(2+)], and pH/mitochondrial membrane potential; Fura-2-FF for mitochondrial [Ca(2+)] and TMRE for Ψm or carboxy-SNARF-1 for pH were used. With this novel method, we found that the resting mitochondrial [Ca(2+)] concentration was 1.03 µM. This 1 µM cytosolic Ca(2+) could theoretically increase to more than 100 mM in mitochondria. However, the mitochondrial [Ca(2+)] increase was limited to ~30 µM in the presence of 1 µM cytosolic Ca(2+). Our method solved the problem of NADH signal contamination during the use of Fura-2 analogs, and therefore the method may be useful when NADH interference is expected.

The complex interplay between mitochondrial dynamics and cardiac metabolism

Mitochondria are highly dynamic organelles, capable of undergoing constant fission and fusion events, forming networks. These dynamic events allow the transmission of chemical and physical messengers and the exchange of metabolites within the cell. In this article we review the signaling mechanisms controlling mitochondrial fission and fusion, and its relationship with cell bioenergetics, especially in the heart. Furthermore we also discuss how defects in mitochondrial dynamics might be involved in the pathogenesis of metabolic cardiac diseases.

Matching ATP supply and demand in mammalian heart: in vivo, in vitro, and in silico perspectives

Although the heart rapidly adapts cardiac output to match the body's circulatory demands, the regulatory mechanisms ensuring that sufficient ATP is available to perform the required cardiac work are not completely understood. Two mechanisms have been suggested to serve as key regulators: (1) ADP and Pi concentrations--ATP utilization/hydrolysis in the cytosol increases ADP and Pi fluxes to mitochondria and hence the amount of available substrates for ATP production increases; and (2) Ca2+ concentration--ATP utilization/hydrolysis is coupled to changes in free cytosolic calcium and mitochondrial calcium, the latter controlling Ca2+-dependent activation of mitochondrial enzymes taking part in ATP production. Here we discuss the evolving perspectives of each of the putative regulatory mechanisms and the precise molecular targets (dehydrogenase enzymes, ATP synthase) based on existing experimental and theoretical evidence. The data synthesis can generate novel hypotheses and experimental designs to solve the ongoing enigma of energy supply-demand matching in the heart.

Regulation of mitochondrial dehydrogenases by calcium ions

Studies in Bristol in the 1960s and 1970s, led to the recognition that four mitochondrial dehydrogenases are activated by calcium ions. These are FAD-glycerol phosphate dehydrogenase, pyruvate dehydrogenase, NAD-isocitrate dehydrogenase and oxoglutarate dehydrogenase. FAD-glycerol phosphate dehydrogenase is located on the outer surface of the inner mitochondrial membrane and is influenced by changes in cytoplasmic calcium ion concentration. The other three enzymes are located within mitochondria and are regulated by changes in mitochondrial matrix calcium ion concentration. These and subsequent studies on purified enzymes, mitochondria and intact cell preparations have led to the widely accepted view that the activation of these enzymes is important in the stimulation of the respiratory chain and hence ATP supply under conditions of increased ATP demand in many stimulated mammalian cells. The effects of calcium ions on FAD-isocitrate dehydrogenase involve binding to an EF-hand binding motif within this enzyme but the binding sites involved in the effects of calcium ions on the three intramitochondrial dehydrogenases remain to be fully established. It is also emphasised in this article that these three dehydrogenases appear only to be regulated by calcium ions in vertebrates and that this raises some interesting and potentially important developmental issues.

Mapping the ruthenium red-binding site of the voltage-dependent anion channel-1

We have previously shown that ruthenium red (RuR) binds to the voltage-dependent anion channel (VDAC) in the outer mitochondrial membrane, decreasing channel conductance and protecting against apoptotic cell death. In this report, we define the murine and yeast VDAC1 amino acid residues involved in the interaction with RuR. Binding of RuR to bilayer-reconstituted mVDAC1 and the resulting channel closure was inhibited upon mutation of specific VDAC1 residues. RuR protection against cell death, as induced by overexpression of native or mutated mVDAC1, was also diminished upon mutation of these amino acids. Moreover, RuR-mediated inhibition of cytochrome c release normally induced by staurosporine was not observed in cells expressing mutants VDAC1. We found that four glutamate residues, two each located in the first and third mVDAC1 cytosolic loops, are required for the interaction of VDAC1 with RuR and subsequent protection against cell death. Similar results were obtained with Q72E-yeast VDAC1, except that only three glutamate residues, located in two cytosolic loops were required. As a hexavalent reagent, RuR is expected to bind to more than one negatively charged group. Our results thus clearly indicate that RuR protects against cell death via a direct interaction with VDAC1 to inhibit cytochrome c release and subsequent cell death.

Cardiac energy metabolism homeostasis: role of cytosolic calcium

The heart is capable of dramatically altering its overall energy flux with minimal changes in the concentrations of metabolites that are associated with energy metabolism. This cardiac energy metabolism homeostasis is discussed with regard to the potential cytosolic control network responsible for controlling the major energy conversion pathway, oxidative phosphorylation in mitochondria. Several models for this cytosolic control network have been proposed, but a cytosolic Ca(2+) dependent parallel activation scheme for metabolism and work is consistent with most of the experimental results. That model proposes that cytosolic Ca(2+) regulates both the utilization of ATP by the work producing ATPases as well as the mitochondrial production of ATP. Recent studies have provided evidence supporting this role of cytosolic Ca(2+). These data include the demonstration that mitochondrial [Ca(2+)] can track cytosolic [Ca(2+)] and that the compartmentation of cytosolic [Ca(2+)] can facilitate this process. On the metabolic side, Ca(2+) has been shown to rapidly activate several steps in oxidative phosphorylation, including F(1)F(0)-ATPase ATP production as well as several dehydrogenases, which results in a homeostasis of mitochondrial metabolites similar to that observed in the cytosol. Numerous problems with the Ca(2+) parallel activation hypothesis remain including the lack of specific mechanisms of mitochondrial Ca(2+) transport and regulation of F(1)F(0)-ATPase, the time dependence of Ca(2+) activation of cytosolic ATPases as well as oxidative phosphorylation, and the role of cytosolic compartmentation. In addition, the lack of cytosolic or mitochondrial [Ca(2+)] measurements under in vivo conditions is problematic. Several lines of investigation to address these issues are suggested. A model of the cardiac energy metabolism control network is proposed that includes a Ca(2+) parallel activation component together with more classical elements including metabolite feedback and cytosolic compartmentation.

