Intracellular free calcium and mitochondrial membrane potential in ischemia/reperfusion and preconditioning

Cardioprotective effect of epigallocatechin gallate in myocardial ischemia/reperfusion injury and myocardial infarction: a meta-analysis in preclinical animal studies

This meta-analysis aims to determine the efficacy of Epigallocatechin gallate (EGCG) in the treatment of myocardial ischemia-reperfusion injury (MIRI) and summarize the mechanisms involved. Literature from six databases including Web of Science, PubMed, Embase, China National Knowledge Infrastructure (CNKI), Wan-Fang database, and VIP database (VIP) were systematically searched. All the analysis were conducted by R. Twenty-five eligible studies involving 443 animals were included in this meta-analysis. The results indicated that compared to controls, EGCG exerts a cardioprotective effect by reducing myocardial infarct size (SMD = -4.06; 95% CI: -5.17, -2.94; P < 0.01; I2 = 77%). The funnel plot revealed publication bias. Moreover, EGCG significantly improves cardiac function, serum myocardial injury enzyme, and oxidative stress levels in MIRI animal models. This meta-analysis demonstrates that EGCG exhibits therapeutic promise in animal models of MIRI. However, further validation is still needed in large animal models and large clinical studies.

The mitochondrial K-ATP channel opener diazoxide upregulates STIM1 and Orai1 via ROS and the MAPK pathway in adult rat cardiomyocytes

Background

Openers of mitochondrial adenosine triphosphate-dependent potassium (mKATP) channels like diazoxide increase reactive oxygen species (ROS) production in cardiac cells and reduce Ca²⁺ elevations produced by ischemia–reperfusion, protecting the heart from damage. In this study we tested the hypothesis that opening mKATP channels regulates expression of the major components of store-operated Ca²⁺ entry (SOCE) STIM1 and Orai1.

Results

Quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) and western blot experiments showed that diazoxide increased expression of STIM1 and Orai1 at the mRNA and protein levels, respectively, in adult rat cardiomyocytes. Immunofluorescence analyses revealed that diazoxide also disrupted the striated distribution pattern of STIM1. These effects were prevented by the ROS scavenger N-acetyl cysteine (NAC), the mKATP channel antagonist 5-hydroxydecanoate (5-HD), or the protein synthesis inhibitor cycloheximide (CHX). Confocal microscopy revealed that diazoxide also led to nuclear translocation of the transcription factors c-Fos and NFκB, which was also blocked by NAC or 5-HD. Finally, the MAPK pathway inhibitor UO126 attenuated diazoxide-induced upregulation of STIM1 and Orai1 expression.

Conclusions

Our results suggest that opening mitochondrial potassium ATP channels with diazoxide upregulates the expression of STIM1 and Orai1 by de novo synthesis by a mechanism that involves NFkB, c-Fos, and ROS via MAPK/ERK signaling.

MiR-484 Protects Rat Myocardial Cells from Ischemia-Reperfusion Injury by Inhibiting Caspase-3 and Caspase-9 during Apoptosis

BACKGROUND AND OBJECTIVES

To reveal the detail mechanism of miR-484 on myocardial ischemia-reperfusion (MI/R) injury.

METHODS

Rats model of MI/R injury was established based on control (Con; sham operate) group, ischemia-reperfusion (I/R) group, miR-484 treatment (miR) group, and I/R-negative control (IR-C) group, followed by pathological and interleukin (IL)-6, tumor necrosis factor (TNF)-α, and IL-1β expression evaluation. Then the myocardial apoptosis, as well as the expression of miR-484, caspase-3, and caspase-9 in myocardium were examined. Finally, the regulatory relation between miR-484 and SMAD family member 7 (SMAD7) was predicated, followed by verification analysis.

RESULTS

Compared with Con group, the expression of miR-484 in I/R and IR-C group was decreased. Compared with I/R and IR-C group, the expression of miR-484 was increased in miR group. Compared with Con group, the expression levels of IL-6, TNF-α, and IL-1β in cardiac myocytes of I/R group and IR-C group were increased. Compared with Con group, the apoptotic index, membrane potential of I/R, and the expression of caspase-3/9 were increased in IR-C group. Compared with the I/R and IR-C groups, the apoptotic index of myocardial cells in the ischemic region was decreased, the membrane potential was increased, and the expression of caspase-3/9 was decreased significantly in the miR group. SMAD7 was the target gene of miR-484.

CONCLUSIONS

MiR-484 protected myocardial cells from I/R injury by suppressing caspase-3 and caspase-9 expression during cardiomyocyte apoptosis. MiR-484 reduced the expression of IL-6, TNF-α, and IL-1β in MI/R. MiR-484 might alleviate the decreasing of mitochondrial membrane potential in MI/R cells.

Real-Time Fluorescence Measurements of ROS and [Ca2+] in Ischemic / Reperfused Rat Hearts: Detectable Increases Occur only after Mitochondrial Pore Opening and Are Attenuated by Ischemic Preconditioning

Mitochondrial permeability transition pore (mPTP) opening is critical for ischemia / reperfusion (I/R) injury and is associated with increased [Ca²⁺] and reactive oxygen species (ROS). Here we employ surface fluorescence to establish the temporal sequence of these events in beating perfused hearts subject to global I/R. A bespoke fluorimeter was used to synchronously monitor surface fluorescence and reflectance of Langendorff-perfused rat hearts at multiple wavelengths, with simultaneous measurements of hemodynamic function. Potential interference by motion artefacts and internal filtering was assessed and minimised. Re-oxidation of NAD(P)H and flavoproteins on reperfusion (detected using autofluorescence) was rapid (t_0.5 < 15 s) and significantly slower following ischemic preconditioning (IP). This argues against superoxide production from reduced Complex 1 being a critical mediator of initial mPTP opening during early reperfusion. Furthermore, MitoPY1 (a mitochondria-targeted H₂O₂-sensitive fluorescent probe) and aconitase activity measurements failed to detect matrix ROS increases during early reperfusion. However, two different fluorescent cytosolic ROS probes did detect ROS increases after 2–3 min of reperfusion, which was shown to be after initiation of mPTP opening. Cyclosporin A (CsA) and IP attenuated these responses and reduced infarct size. [Ca²⁺]_i (monitored with Indo-1) increased progressively during ischemia, but dropped rapidly within 90 s of reperfusion when total mitochondrial [Ca²⁺] was shown to be increased. These early changes in [Ca²⁺] were not attenuated by IP, but substantial [Ca²⁺] increases were observed after 2–3 min reperfusion and these were prevented by both IP and CsA. Our data suggest that the major increases in ROS and [Ca²⁺] detected later in reperfusion are secondary to mPTP opening. If earlier IP-sensitive changes occur that might trigger initial mPTP opening they are below our limit of detection. Rather, we suggest that IP may inhibit initial mPTP opening by alternative mechanisms such as prevention of hexokinase 2 dissociation from mitochondria during ischemia.

Mechanisms underlying effect of the mycotoxin cytochalasin B on induction of cytotoxicity, modulation of cell cycle, Ca2+ homeostasis and ROS production in human breast cells

Cytochalasin B, a cell-permeable mycotoxin isolated from the fungus Phoma spp., shows a wide range of biological effects, among which its potent antitumor activity has raised great interests in different models. However, the cytotoxic activity of cytochalasin B and its underlying mechanisms have not been elucidated in breast cells. This study examined the effect of cytochalasin B on MCF 10A human breast epithelial cells and ZR-75-1 human breast cancer cells. Cytochalasin B (10-20μM) concentration-dependently induced cytotoxicity, cell cycle arrest, and [Ca²⁺]_i rises in ZR-75-1 cells but not in MCF 10A cells. In ZR-75-1 cells, cytochalasin B triggered G2/M phase arrest through the modulation of CDK1, cyclin B1, p53, p27 and p21 expressions. The Ca²⁺ signal response induced by cytochalasin B was reduced by removing extracellular Ca²⁺ and was inhibited by the store-operated Ca²⁺ channel blocker 2-APB and SKF96365. In Ca²⁺-free medium, cytochalasin B induced Ca²⁺ release through thapsigargin-sensitive endoplasmic reticulum stores. Moreover, cytochalasin B increased H₂O₂ levels but reduced GSH levels. The apoptotic effects evoked by cytochalasin B were partially inhibited by prechelating cytosolic Ca²⁺ with BAPTA-AM and the antioxidant NAC. Together, in ZR-75-1 cells but not in MCF 10A cells, cytochalasin B activated Ca²⁺-associated mitochondrial apoptotic pathways that involved G2/M phase arrest and ROS signaling. Furthermore, cytochalasin B induced [Ca²⁺]_i rises by releasing Ca²⁺ from the endoplasmic reticulum and causing Ca²⁺ influx through 2-APB or SKF96365-sensitive store-operated Ca²⁺ entry. Our findings provide new insights into the possible application of cytochalasin B in human breast cancer therapy.

