Mitochondrial calcium transporting pathways during hypoxia and reoxygenation in single rat cardiomyocytes

Metabolic inhibition-induced transient Ca2+ increase depends on mitochondria in a human proximal renal cell line

During ischemia or hypoxia an increase in intracellular cytosolic Ca(2+) induces deleterious events but is also implicated in signaling processes triggered in such conditions. In MDCK cells (distal tubular origin), it was shown that mitochondria confer protection during metabolic inhibition (MI), by buffering the Ca(2+) overload via mitochondrial Na(+)-Ca(2+) exchanger (NCX). To further assess this process in cells of human origin, human cortical renal epithelial cells (proximal tubular origin) were subjected to MI and changes in cytosolic Ca(2+) ([Ca(2+)](i)), Na(+), and ATP concentrations were monitored. MI was accomplished with both antimycin A and 2-deoxyglucose and induced a 3.5-fold increase in [Ca(2+)](i), reaching 136.5 +/- 15.8 nM in the first 3.45 min. Subsequently [Ca(2+)](i) dropped and stabilized to 62.7 +/- 7.3 nM by 30 min. The first phase of the transient increase was La(3+) sensitive, not influenced by diltiazem, and abolished when mitochondria were deenergized with the protonophore carbonylcyanide p-trifluoromethoxyphenylhydrazone. The subsequent recovery phase was impaired in a Na(+)-free medium and weakened when the mitochondrial NCX was blocked with 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP-37157). Thus Ca(2+) entry is likely mediated by store-operated Ca(2+) channels and depends on energized mitochondria, whereas [Ca(2+)](i) recovery relied partially on the activity of mitochondrial NCX. These results indicate a possible mitochondrial-mediated signaling process triggered by MI, support the hypothesis that mitochondrial NCX has an important role in the Ca(2+) clearance, and overall suggest that mitochondria play a preponderant role in the regulation of responses to MI in human renal epithelial cells.

Mitochondrial channelopathies in aging

Defects in ion channels (channelopathies) are increasingly found in a large spectrum of human pathologies including aging. Mutations in genes encoding ion channel proteins, which disrupt channel function, are the most commonly identified cause of channelopathies. Mutations in associated proteins, alterations in the expression of ion channels, or changes in the activity of non-mutated channel genes or associated proteins can also produce acquired channelopathies. Mitochondria, the powerhouse of the cells, are considered to be the most important cellular organelles to contribute to aging mainly because of their role in the production of reactive oxygen species in the initiation of apoptotic cell remodeling and in efficient ATP synthesis. During the past 50 years, multiple ion channels or transporters have been found in mitochondria, and the relationship between the activity of these channels and cellular aging, as well as the overall cellular biological function, has been intensively studied in a number of cell types and animal models. In this review, we discuss the better characterized mitochondrial ion channels whose dysfunction (mitochondrial channelopathies) may affect or accelerate the aging processes. These channels include the mitochondrial ATP-sensitive potassium channel (mitoK(ATP)), Ca(2+) transporters, voltage-dependent anion channel, and the mitochondrial permeability transition pore (mitoPTP).

Attenuation of mitochondrial, but not cytosolic, Ca2+ overload reduces myocardial injury induced by ischemia and reperfusion

AIM

Attenuation of mitochondrial Ca2+ ([Ca2+]m), but not cytosolic Ca2+ ([Ca2+]c), overload improves contractile recovery. We hypothesized that attenuation of [Ca2+]m, but not [Ca2+]c, overload confers cardioprotection against ischemia/reperfusion-induced injury.

METHODS

Infarct size from isolated perfused rat heart, cell viability, and electrically-induced Ca2+ transient in isolated rat ventricular myocytes were measured. We determined the effects of BAPTA-AM, a Ca2+ chelator, at concentrations that abolish the overload of both [Ca2+]c and [Ca2+]m, and ruthenium red, an inhibitor of mitochondrial uniporter of Ca2+ transport, at concentrations that abolish the overload of [Ca2+]m, but not [Ca2+]c, on cardiac injury induced by ischemia/reperfusion.

RESULTS

Attenuation of both [Ca2+]m and [Ca2+]c by BAPTA-AM, and attenuation of [Ca2+]m, but not [Ca2+]c, overload by ruthenium red, reduced the cardiac injury observations, indicating the importance of [Ca2+]m in cardioprotection and contractile recovery in response to ischemia/reperfusion.

CONCLUSION

The study has provided unequivocal evidence using a cause-effect approach that attenuation of [Ca2+]m, but not [Ca2+]c, overload is responsible for cardioprotection against ischemia/reperfusion-induced injury. We also confirmed the previous observation that attenuation of [Ca2+]m, but not [Ca2+]c, by ruthenium red improves contractile recovery following ischemia/reperfusion.

