Reduced cytosolic Ca(2+) loading and improved cardiac function after cardioplegic cold storage of guinea pig isolated hearts

Physiological Impact of Hypothermia: The Good, the Bad and the Ugly

Hypothermia is defined as a core body temperature of < 35°C, and as body temperature is reduced the impact on physiological processes can be beneficial or detrimental. The beneficial effect of hypothermia enables circulation of cooled experimental animals to be interrupted for 1-2 h without creating harmful effects, while tolerance of circulation arrest in normothermia is between 4 and 5 min. This striking difference has attracted so many investigators, experimental as well as clinical, to this field, and this discovery was fundamental for introducing therapeutic hypothermia in modern clinical medicine in the 1950's. Together with the introduction of cardiopulmonary bypass, therapeutic hypothermia has been the cornerstone in the development of modern cardiac surgery. Therapeutic hypothermia also has an undisputed role as a protective agent in organ transplantation and as a therapeutic adjuvant for cerebral protection in neonatal encephalopathy. However, the introduction of therapeutic hypothermia for organ protection during neurosurgical procedures or as a scavenger after brain and spinal trauma has been less successful. In general, the best neuroprotection seems to be obtained by avoiding hyperthermia in injured patients. Accidental hypothermia occurs when endogenous temperature control mechanisms are incapable of maintaining core body temperature within physiologic limits and core temperature becomes dependent on ambient temperature. During hypothermia spontaneous circulation is considerably reduced and with deep and/or prolonged cooling, circulatory failure may occur, which may limit safe survival of the cooled patient. Challenges that limit safe rewarming of accidental hypothermia patients include cardiac arrhythmias, uncontrolled bleeding, and "rewarming shock".

Maintaining intravenous volume mitigates hypothermia-induced myocardial dysfunction and accumulation of intracellular Ca2

NEW FINDINGS

What is the central question of this study? Detailed guidelines for volume replacement to counteract hypothermia-induced intravascular fluid loss are lacking. Evidence suggests colloids might have beneficial effects compared to crystalloids. Are central haemodynamic function and level of hypothermia-induced calcium overload, as a marker of cardiac injury, restored by fluid substitution during rewarming, and are colloids favourable to crystalloids? What is the main finding and its importance? Infusion with crystalloid or dextran during rewarming abolished post-hypothermic cardiac dysfunction, and partially mitigated myocardial calcium overload. The effects of volume replacement to support haemodynamic function are comparable to those using potent cardio-active drugs. These findings underline the importance of applying intravascular volume replacement to maintain euvolaemia during rewarming.

ABSTRACT

Previous research exploring pathophysiological mechanisms underlying circulatory collapse after rewarming victims of severe accidental hypothermia has documented post-hypothermic cardiac dysfunction and hypothermia-induced elevation of intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) in myocardial cells. The aim of the present study was to examine if maintaining euvolaemia during rewarming mitigates cardiac dysfunction and/or normalizes elevated myocardial [Ca²⁺ ]_i . A total of 21 male Wistar rats (300 g) were surface cooled to 15°C, then maintained at 15°C for 4 h, and subsequently rewarmed to 37°C. The rats were randomly assigned to one of three groups: (1) non-intervention control (n = 7), (2) dextran treated (i.v. 12 ml/kg dextran 70; n = 7), or (3) crystalloid treated (24 ml/kg 0.9% i.v. saline; n = 7). Infusions occurred during the first 30 min of rewarming. Arterial blood pressure, stroke volume (SV), cardiac output (CO), contractility (dP/dt_max ) and blood gas changes were measured. Post-hypothermic changes in [Ca²⁺ ]_i were measured using the method of radiolabelled Ca²⁺ (⁴⁵ Ca²⁺ ). Untreated controls displayed post-hypothermic cardiac dysfunction with significantly reduced CO, SV and dP/dt_max . In contrast, rats receiving crystalloid or dextran treatment showed a return to pre-hypothermic control levels of CO and SV after rewarming, with the dextran group displaying significantly better amelioration of post-hypothermic cardiac dysfunction than the crystalloid group. Compared to the post-hypothermic increase in myocardial [Ca²⁺ ]_i in non-treated controls, [Ca²⁺ ]_i values with crystalloid and dextran did not increase to the same extent after rewarming. Volume replacement with crystalloid or dextran during rewarming abolishes post-hypothermic cardiac dysfunction, and partially mitigates the hypothermia-induced elevation of [Ca²⁺ ]_i .

