The signaling pathway for aldosterone-induced mitochondrial production of superoxide anion in the myocardium.

Mineralocorticoid receptor (MR) antagonists decrease morbidity and mortality in heart failure patients for whom oxidative stress is usual; however, the underlying mechanism for this protection is unclear. Since aldosterone stimulates reactive oxygen species (ROS) production in several tissues, we explored its effect and the intracellular pathway involved in the rat myocardium. Aldosterone dose-dependently increased O2(-) production in myocardial slices. At 10 nmol/L, aldosterone increased O2(-) to 165 ± 8.8% of control, an effect prevented not only by the MR antagonists eplerenone and spironolactone (107 ± 7.8 and 103 ± 5.3%, respectively) but also by AG1478 (105 ± 8.0%), antagonist of the EGF receptor (EGFR). Similar results were obtained by silencing MR expression through the direct intramyocardial injection of a lentivirus coding for a siRNA against the MR. The aldosterone effect on O2(-) production was mimicked by the mKATP channel opener diazoxide and blocked by preventing its opening with 5-HD and glibenclamide, implicating the mitochondria as the source of O2(-). Inhibiting the respiratory chain with rotenone or mitochondrial permeability transition (MPT) with cyclosporine A or bongkrekic acid also canceled aldosterone-induced O2(-) production. In addition, aldosterone effect depended on NADPH oxidase and phosphoinositide 3-kinase activation, as apocynin and wortmannin, respectively, inhibited it. EGF (0.1 µg/mL) similarly increased O2(-), although in this case MR antagonists had no effect, suggesting that EGFR transactivation occurred downstream from MR activation. Inhibition of mKATP channels, the respiratory chain, or MPT did not prevent Akt phosphorylation, supporting that it happened upstream of the mitochondria. Importantly, cardiomyocytes were confirmed as a source of aldosterone induced mitochondrial ROS production in experiments performed in isolated cardiac myocytes. These results allow us to speculate that the beneficial effects of MR antagonists in heart failure may be related to a decrease in oxidative stress.

Mitochondrial reactive oxygen species (ROS) as signaling molecules of intracellular pathways triggered by the cardiac renin-angiotensin II-aldosterone system (RAAS).

Mitochondria represent major sources of basal reactive oxygen species (ROS) production of the cardiomyocyte. The role of ROS as signaling molecules that mediate different intracellular pathways has gained increasing interest among physiologists in the last years. In our lab, we have been studying the participation of mitochondrial ROS in the intracellular pathways triggered by the renin-angiotensin II-aldosterone system (RAAS) in the myocardium during the past few years. We have demonstrated that acute activation of cardiac RAAS induces mitochondrial ATP-dependent potassium channel (mitoKATP) opening with the consequent enhanced production of mitochondrial ROS. These oxidant molecules, in turn, activate membrane transporters, as sodium/hydrogen exchanger (NHE-1) and sodium/bicarbonate cotransporter (NBC) via the stimulation of the ROS-sensitive MAPK cascade. The stimulation of such effectors leads to an increase in cardiac contractility. In addition, it is feasible to suggest that a sustained enhanced production of mitochondrial ROS induced by chronic cardiac RAAS, and hence, chronic NHE-1 and NBC stimulation, would also result in the development of cardiac hypertrophy.

The positive inotropic effect of endothelin-1 is mediated by mitochondrial reactive oxygen species.