Mitochondria regulate inactivation of L-type Ca2+ channels in rat heart

1. L-type Ca2+ channels play an important role in vital cell functions such as muscle contraction and hormone secretion. Both a voltage-dependent and a Ca2+-dependent process inactivate these channels. Here we present evidence that inhibition of the mitochondrial Ca2+ import mechanism in rat (Sprague-Dawley) ventricular myocytes by ruthenium red (RR), by Ru360 or by carbonyl cyanide m-chlorophenylhydrazone (CCCP) decreases the magnitude of electrically evoked transient elevations of cytosolic Ca2+ concentration ([Ca2+]c). These agents were most effective at stimulus rates greater than 1 Hz. 2. RR and CCCP also caused a significant delay in the recovery from inactivation of L-type Ca2+ currents (I(Ca)). This suggests that sequestration of cytosolic Ca2+, probably near the mouth of L-type Ca2+ channels, into mitochondria during cardiac contractile cycles, helps to remove the Ca2+-dependent inactivation of L-type Ca2+ channels. 3. We conclude that impairment of mitochondrial Ca2+ transport has no impact on either L-type Ca2+ currents or SR Ca2+ release at low stimulation frequencies (e.g. 0.1 Hz); however, it causes a depression of cytosolic Ca2+ transients attributable to an impaired recovery of L-type Ca2+ currents from inactivation at high stimulation frequencies (e.g. 3 Hz). The impairment of mitochondrial Ca2+ uptake and subsequent effects on Ca2+ transients at high frequencies at room temperature could be physiologically relevant since the normal heart rate of rat is around 5 Hz at body temperature. The role of mitochondria in clearing Ca2+ in the micro-domain near L-type Ca2+ channels could be impaired during high frequencies of heart beats such as in ventricular tachycardia, explaining, at least in part, the reduction of muscle contractility.

Alterations of intracellular calcium homeostasis and mitochondrial function are involved in ruthenium red neurotoxicity in primary cortical cultures

Ruthenium red (RR) is a polycationic dye that induces neuronal death in vivo and in primary cultures. To characterize this neurotoxic action and to determine the mechanisms involved, we have analyzed the ultrastructural alterations induced by RR in rat cortical neuronal cultures and measured its effect on cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) and on mitochondrial function. RR produced a dose-dependent, progressive disruption of neurites and plasma membrane of neuronal somata after 8-24 hr of incubation. RR caused also an elevation of both the basal [Ca(2+)](i) and its maximal levels after K(+) depolarization. Mitochondrial oxidative function, assessed by reduction of 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide and by changes in dihydrorhodamine-123 fluorescence, was significantly diminished after treatment with RR, both in cultured neurons and in isolated brain mitochondria. La(3+) did not prevent but rather potentiated RR-induced cell death. Glutamate receptor antagonists also failed to prevent RR neurotoxicity. Apoptotic electron microscope images were not observed, and protein synthesis inhibitors did not show any protective effect. It is concluded that RR penetrates neurons and that its neurotoxic damage probably is due to intracellular Ca(2+) dishomeostasis and disruption of mitochondrial oxidative function. These results enhance our understanding of the intracellular mechanisms underlying neuronal death.

Pyruvate: metabolic protector of cardiac performance

Pyruvate, a metabolic product of glycolysis and an oxidizable fuel in myocardium, increases cardiac mechanical performance and energy reserves, especially when supplied at supraphysiological concentrations. The inotropic effects of pyruvate are most impressive in hearts that have been reversibly injured (stunned) by ischemia/reperfusion stress. Glucose appears to be an essential co-substrate for pyruvate's salutary effects in stunned hearts, but other fuels including lactate, acetate, fatty acids, and ketone bodies produce little or no improvement in postischemic function over glucose alone. In contrast to pharmacological inotropism by catecholamines, metabolic inotropism by pyruvate increases cardiac energy reserves and bolsters the endogenous glutathione antioxidant system. Pyruvate enhancement of cardiac function may result from one or more of the following mechanisms: increased cytosolic ATP phosphorylation potential and Gibbs free energy of ATP hydrolysis, enhanced sarcoplasmic reticular calcium ion uptake and release, decreased cytosolic inorganic phosphate concentration, oxyradical scavenging via direct neutralization of peroxides and/or enhancement of the intracellular glutathione/NADPH antioxidant system, and/or closure of mitochondrial permeability transition pores. This review aims to summarize evidence for each of these mechanisms and to consider the potential utility of pyruvate as a therapeutic intervention for clinical management of cardiac insufficiency.