Allopurinol modulates reactive oxygen species generation and Ca2+ overload in ischemia-reperfused heart and hypoxia-reoxygenated cardiomyocytes

Myocardial oxidative stress and Ca2+ overload induced by ischemia-reperfusion may be involved in the development and progression of myocardial dysfunction in heart failure. Xanthine oxidase, which is capable of producing reactive oxygen species, is considered as a culprit regarding ischemia-reperfusion injury of cardiomyocytes. Even though inhibition of xanthine oxidase by allopurinol in failing hearts improves cardiac performance, the regulatory mechanisms are not known in detail. We therefore hypothesized that allopurinol may prevent the xanthine oxidase-induced reactive oxygen species production and Ca2+ overload, leading to decreased calcium-responsive signaling in myocardial dysfunction. Allopurinol reversed the increased xanthine oxidase activity in ischemia-reperfusion injury of neonatal rat hearts. Hypoxia-reoxygenation injury, which simulates ischemia-reperfusion injury, of neonatal rat cardiomyocytes resulted in activation of xanthine oxidase relative to that of the control, indicating that intracellular xanthine oxidase exists in neonatal rat cardiomyocytes and that hypoxia-reoxygenation induces xanthine oxidase activity. Allopurinol (10 microM) treatment suppressed xanthine oxidase activity induced by hypoxia-reoxygenation injury and the production of reactive oxygen species. Allopurinol also decreased the concentration of intracellular Ca2+ increased by enhanced xanthine oxidase activity. Enhanced xanthine oxidase activity resulted in decreased expression of protein kinase C and sarcoendoplasmic reticulum calcium ATPase and increased the phosphorylation of extracellular signal-regulated protein kinase and p38 kinase. Xanthine oxidase activity was increased in both ischemia-reperfusion-injured rat hearts and hypoxia-reoxygenation-injured cardiomyocytes, leading to reactive oxygen species production and intracellular Ca2+ overload through mechanisms involving p38 kinase and extracellular signal-regulated protein kinase (ERK) via sarcoendoplasmic reticulum calcium ATPase (SERCA) and protein kinase C (PKC). Xanthine oxidase inhibition with allopurinol modulates reactive oxygen species production and intracellular Ca2+ overload in hypoxia-reoxygenation-injured neonatal rat cardiomyocytes.

Calcium ionophore A23187 action on cardiac myocytes is accompanied by enhanced production of reactive oxygen species

We show that rat neonatal cardiac myocytes exposed to 1 micromol/l of the calcium ionophore A23187 respond with an enhanced production of reactive oxygen species (ROS). This dose is not cytotoxic to the myocytes. A higher concentration (10 micromol/l) evokes less ROS production and is significantly cytotoxic 24 h after exposure, but not immediately after removal of the A23187, when ROS are measured. Both cell death and the decrease in mitochondrial potential are only partially sensitive to MPT inhibitor cyclosporin A. Experiments performed to elucidate the sources of ROS included use of the nitric oxide synthase (NOS) inhibitor L-NAME; NOS involvement was excluded. Experiments with the oxidative phosphorylation uncoupler CCCP revealed that mitochondria are at least partially responsible for the observed effect. Further studies with cyclooxygenase (COX) and lipoxygenase (LOX) inhibitors (indomethacin and MK886, respectively) showed that these enzymes could also be sources of ROS when the calcium level is elevated. Their effect appeared to be independent of phospholipase A(2) inhibition, suggesting that COX and LOX stimulation is not due to elevated substrate (arachidonic acid) concentration but rather to a direct effect of calcium.

Involvement of Na+/H+ exchanger in hypoxia/re-oxygenation-induced neonatal rat cardiomyocyte apoptosis