Effects of luteolin on regulatory proteins and enzymes for myocyte calcium circulation in hypothermic preserved rat heart

Heart transplantation has been applied in the clinic as an optimal solution for patients with end stage cardiac failure for a number of years. However, hypothermic preservation of the heart remains limited to 4–6 h and calcium accumulation over time is an important factor resulting in cell death. To provide longer and safer storage for donor hearts, it was demonstrated in our previous study that luteolin, a traditional Chinese medicine used to treat cardiovascular diseases, inhibits cell death and L-type calcium currents during hypothermic preservation. In the current study, the protective role of luteolin in modulating cardiomyocyte calcium cycling was further investigated. Intracellular calcium overload has already been implicated in hypothermia-induced dysfunction of cardiomyocytes. University of Wisconsin (UW) solution supplemented with 7.5, 15 or 30 µmol/l luteolin was used to preserve fresh isolated cardiomyocytes at 4°C. The results demonstrated that all three doses of luteolin supplementation attenuated calcium overload over a 6 h preservation period. Luteolin also suppressed the accumulation of important regulatory proteins and enzymes for cardiomyocyte calcium circulation, mitochondria Ca²⁺ uniporter and calmodulin, which are normally induced by cold storage in UW solution. Protein Kinase A activity was also suppressed in cardiomyocytes preserved in luteolin supplemented UW solution, while Ca²⁺-Mg²⁺-ATPase activity was increased. The results demonstrated that luteolin confers a cardioprotective effect through inhibiting the changes of calcium regulators during cold storage and therefore ameliorates Ca²⁺ overload in rat cardiomyocytes.

Stretch-induced increase in cardiac contractility is independent of myocyte Ca2+ while block of stretch channels by streptomycin improves contractility after ischemic stunning

Stretching the cardiac left ventricle (LV) enhances contractility but its effect on myoplasmic [Ca²⁺] is controversial. We measured LV pressure (LVP) and [Ca²⁺] as a function of intra-LV stretch in guinea pig intact hearts before and after 15 min global stunning ± perfusion with streptomycin (STM), a stretch-activated channel blocker. LV wall [Ca²⁺] was measured by indo-1 fluorescence and LVP by a saline-filled latex balloon inflated in 50 μL steps to stretch the LV. We implemented a mathematical model to interpret cross-bridge dynamics and myofilament Ca²⁺ responsiveness from the instantaneous relationship between [Ca²⁺] and LVP ± stretching. We found that: (1) stretch enhanced LVP but not [Ca²⁺] before and after stunning in either control (CON) and STM groups, (2) after stunning [Ca²⁺] increased in both groups although higher in STM versus CON (56% vs. 39%), (3) STM-enhanced LVP after stunning compared to CON (98% vs. 76% of prestunning values), and (4) stretch-induced effects on LVP were independent of [Ca²⁺] before or after stunning in both groups. Mathematical modeling suggested: (1) cooperativity in cross-bridge kinetics and myofilament Ca²⁺ handling is reduced after stunning in the unstretched heart, (2) stunning results in depressed myofilament Ca²⁺ sensitivity in the presence of attached cross-bridges regardless of stretch, and (3) the initial mechanism responsible for increased contractility during stretch may be enhanced formation of cross-bridges. Thus stretch-induced enhancement of contractility is not due to increased [Ca²⁺], whereas enhanced contractility after stunning in STM versus CON hearts results from improved Ca²⁺ handling and/or enhanced actinomyosin cross-bridge cycling.

Remote ischaemic preconditioning: is it a flag on the field?

Ischaemic preconditioning is one of several different techniques that have been proposed to render the heart more resistant to ischaemia/reperfusion injuries. A significant reduction of troponin release is ‘proof of concept’, however, whether ischaemic preconditioning leads to improved clinical outcomes is still to be proven. Moreover, the exact mechanism of action still remains unknown since very few studies have investigated the signal transmission in humans.