A technical review of optical mapping of intracellular calcium within myocardial tissue

Optical mapping of Ca(2+)-sensitive fluorescence probes has become an extremely useful approach and adopted by many cardiovascular research laboratories to study a spectrum of myocardial physiology and disease conditions. Optical mapping data are often displayed as detailed pseudocolor images, providing unique insight for interpreting mechanisms of ectopic activity, action potential and Ca(2+) transient alternans, tachycardia, and fibrillation. Ca(2+)-sensitive fluorescent probes and optical mapping systems continue to evolve in the ongoing effort to improve therapies that ease the growing worldwide burden of cardiovascular disease. In this technical review we provide an updated overview of conventional approaches for optical mapping of Cai (2+) within intact myocardium. In doing so, a brief history of Cai (2+) probes is provided, and nonratiometric and ratiometric Ca(2+) probes are discussed, including probes for imaging sarcoplasmic reticulum Ca(2+) and probes compatible with potentiometric dyes for dual optical mapping. Typical measurements derived from optical Cai (2+) signals are explained, and the analytics used to compute them are presented. Last, recent studies using Cai (2+) optical mapping to study arrhythmias, heart failure, and metabolic perturbations are summarized.

Oxygen demand of perfused heart preparations: how electromechanical function and inadequate oxygenation affect physiology and optical measurements

NEW FINDINGS

What is the topic of this review? This review discusses how the function and electrophysiology of isolated perfused hearts are affected by oxygenation and energy utilization. The impact of oxygenation on fluorescence measurements in perfused hearts is also discussed. What advances does it highlight? Recent studies have illuminated the inherent differences in electromechanical function, energy utilization rate and oxygen requirements between the primary types of excised heart preparations. A summary and analysis of how these variables affect experimental results are necessary to elevate the physiological relevance of these approaches in order to advance the field of whole-heart research. The ex vivo perfused heart recreates important aspects of in vivo conditions to provide insight into whole-organ function. In this review we discuss multiple types of ex vivo heart preparations, explain how closely each mimic in vivo function, and discuss how changes in electromechanical function and inadequate oxygenation of ex vivo perfused hearts may affect measurements of physiology. Hearts that perform physiological work have high oxygen demand and are likely to experience hypoxia when perfused with a crystalloid perfusate. Adequate myocardial oxygenation is critically important for obtaining physiologically relevant measurements, so when designing experiments the type of ex vivo preparation and the capacity of perfusate to deliver oxygen must be carefully considered. When workload is low, such as during interventions that inhibit contraction, oxygen demand is also low, which could dramatically alter a physiological response to experimental variables. Changes in oxygenation also alter the optical properties of cardiac tissue, an effect that may influence optical signals measured from both endogenous and exogenous fluorophores. Careful consideration of oxygen supply, working condition, and wavelengths used to acquire optical signals is critical for obtaining physiologically relevant measurements during ex vivo perfused heart studies.

Triplet state quenching of phenosafranine dye by indolic compounds studied by transient absorption spectroscopy

The interaction of the triplet state of the synthetic dye phenosafranine (3,7-diamino-5-phenylphenazinium chloride) with indolic compounds of biological relevance was investigated in water by means of laser flash photolysis.

Remote ischemic preconditioning of cardiomyocytes inhibits the mitochondrial permeability transition pore independently of reduced calcium-loading or sarcKATP channel activation

Ischemic preconditioning (IPC) inhibits Ca²⁺‐loading during ischemia which contributes to cardioprotection by inhibiting mechanical injury due to hypercontracture and biochemical injury through mitochondrial permeability transition (MPT) pores during reperfusion. However, whether remote‐IPC reduced Ca²⁺‐loading during ischemia and its subsequent involvement in inhibiting MPT pore formation during reperfusion has not been directly shown. We have developed a cellular model of remote IPC to look at the impact of remote conditioning on Ca²⁺‐regulation and MPT pore opening during simulated ischemia and reperfusion, using fluorescence microscopy. Ventricular cardiomyocytes were isolated from control rat hearts, hearts preconditioned with three cycles of ischemia/reperfusion or naïve myocytes remotely conditioned with effluent collected from preconditioned hearts. Both conventional‐IPC and remote‐IPC reduced the loss of Ca²⁺‐homeostasis and contractile function following reenergization of metabolically inhibited cells and protected myocytes against ischemia/reperfusion injury. However, only conventional‐IPC reduced the Ca²⁺‐loading during metabolic inhibition and this was independent of any change in sarcK_ATP channel activity but was associated with a reduction in Na⁺‐loading, reflecting a decrease in Na/H exchanger activity. Remote‐IPC delayed opening of the MPT pores in response to ROS, which was dependent on PKCε and NOS‐signaling. These data show that remote‐IPC inhibits MPT pore opening to a similar degree as conventional IPC, however, the contribution of MPT pore inhibition to protection against reperfusion injury is independent of Ca²⁺‐loading in remote IPC. We suggest that inhibition of the MPT pore and not Ca²⁺‐loading is the common link in cardioprotection between conventional and remote IPC.

Remote ischemic preconditioning (IPC) provides a similar level of protection against ischemia–reperfusion injury to that of conventional‐IPC. This study shows that unlike conventional‐IPC, this was independent of any reduction in Na or Ca²⁺‐loading during the simulated ischemic event but results from a direct PKCε‐dependent inhibition of the mitochondrial permeability transition pore.

Exenatide protects against hypoxia/reoxygenation-induced apoptosis by improving mitochondrial function in H9c2 cells

Glucagon-like peptide-1 (GLP-1) analogues might exert the cardioprotective effects via attenuating apoptosis. This study aimed to determine the protective effects and mechanism of exenatide, a GLP-1 analogue, on cardiomyocyte apoptosis using an in vitro model of hypoxia/reoxygenation (H/R). H9c2 cells were employed to establish an in vitro model of H/R. 200 nM exenatide pretreatment significantly reduced apoptosis measured by flow cytometry. To further study the antiapoptotic mechanism of exenatide, we used flow cytometry in combination with laser confocal microscopy to determine the interaction between exenatide and the process of mitochondria-mediated apoptosis. We found that exenatide pretreatment reduced the intracellular reactive oxygen species (ROS) levels and decreased the mitochondrial calcium overload caused by H/R. Furthermore, an increase of total superoxide dismutase (T-SOD) levels, a decrease of malondialdehyde (MDA) levels, a preservation of mitochondrial membrane potential (ΔΨm), a reduction of cytochrome-c release, a decline of cleaved caspase-3 expression, and caspase-3 activation were observed in exenatide-pretreated cultures. These results suggest that exenatide exerts a protective effect on preventing against H/R-induced apoptosis. Importantly, the protective effects of exenatide may be attributed to its role in improving mitochondrial function in H9c2 cells subjected to H/R.

Ratiometric imaging of calcium during ischemia-reperfusion injury in isolated mouse hearts using Fura-2

Background

We present an easily implementable method for measuring Fura-2 fluorescence from isolated mouse hearts using a commercially available switching light source and CCD camera. After calibration, it provides a good estimate of intracellular [Ca²⁺] with both high spatial and temporal resolutions, permitting study of changes in dispersion of diastolic [Ca²⁺], Ca²⁺ transient dynamics, and conduction velocities in mouse hearts. In a proof-of-principle study, we imaged isolated Langendorff-perfused mouse hearts with reversible regional myocardial infarctions.

Methods

Isolated mouse hearts were perfused in the Landendorff-mode and loaded with Fura-2. Hearts were then paced rapidly and subjected to 15 minutes of regional ischemia by ligation of the left anterior descending coronary artery, following which the ligation was removed to allow reperfusion for 15 minutes. Fura-2 fluorescence was recorded at regular intervals using a high-speed CCD camera. The two wavelengths of excitation light were interleaved at a rate of 1 KHz with a computer controlled switching light source to illuminate the heart.

Results

Fura-2 produced consistent Ca²⁺ transients from different hearts. Ligating the coronary artery rapidly generated a well defined region with a dramatic rise in diastolic Ca²⁺ without a significant change in transient amplitude; Ca²⁺ handling normalized during reperfusion. Conduction velocity was reduced by around 50% during ischemia, and did not recover significantly when monitored for 15 minutes following reperfusion.