Although increased Na(+)/H(+) exchanger type-1 (NHE-1) activity has been implicated in the pathogenesis of myocardial infarction, the role of NHE-1 in induction of apoptosis, and the potential mechanisms involved have not been fully characterized. This study tested the hypothesis that NHE-1 activity is involved in hypoxia (H)/re-oxygenation (Re)-induced cardiomyocyte apoptosis by increasing mitochondrial Ca(2+) ([Ca(2+)]m). Primary cultured neonatal rat cardiomyocytes were subjected to 4.5 h of H followed by 12 h of Re. Relative to H alone, the level of X-rhod-1 acetoxymethyl (AM)-labeled [Ca(2+)]m was increased, and the frequency of cell death (propidium iodide (PI) staining) and apoptotic cells (terminal deoxynucleotidyl transferase (TdT)-mediated-UTP nick end labeling [TUNEL]), confirmed by Annexin-V, were augmented at the end of Re, along with appearance of cytosolic cytochrome c, activation of caspase-3, and increased ratio of Bax and Bcl-2. Addition of cariporide (20 micromol/l), a well-known NHE-1 inhibitor, to cultured cells before H significantly reduced [Ca(2+)]m, the number of PI and TUNEL positive cells relative to the levels at end of Re, but did not completely eliminate these changes compared to Sham control. There was a strong trend for attenuation in increased levels of [Ca(2+)]m, and the number of PI and TUNEL positive cells when same dose of cariporide was added only at Re, but the difference in these variables did not reach significance. In contrast, the levels of [Ca(2+)]m and the number of PI and TUNEL positive cells were significantly reduced to a level comparable to Sham control when cariporide (20 micromol/l) was administered before H and during Re, respectively, associated with a reduction in cytosolic cytochrome c, caspase-3 activity and ratio of Bax and Bcl-2. In conclusion, these data suggest that NHE-1 is involved in induction of cardiomyocyte apoptosis during both H and Re through a [Ca(2+)]m-dependent manner, thereby resulting in activation of cytochrome c-caspase-3 signaling pathways.

Ca2+ uptake in mitochondria occurs via the reverse action of the Na+/Ca2+ exchanger in metabolically inhibited MDCK cells

In ischemic or hypoxic tissues, elevated Ca2+ levels have emerged as one of the main damaging agents among other Ca2+-independent mechanisms of cellular injury. Because mitochondria, besides the endoplasmic reticulum, play a key role in the maintainance of cellular Ca2+ homeostasis, alterations in the mitochondrial Ca2+ content ([Ca2+]m) were monitored in addition to changes in cytosolic Ca2+ concentration ([Ca2+]i) during metabolic inhibition (MI) in renal epithelial Madin-Darby canine kidney (MDCK) cells. [Ca2+]i and [Ca2+]m were monitored via, respectively, fura 2 and rhod 2 measurements. MI induced an increase in [Ca2+]i reaching 631+/-78 nM in approximately 20 min, followed by a decrease to 118+/-9 nM in the next approximately 25 min. A pronounced drop in cellular ATP levels and a rapid increase in intracellular Na+ concentrations in the first 20 min of MI excluded Ca2+ efflux in the second phase via plasma membrane ATPases or Na+/Ca2+ exchangers (NCE). Mitochondrial rhod 2 intensities increased to 434+/-46% of the control value during MI, indicating that mitochondria sequester Ca2+ during MI. The mitochondrial potential (deltapsim) was lost in 20 min of MI, excluding mitochondrial Ca2+ uptake via the deltapsim-dependent mitochondrial Ca2+ uniporter after 20 min of MI. Under Na+-free conditions, or when CGP-37157, a specific inhibitor of the mitochondrial NCE, was used, no drop in [Ca2+]i was seen during MI, whereas the MI-induced increase in mitochondrial rhod 2 fluorescence was strongly reduced. To our knowledge, this study is the first to report that in metabolically inhibited renal epithelial cells mitochondria take up Ca2+ via the NCE acting in the reverse mode.

Time related changes in calcium handling in the isolated ischemic and reperfused rat heart

The main aim of this study was to assess the kinetics of intracellular free calcium (Ca(2+)i) handling by isolated rat hearts rendered ischemic for 30 min followed by 30 min of reperfusion analyzing the upstroke and downslope of the Ca(2+)i transient. Changes in mechanical performance and degradation of membrane phospholipids--estimated by tissue arachidonic acid content--were correlated with Ca(2+)i levels of the heart. The fluorescence ratio technique was applied to estimate Ca(2+)i. The disappearance of mechanical activity of the heart preceded that of the Ca(2+)i transient in the first 2 min of ischemia. The slope of upstroke of the Ca(2+)i transient, reflecting Ca2+ release, decreased by 60%, while the duration of the downslope of the transient, reflecting Ca2+ sequestration, expressed a significant prolongation (105 +/- 17 vs. 149 +/- 39 msec) during the first 3 min of ischemia. At about 20 min of ischemia end-diastolic pressure expressed a 3.5-fold increase (contracture) when the fluorescence ratio showed a 2-fold elevation. Reperfusion was accompanied with a further precipitous increase in end-diastolic pressure, while resting Ca(2+)i remained at end-ischemic levels. Increases in the arachidonic acid (AA) content of the ischemic and postischemic hearts were proportional to Ca(2+)i levels. In summary, the present findings indicate that both calcium release and removal are hampered during the early phase of ischemia. Moreover, a critical level of Ca(2+)i and a critical duration of ischemia may exist to provoke contracture of the heart. Upon reperfusion the hearts show membrane phospholipid degradation and signs of stunning exemplified by elevated AA levels, partial recovery of Ca(2+)i handling and sustained depression of mechanical performance.