We have previously demonstrated the participation of reactive oxygen species (ROS) in the positive inotropic effect of a physiological concentration of Angiotensin II (Ang II, 1 nM). The objective of the present work was to evaluate the role and source of ROS generation in the positive inotropic effect produced by an equipotent concentration of endothelin-1 (ET-1, 0.4 nM). Isolated cat ventricular myocytes were used to measure sarcomere shortening with a video-camera, superoxide anion (()O(2)(-)) with chemiluminescence, and ROS production and intracellular pH (pH(i)) with epifluorescence. The ET-1-induced positive inotropic effect (40.4+/-3.1%, n=10, p<0.05) was associated to an increase in ROS production (105+/-29 fluorescence units above control, n=6, p<0.05). ET-1 also induced an increase in ()O(2)(-) production that was inhibited by the NADPH oxidase blocker, apocynin, and by the blockers of mitochondrial ATP-sensitive K(+) channels (mK(ATP)), glibenclamide and 5 hydroxydecanoic acid. The ET-1-induced positive inotropic effect was inhibited by apocynin (0.3 mM; 6.3+/-6.6%, n=13), glibenclamide (50 microM; 8.8+/-3.5%, n=6), 5 hydroxydecanoic acid (500 microM; 14.1+/-8.1, n=9), and by scavenging ROS with MPG (2 mM; 0.92+/-5.6%, n=8). ET-1 enhanced proton efflux (J(H)) carried by the Na(+)/H(+) exchanger (NHE) after an acid load, effect that was blocked by MPG. Consistently, the ET-induced positive inotropic effect was also inhibited by the NHE selective blocker HOE642 (5 microM; 9.37+/-6.07%, n=7). The data show that the effect of a concentration of ET-1 that induces an increase in contractility of about 40% is totally mediated by an intracellular pathway triggered by mitochondrial ROS formation and stimulation of the NHE.

An autocrine/paracrine mechanism triggered by myocardial stretch induces changes in contractility.

An autocrine/paracrine mechanism is triggered by stretching the myocardium. This mechanism involves release of angiotensin II, release/increased formation of endothelin, activation of the Na(+)/H(+) exchanger, increase in intracellular Na(+), and the increase in the Ca(2+) transient that underlies the slow force response to stretch. The autocrine/paracrine mechanism could explain how changes in afterload alter cardiac contractility.

Hyperactivity and altered mRNA isoform expression of the Cl(-)/HCO(3)(-) anion-exchanger in the hypertrophied myocardium.

OBJECTIVE: The aim was to examine the regulation of the cardiac Na(+)-independent Cl(-)/HCO(3)(-) exchanger (AE) mRNA isoform expression in association to the enhanced AE activity in the hypertrophied myocardium of spontaneously hypertensive rats (SHR). METHODS: AE activity was determined by the initial rates of the pH(i) recovery from imposed intracellular alkalinization (forward mode of exchange) and the pH(i) rise induced by Cl(-) removal (reverse mode). Net HCO(3)(-) (J(HCO(3)(-))) efflux and influx were respectively determined. AE mRNA isoforms were analyzed by Northern blot with specific probes to detect AE1, AE2 and AE3 mRNAs. RESULTS: Initial J(HCO(3)(-)) efflux after imposed alkaline load (pH(i) congruent with 7.5) was higher in SHR than in normotensive WKY rats (3.01+/-0.33, n=7, vs. 0.64+/-0.29 mM/min, n=5, P<0.05). J(HCO(3)(-)) influx induced by Cl(-) deprivation was also increased in SHR, 4.24+/-0.56 mM/min (n=10) versus 2.31+/-0.26 (n=10, P<0.05) in WKY. In arbitrary units, the 4.1-kb AE1 mRNA decreased in SHR (0.15+/-0.01, n=7) compared to WKY (0.29+/-0.06, n=7, P<0.05), whereas the 3.6-kb mRNA did not change. AE2 mRNAs were similarly expressed in WKY and SHR. Cardiac specific AE3 (cAE3) mRNA decreased in SHR, 1.10+/-0.16 arbitrary units (n=8) versus 1.79+/-0.24, (n=8, P<0.05) in WKY. Full length AE3 (flAE3) mRNA increased from 0.69+/-0.06 (WKY, n=8) to 1.25+/-0.19 arbitrary units in SHR (n=8, P<0.05). CONCLUSIONS: The increase in flAE3 mRNA expression in cardiac tissue from the SHR is an adaptive change of the hypertrophied myocardium that might be in connection with the increased activity of the AE.

[The Na+/Ca2+ exchanger as responsible for myocardial stunning]. / El intercambiador Na+/Ca2+ como responsable del atontamiento miocárdico.