Conclusions

Our method of imaging Fura-2 from isolated whole hearts is capable of detecting pathological changes in intracellular Ca²⁺ levels in cardiac tissue. The persistent change in the conduction velocities indicates that changes to tissue connectivity rather than altered intracellular Ca²⁺ handling may be underlying the electrical instabilities commonly seen in patients following a myocardial infarction.

Slow calcium waves and redox changes precede mitochondrial permeability transition pore opening in the intact heart during hypoxia and reoxygenation

AIMS

Opening of the mitochondrial permeability transition pore (mPTP) is an important step on the pathway towards cardiomyocyte death, defining the extent of injury following cardiac ischaemia and reperfusion. In isolated mitochondria, mPTP opening is triggered by calcium overload facilitated by oxidative stress. In isolated cells, however, it has been suggested that mPTP opening occurs before calcium overload and is stimulated by oxidative stress. Our objective was to establish the events that cause mPTP opening in the intact heart.

METHODS AND RESULTS

We performed multiphoton imaging of Langendorff-perfused mouse hearts expressing an inducible, Ca(2+)-sensitive reporter (circularly Permuted GFP and calmodulin (CaM), version 2), to examine the spatiotemporal relationship between [Ca(2+)](c), redox state, and mPTP opening in the intact heart during hypoxia and reoxygenation at sub-myocyte resolution. We found that during reperfusion, calcium waves propagated across multiple cells at 3.3 µm/s. mPTP opening caused an abrupt loss of mitochondrial membrane potential, measured using a potentiometric dye, which was invariably preceded by a rise in [Ca(2+)](c). The probability that localized [Ca(2+)](c) waves led to mPTP opening was greater early during reoxygenation. During reoxygenation, coordinated redox changes also occurred across large regions and preceded mPTP opening on average by 122 ± 38 s. Fewer [Ca(2+)] waves led to mPTP opening in the presence of mPTP inhibitor cyclosporin A or mitochondrial-targeted scavenger of reactive oxygen species, MitoQ.

CONCLUSION

These experiments define the spatiotemporal relationship between changes in [Ca(2+)](c), redox state and mPTP opening during reoxygenation in the intact heart. Tissue oxidation coincident with localized calcium waves together conspire to cause mPTP opening and subsequent cell death.

Does the voltage dependent anion channel modulate cardiac ischemia-reperfusion injury?

The voltage dependent anion channel (VDAC) provides exchange of metabolites, anions, and cations across the outer mitochondrial membrane. VDAC provides substrates and adenine nucleotides necessary for electron transport and therefore plays a key role in regulating mitochondrial bioenergetics. VDAC has also been suggested to regulate the response to cell death signaling. Emerging data show that VDAC is regulated by protein-protein interactions as well as by post-translational modifications. This review will focus on the regulation of VDAC and its potential role in regulating cell death in cardiac ischemia-reperfusion. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

What makes the mitochondria a killer? Can we condition them to be less destructive?

Cardioprotection, such as preconditioning and postconditioning, has been shown to result in a significant reduction in cell death. Many of the signaling pathways activated by cardioprotection have been elucidated, but there is still a lack of understanding of the mechanisms by which these signaling pathways reduce cell death. Mitochondria have been reported to be an important player in many types of apoptotic and necrotic cell death. If mitochondria play an important role in cell death, then it seems reasonable to consider that cardioprotective mechanisms might act, at least in part, by opposing mitochondrial cell death pathways. One of the major mechanisms of cell death in ischemia-reperfusion is suggested to be the opening of a large conductance pore in the inner mitochondrial membrane, known as the mitochondrial permeability transition pore. Inhibition of this mitochondrial pore appears to be one of the major mechanisms by which cardioprotection reduces cell death. Cardioprotection activates a number of signaling pathways that reduce the level of triggers (reactive oxygen species and calcium) or enhances inhibitors of the mitochondrial permeability transition pore at the start of reperfusion. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

Polyphenol (-)-epigallocatechin gallate during ischemia limits infarct size via mitochondrial K(ATP) channel activation in isolated rat hearts

Polyphenol (-)-epigallocatechin gallate (EGCG), the most abundant catechin of green tea, appears to attenuate myocardial ischemia/reperfusion injury. We investigated the involvement of ATP-sensitive potassium (K(ATP)) channels in EGCG-induced cardioprotection. Isolated rat hearts were subjected to 30 min of regional ischemia and 2 hr of reperfusion. EGCG was perfused for 40 min, from 10 min before to the end of index ischemia. A nonselective K(ATP) channel blocker glibenclamide (GLI) and a selective mitochondrial K(ATP) (mK(ATP)) channel blocker 5-hydroxydecanoate (HD) were perfused in EGCG-treated hearts. There were no differences in coronary flow and cardiodynamics including heart rate, left ventricular developed pressure, rate-pressure product, +dP/dt(max), and -dP/dt(min) throughout the experiments among groups. EGCG-treatment significantly reduced myocardial infarction (14.5+/-2.5% in EGCG 1 microM and 4.0+/-1.7% in EGCG 10 microM, P<0.001 vs. control 27.2+/-1.4%). This anti-infarct effect was totally abrogated by 10 microM GLI (24.6+/-1.5%, P<0.001 vs. EGCG). Similarly, 100 microM HD also aborted the anti-infarct effect of EGCG (24.1+/-1.2%, P<0.001 vs. EGCG ). These data support a role for the K(ATP) channels in EGCG-induced cardioprotection. The mK(ATP) channels play a crucial role in the cardioprotection by EGCG.

Calcium signaling-mediated endogenous protection of cell energetics in the acutely diabetic myocardium

In acute diabetic myocardium, calcium signals propagated by intracellular calcium transients participate in the protection of cell energetics via upregulating the formation of mitochondrial energy transition pores (ETP). Mechanisms coupling ETP formation with an increase in membrane fluidity and a decrease in transmembrane potential of the mitochondria are discussed. Our results indicate that the amplification of calcium transients in the diabetic heart is associated with an increase in their amplitude. Moreover, the signals transferred by calcium transients also regulated ETP formation in nondiabetic myocardium. Evidence for the indispensable role of calcium in the regulation of transition pore formation is provided whereby an exchange of cadmium for calcium ions led to a rapid and dramatic decrease in the amount of ETP. Another possible regulatory factor of the mitochondrial function may be radical-induced damage to the diabetic heart. Nevertheless, our data indicate that radical-induced changes in mitochondria predominantly concern the respiratory chain and have no appreciable effect on the fluidity of the mitochondrial membranes. The residual mitochondrial production of ATP owing to its augmented transfer to the cytosol proved to be adequate to preserve sufficient levels of adenine nucleotides in the acute diabetic myocardium.

Mitochondrial and cell-surface F0F1ATPsynthase in innate and acquired cardioprotection

Mitochondria are central to heart function and dysfunction, and the pathways activated by different cardioprotective interventions mostly converge on mitochondria. In a context of perspectives in innate and acquired cardioprotection, we review some recent advances in F(0)F(1)ATPsynthase structure/function and regulation in cardiac cells. We focus on three topics regarding the mitochondrial F(0)F(1)ATPsynthase and the plasma membrane enzyme, i.e.: i) the crucial role of cardiac mitochondrial F(0)F(1)ATPsynthase regulation by the inhibitory protein IF(1) in heart preconditioning strategies; ii) the structure and function of mitochondrial F(0)F(1)ATPsynthase oligomers in mammalian myocardium as possible endogenous factors of mitochondria resistance to ischemic insult; iii) the external location and characterization of plasma membrane F(0)F(1) ATP synthase in search for possible actors of its regulation, such as IF(1) and calmodulin, at cell surface.

The mitochondrial permeability transition pore as a target for preconditioning and postconditioning

The experimental evidence supporting the mitochondrial permeability transition pore (mPTP) as a major mediator of lethal myocardial reperfusion injury and therefore a critical target for cardioprotection is persuasive. Although, its molecular identity eludes investigators, it is generally accepted that mitochondrial cyclophilin-D, the target for the inhibitory effects of cyclosporine-A on the mPTP, is a regulatory component of the mPTP. Animal myocardial infarction studies and a recent clinical proof-of-concept study have demonstrated that pharmacologically inhibiting its opening at the onset of myocardial reperfusion reduces myocardial infarct size in the region of 30-50%. Interestingly, the inhibition of mPTP opening at this time appears to underpin the infarct-limiting effects of the endogenous cardioprotective strategies of ischemic preconditioning (IPC) and postconditioning (IPost). However, the mechanism underlying this inhibitory action of IPC and IPost on mPTP opening is unclear. The objectve of this review article will be to explore the potential mechanisms which link IPC and IPost to mPTP inhibition in the reperfused heart.