Effects of fatty acids in isolated mitochondria: implications for ischemic injury and cardioprotection

Fatty acids accumulate during myocardial ischemia and are implicated in ischemia-reperfusion injury and mitochondrial dysfunction. Because functional recovery after ischemia-reperfusion ultimately depends on the ability of the mitochondria to recover membrane potential (DeltaPsim), we studied the effects of fatty acids on DeltaPsim regulation, cytochrome c release, and Ca2+ handling in isolated mitochondria under conditions that mimicked aspects of ischemia-reperfusion. Long-chain but not short-chain free fatty acids caused a progressive and reversible (with BSA) increase in inner membrane leakiness (proton leak), which limited mitochondrial ability to support DeltaPsim. In comparison, long-chain activated fatty acids promoted 1). a slower depolarization that was not reversible with BSA, 2). cytochrome c loss that was unrelated to permeability transition pore opening, and 3). inhibition of the adenine nucleotide translocator. Together, these results impaired both mitochondrial ATP production and Ca2+ handling. Diazoxide, a selective opener of mitochondrial ATP-dependent potassium (KATP) channels, partially protected against these effects. These findings indicate that long-chain fatty acid accumulation during ischemia-reperfusion may predispose mitochondria to cytochrome c loss and irreversible injury and identify a novel cardioprotective action of diazoxide.

An integrated model of cardiac mitochondrial energy metabolism and calcium dynamics

We present an integrated thermokinetic model describing control of cardiac mitochondrial bioenergetics. The model describes the tricarboxylic acid (TCA) cycle, oxidative phosphorylation, and mitochondrial Ca(2+) handling. The kinetic component of the model includes effectors of the TCA cycle enzymes regulating production of NADH and FADH(2), which in turn are used by the electron transport chain to establish a proton motive force (Delta mu(H)), driving the F(1)F(0)-ATPase. In addition, mitochondrial matrix Ca(2+), determined by Ca(2+) uniporter and Na(+)/Ca(2+) exchanger activities, regulates activity of the TCA cycle enzymes isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase. The model is described by twelve ordinary differential equations for the time rate of change of mitochondrial membrane potential (Delta Psi(m)), and matrix concentrations of Ca(2+), NADH, ADP, and TCA cycle intermediates. The model is used to predict the response of mitochondria to changes in substrate delivery, metabolic inhibition, the rate of adenine nucleotide exchange, and Ca(2+). The model is able to reproduce, qualitatively and semiquantitatively, experimental data concerning mitochondrial bioenergetics, Ca(2+) dynamics, and respiratory control. Significant increases in oxygen consumption (V(O(2))), proton efflux, NADH, and ATP synthesis, in response to an increase in cytoplasmic Ca(2+), are obtained when the Ca(2+)-sensitive dehydrogenases are the main rate-controlling steps of respiratory flux. These responses diminished when control is shifted downstream (e.g., the respiratory chain or adenine nucleotide translocator). The time-dependent behavior of the model, under conditions simulating an increase in workload, closely reproduces experimentally observed mitochondrial NADH dynamics in heart trabeculae subjected to changes in pacing frequency. The steady-state and time-dependent behavior of the model support the hypothesis that mitochondrial matrix Ca(2+) plays an important role in matching energy supply with demand in cardiac myocytes.

Changes of myocardial mitochondrial Ca(2+) transport and mechanism in the early stage after severe burns

OBJECTIVE

To investigate the change of myocardial mitochondrial Ca(2+) transport and its mechanism in the early stage after burns.

METHODS

Forty-eight Wistar rats were randomized into a normal control group (n=8) and a burns group (n=40). The rats of the burns group were given a 30%TBSA full-thickness. Myocardial mitochondria were isolated from normal and scalded rats which were sacrificed at the 1st, 3rd, 6th, 12th and 24th hour post-burn. Mitochondrial Ca(2+) transport velocity, membrane potential (MP), ATP content and cytosolic Ca(2+) concentration [Ca(2+)](c) were determined. The effects of exogenous ATP on mitochondrial Ca(2+) transport velocity were also investigated.

RESULTS

Mitochondrial Ca(2+) uptake velocity of the 1st hour post-burn was higher than that of the control, and Ca(2+) release velocity did not change significantly, but mitochondrial Ca(2+) transport velocity, MP and ATP content were all decreased at the 3rd, 6th, 12th and 24th hour post-burn. Mitochondrial Ca(2+) uptake velocity was positively correlated with MP after burn, and Ca(2+) release velocity with mitochondrial ATP content. [Ca(2+)](c) was increased at the 3rd, 6th, 12th and 24th hour post-burn. Exogenous ATP increased myocardial mitochondrial Ca(2+) uptake velocity of rats at the 3rd and 6th hour post-burn and Ca(2+) release velocity at the 3rd, 6th and 12th hour post-burn.

