Effects of cytosolic ATP on spontaneous and triggered Ca2+-induced Ca2+ release in permeabilised rat ventricular myocytes

Flecainide induces a sustained countercurrent dependent effect on RyR2 in permeabilized WT ventricular myocytes but not in intact cells

Background and purpose: While flecainide is now an accepted treatment for arrhythmias associated with catecholaminergic polymorphic ventricular tachycardia (CPVT), its mechanism of action remains controversial. In studies on myocytes from CPVT mice, inhibition of proarrhythmic Ca²⁺ waves was initially attributed to a novel action on the type-2 ryanodine receptor (RyR2). However, subsequent work on wild type (WT) myocytes questioned the conclusion that flecainide has a direct action on RyR2. In the present study, the effects of flecainide were compared in intact and permeabilized WT myocytes. Experimental approach: Intracellular Ca²⁺ was measured using confocal microscopy in intact or saponin permeabilized adult rat ventricular myocytes (ARVM). In some experiments on permeabilized cells, flecainide was studied following partial inhibition of the sarcoplasmic reticulum (SR) counter-current. Key results: Flecainide induced sustained changes Ca²⁺ sparks and waves in permeabilized ARVM, which were comparable to those reported in intact or permeabilized myocytes from CPVT mice. However, a relatively high level of flecainide (25 μM) was required to induce these effects. Inhibition of the SR counter-current potentiated the effects of flecainide on SR Ca²⁺ waves. In intact field stimulated ARVM, prolonged exposure to 15 μM flecainide decreased wave frequency but RyR2 dependent effects on Ca²⁺ sparks were absent; higher drug concentrations blocked field stimulation, consistent with inhibition of Nav1.5. Conclusions and implications: In intact ARVM, the absence of effects on Ca²⁺ sparks suggests that the intracellular flecainide concentration was insufficient to influence RyR2. Wave inhibition in intact ARVM may reflect secondary effects of Nav1.5 inhibition. Potentiation of flecainide's action by counter-current inhibition can be explained if transient polarization of the SR membrane during SR Ca²⁺ release facilitates its action on RyR2.

Optimizing mitochondrial maintenance in extended neuronal projections

Neurons rely on localized mitochondria to fulfill spatially heterogeneous metabolic demands. Mitochondrial aging occurs on timescales shorter than the neuronal lifespan, necessitating transport of fresh material from the soma. Maintaining an optimal distribution of healthy mitochondria requires an interplay between a stationary pool localized to sites of high metabolic demand and a motile pool capable of delivering new material. Interchange between these pools can occur via transient fusion / fission events or by halting and restarting entire mitochondria. Our quantitative model of neuronal mitostasis identifies key parameters that govern steady-state mitochondrial health at discrete locations. Very infrequent exchange between stationary and motile pools optimizes this system. Exchange via transient fusion allows for robust maintenance, which can be further improved by selective recycling through mitophagy. These results provide a framework for quantifying how perturbations in organelle transport and interactions affect mitochondrial homeostasis in neurons, a key aspect underlying many neurodegenerative disorders.

Inter and Intracellular mitochondrial trafficking in health and disease

Neurons and glia maintain central nervous system (CNS) homeostasis through diverse mechanisms of intra- and intercellular signaling. Some of these interactions include the exchange of soluble factors between cells via direct cell-to-cell contact for both short and long-distance transfer of biological materials. Transcellular transfer of mitochondria has emerged as a key example of this communication. This transcellular transfer of mitochondria are dynamically involved in the cellular and tissue response to CNS injury and play beneficial roles in recovery. This review highlights recent research addressing the cause and effect of intra- and intercellular mitochondrial transfer with a specific focus on the future of mitochondrial transplantation therapy. We believe that mitochondrial transfer plays a crucial role during bioenergetic crisis/deficit, but the quality, quantity and mode of mitochondrial transfer determines the protective capacity for the receiving cells. Mitochondrial transplantation is a new treatment paradigm and will overcome the major bottleneck of traditional approach of correcting mitochondria-related disorders.

Enhanced expansion microscopy to measure nanoscale structural and biochemical remodeling in single cells

Resolution is a key feature in microscopy which allows the visualization of the fine structure of cells. Much of the life processes within these cells depend on the three-dimensional (3D) complexity of these structures. Optical super-resolution microscopies are currently the preferred choice of molecular and cell biologists who seek to visualize the organization of specific protein species at the nanometer scale. Traditional super-resolution microscopy techniques have often been limited by sample thickness, axial resolution, specialist optical instrumentation and computationally-demanding software for assembling the images. In this chapter we detail the protocol, "enhanced expansion microscopy" (EExM), which combines X10 expansion microscopy with Airyscan confocal microscopy. EExM enables 15nm lateral (and 35nm axial) resolution, and is a relatively cheap, accessible option allowing single protein resolution for the non-specialist optical microscopists. We illustrate how EExM has been utilized for mapping the 3D topology of intracellular protein arrays at sample depths which are not always compatible with some of the traditional super-resolution techniques. We demonstrate that antibody markers can recognize and map post-translational modifications of individual proteins in addition to their 3D positions. Finally, we discuss the current uncertainties and validations in EExM which include the isotropy in gel expansion and assessment of the expansion factor (of resolution improvement).