Our objective was to assess the participation of Na+/H+ exchanger (NHE) and Na+/Ca2+ exchanger (NCX) on systolic and diastolic alterations of myocardial stunning. Isolated perfused rat hearts were submitted to 20 min of global ischemia (Is) followed by 30 min of reperfusion (R). This protocol was repeated after treatment before ischemia and/or early in R. with HOE 642 1 microM, a specific blocker of NHE-1 and KB-R7943 1 microM the novel inhibitor of the reverse mode of NCX. In control ischemic hearts the contractility assessed through +dP/dtmax recovered approximately 60%. When the NHE blockade was performed before is or early in R the postischemic recovery reached 100%. The blockade of the reverse mode of NCX only improved significantly the recovery when administered before is and early in R (95 +/- 7%). The ischemic contracture decreased when the treatment with both blockers was performed before Is. During R the increase of end diastolic pressure (EDP) observed in control ischemic hearts (at 30 min of R, EDP value was 44 +/- 4 mmHg) diminished significantly by NHE (24 +/- 6 and 12 +/- 2 mmHg when the blocker was administered before or after Is) and NCX blockade performed before and after is (12 +/- 6 mmHg). These results indicate that the activation of the reverse mode of NCX secondary to the NHE activation during ischemia and reperfusion is the mechanism responsible for the Ca2+ overload involved in the diminution of contractility that characterizes myocardial stunning.

Angiotensin II stimulates cardiac L-type Ca(2+) current by a Ca(2+)- and protein kinase C-dependent mechanism.

Angiotensin II (ANG II) evokes positive inotropic responses in various species. However, the effects of this peptide on L-type Ca(2+) currents (I(Ca)) are still controversial. We report in this study that the effects of ANG II on I(Ca) differ depending on the mode of patch-clamp technique used, standard whole cell (WC) or perforated patch (PP). No significant effects of ANG II (0.5 microM) were observed when WC in cells dialyzed with high EGTA was used. However, when the intracellular milieu was preserved using PP, ANG II induced a significant 77 +/- 6% increase in I(Ca) (-2.2 +/- 0.3 in control and -3.9 +/- 0.6 pA/pF in ANG II, n = 8, P < 0.05). When WC was used in cells dialyzed with low Ca(2+) buffer capacity (EGTA 0.1 mM), ANG II was able to induce an increase in I(Ca) (-3.5 +/- 0.3 in control vs. -4.8 +/- 0.4 pA/pF in ANG II, n = 13, P < 0.05). This increase was prevented when the cells were also dialyzed with the protein kinase C (PKC) inhibitor chelerythrine (50 microM) or calphostin C (1 microM). The above results allow us to conclude that strong intracellular Ca(2+) buffering prevents the physiological actions of ANG II on cardiac I(Ca), which are also dependent on activation of PKC.

Reverse mode of the Na+-Ca2+ exchange after myocardial stretch: underlying mechanism of the slow force response.

This study was designed to gain additional insight into the mechanism of the slow force response (SFR) to stretch of cardiac muscle. SFR and changes in intracellular Na(+) concentration ([Na(+)](i)) were assessed in cat papillary muscles stretched from 92% to approximately 98% of L(max). The SFR was 120+/-0.6% (n=5) of the rapid initial phase and coincided with an increase in [Na(+)](i). The SFR was markedly depressed by Na(+)-H(+) exchanger inhibition, AT(1) receptor blockade, nonselective endothelin-receptor blockade and selective ET(A)-receptor blockade, extracellular Na(+) removal, and inhibition of the reverse mode of the Na(+)-Ca(2+) exchange by KB-R7943. KB-R7943 prevented the SFR but not the increase in [Na(+)](i). Inhibition of endothelin-converting enzyme activity by phosphoramidon suppressed both the SFR and the increase in [Na(+)](i). The SFR and the increase in [Na(+)](i) after stretch were both present in muscles with their endothelium (vascular and endocardial) made functionally inactive by Triton X-100. In these muscles, phosphoramidon also suppressed the SFR and the increase in [Na(+)](i). The data provide evidence that the last step of the autocrine-paracrine mechanism leading to the SFR to stretch is Ca(2+) entry through the reverse mode of Na(+)-Ca(2+) exchange.

Is the protection against ischemia induced by red wine linked to its antioxidant capacity?