CONCLUSIONS

Increase of [Ca(2+)](c) led to reinforcement of mitochondrial Ca(2+) uptake at the beginning of the post-burns period. ATP depletion and MP collapse cause myocardial mitochondrial Ca(2+) transport disorder in the following stages.

Reactive species mechanisms of cellular hypoxia-reoxygenation injury

Exacerbation of hypoxic injury after restoration of oxygenation (reoxygenation) is an important mechanism of cellular injury in transplantation and in myocardial, hepatic, intestinal, cerebral, renal, and other ischemic syndromes. Cellular hypoxia and reoxygenation are two essential elements of ischemia-reperfusion injury. Activated neutrophils contribute to vascular reperfusion injury, yet posthypoxic cellular injury occurs in the absence of inflammatory cells through mechanisms involving reactive oxygen (ROS) or nitrogen species (RNS). Xanthine oxidase (XO) produces ROS in some reoxygenated cells, but other intracellular sources of ROS are abundant, and XO is not required for reoxygenation injury. Hypoxic or reoxygenated mitochondria may produce excess superoxide (O) and release H(2)O(2), a diffusible long-lived oxidant that can activate signaling pathways or react vicinally with proteins and lipid membranes. This review focuses on the specific roles of ROS and RNS in the cellular response to hypoxia and subsequent cytolytic injury during reoxygenation.

Calcium and oxidative stress: from cell signaling to cell death

Reactive oxygen and nitrogen species can be used as a messengers in normal cell functions. However, at oxidative stress levels they can disrupt normal physiological pathways and cause cell death. Such a switch is largely mediated through Ca(2+) signaling. Oxidative stress causes Ca(2+) influx into the cytoplasm from the extracellular environment and from the endoplasmic reticulum or sarcoplasmic reticulum (ER/SR) through the cell membrane and the ER/SR channels, respectively. Rising Ca(2+) concentration in the cytoplasm causes Ca(2+) influx into mitochondria and nuclei. In mitochondria Ca(2+) accelerates and disrupts normal metabolism leading to cell death. In nuclei Ca(2+) modulates gene transcription and nucleases that control cell apoptosis. Both in nuclei and cytoplasm Ca(2+) can regulate phosphorylation/dephosphorylation of proteins and can modulate signal transduction pathways as a result. Since oxidative stress is associated with many diseases and the aging process, understanding how oxidants alter Ca(2+) signaling can help to understand process of aging and disease, and may lead to new strategies for their prevention.

Mitochondrial ATP-sensitive potassium channels attenuate matrix Ca(2+) overload during simulated ischemia and reperfusion: possible mechanism of cardioprotection

Mitochondrial ATP-sensitive potassium (mitoK(ATP)) channels play a key role in ischemic preconditioning of the heart. However, the mechanism of cardioprotection remains controversial. We measured rhod-2 fluorescence in adult rabbit ventricular cardiomyocytes as an index of mitochondrial matrix Ca(2+) concentration ([Ca(2+)](m)), using time-lapse confocal microscopy. To simulate ischemia and reperfusion (I/R), cells were exposed to metabolic inhibition (50 minutes) followed by washout with control solution. Rhod-2 fluorescence gradually increased during simulated ischemia and rose even further with reperfusion. The mitoK(ATP) channel opener diazoxide attenuated the accumulation of [Ca(2+)](m) during simulated I/R (EC(50)=18 micromol/L). These effects of diazoxide were blocked by the mitoK(ATP) channel antagonist 5-hydroxydecanoate (5HD). In contrast, inhibitors of the mitochondrial permeability transition (MPT), cyclosporin A and bongkrekic acid, did not alter [Ca(2+)](m) accumulation during ischemia, but markedly suppressed the surge in rhod-2 fluorescence during reperfusion. Measurements of mitochondrial membrane potential, DeltaPsi(m), in permeabilized myocytes revealed that diazoxide depolarized DeltaPsi(m) (by 12% at 10 micromol/L, P<0.01) in a 5HD-inhibitable manner. Our data support the hypothesis that attenuation of mitochondrial Ca(2+) overload, as a consequence of partial mitochondrial membrane depolarization by mitoK(ATP) channels, underlies cardioprotection. Furthermore, mitoK(ATP) channels and the MPT differentially affect mitochondrial calcium homeostasis: mitoK(ATP) channels suppress calcium accumulation during I/R, while the MPT comes into play only upon reperfusion.