Activation of RyR2 by class I kinase inhibitors

BACKGROUND AND PURPOSE

Kinase inhibitors are a common treatment for cancer. Class I kinase inhibitors that target the ATP-binding pocket are particularly prevalent. Many of these compounds are cardiotoxic and can cause arrhythmias. Spontaneous release of Ca²⁺ via cardiac ryanodine receptors (RyR2), through a process termed store overload-induced Ca²⁺ release (SOICR), is a common mechanism underlying arrhythmia. We explored whether class I kinase inhibitors could modify the activity of RyR2 and trigger SOICR to determine if this contributes to the cardiotoxic nature of these compounds.

EXPERIMENTAL APPROACH

The impact of class I and II kinase inhibitors on SOICR was studied in HEK293 cells and ventricular myocytes using single-cell Ca²⁺ imaging. A specific effect on RyR2 was confirmed using single channel recordings. Ventricular myocytes were also used to determine if drug-induced changes in SOICR could be reversed using anti-SOICR agents.

KEY RESULTS

Class I kinase inhibitors increased the propensity of SOICR. Single channel recording showed that this was due to a specific effect on RyR2. Class II kinase inhibitors decreased the activity of RyR2 at the single channel level but had little effect on SOICR. The promotion of SOICR mediated by class I kinase inhibitors could be reversed using the anti-SOICR agent VK-II-86.

CONCLUSIONS AND IMPLICATIONS

Part of the cardiotoxicity of class I kinase inhibitors can be assigned to their effect on RyR2 and increase in SOICR. Compounds with anti-SOICR activity may represent an improved treatment option for patients.

Epac2-Rap1 Signaling Regulates Reactive Oxygen Species Production and Susceptibility to Cardiac Arrhythmias

AIMS

In the heart, β1-adrenergic signaling involves cyclic adenosine monophosphate (cAMP) acting via both protein kinase-A (PKA) and exchange protein directly activated by cAMP (Epac): a guanine nucleotide exchange factor for the small GTPase Rap1. Inhibition of Epac-Rap1 signaling has been proposed as a therapeutic strategy for both cancer and cardiovascular disease. However, previous work suggests that impaired Rap1 signaling may have detrimental effects on cardiac function. The aim of the present study was to investigate the influence of Epac2-Rap1 signaling on the heart using both in vivo and in vitro approaches.

RESULTS

Inhibition of Epac2 signaling induced early afterdepolarization arrhythmias in ventricular myocytes. The underlying mechanism involved an increase in mitochondrial reactive oxygen species (ROS) and activation of the late sodium current (INalate). Arrhythmias were blocked by inhibition of INalate or the mitochondria-targeted antioxidant, mitoTEMPO. In vivo, inhibition of Epac2 caused ventricular tachycardia, torsades de pointes, and sudden death. The in vitro and in vivo effects of Epac2 inhibition were mimicked by inhibition of geranylgeranyltransferase-1, which blocks interaction of Rap1 with downstream targets.

INNOVATION

Our findings show for the first time that Rap1 acts as a negative regulator of mitochondrial ROS production in the heart and that impaired Epac2-Rap1 signaling causes arrhythmias due to ROS-dependent activation of INalate. This has implications for the use of chemotherapeutics that target Epac2-Rap1 signaling. However, selective inhibition of INalate provides a promising strategy to prevent arrhythmias caused by impaired Epac2-Rap1 signaling.

CONCLUSION

Epac2-Rap1 signaling attenuates mitochondrial ROS production and reduces myocardial arrhythmia susceptibility. Antioxid. Redox Signal. 27, 117-132.

Astrocytic dynamin-like protein 1 regulates neuronal protection against excitotoxicity in Parkinson disease

Mitochondrial dynamics has recently become an area of piqued interest in neurodegenerative disorders, including Parkinson disease (PD); however, the contribution of astrocytes to these disorders remains unclear. Here, we show that the level of dynamin-like protein 1 (Dlp1; official name DNM1L), which promotes mitochondrial fission, is lower in astrocytes from the brains of PD patients, and that decreased astrocytic Dlp1 likely represents a relatively early event in PD pathogenesis. In support of this conclusion, we show that Dlp1 knockdown dramatically affects mitochondrial morphological characteristics and localization in astrocytes, impairs the ability of astrocytes to adequately protect neurons from the excitotoxic effects of glutamate, and increases intracellular Ca(2+) in response to extracellular glutamate, resulting from compromised intracellular Ca(2+) buffering. Taken together, our results suggest that astrocytic mitochondrial Dlp1 is a key protein in mitochondrial dynamics and decreased Dlp1 may interfere with neuron survival in PD by disrupting Ca(2+)-coupled glutamate uptake.

