Mitochondrial Ca2+ fluxes and levels during ischaemia and reperfusion: possible mechanisms

L-type Ca(2+) channel facilitation mediated by H(2)O(2)-induced activation of CaMKII in rat ventricular myocytes

The Ca(2+)-dependent facilitation (CDF) of L-type Ca(2+) channels, a major mechanism for force-frequency relationship of cardiac contraction, is mediated by Ca(2+)/CaM-dependent kinase II (CaMKII). Recently, CaMKII was shown to be activated by methionine oxidation. We investigated whether oxidation-dependent CaMKII activation is involved in the regulation of L-type Ca(2+) currents (I(Ca,L)) by H(2)O(2) and whether Ca(2+) is required in this process. Using patch clamp, I(Ca)(,L) was measured in rat ventricular myocytes. H(2)O(2) induced an increase in I(Ca,L) amplitude and slowed inactivation of I(Ca)(,L). This oxidation-dependent facilitation (ODF) of I(Ca)(,L) was abolished by a CaMKII blocker KN-93, but not by its inactive analog KN-92, indicating that CaMKII is involved in ODF. ODF was not affected by replacement of external Ca(2+) with Ba(2+) or presence of EGTA in the internal solutions. However, ODF was abolished by adding BAPTA to the internal solution or by depleting sarcoplasmic reticulum (SR) Ca(2+) stores using caffeine and thapsigargin. Alkaline phosphatase, beta-iminoadenosine 5'-triphosphate (AMP-PNP), an autophosphorylation inhibitor autocamtide-2-related inhibitory peptide (AIP), or a catalytic domain blocker (CaM-KIINtide) did not affect ODF. In conclusion, oxidation-dependent facilitation of L-type Ca(2+) channels is mediated by oxidation-dependent CaMKII activation, in which local Ca(2+) increases induced by SR Ca(2+) release is required.

Sodium ferulate attenuates anoxia/reoxygenation-induced calcium overload in neonatal rat cardiomyocytes by NO/cGMP/PKG pathway

Development of intracellular calcium overload is an important pathophysiological factor in myocardial ischemia/reperfusion or anoxia/reoxygenation injury. Recent studies have shown that Sodium Ferulate (SF) stimulates nitric oxide (NO) production and exerts a cardioprotective effect in the ischemia-reperfused heart. However, it has not been determined whether the cardioprotection of SF is associated with suppression of Ca(2+) overload via NO/cyclic GMP (cGMP)/cGMP-dependent protein kinase (PKG) pathway. In this work, after cardiomyocytes were incubated with 100, 200, 400, or 800 microM SF for 3 h, anoxia/reoxygenation injury was induced and intracellular Ca(2+) concentration, NO synthase (NOS) activity, guanylate cyclase activity, NO, and cGMP formation were measured appropriately. The results showed that treatment with SF concentration-dependently inhibited calcium overload induced by anoxia/reoxygenation. We also demonstrated that SF (100-800 microM) concentration dependently enhanced NO and cGMP formation through increasing NOS activity and guanylate cyclase activity in the cardiomyocytes. On the contrary, inhibition of calcium overload by SF was markedly attenuated by addition of an NOS inhibitor, an NO scavenger, an soluble guanylate cyclase inhibitor, and a PKG inhibitor: N(G)-nitro-l-arginine methyl ester (L-NAME, 100 microM), 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazole-1-oxyl-3-oxide (c-PTIO, 1.0 microM), 1H-[1, 2, 4] oxadiazolo [4, 3-alpha] quinoxalin-1-one (ODQ, 20 microM) and KT5823 (0.2 microM), respectively. Our findings indicate that SF significantly attenuates anoxia/reoxygenation-induced Ca(2+) overload and improves cell survival in cultured cardiomyocytes through NO/cGMP/PKG signal pathway.