Selective inhibition of potassium-stimulated rat adrenal glomerulosa cells by ruthenium red

The effect of the cationic dye, ruthenium red (RR), on ionic fluxes, Ca2+ signal generation, and stimulation of aldosterone production was studied in isolated rat adrenal glomerulosa cells. In these cells, increased extracellular [K+] as well as angiotensin II (Ang II) elevate cytoplasmic Ca2+ concentration and thereupon activate steroidogenesis. However, the mode of action of the two stimuli are different: while a dihidropyridine-sensitive mechanism contributes to the response to both agonists, Ang II induces Ca2+ release from intracellular stores and causes capacitative Ca2+ influx, whereas K+ was recently shown to activate a plasma membrane Ca2+ current (Igl) independently of membrane depolarization. The difference is reflected in the sensitivity of the response of the cells to RR. The Ang II-induced Ca2+ signal and aldosterone production were not inhibited, but rather slightly potentiated by the dye. This potentiation was probably the consequence of the membrane-depolarizing effect of RR, due to the observed inhibition of the resting K+ conductance. Conversely, Ca2+ signal and aldosterone production were significantly reduced by RR when the cells were stimulated by moderately elevated [K+] (6-8 mM). Our patch clamp studies suggest that this effect was related to the inhibition of different voltage-dependent and -independent inward Ca2+ currents and indicates the functional importance of the latter in the signal transduction of the potassium-stimulated glomerulosa cell.

PUFA and aging modulate cardiac mitochondrial membrane lipid composition and Ca2+ activation of PDH

Aberrations in cell Ca2+ homeostasis have been known to parallel both changes in membrane lipid composition and aging. Previous work has shown that the lowered efficiency of work performance, which occurs in isolated hearts from rats fed a diet rich in n-6 polyunsaturated fatty acids (PUFA), relative to those fed n-3 PUFA, could be raised by mitochondrial (Mito) Ca2+ transport inhibition. We tested whether, after Ca2+-dependent stress, the Ca2+-dependent activation of pyruvate dehydrogenase (PDHA/PDHTotal) and Mito Ca2+ cycling could be manipulated by varying the ratio of n-3 to n-6 PUFA in Mito membranes in young (6 mo) and aged (24 mo) isolated rat hearts treated to n-3 or n-6 PUFA-rich diet. Inotropic stimulation by 1 microM norepinephrine (NE) of 24-mo n-6 PUFA-rich hearts elevated total Mito Ca2+ content 38% more than in 6-mo hearts (P < 0. 05). However, both the NE-induced rise in Mito Ca2+ and the difference in response between 6- and 24-mo hearts were partially abolished by n-3 PUFA treatment. NE increased the fractional activation of PDH by 44% above control levels in the 6-mo group compared with 49% in the 24-mo group after n-6 PUFA diet. However, NE stimulation of PDHA was attenuated by n-3 PUFA diet, attaining values only 29 and 23% above control levels in 6- and 24-mo mitochondria, respectively (P < 0.05). Global ischemia and reperfusion (I/R) in n-6 PUFA hearts gave rise to higher levels of total Mito Ca2+ concentration (P < 0.0001) and PDHA (P < 0.0001) compared with n-3 PUFA. Ruthenium red (3.4 microM) abolished the effects of I/R in all groups. With aging, heart Mito membrane phosphatidylcholine was increased after n-6 PUFA-rich diet (by approximately 15%, P < 0.05), whereas cardiolipin and n-3 PUFA content were diminished by 31% (P < 0.05) and 73% (P < 0.05), respectively. These effects were prevented by n-3 PUFA-rich diet. The present study, by directly manipulating the cardiac Mito membrane n-3-to-n-6 PUFA ratio, shows that the activation of Ca2+-dependent PDH can be augmented when the n-3-to-n-6 PUFA ratio is low (n-6 PUFA-rich diet; 24-mo hearts) or attenuated when this ratio is relatively high (n-3 PUFA-rich diet). We propose that one of the consequences of dietary-induced manipulation of membrane phospholipids and PUFAs may be the altered flux of Ca2+ across the Mito membrane and thus altered intramitochondrial Ca2+-dependent processes.

Ruthenium red-catalyzed degradation of peroxides can prevent mitochondrial oxidative damage induced by either tert-butyl hydroperoxide or inorganic phosphate

We have recently shown that ruthenium red, a non-competitive inhibitor of the mitochondrial Ca2+ uniporter, can reduce tert-butyl hydroperoxide via a Fenton-type reaction. In respiring mitochondrial preparations containing tert-butyl hydroperoxide, redox cycling of ruthenium red occurs and causes the amplification of methyl radical generation (Meinicke, A. R., Zavan, S. S., Ferreira, A. M. C., Vercesi, A. E., and Bechara, E. J. H. (1996) Arch. Biochem. Biophys. 328, 239-244). In this study we show that ruthenium red can act as an antioxidant preventing mitochondrial damage when the respiratory chain is reduced or when ascorbate is present. Ruthenium red can catalyze the degradation of hydrogen peroxide into H2O and O2. We show here that ruthenium red prevents both accumulation of mitochondrial generated H2O2 and swelling in the presence of the Ca2+ ionophore A23187. Under these conditions the damage induced by Ca2+ ions and either tert-butyl hydroperoxide or inorganic phosphate is promoted by mitochondrial-generated reactive oxygen species. Swelling induced by phenylarsine oxide, a thiol cross-linker, by a mechanism independent of free radicals is not inhibited by ruthenium red. These data provide evidence that the antioxidant behavior of ruthenium red under our conditions is due to its ability to destroy peroxides, which is related to its redox cycling and is prevalent over the Fenton-type reaction.