Phenylarsine oxide induces mitochondrial permeability transition, hypercontracture, and cardiac cell death

The mitochondrial permeability transition (MPT) is implicated in cardiac reperfusion/reoxygenation injury. In isolated ventricular myocytes, the sulfhydryl (SH) group modifier and MPT inducer phenylarsine oxide (PAO) caused MPT, severe hypercontracture, and irreversible membrane injury associated with increased cytoplasmic free [Ca(2+)]. Removal of extracellular Ca(2+) or depletion of nonmitochondrial Ca(2+) pools did not prevent these effects, whereas the MPT inhibitor cyclosporin A was partially protective and the SH-reducing agent dithiothreitol fully protective. In permeabilized myocytes, PAO caused hypercontracture at much lower free [Ca(2+)] than in its absence. Thus PAO induced hypercontracture by both increasing myofibrillar Ca(2+) sensitivity and promoting mitochondrial Ca(2+) efflux during MPT. Hypercontracture did not directly cause irreversible membrane injury because lactate dehydrogenase (LDH) release was not prevented by abolishing hypercontracture with 2,3-butanedione monoxime. However, loading myocytes with the membrane-permeable Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA-AM) prevented PAO-induced LDH release, thus implicating the PAO-induced rise in cytoplasmic [Ca(2+)] as obligatory for irreversible membrane injury. In conclusion, PAO induces MPT and enhanced susceptibility to hypercontracture in isolated cardiac myocytes, both key features also implicated in cardiac reperfusion and reoxygenation injury.

Regulation of the mitochondrial permeability transition by matrix Ca(2+) and voltage during anoxia/reoxygenation

We studied the interplay between matrix Ca(2+) concentration ([Ca(2+)]) and mitochondrial membrane potential (Deltapsi) in regulation of the mitochondrial permeability transition (MPT) during anoxia and reoxygenation. Without Ca(2+) loading, anoxia caused near-synchronous Deltapsi dissipation, mitochondrial Ca(2+) efflux, and matrix volume shrinkage when a critically low PO(2) was reached, which was rapidly reversible upon reoxygenation. These changes were related to electron transport inhibition, not MPT. Cyclosporin A-sensitive MPT did occur when extramitochondrial [Ca(2+)] was increased to promote significant Ca(2+) uptake during anoxia, depending on the Ca(2+) load size and ability to maintain Deltapsi. However, when [Ca(2+)] was increased after complete Deltapsi dissipation, MPT did not occur until reoxygenation, at which time reactivation of electron transport led to partial Deltapsi regeneration. In the setting of elevated extramitochondrial Ca(2+), this enhanced matrix Ca(2+) uptake while promoting MPT because of less than full recovery of Deltapsi. The interplay between Deltapsi and matrix [Ca(2+)] in accelerating or inhibiting MPT during anoxia/reoxygenation has implications for preventing reoxygenation injury associated with MPT.

Use of ruthenium red as an inhibitor of mitochondrial Ca(2+) uptake in single rat cardiomyocytes

With the current resurgence of interest in the role of mitochondrial [Ca(2+)] in energy production and cellular Ca(2+) signalling, ruthenium red (RR) is being increasingly used as an inhibitor of mitochondrial Ca(2+) uptake. In the present study, the effects of RR on cell and mitochondrial [Ca(2+)], and on cell contractility were determined in isolated rat ventricular myocytes subjected to adrenergic and electrical stimulation. At low concentrations, 0-1 microM, RR inhibited mitochondrial Ca(2+) uptake but this was a secondary effect due to a reduced total intracellular [Ca(2+)], a conclusion supported by the ability of RR to inhibit cell shortening. 5 microM RR completely inhibited cell contraction, whereas higher concentrations, 10-25 microM, induced spontaneous Ca(2+) oscillations and contractile waves. These results indicate that great care must be taken when using RR in intact cells, and in interpreting any effects as resulting from a primary inhibition of mitochondrial Ca(2+) uptake.