Effects of pyruvate on the energetics of rat ventricles stunned by ischemia–reperfusion

Pyruvate (Pyr) was proposed as an additive to cold high-K+–low-Ca2+ cardioplegia (CPG) to protect the heart during surgery. We explored whether Pyr and CPG would work synergistically to protect rat hearts from stunning during ischemia–reperfusion (I/R). We measured the heat release and contractility of perfused ventricles during I/R, and the cytosolic and mitochondrial [Ca2+] in cardiomyocytes by confocal microscopy. We found that under cold-CPG (30 °C), 10 mmol·L−1 Pyr reduced the post-ischemic contractile recovery (PICR) as well as muscle economy, when added either before ischemia or during I/R, which was reversed by blockade of UCam. In noncardioplegic hearts, Pyr was cardioprotective when it was present during I/R, more so at 37 °C than at 30 °C, with improved economy. In cardiomyocytes, the addition of Pyr to CPG slightly increased the mitochondrial [Ca2+] but decreased cytosolic [Ca2+]. The results suggest that Pyr only protects hearts from stunning when present before ischemia and during reperfusion, and that it dampens the cardioprotective properties of CPG. The mechanisms underlying such different behavior depend on the dynamic balance between Pyr stimulation of the energetic state and mitochondrial Ca2+ uptake. Our results support the use of Pyr in stunned hearts, but not in cold high-K+ cardioplegia.

Hyperkalemic cardioplegia for adult and pediatric surgery: end of an era?

Despite surgical proficiency and innovation driving low mortality rates in cardiac surgery, the disease severity, comorbidity rate, and operative procedural difficulty have increased. Today's cardiac surgery patient is older, has a "sicker" heart and often presents with multiple comorbidities; a scenario that was relatively rare 20 years ago. The global challenge has been to find new ways to make surgery safer for the patient and more predictable for the surgeon. A confounding factor that may influence clinical outcome is high K(+) cardioplegia. For over 40 years, potassium depolarization has been linked to transmembrane ionic imbalances, arrhythmias and conduction disturbances, vasoconstriction, coronary spasm, contractile stunning, and low output syndrome. Other than inducing rapid electrochemical arrest, high K(+) cardioplegia offers little or no inherent protection to adult or pediatric patients. This review provides a brief history of high K(+) cardioplegia, five areas of increasing concern with prolonged membrane K(+) depolarization, and the basic science and clinical data underpinning a new normokalemic, "polarizing" cardioplegia comprising adenosine and lidocaine (AL) with magnesium (Mg(2+)) (ALM™). We argue that improved cardioprotection, better outcomes, faster recoveries and lower healthcare costs are achievable and, despite the early predictions from the stent industry and cardiology, the "cath lab" may not be the place where the new wave of high-risk morbid patients are best served.

Effects of Kaempferia parviflora Wall. Ex. Baker and sildenafil citrate on cGMP level, cardiac function, and intracellular Ca2+ regulation in rat hearts

Although Kaempferia parviflora extract (KPE) and its flavonoids have positive effects on the nitric oxide (NO) signaling pathway, its mechanisms on the heart are still unclear. Because our previous studies demonstrated that KPE decreased defibrillation efficacy in swine similar to that of sildenafil citrate, the phosphodiesterase-5 inhibitor, it is possible that KPE may affect the cardiac NO signaling pathway. In the present study, the effects of KPE and sildenafil citrate on cyclic guanosine monophosphate (cGMP) level, modulation of cardiac function, and Ca transients in ventricular myocytes were investigated. In a rat model, cardiac cGMP level, cardiac function, and Ca transients were measured before and after treatment with KPE and sildenafil citrate. KPE significantly increased the cGMP level and decreased cardiac function and Ca transient. These effects were similar to those found in the sildenafil citrate-treated group. Furthermore, the nonspecific NOS inhibitor could abolish the effects of KPE and sildenafil citrate on Ca transient. KPE has positive effect on NO signaling in the heart, resulting in an increased cGMP level, similar to that of sildenafil citrate. This effect was found to influence the physiology of normal heart via the attenuation of cardiac function and the reduction of Ca transient in ventricular myocytes.