Mitochondrial free calcium transients during excitation-contraction coupling in rabbit cardiac myocytes

Mitochondrial free Ca2+ may regulate mitochondrial ATP production during cardiac exercise. Here, using laser scanning confocal microscopy of adult rabbit cardiac myocytes co-loaded with Fluo-3 to measure free Ca2+ and tetramethylrhodamine methylester to identify mitochondria, we measured cytosolic and mitochondrial Ca2+ transients during the contractile cycle. In resting cells, cytosolic and mitochondrial Fluo-3 signals were similar. During electrical pacing, transients of Fluo-3 fluorescence occurred in both the cytosolic and mitochondrial compartments. Both the mitochondrial and the cytosolic transients were potentiated by isoproterenol. These experiments show directly that mitochondrial free Ca2+ rises and falls during excitation-contraction coupling in cardiac myocytes and that changes of mitochondrial Ca2+ are kinetically competent to regulate mitochondrial metabolism on a beat-to-beat basis.

Effects of different oxidative insults on intermediary metabolism in isolated perfused rat hearts

13C and 31P NMR were used to evaluate exogenous substrate utilization and endogenous phosphate metabolites in perfused rat hearts exposed to tert-butylhydroperoxide (tert-BOOH) and hydrogen peroxide (H2O2). Both reagents caused a reduction in developed pressure compared to controls and, in agreement with previous 31P NMR data, had different effects on intracellular high-energy phosphates and glycolysis. 13C Isotopomer analysis of tissue extracts showed that H2O2 and tert-BOOH also had significantly different effects on substrate utilization by the citric acid cycle. The contribution of exogenous lactate and glucose to acetyl-CoA was 43% in controls and increased to over 80% in the presence of either oxidant. With tert-BOOH, exogenous glucose and lactate were both significant contributors to acetyl-CoA (44 +/- 2 and 41 +/- 3%). However, with H2O2, exogenous lactate supplied a much higher fraction of acetyl-CoA (72 +/- 2%) than glucose (9 +/- 1%). Also, when [2-(13)C] glucose was supplied, accumulation of [2-(13)C] and [5-(13)C] fructose 1,6-bisphosphate was observed in the presence of H2O2, indicating inhibition of glyceraldehyde-3-phosphate dehydrogenase. These results indicate that despite this glycolytic inhibition, H2O2 increased the utilization of pyruvate precursors when lactate was present as an alternative carbohydrate substrate.

Effects of calcium on mitochondrial NAD(P)H in paced rat ventricular myocytes

The response of the steady-state level of mitochondrial NAD(P)H of individual cardiac myocytes to substrate and to pharmacological alteration of intracellular calcium was investigated using a defined pacing protocol. Rapid pacing (5 Hz) reversibly decreased the NAD(P)H level and increased oxygen consumption whereas phosphocreatine and ATP levels did not change significantly. Verapamil plus NiCl2 blockade of calcium channels abolished contractions. Ryanodine, which prevents calcium-induced calcium release, also stopped cell contraction. NAD(P)H levels do not change in the absence of contraction. Blockade of sarcolemmal K+ channels did not stop contraction, and NAD(P)H levels reversibly decreased during rapid pacing. Thus rapid contractions are associated with a reversible decrease in NAD(P)H levels. Ruthenium red blockade of Ca2+ entry into mitochondria did not block contraction but significantly decreased NAD(P)H levels in both slowly paced (0.5 Hz) and rapidly paced cells. The simplest explanation of these data is that the steady-state reduction of NAD(P)H is strongly dependent on the rate of ATP utilization and not on sarcoplasmic Ca2+ levels when the oxygen and substrate supplies are not limiting and the intracellular calcium regulation is maintained. An effect of intracellular Ca2+ on NAD(P)H is observed only when Ca2+ entry into mitochondria is blocked with ruthenium red.

Ca(2+)- and GTP[gamma S]-induced translocation of the glucose transporter, GLUT-4, to the plasma membrane of permeabilized cardiomyocytes determined using a novel immunoprecipitation method

In cardiomyocytes glucose transport is activated not only by insulin but also by contractile activity that causes translocation of the glucose transporter, GLUT-4, from intracellular vesicles to the plasma membrane. The latter effect may possibly be mediated by intracellular Ca2+, as suggested by previous studies. To investigate the role of Ca2+, we permeabilized neonatal rat myocytes with alpha-toxin and incubated them for 1 h either at a pCa (i.e.--log10 [Ca2+]) of 8 (control) or at a pCa of 5 in the presence of adenosine 5'-triphosphate (ATP). Translocation of GLUT-4 was then monitored by a novel immunoprecipitation method using a peptide antibody directed against an exofacial (extracellular) loop of GLUT-4 (residues 58-80). Incorporation of GLUT-4 into the plasmalemma was stimulated 1.8-fold by 10 microM Ca2+ and 1.7-fold by insulin (as in the case of intact cells). The insulin effect was Ca2+ independent, i.e. it was identical in the absence and presence of Ca2+ (10 microM). Guanosine 5'-O-(3-thio-triphosphate) (GTP[gamma S]), which was inactive in intact cells, also caused translocation of GLUT-4 in permeabilized cardiomyocytes. Thus, incorporation of GLUT-4 into the plasma membrane was enhanced 2.5-fold by 200 microM GTP[gamma S] in the virtual absence of Ca2+ (pCa 8) and even 3.5-fold at 10 microM free Ca2+. We conclude that an increase in intracellular Ca2+ concentration increases GLUT-4 translocation of (permeabilized) cardiomyocytes to a similar extent as do insulin and GTP[gamma S] in the absence of Ca2+, but that the effects of Ca2+ and GTP[gamma S] may be additive.