Effect of hydrogen peroxide on reoxygenation-induced Ca2+ accumulation in rat cardiomyocytes

Reactive oxygen species (ROS) contribute to cell damage during reperfusion of the heart. ROS may exert their effects partly by interfering with Ca(2+) homeostasis of the myocardium. The purpose of this study was to investigate the effects of hydrogen peroxide (H(2)O(2)) on Ca(2+) accumulation during reoxygenation of isolated adult rat cardiomyocytes exposed to 1 h of hypoxia and to relate the effects to possible changes in release of lactate dehydrogenase (LDH), free intracellular Ca(2+) ([Ca(2+)](i)) and Mg(2+)([Mg(2+)](i)), and mitochondrial membrane potential (Deltapsim). Cell Ca(2+) was determined by (45)Ca(2+) uptake. Free [Mg(2+)](i) and [Ca(2+)](i) and Deltapsim were measured by flow cytometry. Reoxygenation-induced Ca(2+) accumulation was attenuated by 23 and 34% by 10 and 25 microM H(2)O(2), respectively, added at reoxygenation. H(2)O(2) at 100 and 250 microM increased cell Ca(2+) by 50 and 83%, respectively, whereas 500 microM H(2)O(2) decreased cell Ca(2+) by 20%. H(2)O(2) at (25 microM) reduced LDH release and [Mg(2+)](i) and increased Deltapsim, indicating cell protection, whereas 250 microM H(2)O(2) increased LDH release and [Mg(2+)](i) and decreased Deltapsim, indicating cell damage. Clonazepam (100 microM) attenuated the increase in Ca(2+) accumulation, the elevation of [Ca(2+)](i), and the decrease in Deltapsim induced by 100 and 250 microM H(2)O(2) during reoxygenation. We report for the first time that 25 microM H(2)O(2) attenuates Ca(2+) accumulation, LDH release, and dissipation of Deltapsim during reoxygenation of hypoxic cardiomyocytes, indicating cell protection.

Increased [Mg2+]o reduces Ca2+ influx and disruption of mitochondrial membrane potential during reoxygenation

Increase in extracellular Mg2+ concentration ([Mg2+]o) reduces Ca2+ accumulation during reoxygenation of hypoxic cardiomyocytes and exerts protective effects. The aims of the present study were to investigate the effect of increased [Mg(2+)](o) on Ca2+ influx and efflux, free cytosolic Ca2+ ([Ca2+]i) and Mg2+ concentrations ([Mg2+]i), Ca2+ accumulation in the presence of inhibitors of mitochondrial or sarcoplasmatic reticulum Ca2+ transport, and finally mitochondrial membrane potential (Delta(psi)m). Isolated adult rat cardiomyocytes were exposed to 1 h of hypoxia and subsequent reoxygenation. Cell Ca2+ was determined by 45Ca2+ uptake, and the levels of [Mg2+]i and [Ca2+]i were determined by flow cytometry as the fluorescence of magnesium green and fluo 3, respectively. Ca2+ influx rate was significantly reduced by approximately 40%, whereas Ca2+ efflux was not affected by increased [Mg2+]o (5 mM) during reoxygenation. [Ca2+]i and [Mg2+]i were increased at the end of hypoxia, fell after reoxygenation, and were unaffected by increased [Mg2+]o. Clonazepam, a selective mitochondrial Na+/Ca2+ exchange inhibitor (100 microM), significantly reduced Ca2+ accumulation by 70% and in combination with increased [Mg2+]o by 90%. Increased [Mg2+]o, clonazepam, and the combination of both attenuated the hypoxia-reoxygenation-induced reduction in Delta(psi)m, determined with the cationic dye JC-1 by flow cytometry. A significant inverse correlation was observed between Delta(psi)m and cell Ca2+ in reoxygenated cells treated with increased [Mg2+]o and clonazepam. In conclusion, increased [Mg2+]o (5 mM) inhibits Ca2+ accumulation by reducing Ca2+ influx and preserves Delta(psi)m without affecting [Ca2+]i and [Mg2+]i during reoxygenation. Preservation of mitochondria may be an important effect whereby increased [Mg2+]o protects the postischemic heart.