Successful transplantation of rat hearts subjected to extended cold preservation with a novel preservation solution

Since prolonged cold preservation of the heart deteriorates the outcome of heart transplantation, a more protective preservation solution is required. We therefore developed a new solution, named Dsol, and examined whether Dsol, in comparison to UW, could better inhibit myocardial injury resulting from prolonged cold preservation. Syngeneic heterotopic heart transplantation in Lewis rats was performed after cold preservation with UW or Dsol for 24 or 36 h. In addition to graft survival, myocardial injury, ATP content, and Ca(2+) -dependent proteases activity were assessed in the 24-h preservation group. The cytosolic Ca(2+) concentration of H9c2 cardiomyocytes after 24-h cold preservation was assessed. Dsol significantly improved 7-day graft survival after 36-h preservation. After 24-h preservation, Dsol was associated with significantly faster recovery of ATP content and less activation of calpain and caspase-3 after reperfusion. Dsol diminished graft injury significantly, as revealed by the lower levels of infarction, apoptosis, serum LDH and AST release, and graft fibrosis at 7-day. Dsol significantly inhibited Ca(2+) overload during cold preservation. Dsol inhibited myocardial injury and improved graft survival by suppressing Ca(2+) overload during the preservation and the activation of Ca(2+) -dependent proteases. Dsol is therefore considered a better alternative to UW to ameliorate the outcome of heart transplantation.

Reduced mitochondrial Ca2+ loading and improved functional recovery after ischemia-reperfusion injury in old vs. young guinea pig hearts

Oxidative damage and impaired cytosolic Ca(2+) concentration ([Ca(2+)](cyto)) handling are associated with mitochondrial [Ca(2+)] ([Ca(2+)](mito)) overload and depressed functional recovery after cardiac ischemia-reperfusion (I/R) injury. We hypothesized that hearts from old guinea pigs would demonstrate impaired [Ca(2+)](mito) handling, poor functional recovery, and a more oxidized state after I/R injury compared with hearts from young guinea pigs. Hearts from young (∼4 wk) and old (>52 wk) guinea pigs were isolated and perfused with Krebs-Ringer solution (2.1 mM Ca(2+) concentration at 37°C). Left ventricular pressure (LVP, mmHg) was measured with a balloon, and NADH, [Ca(2+)](mito) (nM), and [Ca(2+)](cyto) (nM) were measured by fluorescence with a fiber optic probe placed against the left ventricular free wall. After baseline (BL) measurements, hearts were subjected to 30 min global ischemia and 120 min reperfusion (REP). In old vs. young hearts we found: 1) percent infarct size was lower (27 ± 9 vs. 57 ± 2); 2) developed LVP (systolic-diastolic) was higher at 10 min (57 ± 11 vs. 29 ± 2) and 60 min (55 ± 10 vs. 32 ± 2) REP; 3) diastolic LVP was lower at 10 and 60 min REP (6 ± 3 vs. 29 ± 4 and 3 ± 3 vs. 21 ± 4 mmHg); 4) mean [Ca(2+)](cyto) was higher during ischemia (837 ± 39 vs. 541 ± 39), but [Ca(2+)](mito) was lower (545 ± 62 vs. 975 ± 38); 5) [Ca(2+)](mito) was lower at 10 and 60 min REP (129 ± 2 vs. 293 ± 23 and 122 ± 2 vs. 234 ± 15); 6) reduced inotropic responses to dopamine and digoxin; and 7) NADH was elevated during ischemia in both groups and lower than BL during REP. Contrary to our stated hypotheses, old hearts showed reduced [Ca(2+)](mito), decreased infarction, and improved basal mechanical function after I/R injury compared with young hearts; no differences were noted in redox state due to age. In this model, aging-associated protection may be linked to limited [Ca(2+)](mito) loading after I/R injury despite higher [Ca(2+)](cyto) load during ischemia in old vs. young hearts.

Mitochondrial and cytosolic calcium in rat hearts under high-K(+) cardioplegia and pyruvate: mechano-energetic performance