Early cellular events couple covalent binding of reactive metabolites to cell killing by nephrotoxic cysteine conjugates

Addition of the nephrotoxic cysteine conjugate, S-(1,2-dichlorovinyl)-L-cysteine (DCVC), to the LLC-PK1 line of renal epithelial cells leads to covalent binding of reactive intermediates followed by thiol depletion, lipid peroxidation, and cell death (Chen et al., 1990, J. Biol. Chem., 265:21603-21611). The present study was designed to determine if increased intracellular free calcium might play a role in this pathway of DCVC-induced toxicity by comparing the temporal relationships among increased intracellular free calcium, lipid peroxidation, and cytotoxicity. Intracellular free calcium increased 1 hr after DCVC treatment, long before LDH release occurred. The elevation of intracellular free calcium and cytotoxicity was prevented by inhibiting DCVC metabolism with AOA. The cell-permeable chelators, Quin-2AM and EGTA-AM, prevented the toxicity. Pretreatment of cells with a nontoxic concentration of ionomycin increased intracellular free calcium and potentiated DCVC-induced LDH release. However, the antioxidant, DPPD, which blocks lipid peroxidation and toxicity, did not affect the increase in intracellular free calcium, whereas buffering intracellular calcium with Quin-2AM or EGTA-AM blocked both lipid peroxidation and toxicity without preventing the depletion of nonprotein sulfhydryls by DCVC. Ruthenium red, an inhibitor of mitochondrial calcium uptake, also blocked cell death. We hypothesize that covalent binding of the reactive fragment from DCVC metabolism leads to deregulation of intracellular calcium homeostasis and elevation of intracellular free calcium. Increased intracellular free calcium may in turn be coupled to mitochondrial damage and the accumulation of endogenous oxidants which cause lipid peroxidation and cell death.

Physiological role of mitochondrial Ca2+ transport

A model has been proposed in which mitochondrial Ca2+ ion transport serves to regulate mitochondrial matrix free Ca2+ ([Ca2+]m), with the advantage to the animal that this allows the regulation of pyruvate dehydrogenase and the tricarboxylate cycle in response to energy demand. This article examines recent evidence for dehydrogenase activation and for increases in [Ca2+]m in response to increased tissue energy demands, especially in cardiac myocytes and in heart. It critiques recent results on beat-to-beat variation in [Ca2+]m in cardiac muscle and also briefly surveys the impact of mitochondrial Ca2+ transport on transient changes in cytosolic free Ca2+ in excitable tissues. Finally, it proposes that a failure to elevate [Ca2+]m sufficiently in response to work load may underlie some cardiomyopathies of metabolic origin.

Effects of oxidant exposure on substrate utilization and high-energy phosphates in isolated rat hearts

The effects of a xanthine oxidase-mediated free radical-generating system containing purine and iron-loaded transferrin or solutions containing hydrogen peroxide and iron-loaded transferrin on substrate utilization and high-energy phosphates were evaluated by nuclear magnetic resonance (NMR) spectroscopy in isolated perfused rat hearts. Hearts were supplied with lactate, acetate, and glucose, and the contribution of each substrate to acetyl coenzyme A was measured in control hearts and in the presence of a free radical-generating system. Perfused hearts were monitored by 31P NMR, and tissue extracts were analyzed by 13C NMR. Free radicals decreased the phosphocreatine and beta-ATP peak areas and reduced contractile function. Under control conditions, lactate, acetate, and endogenous sources were the major contributors of acetyl coenzyme A units, with only 5% originating from glucose. In the presence of a xanthine oxidase-mediated free radical-generating system, the glucose contribution increased to 54%, while contributions from acetate and endogenous sources were significantly reduced. Both 13C and 31P NMR analyses showed no significant accumulation of glycolytic sugar phosphates, suggesting little inhibition of glyceraldehyde-3-phosphate dehydrogenase. The increased contribution of glucose to the tricarboxylic acid cycle relative to acetate and endogenous sources is consistent with activation of pyruvate dehydrogenase. In contrast, hearts exposed to a hydrogen peroxide-based free radical-generating system showed an increase in lactate utilization, a decrease in acetate utilization, and no change in glucose utilization compared with control hearts. Glycolytic sugar phosphates were found to accumulate, suggesting possible inhibition of glyceraldehyde-3-phosphate. Thus, different radicals or their metabolites may have varying effects on myocardial metabolism.

The role of cytosolic free calcium in the regulation of pyruvate dehydrogenase in synaptosomes

Calcium homeostasis and mitochondrial oxidative metabolism interact closely in brain and both processes are impaired during hypoxia. Since the regulation of the pyruvate dehydrogenase complex (PDHC) may link these two processes, the relation of cytosolic free calcium ([Ca2+]i) to the activation state of PDHC (PDHa) was assessed in isolated nerve terminals (i.e. synaptosomes) under conditions that alter [Ca2+]i. K+ depolarization elevated [Ca2+]i and PDHa and both responses required external calcium. Treatment with KCN, an in vitro model of hypoxia decreased ATP and elevated [Ca2+]i and PDHa. Furthermore, in the presence of KCN, PDHa became more sensitive to K+ depolarization as indicated by larger changes in PDHa than in [Ca2+]i. The calcium ionophore Br-A23187 elevated [Ca2+]i, but did not affect PDHa. K+ depolarization elevated [Ca2+]i and PDHa even if [Ca2+]i was elevated by prior addition of ionophore or KCN. Previous in vivo studies by others show that PDHa is altered during and after ischemia. The current in vitro results suggest that hypoxia, only one component of ischemia, is sufficient to increase PDHa. These data also further support the notion that PDHa is regulated by [Ca2+]i as well as by other factors such as ATP. Our results are consistent with the concept that PDHa in nerve endings may be affected by [Ca2+]i and that these two processes are clearly linked.