Mitochondrial trafficking in neurons

Neurons, perhaps more than any other cell type, depend on mitochondrial trafficking for their survival. Recent studies have elucidated a motor/adaptor complex on the mitochondrial surface that is shared between neurons and other animal cells. In addition to kinesin and dynein, this complex contains the proteins Miro (also called RhoT1/2) and milton (also called TRAK1/2) and is responsible for much, although not necessarily all, mitochondrial movement. Elucidation of the complex has permitted inroads for understanding how this movement is regulated by a variety of intracellular signals, although many mysteries remain. Regulating mitochondrial movement can match energy demand to energy supply throughout the extraordinary architecture of these cells and can control the clearance and replenishing of mitochondria in the periphery. Because the extended axons of neurons contain uniformly polarized microtubules, they have been useful for studying mitochondrial motility in conjunction with biochemical assays in many cell types.

The meaning of mitochondrial movement to a neuron's life

Cells precisely regulate mitochondrial movement in order to balance energy needs and avoid cell death. Neurons are particularly susceptible to disturbance of mitochondrial motility and distribution due to their highly extended structures and specialized function. Regulation of mitochondrial motility plays a vital role in neuronal health and death. Here we review the current understanding of regulatory mechanisms that govern neuronal mitochondrial transport and probe their implication in health and disease. This article is part of a Special Issue entitled: Mitochondrial dynamics and physiology.

Flecainide inhibits arrhythmogenic Ca2+ waves by open state block of ryanodine receptor Ca2+ release channels and reduction of Ca2+ spark mass

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is linked to mutations in the cardiac ryanodine receptor (RyR2) or calsequestrin. We recently found that the drug flecainide inhibits RyR2 channels and prevents CPVT in mice and humans. Here we compared the effects of flecainide and tetracaine, a known RyR2 inhibitor ineffective in CPVT myocytes, on arrhythmogenic Ca(2+) waves and elementary sarcoplasmic reticulum (SR) Ca(2+) release events, Ca(2+) sparks. In ventricular myocytes isolated from a CPVT mouse model, flecainide significantly reduced spark amplitude and spark width, resulting in a 40% reduction in spark mass. Surprisingly, flecainide significantly increased spark frequency. As a result, flecainide had no significant effect on spark-mediated SR Ca(2+) leak or SR Ca(2+) content. In contrast, tetracaine decreased spark frequency and spark-mediated SR Ca(2+) leak, resulting in a significantly increased SR Ca(2+) content. Measurements in permeabilized rat ventricular myocytes confirmed the different effects of flecainide and tetracaine on spark frequency and Ca(2+) waves. In lipid bilayers, flecainide inhibited RyR2 channels by open state block, whereas tetracaine primarily prolonged RyR2 closed times. The differential effects of flecainide and tetracaine on sparks and RyR2 gating can explain why flecainide, unlike tetracaine, does not change the balance of SR Ca(2+) fluxes. We suggest that the smaller spark mass contributes to flecainide's antiarrhythmic action by reducing the probability of saltatory wave propagation between adjacent Ca(2+) release units. Our results indicate that inhibition of the RyR2 open state provides a new therapeutic strategy to prevent diastolic Ca(2+) waves resulting in triggered arrhythmias, such as CPVT.

Mitochondria in cardiomyocyte Ca2+ signaling

Ca(2+) signaling is of vital importance to cardiac cell function and plays an important role in heart failure. It is based on sarcolemmal, sarcoplasmic reticulum and mitochondrial Ca(2+) cycling. While the first two are well characterized, the latter remains unclear, controversial and technically challenging. In mammalian cardiac myocytes, Ca(2+) influx through L-type calcium channels in the sarcolemmal membrane triggers Ca(2+) release from the nearby junctional sarcoplasmic reticulum to produce Ca(2+) sparks. When this triggering is synchronized by the cardiac action potential, a global [Ca(2+)](i) transient arises from coordinated Ca(2+) release events. The ends of intermyofibrillar mitochondria are located within 20 nm of the junctional sarcoplasmic reticulum and thereby experience a high local [Ca(2+)] during the Ca(2+) release process. Both local and global Ca(2+) signals may thus influence calcium signaling in mitochondria and, reciprocally, mitochondria may contribute to the local control of calcium signaling. In addition to the intermyofibrillar mitochondria, morphologically distinct mitochondria are also located in the perinuclear and subsarcolemmal regions of the cardiomyocyte and thus experience a different local [Ca(2+)]. Here we review the literature in regard to several issues of broad interest: (1) the ultrastructural basis for mitochondrion - sarcoplasmic reticulum cross-signaling; (2) mechanisms of sarcoplasmic reticulum signaling; (3) mitochondrial calcium signaling; and (4) the possible interplay of calcium signaling between the sarcoplasmic reticulum and adjacent mitochondria. Finally, this review discusses experimental findings and mathematical models of cardiac calcium signaling between the sarcoplasmic reticulum and mitochondria, identifies weaknesses in these models, and suggests strategies and approaches for future investigations.