High-K(+)-cardioplegia (CPG) and pyruvate (Pyr) are used as cardioprotective agents. Considering that mitochondria play a critical role in cardiac dysfunction, we investigated the effect of CPG on mitochondrial Ca(2+) uptake and sarcorreticular (SR) calcium handling. Cytosolic and mitochondrial Ca(2+), as well as mitochondrial membrane potential (ΔΨm) were assessed in rat cardiomyocytes by confocal microscopy. Mechano-calorimetrical correlation was studied in perfused hearts. CPG did not modify JC-1 (ΔΨm), but transiently increased, by up to 1.8 times, the Fura-2 (intracellular Ca concentration, [Ca(2+)]i) and Rhod-2 (mitochondrial free Ca concentration [Ca(2+)]m) fluorescence of resting cells, with exponential decays. The addition of 5 µmol·L(-1) thapsigargin (Tpg) increased the Rhod-2 fluorescence in a group of cells without any effect on the Fura-2 signal. In rat hearts perfused with CPG, 1 µmol·L(-1) Tpg decreased resting heat rate (ΔH(r): -0.44 ± 0.07 mW·g(-1)), while the addition of 5 µmol·L(-1) KB-R7943 increased resting pressure (ΔrLVP by +5.26 ± 1.10 mm Hg; 1 mm Hg = 133.322 Pa). The addition of 10 mmol·L(-1) Pyr to CPG increased H(r) (+3.30 ± 0.24 mW·g(-1)) and ΔrLVP (+2.2 ± 0.4 mm Hg), which are effects potentiated by KB-R7943. The results suggest that under CPG, (i) there was an increase in [Ca(2+)]i and [Ca(2+)]m (without changing ΔΨm) that decayed by exothermic removal mechanisms; (ii) mitochondrial Ca(2+) uptake contributed to the removal of cytosolic Ca(2+), in a process that was potentiated by inhibition of sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA), and reduced by KB-R7943; (iii) under these conditions, SERCA represents the main energetic consumer; (iv) Pyr increased the energetic performance of hearts,mainly by inducing mitochondrial metabolism.

Myocardial protection with mild hypothermia

Mild hypothermia, 32-35° C, is very potent at reducing myocardial infarct size in rabbits, dogs, sheep, pigs, and rats. The benefit is directly related to reduction in normothermic ischaemic time, supporting the relevance of early and rapid cooling. The cardioprotective effect of mild hypothermia is not limited to its recognized reduction of infarct size, but also results in conservation of post-ischaemic contractile function, prevention of no-reflow or microvascular obstruction, and ultimately attenuation of left ventricular remodelling. The mechanism of the anti-infarct effect does not appear to be related to diminished energy utilization and metabolic preservation, but rather to survival signalling that involves either the extracellular signal-regulated kinases and/or the Akt/phosphoinositide 3-kinase/mammalian target of rapamycin pathways. Initial clinical trials of hypothermia in patients with ST-segment elevation myocardial infarction were disappointing, probably because cooling was too slow to shorten normothermic ischaemic time appreciably. New approaches to more rapid cooling have recently been described and may soon be available for clinical use. Alternatively, it may be possible to pharmacologically mimic the protection provided by cooling soon after the onset of ischaemia with an activator of mild hypothermia signalling, e.g. extracellular signal-regulated kinase activator, that could be given by emergency medical personnel. Finally, the protection afforded by cooling can be added to that of pre- and post-conditioning because their mechanisms differ. Thus, myocardial salvage might be greatly increased by rapidly cooling patients as soon as possible and then giving a pharmacological post-conditioning agent immediately prior to reperfusion.

Ranolazine reduces Ca2+ overload and oxidative stress and improves mitochondrial integrity to protect against ischemia reperfusion injury in isolated hearts

Ranolazine is a clinically approved drug for treating cardiac ventricular dysrhythmias and angina. Its mechanism(s) of protection is not clearly understood but evidence points to blocking the late Na+ current that arises during ischemia, blocking mitochondrial complex I activity, or modulating mitochondrial metabolism. Here we tested the effect of ranolazine treatment before ischemia at the mitochondrial level in intact isolated hearts and in mitochondria isolated from hearts at different times of reperfusion. Left ventricular (LV) pressure (LVP), coronary flow (CF), and O2 metabolism were measured in guinea pig isolated hearts perfused with Krebs-Ringer's solution; mitochondrial (m) superoxide (O2·-), Ca2+, NADH/FAD (redox state), and cytosolic (c) Ca2+ were assessed on-line in the LV free wall by fluorescence spectrophotometry. Ranolazine (5 μM), infused for 1 min just before 30 min of global ischemia, itself did not change O2·-, cCa2+, mCa2+ or redox state. During late ischemia and reperfusion (IR) O2·- emission and m[Ca2+] increased less in the ranolazine group vs. the control group. Ranolazine decreased c[Ca2+] only during ischemia while NADH and FAD were not different during IR in the ranolazine vs. control groups. Throughout reperfusion LVP and CF were higher, and ventricular fibrillation was less frequent. Infarct size was smaller in the ranolazine group than in the control group. Mitochondria isolated from ranolazine-treated hearts had mild resistance to permeability transition pore (mPTP) opening and less cytochrome c release than control hearts. Ranolazine may provide functional protection of the heart during IR injury by reducing cCa2+ and mCa2+ loading secondary to its effect to block the late Na+ current. Subsequently it indirectly reduces O2·- emission, preserves bioenergetics, delays mPTP opening, and restricts loss of cytochrome c, thereby reducing necrosis and apoptosis.