OBJECTIVE: To establish whether the total antioxidant capacity of nonalcoholic extracts of three Argentine red wines (RWE) is correlated with their protection against ischemia-reperfusion injury. ANIMALS AND METHODS: The antioxidant properties of three RWE were determined using different free radical-generating systems. To examine the effects of these RWE during a 20 min global ischemic period followed by 30 min of reperfusion, isolated rat hearts received 50 mug/mL of RWE 1 (cabernet-sauvignon), RWE 2 (malbec) or RWE 3 (a commercial mixture of cabernet-sauvignon, malbec and merlot) 10 min before and after ischemia. Left ventricular developed pressure (LVDP), maximal velocity of rise of left ventricular pressure (+dP/dt(max)) and left ventricular end-diastolic pressure (LVEDP) were used to assess contractility and diastolic function. RESULTS: All RWE inhibited lipid peroxidation induced by the Cl(4)C/NADPH system in a similar proportion (42+/-4%, 47+/-9% and 43+/-14% for RWE 1, RWE 2 and RWE 3, respectively). The scavenging activity of superoxide anion and 2,2-diphenyl-1-picryl-hydrazyl radical was about the same with the three RWE. In hearts without RWE treatment, LVDP and +dP/dt(max) were 61+/-4% and 62+/-5%, respectively, at the end of the reperfusion period. Infusion of RWE 1 and RWE 2 significantly improved postischemic recovery (LVDP and +dP/dt(max) were 102+/-4% and 101+/-4% for RWE 1 and 92+/-5% and 91+/-5% for RWE 2, respectively) and attenuated the increase of LVEDP. RWE 3 did not improve either systolic or diastolic dysfunction. CONCLUSION: These data show that although the three non-alcoholic RWE exhibit a similar total antioxidant capacity, only two of them protect the heart against myocardial stunning, suggesting that the protective effect is not primarily linked to the anti-oxidant properties of the extracts.

El intercambiador Na+/Ca2+ como responsable del atontamiento miocárdico. / [The Na+/Ca2+ exchanger as responsible for myocardial stunning]

Our objective was to assess the participation of Na+/H+ exchanger (NHE) and Na+/Ca2+ exchanger (NCX) on systolic and diastolic alterations of myocardial stunning. Isolated perfused rat hearts were submitted to 20 min of global ischemia (Is) followed by 30 min of reperfusion (R). This protocol was repeated after treatment before ischemia and/or early in R. with HOE 642 1 microM, a specific blocker of NHE-1 and KB-R7943 1 microM the novel inhibitor of the reverse mode of NCX. In control ischemic hearts the contractility assessed through +dP/dtmax recovered approximately 60

. When the NHE blockade was performed before is or early in R the postischemic recovery reached 100

. The blockade of the reverse mode of NCX only improved significantly the recovery when administered before is and early in R (95 +/- 7

). The ischemic contracture decreased when the treatment with both blockers was performed before Is. During R the increase of end diastolic pressure (EDP) observed in control ischemic hearts (at 30 min of R, EDP value was 44 +/- 4 mmHg) diminished significantly by NHE (24 +/- 6 and 12 +/- 2 mmHg when the blocker was administered before or after Is) and NCX blockade performed before and after is (12 +/- 6 mmHg). These results indicate that the activation of the reverse mode of NCX secondary to the NHE activation during ischemia and reperfusion is the mechanism responsible for the Ca2+ overload involved in the diminution of contractility that characterizes myocardial stunning.

Comparison of the protective effects of ischemic preconditioning and the Na+/H+ exchanger blockade.

The protective effects of ischemic preconditioning (IP) and Na+/H+ exchanger blockade (NHEb) by two blockers [ethylisopropylamiloride (EIPA) and HOE 642] were compared in the isovolumic perfused rat heart. The impairment in systolic and diastolic function detected in control ischemic hearts (C) exposed to 20 min of ischemia and 30 min of reperfusion was diminished in similar extent by IP and by NHEb with EIPA and HOE 642. At the end of the reperfusion period +dP/dtmax values were 57+/-9% in C hearts and 94+/-6%, 82+/-6% and 104+/-6% after IP and NHEb with EIPA and HOE 642, respectively. A depletion of ATP levels detected in C hearts after reperfusion (from 20.2+/-0.8 micromol/g dry weight before ischemia to 6.9+/-0.7 micromol/g dry weight) was partially prevented by both IP and NHEb with EIPA (9.2+/-0.7 micromol/g dry weight and 11.1+/-0.5 micromol/g dry weight, respectively). The ischemic contracture (IC), assessed by the left ventricular end diastolic pressure (LVEDP), observed in C hearts (35+/-4 mmHg) was not decreased by IP (40+/-4 mmHg) but it was prevented by NHEb (18+/-4 mmHg and 10+/-3 mmHg with EIPA and HOE 642, respectively). The ATP levels at the end of the ischemic period were similar in C and IP hearts (4.1+/-0.2 micromol/g dry wt vs. 3.3+/-0.4 micromol/g dry wt) but they were significantly higher after NHEb with HOE 642 (7.0+/-1.0 micromol/g dry wt). PKC inhibition by chelerythrine abolished the protection induced by IP after reperfusion although not the improvement induced by NHEb with EIPA. According to the present results, we can conclude that despite the fact that IP and NHEb are protecting the postischemic function in a similar magnitude, both interventions are different in terms of modifying IC that develops during the ischemic period. IC was prevented by NHEb whereas it was not by IP. Furthermore, IP protection and not that obtained by NHEb is abolished by PKC.