The mechanism of Ca2+ -dependent regulation of kinesin-mediated mitochondrial motility

Mitochondria are mobile organelles and cells regulate mitochondrial movement in order to meet the changing energy needs of each cellular region. Ca(2+) signaling, which halts both anterograde and retrograde mitochondrial motion, serves as one regulatory input. Anterograde mitochondrial movement is generated by kinesin-1, which interacts with the mitochondrial protein Miro through an adaptor protein, milton. We show that kinesin is present on all axonal mitochondria, including those that are stationary or moving retrograde. We also show that the EF-hand motifs of Miro mediate Ca(2+)-dependent arrest of mitochondria and elucidate the regulatory mechanism. Rather than dissociating kinesin-1 from mitochondria, Ca(2+)-binding permits Miro to interact directly with the motor domain of kinesin-1, preventing motor/microtubule interactions. Thus, kinesin-1 switches from an active state in which it is bound to Miro only via milton, to an inactive state in which direct binding to Miro prevents its interaction with microtubules. Disrupting Ca(2+)-dependent regulation diminishes neuronal resistance to excitotoxicity.

Trimetazidine improved Ca2+ handling in isoprenaline-mediated myocardial injury of rats

Dysregulation of intracellular Ca2+ homeostasis plays an important role in mediating myocardial injury. We tested the hypothesis that treatment with trimetazidine (TMZ) would improve intracellular Ca2+ handling in myocardial injury of rats. The control group received saline only (10 ml kg(-1) day(-1), i.p.) for 7 days. In a second group, isoprenaline (ISO; 5 mg kg(-1) day(-1), s.c.) was administered to rats for 2 days to induce an acute injury of the myocardium. In a third group, treatment with TMZ (10 mg kg(-1) day(-1), i.p.) was initiated 1 day before ISO administration and continued for 7 days (n = 7 rats in each group). Histopathological evaluation showed that TMZ prevented ISO-induced myocardial damage. TMZ preserved the ATP levels and decreased the maleic dialdehyde (MDA) content in the hearts compared with ISO-treated rats. The diastolic [Ca2+]i measured by loading with fura-2 AM in isolated cardiomyocytes was increased significantly in ISO-treated rats compared to the control animals. TMZ prevented the rise of diastolic [Ca2+]i and the depression of caffeine-induced Ca2+ transients caused by ISO administration. The reduction in sarcoplasmic reticulum (SR) Ca2+ content in the heart cells and in cardiac SR Ca2+-ATPase activity in ISO-treated rats was abolished by TMZ, although there were no differences in SR Ca2+-ATPase protein levels between the control, ISO and ISO + 7 mz-treated rats. In addition, TMZ prevented the reduction in sarcolemmal L-type Ca2+ current density in the heart cells induced by ISO treatment. These results demonstrate that the treatment of rats with TMZ inhibited the increase of diastolic [Ca2+]i and prevented the decrease of SR Ca2+ content, SR Ca2+-ATPase activity and L-type Ca2+ current density in cardiomyocytes in ISO-mediated myocardial injury of rats. These changes in Ca2+ handling could help to explain the favourable action of TMZ in myocardial injury.

Temporal and spatial properties of cellular Ca2+flux in trout ventricular myocytes

Confocal microscopy was used to investigate the temporal and spatial properties of Ca2+transients and Ca2+sparks in ventricular myocytes of the rainbow trout ( Oncorhynchus mykiss). Confocal imaging confirmed the absence of T tubules and the long (∼160 μm), thin (∼8 μm) morphology of trout myocytes. Line scan imaging of Ca2+transients evoked by electrical stimulation in cells loaded with fluo 4 revealed spatial inhomogeneities in the temporal properties of Ca2+transients across the width of the myocytes. The Ca2+wavefront initiated faster, rose faster, and reached larger peak amplitudes in the periphery of the myocyte compared with the center. These differences were exacerbated by stimulation with the L-type Ca2+channel agonist (−)BAY K 8644 or by sarcoplasmic reticulum (SR) inhibition with ryanodine and thapsigargin. Results reveal that the shape of the trout myocyte allows for rapid diffusion of Ca2+from the cell periphery to the cell center, with SR Ca2+release contributing to the cytosolic Ca2+rise in a time-dependent manner. Spontaneous Ca2+sparks were exceedingly rare in trout myocytes under control conditions (1 sparking cell from 238 cells examined). This is in marked contrast to the rat where a total of 56 spontaneous Ca2+sparks were observed in 9 of 11 myocytes examined. Ca2+sparklike events were observed in a very small number of trout myocytes (15 sparks from 9 of 378 cells examined) after stimulation with either (−)BAY K 8644 or high Ca2+(6 mM). Reducing temperature to 15°C in intact myocytes or permeabilizing myocytes to adjust intracellular conditions to favor Ca2+spark detection was without significant effects. Possible reasons for the rarity of Ca2+sparks in a cardiac myocyte with an active SR are discussed.