Mechanisms underlying acute protection from cardiac ischemia-reperfusion injury

Mitochondria play an important role in cell death and cardioprotection. During ischemia, when ATP is progressively depleted, ion pumps cannot function resulting in a rise in calcium (Ca(2+)), which further accelerates ATP depletion. The rise in Ca(2+) during ischemia and reperfusion leads to mitochondrial Ca(2+) accumulation, particularly during reperfusion when oxygen is reintroduced. Reintroduction of oxygen allows generation of ATP; however, damage to the electron transport chain results in increased mitochondrial generation of reactive oxygen species (ROS). Mitochondrial Ca(2+) overload and increased ROS can result in opening of the mitochondrial permeability transition pore, which further compromises cellular energetics. The resultant low ATP and altered ion homeostasis result in rupture of the plasma membrane and cell death. Mitochondria have long been proposed as central players in cell death, since the mitochondria are central to synthesis of both ATP and ROS and since mitochondrial and cytosolic Ca(2+) overload are key components of cell death. Many cardioprotective mechanisms converge on the mitochondria to reduce cell death. Reducing Ca(2+) overload and reducing ROS have both been reported to reduce ischemic injury. Preconditioning activates a number of signaling pathways that reduce Ca(2+) overload and reduce activation of the mitochondrial permeability transition pore. The mitochondrial targets of cardioprotective signals are discussed in detail.

Increased mitochondrial calcium coexists with decreased reperfusion injury in postconditioned (but not preconditioned) hearts

Ca(2+) is the main trigger for mitochondrial permeability transition pore opening, which plays a key role in cardiomyocyte death after ischemia-reperfusion. We investigated whether a reduced accumulation of mitochondrial Ca(2+) might explain the attenuation of lethal reperfusion injury by postconditioning. Anesthetized New Zealand White rabbits underwent 30 min of ischemia, followed by either 240 (infarct size protocol) or 60 (mitochondria protocol) min of reperfusion. They received either no intervention (control), preconditioning by 5-min ischemia and 5-min reperfusion, postconditioning by four cycles of 1-min reperfusion and 1-min ischemia at the onset of reflow, or pharmacological inhibition of the transition pore opening by N-methyl-4-isoleucine-cyclosporin (NIM811; 5 mg/kg iv) given at reperfusion. Area at risk and infarct size were assessed by blue dye injection and triphenyltetrazolium chloride staining. Mitochondria were isolated from the risk region for measurement of 1) Ca(2+) retention capacity (CRC), and 2) mitochondrial content of total (atomic absorption spectrometry) and ionized (potentiometric technique) calcium concentration. CRC averaged 0.73 +/- 0.16 in control vs. 4.23 +/- 0.17 mug Ca(2+)/mg proteins in shams (P < 0.05). Postconditioning, preconditioning, or NIM811 significantly increased CRC (P < 0.05 vs. control). In the control group, total and free mitochondrial calcium significantly increased to 2.39 +/- 0.43 and 0.61 +/- 0.10, respectively, vs. 1.42 +/- 0.09 and 0.16 +/- 0.01 mug Ca(2+)/mg in sham (P < 0.05). Surprisingly, whereas total and ionized mitochondrial Ca(2+) decreased in preconditioning, it significantly increased in postconditioning and NIM811 groups. These data suggest that retention of calcium within mitochondria may explain the decreased reperfusion injury in postconditioned (but not preconditioned) hearts.

Ischemic preconditioning of the whole heart confers protection on subsequently isolated ventricular myocytes

Current cellular models of ischemic preconditioning (IPC) rely on inducing preconditioning in vitro and may not accurately represent complex pathways triggered by IPC in the intact heart. Here, we show that it is possible to precondition the intact heart and to subsequently isolate individual ventricular myocytes that retain the protection triggered by IPC. Myocytes isolated from Langendorff-perfused hearts preconditioned with three cycles of ischemia-reperfusion were exposed to metabolic inhibition and reenergization. Injury was assessed from induction of hypercontracture and loss of Ca2+ homeostasis and contractile function. IPC induced an immediate window of protection in isolated myocytes, with 64.3 ± 7.6% of IPC myocytes recovering Ca2+ homeostasis compared with 16.9 ± 2.4% of control myocytes ( P < 0.01). Similarly, 64.1 ± 5.9% of IPC myocytes recovered contractile function compared with 15.3 ± 2.2% of control myocytes ( P < 0.01). Protection was prevented by the presence of 0.5 mM 5-hydroxydecanoate during the preconditioning stimulus. This early protection disappeared after 6 h, but a second window of protection developed 24 h after preconditioning, with 54.9 ± 4.7% of preconditioned myocytes recovering Ca2+ homeostasis compared with 12.6 ± 2.9% of control myocytes ( P < 0.01). These data show that “true” IPC of the heart confers both windows of protection in the isolated myocytes, with a similar temporal relationship to in vivo preconditioning of the whole heart. The model should allow future studies in isolated cells of the protective mechanisms induced by true ischemia.

Calcium in cell injury and death

Loss of Ca(2+) homeostasis, often in the form of cytoplasmic increases, leads to cell injury. Depending upon cell type and the intensity of Ca(2+) toxicity, the ensuing pathology can be reversible or irreversible. Although multiple destructive processes are activated by Ca(2+), lethal outcomes are determined largely by Ca(2+)-induced mitochondrial permeability transition. This form of damage is primarily dependent upon mitochondrial Ca(2+) accumulation, which is regulated by the mitochondrial membrane potential. Retention of the mitochondrial membrane potential during Ca(2+) increases favors mitochondrial Ca(2+) uptake and overload, resulting in mitochondrial permeability transition and cell death. In contrast, dissipation of mitochondrial membrane potential reduces mitochondrial Ca(2+) uptake, retards mitochondrial permeability transition, and delays death, even in cells with large Ca(2+) increases. The rates of mitochondrial membrane potential dissipation and mitochondrial Ca(2+) uptake may determine cellular sensitivity to Ca(2+) toxicity under pathological conditions, including ischemic injury.

The role of mitochondria in protection of the heart by preconditioning

A prolonged period of ischaemia followed by reperfusion irreversibly damages the heart. Such reperfusion injury (RI) involves opening of the mitochondrial permeability transition pore (MPTP) under the conditions of calcium overload and oxidative stress that accompany reperfusion. Protection from MPTP opening and hence RI can be mediated by ischaemic preconditioning (IP) where the prolonged ischaemic period is preceded by one or more brief (2–5 min) cycles of ischaemia and reperfusion. Following a brief overview of the molecular characterisation and regulation of the MPTP, the proposed mechanisms by which IP reduces pore opening are reviewed including the potential roles for reactive oxygen species (ROS), protein kinase cascades, and mitochondrial potassium channels. It is proposed that IP-mediated inhibition of MPTP opening at reperfusion does not involve direct phosphorylation of mitochondrial proteins, but rather reflects diminished oxidative stress during prolonged ischaemia and reperfusion. This causes less oxidation of critical thiol groups on the MPTP that are known to sensitise pore opening to calcium. The mechanisms by which ROS levels are decreased in the IP hearts during prolonged ischaemia and reperfusion are not known, but appear to require activation of protein kinase Cε, either by receptor-mediated events or through transient increases in ROS during the IP protocol. Other signalling pathways may show cross-talk with this primary mechanism, but we suggest that a role for mitochondrial potassium channels is unlikely. The evidence for their activity in isolated mitochondria and cardiac myocytes is reviewed and the lack of specificity of the pharmacological agents used to implicate them in IP is noted. Some K⁺ channel openers uncouple mitochondria and others inhibit respiratory chain complexes, and their ability to produce ROS and precondition hearts is mimicked by bona fide uncouplers and respiratory chain inhibitors. IP may also provide continuing protection during reperfusion by preventing a cascade of MPTP-induced ROS production followed by further MPTP opening. This phase of protection may involve survival kinase pathways such as Akt and glycogen synthase kinase 3 (GSK3) either increasing ROS removal or reducing mitochondrial ROS production.