Stimulation of myocardial Na(+)-independent Cl(-)-HCO(3)(-) exchanger by angiotensin II is mediated by endogenous endothelin.

Experiments were performed in isolated cat papillary muscles loaded with the pH-sensitive dye 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein in the esterified form to study the effect of endothelin-1 (ET-1) on the activity of the Na(+)-independent Cl(-)-HCO(3)(-) exchanger. Exposure to ET-1 (10 nmol/L) raised pH(i) by 0.13+/-0.03 U (P<0.05) in papillary muscles superfused with nominally HCO(3)(-)-free solution, whereas no significant change was detected under CO(2)/HCO(3)(-)-buffered medium. However, if ET-1 was applied to muscles pretreated with the anion exchanger inhibitor 4-acetamido-4'-isothiocyanato-stilbene-2, 2'-disulfonic acid, pH(i) increased by 0.09+/-0.02 U (P<0.05) in the presence of CO(2)/HCO(3)(-) buffer. The rate of pH(i) recovery from trimethylamine hydrochloride-induced intracellular alkaline load was enhanced so that net HCO(3) efflux increased about three times in the presence of ET-1 (2.74+/-0.25 versus 9.66+/-1.29 mmol. L(-1). min(-1) at pH(i) 7.55, P<0.05). This effect was canceled by previous exposure to either 50 nmol/L PD 142,893 (nonselective endothelin receptor blocker) or 300 nmol/L BQ 123 (selective blocker of ET(A) receptors). BQ 123 also abolished angiotensin II-induced activation of the Na(+) independent Cl(-)-HCO(3)(-) exchanger. These results show that ET-1 increases the activity of the Na(+)-independent Cl(-)-HCO(3)(-) exchanger in cardiac tissue through the ET(A) receptors. Furthermore, our data suggest that the previously described angiotensin II-induced stimulation of the anion exchanger activity is mediated by endogenous ET-1.

[Post-ischemic myocardial protection with Cabernet-Sauvignon red wine]. / Protección de la función miocárdica post-isquemia por el vino tinto Cabernet-Sauvignon Argentino.

The objective was to assess the action of two Argentine (from Mendoza) non-alcoholic red wine extracts (Cabernet-Sauvignon (CS) and a generic control (G)) on myocardial stunning that follows the reperfusion period. In the isolated isovolumically perfused rat heart the recovery of systolic and diastolic functions after 20 min of global ischemia were assessed through left ventricular developed pressure (LVDP) and end diastolic pressure (EDP), respectively. After 30 min of reperfusion LVDP recovered to 66 +/- 7% in control ischemic hearts. The administration of non-alcoholic extract of CS not of G wine significantly increased the post-ischemic recovery (101 +/- 4%, p < 0.05) of the hearts. The ischemic contracture was not modified by either of the wines. However, during reperfusion the CS, not the G wine, decreased significantly the increase in EDP observed in control ischemic hearts. These results present experimental evidence that the non-alcoholic extracts of CS and not of G Argentine red wine induces protection of postischemic myocardial function.