Mitochondrial pyruvate transport in working guinea-pig heart. Work-related vs. carrier-mediated control of pyruvate oxidation

Myocardial pyruvate oxidation is work- or calcium-load-related, but control of pyruvate dehydrogenase (PDH) by the specific mitochondrial pyruvate transporter has also been proposed. To test the transport hypothesis distribution of pyruvate across the cell membrane as well as rates of mitochondrial pyruvate net transport plus oxidation were examined in isolated perfused but stable and physiologically working guinea-pig hearts. 150 microM-1.2 mM alpha-cyanohydroxycinnamate proved to specifically block mitochondrial pyruvate uptake in these hearts. When perfusate glucose as cytosolic pyruvate precursor was supplied in combination with octanoate (0.2 or 0.5 mM) as diffusible alternative fatty acid substrate, alpha-cyanohydroxycinnamate produced up to 20- and 3-fold increases in pyruvate and lactate efflux, respectively. Cinnamates did not alter myocardial hemodynamics nor sarcolemmal pyruvate and lactate export. In contrast the tested concentrations of cinnamate produced reversible, dose-dependent decreases in 14CO2 production from [1-14C]pyruvate or [U-14C]glucose by inhibiting mitochondrial pyruvate uptake. Linear least-squares estimates of available cinnamate-sensitive total pyruvate transport potential yielded rates close to 110 mumol/min per g dry mass at S0.5 approximately 120 microM, which compared reasonably well with literature values from isolated cardiac mitochondria. This transport potential was severalfold larger than total extractable myocardial PDH activity of approximately 32 mumol/min per g dry mass at 37 degrees C. Even when cytosolic pyruvate levels were in the lower physiologic range of about 90 microM, pyruvate oxidation readily kept pace with mitochondrial respiration over a wide range of workload and inotropism. Furthermore, dichloroacetate, a selective activator of PDH, stimulated pyruvate oxidation without affecting myocardial O2 consumption, regardless of the metabolic or inotropic state of the hearts. Consequently, little or no regulatory function with regard to pyruvate oxidation could be assigned to the native mitochondrial pyruvate carrier of the working heart. Therefore, mitochondrial pyruvate-H+ symport was the normal, highly efficient (rather than controlling) mechanism for pyruvate entry into the mitochondria where PDH regulation controlled pyruvate oxidation.

Retarded diffusion of ADP in cardiomyocytes: possible role of mitochondrial outer membrane and creatine kinase in cellular regulation of oxidative phosphorylation

Possible reasons for retarded intracellular diffusion of ADP were investigated. The isolated skinned cardiac fibers were used to study apparent kinetic parameters for externally added ADP in control of mitochondrial respiration. Participation of myosin-ATPase in binding of ADP within cells as it was supposed earlier (Saks, V.A., Belikova, Yu.O. and Kuznetsov, A.V. (1991) Biochim. Biophys. Acta 1074, 302-311) was completely excluded, since myosin-deprived skinned cardiac fibers ('ghosts') displayed the same kinetic parameters as intact ones (Kmapp for ADP about 300 microM). Significantly lower apparent Km values were obtained for fibers with osmotically disrupted outer mitochondrial membrane (25-35 microM), which was close to that observed for isolated heart mitochondria. The data obtained are in favor of limitation of ADP movement via anion-selective low-conductance porine channels in the outer membrane of mitochondria. It is proposed that the permeability of this membrane is controlled by some unknown intracellular factor(s). In the presence of saturating concentrations of creatine (25 mM) the apparent Km for ADP significantly decreases due to coupling of creatine kinase and oxidative phosphorylation reactions in mitochondria. This coupling is not observed in KCl medium in which mitochondrial creatine kinase is detached from the membrane. It is concluded that in the cells in-vivo ADP movement between cytoplasm and intramitochondrial space is controlled by low-conductivity anion channels in the outer membrane. Thus, the mitochondrial creatine kinase reaction coupled to the adenine nucleotide translocase is an important mechanism in control of oxidative phosphorylation in vivo due to its ability to manifold amplify these very weak ADP signals from cytoplasm.

Pyruvate dehydrogenase dephosphorylation in rat brain synaptosomes and mitochondria: evidence for a calcium-mediated effect in response to depolarization, and variations due to ageing

The phosphorylation state of P42, the phosphorylated, catalytically inactive, alpha-subunit of pyruvate dehydrogenase (PDH), decreased markedly (42.4%) in response to K(+)-depolarization of synaptosomes. The dephosphorylation was rapid (5-15 s), calcium-dependent and could also be observed in isolated mitochondria exposed to a rise in extramitochondrial calcium, suggesting that P42 dephosphorylation may act as a calcium sensor in the mitochondrial matrix. The depolarization-dependent dephosphorylation rate of P42 was decreased in synaptosomes derived from 24-month-old animals with respect to 3-month-old adults. The relevance of these results in terms of PDH activation during ageing is discussed.

Divergent kinetics of 201Tl and 99mTc-SESTAMIBI in cultured chick ventricular myocytes during ATP depletion

BACKGROUND

Thallous chloride (201Tl) and hexakis(2-methoxyisobutyl isonitrile) technetium (I) (99mTc-SESTAMIBI) are myocardial perfusion imaging agents with biological properties that also reflect tissue viability. Initial myocellular uptake rates of 201Tl reflect activity of Na,K-ATPase, whereas those of 99mTc-SESTAMIBI reflect mean plasma membrane potential.