Dual probe fluorescence monitoring of intracellular free calcium during ischemia in mouse heart by using continuous compensation for pH dependence of the dissociation constant of Fura-2, and the interference of myoglobin

Mitochondrial damage is the main source of cellular injury upon ischemia-reperfusion, and calcium loading has been implicated in this phenomenon. The use of optical probes for calcium monitoring of the intact heart is hampered by internal filter effects of intracellular hemoproteins, endogenous fluorescence, and their sensitivity to pH. We describe here a method for measurement of intracellular free calcium in isolated myoglobin-deficient perfused mouse hearts under conditions of large intracellular pH fluctuations by simultaneous fluorescence monitoring of the calcium-probe Fura-2 and the pH probe BCECF through dual wavelength excitation of both probes. In myoglobin-containing mouse heart endogenous chromophores interfere with Fura-2 fluorometry. It is shown that a paradoxical decrease in Fura-2 fluorescence occurs during ischemia in isolated mouse hearts. Simultaneous recording of BCECF fluorescence (calibrated against pH measurement with phosphorus NMR) and data reduction based on continual recalculation of the apparent dissociation constant of the calcium-probe complex revealed that a marked increase in intracellular free calcium occurs, and that the Fura-2 fluorescence decrease was caused by an increase in dissociation constant due to intracellular acidification. Intracellular free calcium rose almost linearly during a 20-min period of ischemia and returned to basal values rapidly upon the commencement of perfusion.

Role of reverse mode Na+/Ca2+ exchanger in the cardioprotection of metabolic inhibition preconditioning in rat ventricular myocytes

This study determined the role of the reverse mode Na(+)/Ca(2+) exchanger (NCX) in cardioprotection of metabolic inhibition preconditioning in isolated ventricular myocyctes. Activity of the reverse mode NCX was assessed by changes of [Ca(2+)](i) upon withdrawal of extracellular Na(+). [Ca(2+)](i) was measured by spectrofluorometry, using Fura-2 as Ca(2+) indicator. The amplitude of contraction and exclusion of trypan blue by myocytes served as indices of contractile function and viability, respectively. Firstly, NCX activity significantly decreased during simulated reperfusion after severe metabolic inhibition (index ischaemia) in myocytes subjected to metabolic inhibition preconditioning. This inhibitory effect on NCX activity correlated with the enhancing effect of metabolic inhibition preconditioning on cell viability following ischaemic insult. Treatment myocytes with E4031, an activator of reverse mode NCX, during index ischaemia and reperfusion attenuated the enhancing effects of metabolic inhibition preconditioning on cell contraction and viability. Secondly, NCX activity was significantly higher at the end of metabolic inhibition preconditioning. More importantly, E4031 pretreatment mimicked the beneficial effects of metabolic inhibition preconditioning in myocytes and ischaemic preconditioning in the isolated perfused heart, respectively, and these effects were abolished by KB-R7943, an inhibitor of reverse mode NCX. The results indicate that increased reverse mode NCX activity during preconditioning triggered cardioprotection, and reduced reverse mode NCX activity during reperfusion after index ischaemia conferred cardioprotection.

Inhibition of phospholipase C attenuates liver mitochondrial calcium overload following cold ischemia

BACKGROUND

Graft failure due to cold ischemia (CI) injury remains a significant problem during liver transplantation. During CI, the consumption of ATP and the increase in cellular Ca concentration may result in mitochondrial Ca (mCa) overload through the mCa uniporter, which can ultimately lead to apoptosis and graft nonfunction. We recently identified phospholipase C-dl (PLC-dl) as a novel regulator of mCa uptake in the liver, and now extend those studies to examine the role of mitochondrial PLC in liver CI injury.

METHODS

Rat livers were perfused with University of Wisconsin (UW) solution. Half was homogenized immediately; the other half was cold-stored for 24 hr in UW. Mitochondria were extracted by differential centrifugation and incubated in buffer containing ATP and 0.1 or 0.2 microM Ca. The selective PLC inhibitor, U-73122, was added to determine the effects of PLC inhibition on mCa uptake following CI. Western blots and densitometry quantified mitochondrial PLC expression. Mito Tracker Red fluorescence microscopy was used to verify mitochondrial transmembrane potential.

RESULTS

Twenty-four hour CI caused a significant increase in mCa uptake (P<0.001), and increasing extramitochondrial Ca potentiated this effect. The PLC inhibitor, U-73122, decreased mCa uptake in nonischemic mitochondria (P<0.001), and had a greater effect on CI mitochondria (P<0.001). Mitochondrial PLC-dl expression increased 175+/-75% following CI (P<0.05).

CONCLUSIONS

These data demonstrate that PLC-dl is essential for mCa uptake following CI, and that the PLC pathway may be sensitized by CI. The CI-induced increase in mitochondrial PLC-delta1 expression represents a novel mechanism whereby mCa uptake can increase independently of cytosolic conditions.

Mitochondrial ion channels: gatekeepers of life and death

Continuous generation of ATP by mitochondrial oxidative phosphorylation is essential to maintain function in mechanically active cells such as cardiomyocytes. Emerging evidence indicates that mitochondrial ion channels activated by reactive oxygen species can induce a mitochondrial "critical" state, which can scale to cause electrical and contractile dysfunction of the cardiac cell and, ultimately, the whole heart. Here we focus on how mitochondrial ion channels participate in life-and-death decisions of the cell and discuss the challenges ahead for translating recent findings into novel therapeutic applications.

Reversible redox-dependent modulation of mitochondrial aconitase and proteolytic activity during in vivo cardiac ischemia/reperfusion

Prooxidents can induce reversible inhibition or irreversible inactivation and degradation of the mitochondrial enzyme aconitase. Cardiac ischemia/reperfusion is associated with an increase in mitochondrial free radical production. In the current study, the effects of reperfusion-induced production of prooxidants on mitochondrial aconitase and proteolytic activity were determined to assess whether alterations represented a regulated response to changes in redox status or oxidative damage. Evidence is provided that ATP-dependent proteolytic activity increased during early reperfusion followed by a time-dependent reduction in activity to control levels. These alterations in proteolytic activity paralleled an increase and subsequent decrease in the level of oxidatively modified protein. In vitro data supports a role for prooxidants in the activation of ATP-dependent proteolytic activity. Despite inhibition during early periods of reperfusion, aconitase was not degraded under the conditions of these experiments. Aconitase activity exhibited a decline in activity followed by reactivation during cardiac reperfusion. Loss and regain in activity involved reversible sulfhydryl modification. Aconitase was found to associate with the iron binding protein frataxin exclusively during reperfusion. In vitro, frataxin has been shown to protect aconitase from [4Fe-4S](2+) cluster disassembly, irreversible inactivation, and, potentially, degradation. Thus, the response of mitochondrial aconitase and ATP-dependent proteolytic activity to reperfusion-induced prooxidant production appears to be a regulated event that would be expected to reduce irreparable damage to the mitochondria.

Mitochondrial K(ATP) channels in cell survival and death

Since the discovery of the mitochondrial ATP-sensitive potassium channel (mitoK(ATP)) more than 13 years ago, it has been implicated in the processes of ischemic preconditioning (IPC), apoptosis and mitochondrial matrix swelling. Different approaches have been employed to characterize the pharmacological profile of the channel, and these studies strongly suggest that cellular protection well correlates with the opening of mitoK(ATP). However, there are many questions regarding mitoK(ATP) that remain to be answered. These include the very existence of mitoK(ATP) itself, its degree of importance in the process of IPC, its response to different pharmacological agents, and how its activation leads to the process of IPC and protection against cell death. Recent findings suggest that mitoK(ATP) may be a complex of multiple mitochondrial proteins, including some which have been suggested to be components of the mitochondrial permeability transition pore. However, the identity of the pore-forming unit of the channel and the details of the interactions between these proteins remain unclear. In this review, we attempt to highlight the recent advances in the physiological role of mitoK(ATP) and discuss the controversies and unanswered questions.

Assessment of mitochondrial membrane potential in proximal tubules after hypoxia-reoxygenation

Proximal tubules develop a severe energetic deficit during hypoxia-reoxygenation (H/R) that previous studies using fluorescent potentiometric probes have suggested is characterized by sustained, partial mitochondrial deenergization. To validate the primary occurrence of mitochondrial deenergization in the process, optimize approaches for estimating changes in mitochondrial membrane potential (DeltaPsim) in the system, and clarify the mechanisms for the defect, we further investigated the behavior of 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazocarbocyanine iodide (JC-1) in these cells and introduce a more dynamic and quantitative approach employing safranin O for use with the tubule system. Although use of JC-1 can be complicated by decreases in the plasma membrane potential that limit cellular uptake of JC-1 and such behavior was demonstrated in ouabain-treated tubules, changes in DeltaPsim entirely accounted for the decreases in the formation of red fluorescent JC-1 aggregates and in the ratio of red/green fluorescence observed after H/R. The red JC-1 aggregates did not readily dissociate when tubules were deenergized after JC-1 uptake, making it unsuitable for dynamic studies of energization. Safranin O uptake by digitonin-permeabilized tubules required very small numbers of tubules, permitted measurements of DeltaPsim for relatively prolonged periods after the end of the experimental maneuvers, was rapidly reversible during deenergization, and allowed for direct assessment of both substrate-dependent, electron transport-mediated DeltaPsim, and ATP hydrolysis-supported DeltaPsim. Both types of energization measured using safranin O in tubules permeabilized after H/R were impaired, but combining substrates and ATP substantially restored DeltaPsim.