Protección de la función miocárdica post-isquemia por el vino tinto Cabernet-Sauvignon Argentino. / [Post-ischemic myocardial protection with Cabernet-Sauvignon red wine]

The objective was to assess the action of two Argentine (from Mendoza) non-alcoholic red wine extracts (Cabernet-Sauvignon (CS) and a generic control (G)) on myocardial stunning that follows the reperfusion period. In the isolated isovolumically perfused rat heart the recovery of systolic and diastolic functions after 20 min of global ischemia were assessed through left ventricular developed pressure (LVDP) and end diastolic pressure (EDP), respectively. After 30 min of reperfusion LVDP recovered to 66 +/- 7

in control ischemic hearts. The administration of non-alcoholic extract of CS not of G wine significantly increased the post-ischemic recovery (101 +/- 4

, p < 0.05) of the hearts. The ischemic contracture was not modified by either of the wines. However, during reperfusion the CS, not the G wine, decreased significantly the increase in EDP observed in control ischemic hearts. These results present experimental evidence that the non-alcoholic extracts of CS and not of G Argentine red wine induces protection of postischemic myocardial function.

Chronic administration of nifedipine induces up-regulation of functional calcium channels in rat myocardium.

Previous studies from our laboratory demonstrated the up-regulation of cardiac dihydropyridine (DHP) receptors in rabbits chronically treated with nifedipine (NIFE). The goal of the present study was to further examine the functionality of this increased number of receptors by analysing different steps of excitation contraction coupling mechanism in adult rats chronically treated with NIFE (a single 10-mg oral dose/kg/day for 28 days). Ca2+ channel density was assessed by specific binding at the DHP receptors with [methyl-(3)H]PN 200-110 in rat ventricular membranes. Chronic NIFE treatment produced up-regulation of Ca2+ channels, being the maximal binding capacities 222+/-19 fmol/mg protein (n=14) and 310+/-21 fmol/mg protein (n=11) in untreated and treated animals, respectively (P<0.05). The functional consequences of this up-regulation of Ca2+ channels were determined in isolated ventricular myocytes by measuring L-type Ca2+ currents (I(Ca)) with the whole-cell configuration of patch-clamp technique and by intracellular Ca2+ (Ca2+(i)) transients estimated by the Indo-1/AM fluorescence ratio (410/482) simultaneously monitored with cell shortening. Peak I(Ca) density recorded at 0 mV was 32% greater in myocytes isolated from the treated group than in those obtained from the untreated group (-10.43+/-0.73 pA/pF (n=13) vs-7.10+/-0.59 pA/pF (n=12) P<0.05). Ca2+(i) transient amplitude and cell shortening, explored at 1 and 2 mM extracellular calcium ([Ca]0) were significantly higher in ventricular myocytes obtained fom NIFE-treated rats than in myocytes isolated from untreated animals. At 2 mM [Ca]0, the values of Ca2+(i) transient and shortening were 460+/-61 nM and 11+/-1 % of resting length (L(0)) in myocytes from treated rats (n=9) and 212+/-22 nM and 5.3+/-0.5% of L(0) in myocytes from control rats (n=6, P<0.05). The results demonstrate an up-regulation of functionally-active cardiac Ca2+ channels after NIFE treatment, and offer a possible explanation for a "withdrawal effect" at myocardial level after the suppression of the treatment with this drug.

Preservation of myofilament calcium responsiveness underlies protection against myocardial stunning by ischemic preconditioning.

OBJECTIVE: Whereas diminution of infarct size by ischemic preconditioning (IP) is well-accepted, protection against stunning is controversial. Since stunning is characterized by decreased myofilament Ca2+ responsiveness, we investigated whether IP would preserve myofilament responsiveness in a model of stunning. METHODS: Rat hearts were retrogradely perfused with Krebs-Henseleit (K-H) solution for 20 min and then subjected to 20 min of no-flow global ischemia, followed by 20 min of reperfusion in the absence (stunning) or in the presence (IP) of a previous 5-min period of ischemia followed by 15 min of reperfusion. A group of hearts perfused under non-ischemic conditions served as control. Thin ventricular trabeculae were dissected from each of the experimental groups and loaded with fura-2 to measure intracellular calcium concentration ([Ca2+]i) and developed force. RESULTS: After 20 min of reperfusion, left ventricular developed pressure decreased in stunned hearts to 61 +/- 5% of control (P < 0.01), whereas recovery was complete in the IP hearts (97 +/- 4%). Steady-state [Ca2+]i-force relationships revealed a decreased maximal Ca(2+)-activated force in stunned hearts relative to control, but no change in the IP group. The Ca2+ required for 50% activation increased in stunning but not in IP. CONCLUSIONS: These results show that the decrease in myofilament responsiveness that characterizes stunning is prevented by ischemic preconditioning.