METHODS AND RESULTS

To better understand the mechanistic responses of these tracers to myocellular injury, cultured chick embryo cardiac myocytes were metabolically inhibited in iodoacetate (1 mM) and rotenone (10 microM) for up to 2 hours, and initial uptake rates of each agent were determined at successive intervals along with correlative cellular contents of ATP, sodium, and potassium and lactate dehydrogenase release. ATP content fell from 30.5 +/- 1.4 to 2.7 +/- 0.9 nmol.(mg protein)-1 within 2 minutes, whereas sodium and potassium contents ran down their thermodynamic gradients more slowly (t 1/2 approximately 60 minutes). Modestly severe cell injury was produced at 2 hours as estimated by lactate dehydrogenase release (18% of total). Initial uptake rates of 201Tl declined from 6.9 +/- 0.8 to 4.0 +/- 0.4 fmol.(mg protein)-1.(nMo)-1.(min)-1 by 20 minutes and remained depressed and ouabain (100 microM)-insensitive at 30 +/- 13% of control. Conversely, initial uptake rates of 99mTc-SESTAMIBI increased from 10.6 +/- 0.8 to 15.0 +/- 0.6 fmol.(mg protein)-1.(nMo)-1.(min)-1 within 10 minutes, remained elevated for 40-60 minutes, and later declined to low values. Injury-induced enhancement of initial uptake rates of 99mTc-SESTAMIBI were insensitive to ouabain (100 microM), carbonyl cyanide-m-chlorophenyl hydrazone (5 microM), and valinomycin (1 microgram/ml) but were significantly inhibited by 130 mM Ko buffer, Ba2+ (1 mM), glybenclamide (100 microM), and quinacrine (10 microM).

CONCLUSIONS

Uptake rates of 201Tl monotonically decline, correlating with Na-K pump inhibition from ATP depletion. Conversely, uptake rates of 99mTc-SESTAMIBI at first increase above control for 40-60 minutes, indicating a mean plasma membrane hyperpolarization possibly resulting from opening of ATP-sensitive and arachidonic acid-activated potassium channels, before declining to low values with more severe cell injury. Correlative non-flow-dependent relations between 201Tl and 99mTc-SESTAMIBI contain information regarding the degree of myocellular injury.

Dehydrogenase activation by Ca2+ in cells and tissues

The activation of intramitochondrial dehydrogenases by Ca2+ provides a link between the intensity of work performance by a tissue and the activity of pyruvate dehydrogenase and the tricarboxylate cycle, and hence the rate of ATP production by the mitochondria. Several aspects of this model of the control of oxidative phosphorylation are examined in this article, with particular emphasis on mitochondrial functioning in situ in cardiac myocytes and in the intact heart. Recent use of the fluorescent Ca2+ chelating agents indo-1 and fura-2 has allowed a more quantitative description of the dependence of dehydrogenase activity upon concentration of free intramitochondrial Ca2+, in experiments with isolated mitochondria. Further, a novel technique developed by Miyata et al. has allowed description of free intramitochondrial Ca2+ within a single cardiac myocyte, and the conclusion that this parameter changes in response to electrical excitation of the cell over a range which would be expected to give substantial modulation of dehydrogenase activity.

The role of mitochondrial Ca2+ transport and matrix Ca2+ in signal transduction in mammalian tissues

The pyruvate, NAD(+)-isocitrate and 2-oxoglutarate dehydrogenases are key regulatory enzymes in intramitochondrial oxidative metabolism in mammalian tissues, and can all be activated by increases in Ca2+ in the micromolar range. There is now mounting evidence that hormones and other stimuli which act by increasing cytosolic Ca2+ also, as a result, cause increases in mitochondrial matrix Ca2+ and hence activation of these enzymes, suggesting that the primary physiological function of mitochondrial Ca2(+)-transport is to be involved in this relay mechanism. This may also explain how in such circumstances rates of ATP production may be increased to meet the greater demand, but without any decreases in ATP/ADP occurring.

Thyroid status and beta-agonistic effects on cytosolic calcium concentrations in single rat cardiac myocytes activated by electrical stimulation or high-K+ depolarization

The effects of the thyroid status on the cytosolic free Ca2+ concentration ([Ca2+]i) in single cardiomyocytes were studied at rest and during contraction. The mean resting [Ca2+]i increased significantly from the hypothyroid (45 +/- 4 nM) through the euthyroid (69 +/- 12 nM) to the hyperthyroid condition (80 +/- 11 nM) at extracellular Ca2+ concentrations ([Ca2+]o) up to 2.5 mM. At [Ca2+]o above 2.5 mM the differences in [Ca2+]i between the groups became less. The amplitude of the Ca2+ transients became higher in all groups with increasing [Ca2+]o (1, 2.5 and 5 mM), and was highest at all [Ca2+]o in hyperthyroid myocytes. The beta-agonist isoprenaline elevated peak [Ca2+]i during contraction and increased the rate of the decay of the Ca2+ transients to a greater extent in hypothyroid myocytes than in hyperthyroid myocytes. Depolarization with high [K+]o induced a large but transient [Ca2+]i overshoot in hypothyroid myocytes, but not in hyperthyroid myocytes, before a new elevated steady-state [Ca2+]i was reached, which was not different between the groups. When isoprenaline was added to K+ o-depolarized myocytes after a steady state was reached, a significantly larger extra increase in [Ca2+]i was measured in the hypothyroid group (28%) compared with the hyperthyroid group (8%). It is concluded that in cardiac tissue exposed to increasing amounts of thyroid hormones (1) [Ca2+]i increases at rest and during contraction in cardiomyocytes and (2) interventions which favour Ca2+ entry into the cytosol [( Ca2+]o elevation, high [K+]o, beta-agonists) tend to have less impact on Ca2+ homoeostasis.