Roles of KATP channels in delayed cardioprotection and intracellular Ca(2+) in the rat heart as revealed by kappa-opioid receptor stimulation with U50488H

The effect of preconditioning with U50488 H (UP), a selective kappa-opioid receptor (kappa-OR) agonist, on infarct size and intracellular Ca2+ ([Ca2+]i) in the heart subjected to ischaemic insults were studied and evaluated. U50488 H administered intravenously reduced the infarct size 18-48 h after administration in isolated hearts subjected to regional ischaemia/reperfusion (I/R). The effect was dose dependent. A peak effect was reached at 10 mg x kg-1 U50488 H and at 24 h after administration. The effect of 10 mg x kg-1 U50488 H at 24 h after administration was abolished by nor-binaltorphimine (nor-BNI), a selective kappa-OR antagonist, indicating the effect was kappa-OR mediated. The infarct reducing effect of U50488 H was attenuated when a selective blocker of mitochondrial (5-hydroxydecanoic acid, 5-HD) or sarcolemmal (HRM-1098) ATP-sensitive potassium channel (KATP) was coadministered with U50488 H 24 h before ischaemia or when 5-HD was administered just before ischaemia. U50488 H also attenuated the elevation in [Ca2+]i and reduction in electrically induced [Ca2+]i transient in cardiomyocytes subjected to ischaemic insults. The effects were reversed by blockade of KATP channel, which abolished the protective effect of preconditioning with U50488 H. The results indicated that mitochondrial KATP channel serves as both a trigger and a mediator, while sarcolemmal KATP channel as a trigger only, of delayed cardioprotection of kappa-OR stimulation. The effects of these channels may result from prevention/attenuation of [Ca2+]i overload induced by ischaemic insults.

In vitro and in vivo studies of F(0)F(1)ATP synthase regulation by inhibitor protein IF(1) in goat heart

A method has been developed to allow the level of F(0)F(1)ATP synthase capacity and the quantity of IF(1) bound to this enzyme be measured in single biopsy samples of goat heart. ATP synthase capacity was determined from the maximal mitochondrial ATP hydrolysis rate and IF(1) content was determined by detergent extraction followed by blue native gel electrophoresis, two-dimensional SDS-PAGE and immunoblotting with anti-IF(1) antibodies. Anaesthetized open-chest goats were subjected to ischemic preconditioning and/or sudden increases of coronary blood flow (CBF) (reactive hyperemia). When hyperemia was induced before ischemic preconditioning, a steep increase in synthase capacity, followed by a deep decrease, was observed. In contrast, hyperemia did not affect synthase capacity when applied after ischemic preconditioning. Similar effects could be produced in vitro by treatment of heart biopsy samples with anoxia (down-regulation of the ATP synthase) or high-salt or high-pH buffers (up-regulation). We show that both in vitro and in vivo the same close inverse correlation exists between enzyme activity and IF(1) content, demonstrating that under all conditions tested the only significant modulator of the enzyme activity was IF(1). In addition, both in vivo and in vitro, 1.3-1.4 mol of IF(1) was predicted to fully inactivate 1 mol of synthase, thus excluding the existence of significant numbers of non-inhibitory binding sites for IF(1) in the F(0) sector.

Separation of fluorescence signals from Ca2+ and NADH during cardioplegic arrest and cardiac ischemia

Determinations of intracellular [Ca(2+)](i) during ischemia using fluorescent indicators are hampered by overlapping cellular autofluorescence (AF), which largely depends on NADH. If Ca(2+) is to be determined under different kinds of ischemia, signal separation merits special attention. We used triple wavelength excitation fluorescence to separate autofluorescence from [Ca(2+)]-dependent fura-2 fluorescence. Excitation at 360 nm served as third, Ca(2+)-insensitive wavelength. Using an appropriate evaluation procedure, we separated Ca(2+)-dependent signals from autofluorescence which is semiquantitatively associated with NADH, an indicator of the cellular redox state. We compared changes of [Ca(2+)](i) in isolated hearts during ischemia following cardioplegic arrest with those after transient stop of nutritive perfusion. We observed [Ca(2+)] transients in spontaneously beating hearts, persisting during ischemic episodes, and an increase of mean [Ca(2+)](i). In contrast, cardioplegic arrest stopped periodical [Ca(2+)](i) transients and heart beats simultaneously. [Ca(2+)](i) remained at diastolic values, tended to decrease during the first minutes of cardioplegic arrest and then increased slowly. Autofluorescence increased under both conditions. During ischemia, this increase was faster than in cardioplegia experiments. It started after the last heart beat despite persisting perfusion. Our measurements demonstrate that rhythmical heart beat is essential for sufficient perfusion. Reduced [Ca(2+)](i) under cardioplegic arrest may influence metabolism.

Novel role of phospholipase C-delta1: regulation of liver mitochondrial Ca2+ uptake

Mitochondrial Ca2+ (mCa2+) handling is an important regulator of liver cell function that controls events ranging from cellular respiration and signal transduction to apoptosis. Cytosolic Ca2+ enters mitochondria through the ruthenium red-sensitive mCa2+ uniporter, but the mechanisms governing uniporter activity are unknown. Activation of many Ca2+ channels in the cell membrane requires PLC. This activation commonly occurs through phosphitidylinositol-4,5-biphosphate (PIP2) hydrolysis and the production of the second messengers inositol 1,4,5-trisphosphate [I(1,4,5)P3] and 1,2-diacylglycerol (DAG). PIP2 was recently identified in mitochondria. We hypothesized that PLC exists in liver mitochondria and regulates mCa2+ uptake through the uniporter. Western blot analysis with anti-PLC antibodies demonstrated the presence of PLC-delta1 in pure preparations of mitochondrial membranes isolated from rat liver. In addition, the selective PLC inhibitor U-73122 dose-dependently blocked mCa2+ uptake when whole mitochondria were incubated at 37 degrees C with 45Ca2+. Increasing extra mCa2+ concentration significantly stimulated mCa2+ uptake, and U-73122 inhibited this effect. Spermine, a uniporter agonist, significantly increased mCa2+ uptake, whereas U-73122 dose-dependently blocked this effect. The inactive analog of U-73122, U-73343, did not affect mCa2+ uptake in any experimental condition. Membrane-permeable I(1,4,5)P3 receptor antagonists 2-aminoethoxydiphenylborate and xestospongin C also inhibited mCa2+ uptake. Although extra mitochondrial I(1,4,5)P3 had no effect on mCa2+ uptake, membrane-permeable DAG analogs 1-oleoyl-2-acetyl-sn-glycerol and DAG-lactone, which inhibit PLC activity, dose-dependently inhibited mCa2+ uptake. These data indicate that PLC-delta1 exists in liver mitochondria and is involved in regulating mCa2+ uptake through the uniporter.

Photophysical properties of safranine O in protic solvents

Spectroscopic and photophysical properties of safranine O (Sf) were investigated in binary water/solvent mixtures. It was found that these properties are strongly solvent-dependent. A blue shift is observed for both the ground-state absorption and the triplet-triplet main absorption band when the solvent polarity augments. At the same time a red shift of the fluorescence emission band takes place. These facts are interpreted in terms of higher dipole moment of the dye molecule in the S(1) state as compared with the S(0) state, while a decrease in the dipole moment of the triplet state T(n) with respect to the triplet state T(1) occurs. The Stokes' shift and the fluorescence lifetime shows a linear correlation with the E(T)(30) parameter, while a non-linear behavior is observed when a correlation with models of a continuous dielectric solvent is attempted. These results suggest the operation of strong specific interactions of Sf with solvent molecules, most likely hydrogen bonding. From fluorescence lifetime and quantum yield determinations, as well as intersystem-crossing quantum yields, the solvent dependence of the photophysical kinetic parameters were obtained. The radiative fluorescence rate constant can be adequately reproduced by calculations based on the UV-Vis absorption and emission spectra, as given by the Strickler-Berg equation.