Characteristics of Prolonged Ca 2+ Release Events Associated With the Nuclei in Adult Cardiac Myocytes

Confocal microscopy was used to study the properties of nuclear Ca 2+ regulation in adult ventricular myocytes. Prolonged nuclear Ca 2+ release (PNCR) events were identified in both intact and permeabilized rat myocytes. PNCR occurred spontaneously and was restricted to localized regions at the ends of the elongated nuclei. Typically, PNCR took the form of a rapid rise in [Ca 2+ ] followed by a maintained plateau. The mean duration of PNCR (1.78±0.19 seconds) was markedly greater than the half decay time for cytosolic Ca 2+ sparks (31.2±0.56 ms) obtained under the same conditions. The PNCR width at half maximum amplitude (5.0±0.2 μm) was also significantly greater than that of cytosolic Ca 2+ sparks (2.6±0.05 μm) obtained under the same conditions. Experiments involving the use of syto-11 to accurately locate the nuclei demonstrated that PNCR originates from the nuclear envelope or a closely associated structure. The spatial spread of PNCR was asymmetrical, with greater diffusion of Ca 2+ toward the center of the nucleus than the cytosol. Both PNCR and Ca 2+ sparks were abolished by interventions that deplete SR Ca 2+ stores or inhibit RYR activation. Experiments on intact, electrically stimulated cells revealed that diffusion of Ca 2+ from the ends of the nucleus toward the center is a prominent feature of the nucleoplasmic Ca 2+ transient. The possibility that recruitment of Ca 2+ release sites involved in PNCR might influence the temporal and spatial characteristics of the nucleoplasmic [Ca 2+ ] transient is considered.

Effects of cytosolic NADH/NAD(+) levels on sarcoplasmic reticulum Ca(2+) release in permeabilized rat ventricular myocytes

In the heart ischaemic conditions induce metabolic changes known to have profound effects on Ca(2+) signalling during excitation-contraction coupling. Ischaemia also affects the redox state of the cell. However, the role of cytosolic redox couples, such as the NADH/NAD(+) redox system, for the regulation of Ca(2+) homeostasis has remained elusive. We studied the effects of NADH and NAD(+) on sarcoplasmic reticulum (SR) Ca(2+) release in permeabilized rat ventricular myocytes as well as on Ca(2+) uptake by SR microsomes and ryanodine receptor (RyR) single channel activity. Exposure of permeabilized myocytes to NADH (2 mm; [Ca(2+)](cyt)= 100nm) decreased the frequency and the amplitude of spontaneous Ca(2+) sparks by 62% and 24%, respectively. This inhibitory effect was reversed by NAD(+) (2 mm) and did not depend on mitochondrial function. The inhibition of Ca(2+) sparks by NADH was associated with a 52% decrease in SR Ca(2+) load. Some of the effects observed with NADH may involve the generation of superoxide anion (O(2)(-).) as they were attenuated to just a transient decrease of Ca(2+) spark frequency by superoxide dismutase (SOD). O(2)(-). generated in situ from the xanthine/xanthine oxidase reaction caused a slowly developing decrease of Ca(2+) spark frequency and SR Ca(2+) load by 44% and 32%, respectively. Furthermore, in studies with cardiac SR microsomes NADH slowed the rate of ATP-dependent Ca(2+) uptake by 39%. This effect also appeared to depend on O(2)(-). formation. Single channel recordings from RyRs incorporated into lipid bilayers revealed that NADH (2 mm) inhibited the activity of RyR channels by 84%. However, NADH inhibition of RyR activity was O(2)(-).-independent. In summary, an increase of the cytoplasmic NADH/NAD(+) ratio depresses SR Ca(2+) release in ventricular cardiomyocytes. The effect appears to be mediated by direct NADH inhibition of RyR channel activity and by indirect NADH inhibition (O(2)(-). mediated) of SR Ca(2+)-ATPase activity with a subsequent decrease in SR Ca(2+) content.