Mitochondria and calcium: from cell signalling to cell death

While a pathway for Ca2+ accumulation into mitochondria has long been established, its functional significance is only now becoming clear in relation to cell physiology and pathophysiology. The observation that mitochondria take up Ca2+ during physiological Ca2+ signalling in a variety of cell types leads to four questions: (i) 'What is the impact of mitochondrial Ca2+ uptake on mitochondrial function?' (ii) 'What is the impact of mitochondrial Ca2+ uptake on Ca2+ signalling?' (iii) 'What are the consequences of impaired mitochondrial Ca2+ uptake for cell function?' and finally (iv) 'What are the consequences of pathological [Ca2+]c signalling for mitochondrial function?' These will be addressed in turn. Thus: (i) accumulation of Ca2+ into mitochondria regulates mitochondrial metabolism and causes a transient depolarisation of mitochondrial membrane potential. (ii) Mitochondria may act as a spatial Ca2+ buffer in many cells, regulating the local Ca2+ concentration in cellular microdomains. This process regulates processes dependent on local cytoplasmic Ca2+ concentration ([Ca2+]c), particularly the flux of Ca2+ through IP3-gated channels of the endoplasmic reticulum (ER) and the channels mediating capacitative Ca2+ influx through the plasma membrane. Consequently, mitochondrial Ca2+ uptake plays a substantial role in shaping [Ca2+]c signals in many cell types. (iii) Impaired mitochondrial Ca2+ uptake alters the spatiotemporal characteristics of cellular [Ca2+]c signalling and downregulates mitochondrial metabolism. (iv) Under pathological conditions of cellular [Ca2+]c overload, particularly in association with oxidative stress, mitochondrial Ca2+ uptake may trigger pathological states that lead to cell death. In the model of glutamate excitotoxicity, microdomains of [Ca2+]c are apparently central, as the pathway to cell death seems to require the local activation of neuronal nitric oxide synthase (nNOS), itself held by scaffolding proteins in close association with the NMDA receptor. Mitochondrial Ca2+ uptake in combination with NO production triggers the collapse of mitochondrial membrane potential, culminating in delayed cell death.

Mitochondrial Ca(2+)homeostasis in the regulation of apoptotic and necrotic cell deaths

Using distinct models of apoptosis and necrosis, we have investigated the effect of mitochondrial Ca(2+)(Ca(m)) homeostasis in the regulation of cell death in neuroblastoma cells as well as cardiac myocytes. The steady state level of Ca(m)was determined as the FCCP-releasable Ca(2+). Culturing cells with low concentration of extracellular Ca(2+)(Ca(o)) or with EGTA triggered an early reduction in both the Ca(m)store and the membrane potential (DeltaPsi(m)). This was followed by the detection of cytochrome c release, caspase activation, and apoptosis. Inhibitors of the mitochondrial permeability transition pore such as cyclosporin A and Bcl-2 blocked the release of Ca(m)and inhibited apoptosis. In contrast, mitochondrial Ca(2+)overload resulted in necrotic cell death. Culturing cells in the presence of excess Ca(o)led to increased Ca(m)load together with a decrease of DeltaPsi(m)that reached maximum at 1 h, with necrosis occurring at 2 h. While the decline of Ca(m)and DeltaPsi(m)was a coupled reaction for apoptosis, this relationship was uncoupled during necrosis. Clonazepam, a relatively specific inhibitor of the mitochondrial Na/Ca exchanger, was able to protect the cells from necrosis by reducing Ca(m)overload. Importantly, combination of clonazepam and cyclosporin showed a cooperative effect in further reducing the Ca(m)overload and abolished cell death. The data imply the participation of Ca(m)homeostasis in the regulation of apoptosis and necrosis.

Mitochondrial calcium transients in adult rabbit cardiac myocytes: inhibition by ruthenium red and artifacts caused by lysosomal loading of Ca(2+)-indicating fluorophores

A cold/warm loading protocol was used to ester-load Rhod 2 into mitochondria and other organelles and Fluo 3 into the cytosol of adult rabbit cardiac myocytes for confocal fluorescence imaging. Transient increases in both cytosolic Fluo 3 and mitochondrial Rhod 2 fluorescence occurred after electrical stimulation. Ruthenium red, a blocker of the mitochondrial Ca(2+) uniporter, inhibited mitochondrial Rhod 2 fluorescence transients but not cytosolic Fluo 3 transients. Thus the ruthenium red-sensitive mitochondrial Ca(2+) uniporter catalyzes Ca(2+) uptake during beat-to-beat transients of mitochondrial free Ca(2+), which in turn may help match mitochondrial ATP production to myocardial ATP demand. After ester loading, substantial amounts of Ca(2+)-indicating fluorophores localized into an acidic lysosomal/endosomal compartment. This lysosomal fluorescence did not respond to electrical stimulation. Because fluorescence arose predominantly from lysosomes after the cold loading/warm incubation procedure, total cellular fluorescence failed to track beat-to-beat changes of mitochondrial fluorescence. Only three-dimensionally resolved confocal imaging distinguished the relatively weak mitochondrial signal from the bright lysosomal fluorescence.