Negative inotropic drugs alter indexes of cytosolic [Ca2+]-left ventricular pressure relationships after ischemia

Negative inotropic agents may differentially modulate indexes of cytosolic [Ca(2+)]-left ventricular (LV) pressure (LVP) relationships when given before and after ischemia. We measured and calculated [Ca(2+)], LVP, velocity ratios [[(d[Ca(2+)]/dt(max))/(dLVP/dt(max)); VR(max)] and [(d[Ca(2+)]/dt(min))/(dLVP/dt(min)); VR(min)]], and area ratio (AR; area [Ca(2+)]/area LVP per beat) before and after global ischemia in guinea pig isolated hearts. Ca(2+) transients were recorded by indo 1-AM fluorescence via a fiberoptic probe placed at the LV free wall. [Ca(2+)]-LVP loops were acquired by plotting LVP as a function of [Ca(2+)] at multiple time points during the cardiac cycle. Hearts were perfused with bimakalim, 2,3-butanedione monoxime (BDM), nifedipine, or lidocaine before and after 30 min of ischemia. Before ischemia, each drug depressed LVP, but only nifedipine decreased both LVP and [Ca(2+)] with a downward and leftward shift of the [Ca(2+)]-LVP loop. After ischemia, each drug depressed LVP and [Ca(2+)] with a downward and leftward shift of the [Ca(2+)]-LVP loop. Each drug except BDM decreased d[Ca(2+)]/dt(max); nifedipine decreased d[Ca(2+)]/dt(min), whereas lidocaine increased it, and bimakalim and BDM had no effect on d[Ca(2+)]/dt(min). Each drug except bimakalim increased VR(max) and VR(min) before ischemia; after ischemia, only BDM and nifedipine increased VR(max) and VR(min). Before and after ischemia, BDM and nifedipine increased AR, whereas lidocaine and bimakalim had no effect. At 30 min of reperfusion, control hearts exhibited marked Ca(2+) overload and depressed LVP. In each drug-pretreated group Ca(2+) overload was reduced on reperfusion, but only the group pretreated with nifedipine exhibited both higher LVP and lower [Ca(2+)]. These results show that negative inotropic drugs are less capable of reducing [Ca(2+)] after ischemia so that there is a relatively larger Ca(2+) expenditure for contraction/relaxation after ischemia than before ischemia. Moreover, the differential effects of pretreatment with negative inotropic drugs on [Ca(2+)]-LVP relationships after ischemia suggest that these drugs, especially nifedipine, can elicit cardiac preconditioning.

Mitochondrial potassium transport: the role of the mitochondrial ATP-sensitive K(+) channel in cardiac function and cardioprotection

Coronary artery disease and its sequelae-ischemia, myocardial infarction, and heart failure-are leading causes of morbidity and mortality in man. Considerable effort has been devoted toward improving functional recovery and reducing the extent of infarction after ischemic episodes. As a step in this direction, it was found that the heart was significantly protected against ischemia-reperfusion injury if it was first preconditioned by brief ischemia or by administering a potassium channel opener. Both of these preconditioning strategies were found to require opening of a K(ATP) channel, and in 1997 we showed that this pivotal role was mediated by the mitochondrial ATP-sensitive K(+) channel (mitoK(ATP)). This paper will review the evidence showing that opening mitoK(ATP) is cardioprotective against ischemia-reperfusion injury and, moreover, that mitoK(ATP) plays this role during all three phases of the natural history of ischemia-reperfusion injury preconditioning, ischemia, and reperfusion. We discuss two distinct mechanisms by which mitoK(ATP) opening protects the heart-increased mitochondrial production of reactive oxygen species (ROS) during the preconditioning phase and regulation of intermembrane space (IMS) volume during the ischemic and reperfusion phases. It is likely that cardioprotection by ischemic preconditioning (IPC) and K(ATP) channel openers (KCOs) arises from utilization of normal physiological processes. Accordingly, we summarize the results of new studies that focus on the role of mitoK(ATP) in normal cardiomyocyte physiology. Here, we observe the same two mechanisms at work. In low-energy states, mitoK(ATP) opening triggers increased mitochondrial ROS production, thereby amplifying a cell signaling pathway leading to gene transcription and cell growth. In high-energy states, mitoK(ATP) opening prevents the matrix contraction that would otherwise occur during high rates of electron transport. MitoK(ATP)-mediated volume regulation, in turn, prevents disruption of the structure-function of the IMS and facilitates efficient energy transfers between mitochondria and myofibrillar ATPases.

How Inotropic Drugs Alter Dynamic and Static Indices of Cyclic Myoplasmic [Ca2+] to Contractility Relationships in Intact Hearts

The authors examined effects of positive (dopamine and digoxin) and negative (nifedipine and lidocaine) inotropic interventions on the instantaneous cyclic relationship between myoplasmic [Ca2+] and simultaneously developed left ventricular pressure (LVP) in intact guinea pig hearts. Novel indices were developed to quantify this relationship based on (1) transient [Ca2+] and LVP signal morphology, ie, maxima and minima, peak derivatives, beat areas, durations, and ratios of indices of LVP to [Ca2+]; (2) temporal delay; and (3) LVP versus [Ca2+] loop morphology, ie, orientation, size, hysteresis, position, shape, and duration. These analyses were used to assess the cost of phasic [Ca2+] for contraction and relaxation over one beat after inotropic intervention. It was found that dopamine and digoxin increased contractile and relaxation responsiveness to phasic [Ca2+], cumulative Ca2+, and net Ca2+ flux. Unlike dopamine, digoxin did not decrease relaxation response time. Nifedipine and lidocaine decreased contractile and relaxation responsiveness to phasic [Ca2+], cumulative Ca2+, and net Ca2+ flux. Unlike lidocaine, nifedipine decreased net available Ca2+ and Ca2+ influx. Positive inotropic agents increased [Ca2+]-LVP loop area and hysteresis and resulted in a more vertically oriented loop. Nifedipine and lidocaine decreased these loop indices and lidocaine exhibited greater loop hysteresis than did nifedipine. These novel indices provide a quantitative assessment of myoplasmic [Ca2+] handling for cardiac contractile function.

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.

Calcium and cardiac arrhythmias: DADs, EADs, and alternans

Rapid progress has been made in understanding the molecular mechanisms by which calcium ions mediate certain cardiac arrhythmias. Principal advances include imaging of cytosolic calcium in isolated cells and in intact tissues, use of fluorescent indicators and monophasic action potentials to record membrane potentials in isolated tissue, and sequencing of the genes that encode critical ion channel proteins. In this review, five types of arrhythmias are discussed where calcium ion currents, or currents controlled by calcium, appear to be responsible for arrythmogenesis. These include: (1) the delayed afterpotential that occurs in conditions of intracellular calcium overload such as digitalis toxicity; (2) the early afterdepolarization that occurs when action potential duration is prolonged; (3) the slowly conducted calcium-dependent action potential (the slow response) in the SA and AV nodes; (4) the phenomenon of calcium transient alternans during ischemia, which is related to action potential duration alternans and t-wave alternans; (5) catecholamine-induced cardiac arrhythmias in families with mutations of the sarcoplasmic reticulum calcium-release channel. For each type of arrhythmia, the clinical implications of emerging knowledge are discussed. An especially important issue is whether ventricular fibrillation during acute coronary artery occlusion is due to calcium transient alternans. Ventricular fibrillation due to acute ischemia is an important subset of the 400,000 sudden cardiac deaths that occur annually in the U.S. Certain drugs, including beta blockers, fish oils, verapamil, and diltiazem, seem to specifically prevent ventricular fibrillation in this setting, and in most cases an effect of the drug on cytosolic calicum appears to be involved.

Reversed activity of mitochondrial adenine nucleotide translocator in ischemia-reperfusion

BACKGROUND

Graft dysfunction as a result of preservation injury remains a major clinical problem in liver transplantation. This is related in part to accumulation of mitochondrial calcium. In an attempt to sustain cell and mitochondrial integrity during ischemia, intramitochondrial F(0)F(1) adenosine triphosphate (ATP) synthase reverses its activity and hydrolyzes ATP to maintain the mitochondrial transmembrane potential (mdeltapsi). It is not known how cytoplasmic ATP becomes available for hydrolysis by this enzyme. The authors hypothesized that mitochondrial adenine nucleotide translocator (ANT) reverses its activity during ischemia, making cytoplasmic ATP available for hydrolysis by F(0)F(1) ATP synthase.