Pyruvate modulates cardiac sarcoplasmic reticulum Ca2+ release in rats via mitochondria-dependent and -independent mechanisms

The glycolytic product pyruvate has beneficial effects on cardiac contractile function. The postulated cellular mechanisms underlying the positive inotropic effect of pyruvate, however, are contradictory or have remained elusive. Therefore, we studied the effects of pyruvate on cardiac Ca2+ regulation, intracellular pH (pHi) and flavoprotein oxidation using fluorescence confocal microscopy in intact and permeabilized rat ventricular myocytes and single channel recordings from rat cardiac ryanodine receptors (RyRs) incorporated into planar lipid bilayers. In intact cells extracellular pyruvate (10 mM) elevated diastolic [Ca2+]i, which was due, at least in part, to a concomitant acidification of the cytosol. Furthermore, pyruvate increased the amplitude and slowed the kinetics of the electrically evoked [Ca2+]i transient, and augmented sarcoplasmic reticulum (SR) Ca2+ content. Recording of flavoprotein (FAD) fluorescence indicated that pyruvate caused a reduction of mitochondrial redox potential, which is proportional to an increase of the rate of ATP synthesis. Inhibitors of mitochondrial monocarboxylate transport (alpha-cyano-4-hydroxycinnamate, 0.5 mM), adenine nucleotide translocation (atractyloside, 0.3 mM) and the electron transport chain (cyanide, 4 mM) abolished or attenuated the pyruvate-mediated increase of the amplitude of the [Ca2+]i transient, but did not change the effect of pyruvate on diastolic [Ca2+]i. Results from experiments with permeabilized myocytes indicated a direct correlation between ATP/ADP ratio and SR Ca2+ content. Furthermore, pyruvate (4 mM) reduced the frequency of spontaneous Ca2+ sparks by approximately 50%. Single RyR channel recordings revealed a approximately 60% reduction of the open probability of the channel by pyruvate (1 mM), but no change in conductance. This effect of pyruvate on RyR channel activity was neither Ca2+ nor ATP dependent. Taken together, these findings suggest that, in cardiac tissue, pyruvate has a dual effect on SR Ca2+ release consisting of a direct inhibition of RyR channel activity and elevation of SR Ca2+ content. The latter effect was most probably mediated by an enhanced SR Ca2+ uptake due to an augmentation of mitochondria-dependent ATP synthesis.

pH-dependent and -independent effects inhibit Ca(2+)-induced Ca2+ release during metabolic blockade in rat ventricular myocytes

We have investigated the role of changes of intracellular pH (pHi) in the effects of metabolic blockade (cyanide plus 2-deoxyglucose) on Ca2+ release from the sarcoplasmic reticulum (SR) in rat ventricular myocytes. pHi and cell length were measured simultaneously. Metabolic blockade decreased the frequency of Ca2+ waves, an effect previously shown to be due to inhibition of Ca2+ release from the SR. This was accompanied by an intracellular acidification. Intracellular acidification was produced in the absence of metabolic inhibition by application of sodium butyrate. A maintained intracellular acidosis produced a decrease of wave frequency. A hysteresis between pHi and wave frequency was observed such that during the onset of the acidification the wave frequency decreased more than in the steady state. Comparison of the steady state relationship between pHi and wave frequency showed that the decrease of wave frequency produced by metabolic blockade was greater than could be accounted for simply by the accompanying decrease of pHi. In other experiments the buffering power of the solution was increased. Under these conditions, metabolic blockade produced no change of pHi but the decrease of wave frequency persisted. We conclude that, although intracellular acidification occurs during metabolic blockade, it is not responsible for most of the inhibition of Ca2+ release from the SR.

Mitochondrial respiration and Ca2+ waves are linked during fertilization and meiosis completion

Fertilization increases both cytosolic Ca(2+) concentration and oxygen consumption in the egg but the relationship between these two phenomena remains largely obscure. We have measured mitochondrial oxygen consumption and the mitochondrial NADH concentration on single ascidian eggs and found that they increase in phase with each series of meiotic Ca(2+) waves emitted by two pacemakers (PM1 and PM2). Oxygen consumption also increases in response to Ins(1,4,5)P(3)-induced Ca(2+) transients. Using mitochondrial inhibitors we show that active mitochondria sequester cytosolic Ca(2+) during sperm-triggered Ca(2+) waves and that they are strictly necessary for triggering and sustaining the activity of the meiotic Ca(2+) wave pacemaker PM2. Strikingly, the activity of the Ca(2+) wave pacemaker PM2 can be restored or stimulated by flash photolysis of caged ATP. Taken together our observations provide the first evidence that, in addition to buffering cytosolic Ca(2+), the egg's mitochondria are stimulated by Ins(1,4,5)P(3)-mediated Ca(2+) signals. In turn, mitochondrial ATP production is required to sustain the activity of the meiotic Ca(2+) wave pacemaker PM2.