Protective effects of low and high doses of cyclosporin A against reoxygenation injury in isolated rat cardiomyocytes are associated with differential effects on mitochondrial calcium levels

In this study we aimed to determine the concentration range of cyclosporin A (CsA) which was effective in protecting against reoxygenation injury in isolated cardiomyocytes, and its effects on intramitochondrial free calcium levels ([Ca2+]m). We also determined whether a high [CsA] had any deleterious effect on normal myocyte function. Isolated adult rat ventricular myocytes were placed in a chamber on the stage of a fluorescence microscope for induction of hypoxia. [Ca2+]m was determined from indo-1/am loaded cells where the cytosolic fluorescence signal had been quenched by superfusion with Mn2+. Cell length was measured using an edge-tracking device. Upon induction of hypoxia, control cells underwent rigor-contracture in 37 +/- 1 min (n = 99) (T1); CsA had no effect on T1. The percentage of control cells which recovered upon reoxygenation depended on the time spent in rigor (T2). With a T2 of 21-30 min, only 36% of control cells recovered compared with 90% and 78% of cells treated with 0.2 microM and 1 microM CsA respectively. After 40 min in rigor, [Ca2+]m was 280 +/- 60 nM in control-recovered cells (50% of cells) and 543 +/- 172 nM and 153 +/- 26 nM in cells treated with 0.2 and 1 microM CsA, respectively (all CsA treated cells recovered). In normoxic studies, CsA had no effect on cell contractility or [Ca2+]m upon rapid pacing, even in presence of an elevated external [Ca2+]. In conclusion, both low and high [CsA] protected against reoxygenation injury to cardiomyocytes despite having opposing effects on [Ca2+]m, suggesting more than one mechanism of action. CsA had no effect on either cell contractility or [Ca2+]m in normoxic cells.

Vasopressin-evoked [Ca2+]i responses in neonatal rat cardiomyocytes

The presence of arginine vasopressin (AVP) V1 receptors on neonatal rat cardiomyocytes (NRCs) linked to processes capable of elevating intracellular free calcium ([Ca2+]i) is now firmly established. This study examined the sources and signaling involved in [Ca2+]i elevations evoked by AVP in NRCs. AVP promoted increases in both [Ca2+]i and 1,4,5-inositoltrisphosphate (IP3) levels in NRCs. The degree of [Ca2+]i elevation was less than that of angiotensin II, but greater than that of endothelin-1. Extracellular Mg2+ depletion led to diminution of the maximal [Ca2+]i response, with a rightward shift in the concentration-response curves to AVP. The phospholipase C inhibitors, D-609, NCDC, or U73122, and the IP3 receptor blocker, heparin, abolished the [Ca2+]i response to AVP. Neither cyclooxygenase inhibition with indomethacin nor PKC inhibition with staurosporine had any effect. Neither ryanodine nor caffeine, which deplete sarcoplasmic reticulum (SR) Ca2+ stores, nor ruthenium red, which inhibits both SR and mitochondrial Ca2+ stores, affected [Ca2+]i responses to AVP. The SR Ca2+ pump inhibitor, cyclopiazonic acid, abolished, and removal of extracellular Ca2+ attenuated, the response to AVP. These data indicate that activation of cardiac V1 receptors by AVP results in mobilization of Ca2+ from a distinct, non-SR, nonmitochondrial, intracellular Ca2+ pool that is Ca2+ pump replenished and IP3 sensitive. This process occurs secondary to phospholipase C (PLC)-mediated generation of IP3, requires the presence of Mg2+ and extracellular Ca2+, and occurs in a manner independent of PKC and cyclooxygenase activation. Such mechanisms of Ca2+ mobilization might indicate a distinct role for AVP in cardiac physiology and disease.

Reversal of mitochondrial Na/Ca exchange during metabolic inhibition in rat cardiomyocytes

During hypoxia of isolated cardiomyocytes, Ca2+ entry into mitochondria may occur via the Na/Ca exchanger, the normal efflux pathway, and not the Ca-uniporter, the normal influx route. If this is the case, then depletion of myocyte Na+ should inhibit Ca2+ uptake, and collapse of the mitochondrial membrane potential (delta psi(m)) would inhibit the uniporter. To test these hypotheses, isolated rat myocytes were exposed to metabolic inhibition, to mimic hypoxia, and [Ca2+]m and [Ca2+]c determined by selective loading of indo-1 into these compartments. Delta psi(m) was determined using rhodamine 123. Following metabolic inhibition, [Ca2+]m was significantly lower in Na-depleted cells than controls (P<0.001), [Ca2+]c was approximately the same in both groups, and mitochondria depolarised completely. Thus Na-depletion inhibited mitochondrial Ca2+ uptake, suggesting that Ca2+ entry occurred via Na/Ca exchange, and the collapse of delta psi(m) during metabolic inhibition is consistent with inactivity of the Ca-uniporter.