Temperature preconditioning is optimal at 26° C and confers additional protection to hypothermic cardioplegic ischemic arrest

We have recently shown that brief episodes of hypothermic perfusion interspersed with periods of normothermic perfusion, referred to as temperature preconditioning (TP), are cardioprotective and can be mimicked by consecutive isoproterenol/adenosine treatment. Here we investigate the optimal temperature for TP and whether TP further enhances protection provided by hypothermic ischemia with or without polarized cardioplegic arrest. Three experimental groups of Langendorff-perfused rat hearts were used. In the first group, hearts were subjected to three episodes of hypothermic perfusion at 7, 17, 26 and 32°C during the TP protocol, followed by 30 min normothermic index ischemia and 60 min reperfusion (37°C). Protein kinase A (PKA) activity and cyclic AMP (cAMP) concentrations were measured prior to index ischemia. In the second group, TP (26°C) hearts were subjected to two hours hypothermic index ischemia at 26°C and two hours normothermic reperfusion. In the third group, TP (26°C) hearts or hearts treated with isoproterenol/adenosine (pharmacological simulation of TP) were subjected to four hours hypothermic index ischemia with procaine-induced polarized cardioplegia at 26°C followed by two hours normothermic reperfusion. Hemodynamic function recovery, lactate dehydrogenase release and infarct size were used to assess cardioprotection. TP at 26°C resulted in highest cardioprotection, increased cAMP concentration and PKA activity, while TP at 7°C exacerbated ischemia/reperfusion damage, and had no effect on cAMP concentration or PKA activity. TP at 26°C also protected hearts during hypothermic ischemia with or without polarized cardioplegia. Isoproterenol/adenosine treatment conferred additional protection similar to TP. In conclusion, the study shows that TP-induced cardioprotection is temperature dependent and is optimal at 26°C; TP confers additional protection to hypothermia and polarized cardioplegia; and that the pharmacological treatment based on the mechanism of TP (consecutive isoproterenol/adenosine treatment) is a potential cardioprotective strategy that can be used during heart surgery and transplantation.

The small chill: mild hypothermia for cardioprotection?

Reducing the heart's temperature by 2-5°C is a potent cardioprotective treatment in animal models of coronary artery occlusion. The anti-infarct benefit depends upon the target temperature and the time at which cooling is instituted. Protection primarily results from cooling during the ischaemic period, whereas cooling during reperfusion or beyond offers little protection. In animal studies, protection is proportional to both the depth and duration of cooling. An optimal cooling protocol must appreciably shorten the normothermic ischaemic time to effectively salvage myocardium. Patients presenting with acute myocardial infarction could be candidates for mild hypothermia since the current door-to-balloon time is typically 90 min. But they would have to be cooled quickly shortly after their arrival. Several strategies have been proposed for ultra-fast cooling, but most like liquid ventilation and pericardial perfusion are too invasive. More feasible strategies might include cutaneous cooling, peritoneal lavage with cold solutions, and endovascular cooling with intravenous thermodes. This last option has been investigated clinically, but the results have been disappointing possibly because the devices lacked capacity to cool the patient quickly or cooling was not implemented soon enough. The mechanism of hypothermia's protection has been assumed to be energy conservation. However, whereas deep hypothermia clearly preserves ATP, mild hypothermia has only a modest effect on ATP depletion during ischaemia. Some evidence suggests that intracellular signalling pathways might be responsible for the protection. It is unknown how cooling could trigger these pathways, but, if true, then it might be possible to duplicate cooling's protection pharmacologically.