Hypoxia-reoxygenation induced increase in cellular Ca2+ in myocytes and perfused hearts: the role of mitochondria

Reoxygenation of isolated rat cardiac myocytes following a period of hypoxia and substrate deprivation resulted in a 1.5-2-fold increase in the total Ca2+ content which could be inhibited by 1 microM antimycin A or ruthenium red (50% inhibition at 2.5 microM). This increase in Ca2+ content was not accompanied by any release of creatine kinase into the medium. Treatment of reoxygenated cells with digitonin also resulted in an antimycin A-sensitive increase in Ca2+ but this was inhibited by a lower concentration of ruthenium red (50% inhibition at 0.25 microM) and was associated with a substantial release of creatine kinase from the cells. It is concluded that the reoxygenation-stimulated increase in Ca2+ is dependent on functioning mitochondria and does not occur as a result of physical damage to the sarcolemma. In a parallel series of experiments, the effects of antimycin A and ruthenium red on the reoxygenation-induced increase in Ca2+ and release of cytosolic contents in the perfused heart (the oxygen paradox) were also investigated. As was observed with the isolated myocytes, each of the compounds significantly reduced the magnitude of the Ca2+ increase that occurred on reoxygenation: the compounds also reduced the extent of release of cell contents in the perfused heart. The implications of these results for the series of events occurring on reoxygenation of the hypoxic myocardium are discussed.

The role of Ca2+ ions in the regulation of intramitochondrial metabolism and energy production in rat heart

In the heart and other mammalian tissues, there are three exclusively intramitochondrial dehydrogenases that occupy key regulatory sites in oxidative metabolism which can be activated by increases in Ca2+ in the approximate range 0.05-5 microM; they are the pyruvate, NAD+-isocitrate and 2-oxoglutarate dehydrogenases. Activation of these enzymes can be demonstrated within intact mitochondria, incubated under expected physiological conditions, when the extramitochondrial concentration of Ca+ is raised within the expected physiological range. Recent studies with fura-2-loaded mitochondria have established that matrix Ca2+ is indeed in the 0.02-2 microM range as the enzymes are activated. There is now good evidence that in the rat heart, increases in cytoplasmic [Ca2+] caused by various inotropic agents result in increases in intramitochondrial Ca2+ and activation of these dehydrogenases. It is argued therefore that matrix Ca2+ may thus be a key regulator of oxidative phosphorylation under such circumstances. The major advantage of such a mechanism of dehydrogenase-based control of this process would be to the energy homeostasis of the cell by allowing stimulated ATP production without the need to decrease the ATP/ADP ratio. Therefore it is also proposed that the major function of the mitochondrial Ca2+-transport system is to regulate matrix Ca2+, and that the ability of mitochondria to buffer the extramitochondrial concentration of Ca2+ may thus only be reserved for pathophysiological conditions of abnormal sarcolemmal Ca2+ influx as perhaps may occur in ischaemia-reperfusion.

Interrelationships between glucose metabolism, energy state, and the cytosolic free calcium concentration in cortical synaptosomes from the guinea pig

The stoichiometries of glycolysis and pyruvate oxidation were determined in cortical synaptosomes under varying rates of ATP consumption. Glycolysis was measured by using D-3-[3H]glucose as a marker and pyruvate oxidation by using D-3,4-[14C]glucose, which has to be metabolized to 1-[14C]pyruvate before being decarboxylated by the pyruvate dehydrogenase complex of intrasynaptosomal mitochondria. Cytosolic free Ca2+ concentration [( Ca2+]c) was determined in parallel and was manipulated by using EGTA in the incubation. The results show that in nonstimulated synaptosomes glycolysis and pyruvate oxidation are tightly coupled and stoichiometric. In the absence of Ca2+, when [Ca2+]c drops from 260 nM to 40 nM, glucose utilization increases, following the increase in energy demand, which has been shown to be due to elevated Na+ cycling. KCl depolarization, veratridine, and a mitochondrial uncoupler, carbonyl cyanide m-chlorophenylhydrazone, all stimulate glycolysis and pyruvate oxidation stoichiometrically, independently of the presence of external Ca2+. A rise in [Ca2+]c, therefore, is not required to regulate mitochondrial pyruvate metabolism. It is concluded that synaptosomes exhibit a high degree of respiratory control, that they rely on glucose oxidation for their energetics, and that stimulation of energy production can be achieved independently of changes in [Ca2+]c.

Direct evidence for a role of intramitochondrial Ca2+ in the regulation of oxidative phosphorylation in the stimulated rat heart. Studies using 31P n.m.r. and ruthenium red

1. The concentrations of free ATP, phosphocreatine (PCr), Pi, H+ and ADP (calculated) were monitored in perfused rat hearts by 31P n.m.r. before and during positive inotropic stimulation. Data were accumulated in 20 s blocks. 2. Administration of 0.1 microM-(-)-isoprenaline resulted in no significant changes in ATP, transient decreases in PCr, and transient increases in ADP and Pi. However, the concentrations of all of these metabolites returned to pre-stimulated values within 1 min, whereas cardiac work and O2 uptake remained elevated. 3. In contrast, in hearts perfused continuously with Ruthenium Red (2.5 micrograms/ml), a potent inhibitor of mitochondrial Ca2+ uptake, administration of isoprenaline caused significant decreases in ATP, and also much larger and more prolonged changes in the concentrations of ADP, PCr and Pi. In this instance values did not fully return to pre-stimulated concentrations. Administration of Ruthenium Red alone to unstimulated hearts had minor effects. 4. It is proposed that, in the absence of Ruthenium Red, the transmission of changes in cytoplasmic Ca2+ across the mitochondrial inner membrane is able to maintain the phosphorylation potential of the heart during positive inotropic stimulation, through activation of the Ca2+-sensitive intramitochondrial dehydrogenases (pyruvate, NAD+-isocitrate and 2-oxoglutarate dehydrogenases) leading to enhanced NADH production. 5. This mechanism is unavailable in the presence of Ruthenium Red, and oxidative phosphorylation must be stimulated primarily by a fall in phosphorylation potential, in accordance with the classical concept of respiratory control. However, the full oxidative response of the heart to stimulation may not be achievable under such circumstances.