Elucidating the molecular mechanism of the permeability transition pore and its role in reperfusion injury of the heart

First, we present a summary of the evidence for our model of the molecular mechanism of the permeability transition (MPT). Our proposal is that the MPT occurs as a result of the binding of mitochondrial cyclophilin (CyP-D) to the adenine nucleotide translocase (ANT) in the inner mitochondrial membrane. This binding is enhanced by thiol modification of the ANT caused by oxidative stress or other thiol reagents. CyP-D binding enhances the ability of the ANT to undergo a conformational change triggered by Ca2+. Binding of ADP or ATP to a matrix site of the ANT antagonises this effect of Ca2+; modification of other ANT thiol groups inhibits ADP binding and sensitises the MPT to [Ca2+]. Increased membrane potential changes the ANT conformation to enhance ATP binding and hence inhibit the MPT. Our most recent data shows that a fusion protein of CyP-D and glutathione-S-transferase immobilised to Sepharose specifically binds the ANT from Triton-solubilised inner mitochondrial membranes in a cyclosporin A (CsA) sensitive manner. Second we summarise the evidence for the MPT being a major factor in the transition from reversible to irreversible injury during reperfusion of a heart following a period of ischaemia. We describe how in the perfused heart [3H]deoxyglucose entrapment within mitochondria can be used to measure the opening of MPT pore in situ. During ischaemia pore opening does not occur, but significant opening does occur during reperfusion, and recovery of the heart is dependent on subsequent pore closure. Pore opening is inhibited by the presence in the perfusion medium of pyruvate and the anaesthetic propofol which both protect the heart from reperfusion injury. Third we discuss how the MPT may be involved in determining whether cell death occurs by necrosis (extensive pore opening and ATP depletion) or apoptosis (transient pore opening with maintenance of ATP).

Reversible ischemia increases levels of Alzheimer amyloid protein precursor without increasing levels of mRNA in the rabbit spinal cord

In a rabbit spinal cord ischemia model (RSCIM), the time courses of neuropathological damage of the spinal cord and neurological impairment of the motor functions are well established, demonstrating that the extent of neuropathological damage and the severity of neurological impairment are closely correlated. We used the RSCIM to elucidate the effects of reversible (15 min) and irreversible (60 min) ischemia on the endogenous levels of amyloid protein precursors (APPs) at both the mRNA and protein levels in the caudolumbar/sacral region of the spinal cord. We speculate that endogenous APPs are induced by ischemia as either trophic factors or stress-induced proteins in the RSCIM. A 15-min occlusion transiently increased the APP protein levels in neurons, which returned to the original levels by the end of 60 min occlusion. The increase in APP protein levels during 15-min ischemic insult does not appear to involve regulation at the mRNA level. The increased level of APPs, particularly of the soluble form, could support the possibility that APPs play a neuroprotective role in the RSCIM as stress-induced proteins. In contrast, failure to maintain the increased APP protein levels or to increase the mRNA, as seen in the 60-min ischemia samples, may be one of the causal factors that induce necrosis and neuronal cell death leading to irreversible neurological impairment.

Mitochondria, oxygen and reperfusion damage

Reperfusion of the ischaemic or hypoxic heart elicits a number of oxygen dependent processes such as cell lysis and Ca2+ uptake. It is known that the energisation of mitochondria, which requires oxygen, plays a key role in these processes and that the organelle actively sequesters Ca2+ under these circumstances. In this brief review we discuss how oxidants derived from mitochondrial electron transport may perturb mitochondrial calcium handling on reoxygenation of the hypoxic myocardium. In addition we show that the immunosuppressive agent cyclosporin has little or no effect on the oxygen dependent increase in total cell Ca2+ which occurs when hypoxic myocytes are reoxygenated. This result suggests that the Ca2+ dependent mitochondrial pore, which is known to function under conditions of oxidative stress, does not play a major role in the perturbation of Ca2+ homeostasis which occurs on reoxygenation of hypoxic hearts.