CARDIAC role of the mitochondrial Ca2+ transporters in the high-[K+](o) cardioprotection of rat hearts under ischemia and reperfusion: a mechano-energetic study

The role of mitochondrial transporters in the cardioprotection of rat hearts exposed to high [K+]-low [Ca2+]-cardioplegia (CPG) and ischemia and reperfusion (I/R) was studied through the mechano-energetic consequences of target drugs. The total heat rate (Ht) and the left intraventricular pressure (LVP) were simultaneously measured in isolated perfused hearts (30 degrees C and 1 Hz) inside a flow-calorimeter during 45 minutes of no-flow I and 45 minutes of R. After stabilization (C) they were pretreated with CPG and 100 microM 5-hydroxidecanoate (5HD, selective mKATP blocker) without and with 10 or 30 microM clonazepam (Clzp, mNCX inhibitor), 30 microM diazoxide (Dzx, selective mKATP opener), 1 microM Ru360 (selective Ca-uniporter blocker), and 0.2 microM cyclosporine-A, (mPTP inhibitor, before I and during R). Before I, 5-hydroxydecanoate in CPG increased the resting heat rate (17.83 +/- 3.55 mW/g) without changing the stunning. Clzp 30 microM + CPG + 5-hydroxydecanoate reduced the postischemic P with diastolic contracture and high Ht. Dzx protected C-hearts from stunning but increased it in CPG hearts with low economy (P/Ht) as well as Ru360. Cyclosporine-A did not modify the stunning of C or CPG ischemic hearts, suggesting that the mPTP was not opened.

CONCLUSIONS

Mitochondria have a precise role for determining cardioprotection or stunning in high-K+ cardioplegic rat hearts under I/R. Known protective drugs, such as Dzx and Ru360, which reduce the mitochondrial Ca2+-uptake, increased the stunning of CPG-rat hearts and reduced muscle economy, whereas 5-hydroxydecanoate and Clzp together increased the stunning by inducing mitochondrial Ca2+ overload.

Antiapoptotic cardioprotective effect of hypothermia treatment against oxidative stress injuries

OBJECTIVES

The effect of hypothermia on cardiomyocyte injury induced by oxidative stress remains unclear. The authors investigated the effects of hypothermia on apoptosis and mitochondrial dysfunction in cardiomyocytes exposed to oxidative stress.

METHODS

Cardiomyocytes (H9c2) derived from embryonic rat heart cell culture were exposed to either normothermic (37 degrees C) or hypothermic (31 degrees C) environments before undergoing oxidative stress via treatment with hydrogen peroxide (H(2)O(2)). The degree of apoptosis was determined by annexin V and terminal deoxynucleotidyl transferase (TUNEL) staining. The amount of reactive oxygen species (ROS) was compared after H(2)O(2) exposure between normo- and hypothermic-pretreated groups. Mitochondrial dysfunction in both groups was measured by differential reductase activity and transmembrane potential (DeltaPsim).

RESULTS

Hydrogen peroxide induced significant apoptosis in both normothermic and hypothermic cardiomyocytes. Hypothermia ameliorated apoptosis as demonstrated by decreased annexin V staining (33 +/- 1% vs. 49 +/- 4%; p < 0.05) and TUNEL staining (27 +/- 17% vs. 80 +/-25%; p < 0.01). The amount of intracellular ROS increased after H(2)O(2) treatment and was higher in the hypothermic group than that in the normothermic group (237.9 +/- 31.0% vs. 146.6 +/- 20.6%; p < 0.05). In the hypothermic group, compared with the normothermic group, after H(2)O(2) treatment mitochondrial reductase activity was greater (72.0 +/- 17.9% vs. 27.0 +/- 13.3%; p < 0.01) and the mitochondria DeltaPsim was higher (101.0 +/- 22.6% vs. 69.7 +/- 12.9%; p < 0.05). Pretreatment of cardiomyocytes with the antioxidant ascorbic acid diminished the hypothermia-induced increase in intracellular ROS and prevented the beneficial effects of hypothermia on apoptosis and mitochondrial function.

CONCLUSIONS

Hypothermia at 31 degrees C can protect cardiomyocytes against oxidative stress-induced injury by decreasing apoptosis and mitochondrial dysfunction through intracellular ROS-dependent pathways.