Effects of phosphocreatine on SR Ca(2+) regulation in isolated saponin-permeabilized rat cardiac myocytes

The effects of phosphocreatine (PCr) on sarcoplasmic reticulum (SR) Ca(2+) regulation were investigated in saponin-permeabilized rat ventricular myocytes. Cells were perfused continuously with weakly Ca(2+)-buffered solutions approximating to the intracellular milieu. Ca(2+) release from the SR was detected using Fura-2 or Fluo-3. Withdrawal of PCr reduced the frequency of spontaneous Ca(2+) release by 12.8 +/- 3.4 % (n = 9) and the amplitude of the spontaneous Ca(2+) transient by 17.4 +/- 3.1 % (n = 9). Stepwise reductions in [PCr] progressively increased the time for the spontaneous Ca(2+) transient to rise from 25 to 100 % of the maximum value (TP75) and to fall by 75 % of the peak level (DT75). Following complete PCr withdrawal, the TP75 and the DT75 were 147.1 +/- 13.2 and 174.8 +/- 23.2 % of the control values, respectively. Experiments involving confocal microscopy showed that PCr withdrawal decreased the propagation velocity of spontaneous Ca(2+) waves. PCr withdrawal also reduced the frequency and amplitude, but increased the duration of spontaneous Ca(2+) sparks. Rapid application of 20 mM caffeine was used to assess the SR Ca(2+) content at the point of spontaneous Ca(2+) release. In the absence of PCr, the amplitude of the caffeine-induced Ca(2+) transient was 18.4 +/- 2.7 % (n = 9) lower than in the presence of 10 mM PCr. This suggests that PCr withdrawal reduces the maximum SR Ca(2+) content that can be sustained before spontaneous Ca(2+) release occurs. These results suggest that local ADP buffering by PCr is essential for normal Ca(2+) regulation by the SR. Prolongation of the descending phase of the spontaneous Ca(2+) transient is consistent with a reduction in the efficiency of the SR Ca(2+) pump due to ADP accumulation. The fact that spontaneous Ca(2+) release occurs at a lower SR Ca(2+) content in the absence of PCr suggests that the Ca(2+) release mechanism may also be affected. These effects may be of relevance in circumstances where PCr depletion and Ca(2+) overload occur, such as myocardial ischaemia or anoxia.

Effects of cytosolic ATP on Ca(2+) sparks and SR Ca(2+) content in permeabilized cardiac myocytes

Confocal imaging was used to study the influence of cytosolic ATP on the properties of spontaneous Ca(2+) sparks in permeabilized ventricular myocytes. Cells were perfused with mock intracellular solutions containing fluo 3. Reducing [ATP] to <0.5 mmol/L decreased the frequency but increased the amplitude of spontaneous Ca(2+) sparks. In the presence of 20 micromol/L ATP, the amplitude increased by 48.7+/-10.9%, and the frequency decreased by 77.07+/-3.8%, relative to control responses obtained at 5 mmol/L ATP. After exposure to a solution containing zero ATP, the frequency of Ca(2+) sparks decreased progressively and approached zero within 90 seconds. As ATP washed out of the cell, the sarcoplasmic reticulum (SR) Ca(2+) content increased, until reaching a maximum after 3 minutes. Subsequent introduction of adenylyl imidodiphosphate precipitated a burst of large-amplitude Ca(2+) sparks. This was accompanied by a rapid decrease in SR Ca(2+) content to 80% to 90% of the steady-state value obtained in the presence of 5 mmol/L ATP. Thereafter, the SR Ca(2+) content declined much more slowly over 5 to 10 minutes. The effects of ATP withdrawal on Ca(2+) sparks may reflect reduced occupancy of the adenine nucleotide site on the SR Ca(2+) channel. These effects may contribute to previously reported changes in SR function during myocardial ischemia and reperfusion, in which ATP depletion and Ca(2+) overload occur.

A comparison of the effects of ATP and tetracaine on spontaneous Ca(2+) release from rat permeabilised cardiac myocytes

1. Fluo-3 fluorescence measurements were made in isolated beta-escin permeablised rat cardiac myocytes using confocal microscopy. Perfusion of a mock intracellular solution containing 0.22-0.23 microM Ca(2+) and 5 mM ATP elicited regular waves of Ca(2+) (approximately every 5 s) due to spontaneous release of Ca(2+) from the sarcoplasmic reticulum (SR). 2. An approximately linear relationship was noted between Ca(2+) wave velocity (v) and amplitude (sigma). Under the control conditions the ratio of velocity to amplitude (v/sigma) varied little and was 99.8 +/- 2.5 m s(-1) microM(-1) (n = 78). 3. Reduction of [ATP] in the bathing solution to 0.5 and 0.2 mM ATP progressively decreased Ca(2+) wave frequency and propagation velocity while increasing the amplitude. The changes in Ca(2+) wave characteristics in 0.5 mM ATP were similar to those observed during perfusion with 50 microM tetracaine. In 0.2 mM ATP the decline of [Ca(2+)] during a Ca(2+) wave was slowed suggesting a lowered rate of Ca(2+) re-uptake by the SR Ca(2+) pump. 4. Reduction of [ATP] to 0.1 mM abolished Ca(2+) waves after 15-20 s. Returning the [ATP] to 5 mM caused a burst of high frequency and large amplitude waves. Mean velocity of the first wave on returning to 5 mM ATP was larger than normal but the v/sigma value was 32 +/- 6 % of control (n = 6). In the similar burst on removal of 100 microM tetracaine v/sigma was higher than control (166 +/- 9 %, n = 6). 5. Rapid application of caffeine (10 mM) was used to assess the SR Ca(2+) content. This showed that SR Ca(2+) increased as [ATP] was reduced or [tetracaine] was increased. The highest SR Ca(2+) content was observed after perfusion with 0.1 mM ATP, which was 245 +/- 15 % of control values. 6. Returning [ATP] from 0.1 mM to 5 mM caused a burst of high frequency, large amplitude Ca(2+) waves. But recovery after incubation with 300 microM tetracaine resulted in SR Ca(2+) release with no coherent wave pattern. The reason for this discrepancy is discussed.