Bay K 8644, modifier of calcium transport and energy metabolism in rat heart mitochondria: a new intracellular site of action

1. The dihydropyridine Ca2+ channel agonist Bay K 8644 (10-200 microM) produced a concentration-dependent increase in State 4 respiration in the rat heart mitochondria with the highest concentration (200 microM) increasing the rate from 33.1 +/- 0.7 to 187.0 +/- 13.3 ng atoms O2 consumed min-1 mg-1 protein. 2. Bay K 8644 (200 microM) reduced State 3 respiration from 247.2 +/- 11.7 to 174.4 +/- 0.06 ng atoms O2 min-1 mg-1 protein, reduced the respiratory control index (RCI) from 5.3 +/- 0.45 to 1.1 +/- 0.03 and reduced the ADP:O ratio from 2.75 +/- 0.03 to 1.3 +/- 0.15. 3. A similar, but smaller, stimulation of State 4 respiration was seen with nitrendipine (25-200 microM), the rate increasing from 22.6 +/- 1.0 to 33.1 +/- 1.8 ng atoms O2 consumed min-1 mg-1 protein in the presence of 200 microM nitrendipine. 4. Bay K 8644 (10-60 microM) increased the total Ca2+ uptake into rat heart mitochondria, the total increasing from 248.8 +/- 8.4 to 406.9 +/- 17.6 ng Ca2+ mg-1 protein at 60 microM Bay K 8644 (EC50 = 18.9 +/- 1.4 microM). 5. Bay K 8644 (10-60 microM) produced a concentration-dependent reduction in the Ca2+ influx rate (IC50 = 52.5 +/- 2.8 microM). Similar effects were seen with (+)-Bay K 8644 and (-)-Bay K 8644. 6. Nitrendipine (40-120 microM) stimulated Ca2+ efflux from mitochondria preloaded with the ion; the efflux rate increasing from 2.9 +/- 0.05 to 114.2 +/- 6.2 nmol Ca2+ min-1 mg-1 protein (EC50 = 57.3 +/- 1.3 microM). 7. These data indicate dihydropyridine-induced changes in the activity of the mitochondrial Na+/Ca2 . antiporter pathway; nitrendipine causing stimulation and Bay K 8644 causing inhibition.

Sarcoplasmic reticulum function in the "stunned" myocardium

Transient ischemia does not induce myocardial necrosis but may be associated with prolonged contractile dysfunction ("stunned" myocardium). It has been suggested that alteration of the excitation-contraction coupling system (sarcoplasmic reticulum) could be responsible for this phenomenon. We tested this hypothesis by characterizing sarcoplasmic reticulum (SR) function in an isolated rat heart model of "stunned" myocardium (hearts reperfused after 10 min of normothermic global ischemia). At the end of the ischemic period oxalate-supported Ca-uptake was depressed either in the whole homogenate or in isolated SR (to 47% and 22% of control values, respectively). During reperfusion Ca-uptake of the whole heart homogenate recovered almost completely whereas slight but significant depression persisted in isolated SR (48 +/- 2 vs 67 +/- 4 nmol/min x mg, P less than 0.01). In the presence of ruthenium red or ryanodine, two inhibitors of SR Ca-release channels, Ca-uptake was stimulated. Both in the whole heart homogenate and in isolated SR, such stimulation was remarkably smaller after reperfusion than in control conditions (P less than 0.001) suggesting reduced conductivity state of the SR Ca-release channels. Ca-stimulated, magnesium-dependent ATPase activity was remarkably reduced during ischemia and postischemic reperfusion induced only incomplete recovery (93 +/- 18 vs 169 +/- 14 nmol ATP/min x mg protein, P less than 0.05). We conclude that complex modifications of SR function occur in the "stunned" myocardium and could contribute to the contractile impairment found in this condition.