METHODS

Rat livers were perfused with cold University of Wisconsin solution at 4 degrees C (39.2 degrees F)through the portal vein and processed immediately or after 24 hr of cold storage. Mitochondria were separated by differential centrifugation. ATP-dependent mitochondrial calcium-45 (45Ca)2+ uptake was determined after incubation with ATP (5 mM) or adenosine diphosphate (ADP) (5 mM) with or without 15 microM of bongkrekic acid (BA), an ANT blocker; the nonhydrolyzable analog of ATP (adenosine 5'-beta,gamma-imidotriphosphate [AMP-PNP]) served as the negative control. All measurements were performed in triplicate. Student t test, P<0.05 was taken as significant.

RESULTS

Inhibition of ANT by BA prevents mitochondrial Ca2+ accumulation in the presence of ATP and high 45Ca2+ concentrations, and increased extramitochondrial 45Ca2+ stimulated mitochondrial 45Ca2+ uptake in the presence of ATP but not ADP, AMP-PNP, or BA.

CONCLUSIONS

These data demonstrate that ANT plays an important role in mitochondrial Ca2+ uptake under ischemic conditions by reversing its activity and allowing transport of extramitochondrial ATP into the matrix for hydrolysis by reversed F(0)F(1) ATP synthase.

Calpain is activated in experimental uremia: is calpain a mediator of uremia-induced myocardial injury?

BACKGROUND

The cysteine proteases calpain and caspase-3 are known mediators of cell death. The aim of this study was to assess their contribution to the tissue damage found in experimental uremia.

METHODS

Calpain and caspase-3 activities were measured in the hearts of rats that were sham-operated (control), sham-operated and spontaneously hypertensive (SHR), and those rendered uremic by 5/6 nephrectomy (uremic). In an in vitro study, heart myoblasts (Girardi) were incubated with human serum from healthy subjects (control serum conditioned media, CSCM) or uremic patients (uremic serum conditioned media, USCM), in the presence and absence of calpain and caspase-3 inhibitors. After 48 hours the activity of calpain and caspase-3 was measured, and cell injury determined by DNA fragmentation (ELISA) and lactate dehydrogenase (LDH) release. An in situ assay was designed to study how USCM affects calpain activity over time.

RESULTS

In the in vivo study, mean calpain activities were almost identical in the control and SHR groups, but calpain and caspase-3 activities were much elevated in the uremic group (P < 0.01 and 0.001 respectively vs. control). The SHR group had significantly higher mean arterial blood pressure (P < 0.001 vs. control, 0.01 vs. uremic). In the in vitro study calpain activity and DNA fragmentation were markedly higher in USCM treated cells compared to CSCM (both P<0.05). Both were reduced in USCM cells containing calpain inhibitors (E64d, calpastatin, or PD 150606). LDH release was raised also in USCM treated cultures (P < 0.05), which only the E64d treatment could significantly reduce (P < 0.02). Caspase-3 activities were similar in USCM and CSCM groups. The in situ assay showed significant increases in calpain activity in USCM treated cells compared to CSCM after just 3.5 hours (P<0.01).

CONCLUSIONS

In vivo results suggest that the increases in calpain and caspase-3 activity in uremic rat hearts were primarily due to uremia and not to hypertension. In vitro data demonstrate that uremia-induced cell injury can be attenuated by calpain inhibition. Therefore, it is likely that calpain is a mediator of uremia-induced myocardial injury.

Impaired Ca2+ Handling in Perfused Hypertrophic Hearts from Dahl Salt-Sensitive Rats

To clarify the correlation between intracellular Ca2+ dynamics and level of Ca2+-regulatory proteins, changes in Ca2+ handling and these proteins were investigated in a whole-heart experimental model of pressure-overload hypertrophy. We used 17-18-week-old male Dahl salt-sensitive rats (DS) and Dahl salt-resistant rats (DR) fed a high-salt diet. We monitored the fura-2 fluorescence ratio, an index of cytoplasmic Ca2+ concentration ([Ca2+]i), using a Ca2+ analyzer in a retrograde perfused heart. Left ventricular pressure (LVP) and an electrocardiogram were simultaneously recorded. Ca2+ handling was assessed by exposing the hearts to 2 min of low-Na+ (70 mmol/l) perfusion to produce an increase in [Ca2+]i (n = 6), which was sensitive to Ni2+, a blocker of the Na+/Ca2+ exchanger (NCX). In another series, the hearts were stimulated at 2.5 to 5 Hz to determine the Ca2+-force-frequency relationship (n = 6). DS rats showed marked cardiac hypertrophy without any signs of failure. The time-to-peak Ca2+ transient was prolonged in DS compared with that in DR during normal beating. During low-Na+ exposure, the time-to-peak diastolic [Ca2+]i (TTP) and the decay-time from peak [Ca2+]i (DT) were prolonged in DS compared with DR (TTP, 43.3 +/- 4.0 vs. 32.5 +/- 2.5 s, p < 0.05; DT, 70.0 +/- 8.8 vs. 29.2 +/- 2.7 s, p < 0.005). Following pretreatment with 10 mmol/l caffeine to inhibit sarcoplasmic reticulum (SR) function, TTP and DT were still prolonged in DS compared with DR (TTP, 64.2 +/- 9.7 vs. 37.0 +/- 5.8 s, p < 0.05; DT, 55.8 +/- 12.6 vs. 26.0 +/- 5.7 s, p < 0.05). The force (LVP)-frequency relationship was initially positive in DR but was negative at all times in DS (%LVP/2.5 Hz: DS, 90.3 +/- 2.0%; DR, 112.2 +/- 4.5%; p < 0.05). Elevation of diastolic [Ca2+]i (percent increase of baseline) was greater in DS than in DR with increased stimulation (5 Hz: DS, 80.7 +/- 6.7%; DR, 52.1 +/- 5.9%; p < 0.05). In Western blot analysis, the protein level of NCX was equivalent, whereas that of SR Ca2+ ATPase (SERCA2) was significantly decreased in DS compared with DR. These results suggest that slowing of cellular Ca2+ mobilization and removal is related to impaired Ca2+ handling in late-phase cardiac hypertrophy. Both the activity of the NCX and that of the SR may be affected. The SR dysfunction may be associated with change in protein level of SERCA2.

Effects of pharmacological preconditioning with U50488H on calcium homeostasis in rat ventricular myocytes subjected to metabolic inhibition and anoxia

1. The effects of pharmacological preconditioning with U50488H (U(50)), a selective kappa-opioid receptor agonist, on Ca(2+) homeostasis in rat ventricular myocytes subjected for 9 min to metabolic inhibition (MI) and anoxia (A), consequences of ischaemia, were studied and compared with those of preconditioning with brief periods of MI/A. 2. Precondition with 30 micro M of U(50) for three cycles of 1 min each cycle separated by 3 min of recovery (UP) significantly increased the percentage of non-blue cells following MI/A. The effect of UP is the same as that of preconditioning with an inhibitor of glycolysis and an oxygen scavenger for three 1-min cycles separated by three-minute recovery (MI/AP). The results indicate that like MI/AP, UP also confers cardioprotection. 3. MI/A increased intracellular Ca(2+) ([Ca(2+)](i)) and reduced the amplitude of caffeine-induced [Ca(2+)](i) transients, an indication of Ca(2+) content in the sarcoplasmic reticulum (SR). MI/A also reduced the electrically-induced [Ca(2+)](i) transient, that indicates Ca(2+)-release during excitation-contraction coupling, and Ca(2+) sparks in unstimulated myocytes, that indicates spontaneous Ca(2+)-release from SR. It also prolonged the decline of the electrically-induced [Ca(2+)](i) transient and slowed down the recovery of the electrically-induced [Ca(2+)](i) transient after administration of caffeine. In addition, MI/A prolonged the decline of caffeine induced [Ca(2+)](i) transient, an indication of Na(+)-Ca(2+) exchange activity, and UP prevented it. So UP, that confers cardioprotection, prevented the changes induced by MI/A. With the exception of Ca(2+)-spark, which was not studied, the effects of MI/AP are the same as those of UP. 4. It is concluded that pharmacological preconditioning with U(50), that confers immediate cardioprotection, prevents changes of Ca(2+) homeostasis altered by MI/A in the rat heart. This may be responsible, at least partly, for the cardioprotective action. 5. The study also provided evidence that MI/A causes mobilization of Ca(2+) from SR to cytoplasm causing Ca(2+)-overload which may be due to reduced Ca(2+)-uptake by SR. MI/A also reduces spontaneous and electrically induced Ca(2+) release from SR.