Involvement of anion exchange in the hypoxia/reoxygenation-induced changes in pH(i) and

The involvement of Cl(-)/HCO(3)(-) exchange in hypoxia/reoxygenation-induced changes in pH(i) and Ca(2+) concentration ([Ca(2+)](i)) was examined in rat ventricular myocytes. During 10-min hypoxia, the initial pH(i) (7.21+/-0.04) fell to below 6.8. Subsequent reperfusion with reoxygenated buffer returned this acidic pH(i) to the neutral range with increases in [Ca(2+)](i). These responses were reduced by the removal of Cl(-) or HCO(3)(-) and by the addition of anion exchange inhibitors, SITS (4-acetamido-4'isothiocyanato-stilbene-2,2'disulfonic acid) and DIDS (4,4'-diisothiocyano-stilbene-2,2'-disulfonic acid), while inhibitors for the Cl(-) channel and Na(+)/K(+)/2Cl(-) cotransport were without effects. The hypoxia-induced acidification was attenuated by protein kinase C inhibitors, calphostin C and chelerythrine, but not by a protein kinase A inhibitor, KT5720. Under normoxic condition, protein kinase C activation induced a SITS-sensitive acidification. Furthermore, in electrically driven rat papillary muscle, SITS and DIDS improved the recovery of developed tension during the reoxygenation. These results suggest that the hypoxia-induced decrease in pH(i) is mediated at least in part by anion exchange stimulation through protein kinase C activation, and this exchange takes part in the reoxygenation-induced Ca(2+) overload as well as contractile dysfunction.

Integrative analysis of calcium cycling in cardiac muscle

The control of intracellular calcium is central to regulation of contractile force in cardiac muscle. This review illustrates how analysis of the control of calcium requires an integrated approach in which several systems are considered. Thus, the calcium content of the sarcoplasmic reticulum (SR) is a major determinant of the amount of Ca(2+) released from the SR and the amplitude of the Ca(2+) transient. The amplitude of the transient, in turn, controls Ca(2+) fluxes across the sarcolemma and thence SR content. This control of SR content influences the response to maneuvers that modify, for example, the properties of the SR Ca(2+) release channel or ryanodine receptor. Specifically, modulation of the open probability of the ryanodine receptor produces only transient effects on the Ca(2+) transient as a result of changes of SR content. These interactions between various Ca(2+) fluxes are modified by the Ca(2+) buffering properties of the cell. Finally, we predict that, under some conditions, the above interactions can result in instability (such as alternans) rather than ordered control of contractility.

Effect of nitric oxide on calcium-induced calcium release in coronary arterial smooth muscle

The present study was designed to determine whether nitric oxide (NO)-induced reduction of [Ca(2+)](i) is associated with Ca(2+)-induced Ca(2+) release (CICR) in coronary arterial smooth muscle cells (CASMCs). Caffeine was used as a CICR activator to induce Ca(2+) release in these cells. The effects of NO donor, sodium nitroprusside (SNP), on caffeine-induced Ca(2+) release were examined in freshly dissociated bovine CASMCs using single cell fluorescence microscopic spectrometry. The effects of NO donor on caffeine-induced coronary vasoconstriction were examined by isometric tension recordings. Caffeine, a CICR or ryanodine receptor (RYR) activator, produced a rapid Ca(2+) release with a 330 nM increase in [Ca(2+)](i). Pretreatment of the CASMCs with SNP, CICR inhibitor tetracaine or RYR blocker ryanodine markedly decreased caffeine-induced Ca(2+) release. Addition of caffeine to the Ca(2+)-free bath solution produced a transient coronary vasoconstriction. SNP, tetracaine and ryanodine, but not guanylyl cyclase inhibitor, ODQ, significantly attenuated caffeine-induced vasoconstriction. These results suggest that CICR is functioning in CASMCs and participates in the vasoconstriction in response to caffeine-induced Ca(2+) release and that inhibition of CICR is of importance in mediating the vasodilator response of coronary arteries to NO.