Energetics of Ca2+ homeostasis during ischemia–reperfusion on neonatal rat hearts under high-[K+] cardioplegia

The mechanocalorimetric consequences and mechanisms involved in Ca2+ homeostasis during ischemia–reperfusion (I/R) as well as the protective role of cardioplegic pretreatment with high [K+] (25 mmol/L) and low or near-normal [Ca2+] (0.5 or 2 mmol/L) were evaluated in a model of neonatal rat heart. Beating hearts from 10–12-day-old rats were perfused with Krebs solution (2 mmol/L Ca2+) under both isotonic and isometric conditions. During pretreatment, hearts were exposed for 20 min to either Krebs (control) or cardioplegia (CPG) before 15 min ischemia and 45 min reperfusion while being continuously measured for either contractility or total heat rate (Ht) in a flow calorimeter. Contractile recovery after reperfusion in hearts exposed to ischemia only (control) was higher in the isometric hearts under optimal length (87.9% ± 8.1%) than in the isotonic hearts (57.3% ± 10.6%). This same behavior was found in hearts pretreated with CPG-0.5 mmol/L Ca2+. Ht in controls was reduced from 11.5 ± 0.8 mW/g in the initial beating condition to 1.11 ± 0.33 mW/g during ischemia and was increased to 13.02 ± 0.93 mW/g (113.8% ± 5.0% of preischemic) after reperfusion. Hearts pretreated with CPG-0.5 mmol/L Ca2+ showed the same behavior. However, when extracellular calcium ([Ca]o) was increased to 2 mmol/L under CPG, isotonic hearts, but not isometric hearts, significantly increased the contractile recovery to a maximum of 88.7% ± 10.8% of preischemic levels. Ht was recovered to 92.1% ± 4.3% of preischemic, suggesting that contractile recovery was less energetically expensive after CPG-2 mmol/L Ca2+ than it was in postischemic hearts exposed to control or CPG-0.5 mmol/L Ca2+. The role of the sarcoplasmic reticulum store was evaluated by pretreating hearts with 10 mmol/L caffeine, which reduced contractile recovery only under isometric conditions or after increasing [Ca]o in CPG under isotonic conditions, suggesting that the contribution of the sarcoplasmic reticulum was dependent on the fibre length or the [Ca]o. The inhibition of the reverse mode of the sarcolemmal Na/Ca exchanger (NCX) and the mitochondrial Ca uniporter (CaU) by KB-R7943 (KBR) at 5 µmol/L in CPG-0.5 mmol/L Ca2+ improved contractile recovery of isotonic hearts, whereas it decreased Ht at the start of reperfusion, suggesting that mitochondria could uptake Ca2+ vía the mitochondrial CaU. Neither the positive inotropism nor Ht were changed by inhibiting the mitochondrial NCX with 10 µmol/L clonazepam in CPG-0.5 mmol/L Ca2+ + 5 µmol/L KBR, which suggests that the mitochondrial NCX does not have a role. Finally, the role of the forward mode of the sarcolemmal NCX was evidenced by the fall in contractile recovery with increased Ht when KBR was increased to 20 µmol/L and added to CPG-2 mmol/L Ca2+ + 10 mmol/L caffeine before I/R. Thus the sarcolemmal NCX was essential for removing the diastolic Ca2+ during the periods of CPG and I/R. In summary, Ca2+ homeostasis during I/R of neonatal rat hearts is different from that of adult rats. High-[K+] CPG protected neonatal hearts only under isotonic conditions, at a near-normal [Ca]o, or by exposure to KBR. Mitochondria were able to uptake Ca2+ via the mitochondrial CaU and reduce the Ca2+ available for contractile recovery. Nevertheless, after increasing [Ca]o in CPG, the sarcoplasmic reticulum had a main role in restoring contractility during reperfusion, as it does in adults. Thus, the degree of maturation of the heart must be taken into account to evaluate the effects of CPG and drugs on I/R.

Mitochondrial role in ischemia-reperfusion of rat hearts exposed to high-K+ cardioplegia and clonazepam: energetic and contractile consequences

The role of the mitochondrial Na/Ca-exchanger (mNCX) in hearts exposed to ischemia-reperfusion (I/R) and pretreated with cardioplegia (CPG) was studied from a mechano-calorimetric approach. No-flow ischemia (ISCH) and reperfusion (REP) were developed in isolated rat hearts pretreated with 10 micromol/L clonazepam (CLZP), an inhibitor of the mNCX, and (or) a high K+ - low Ca2+ solution (CPG). Left ventricular end diastolic pressure (LVEDP), pressure development during beats (P), and the steady heat release (Ht) were continuously measured and muscle contents of ATP and PCr were analyzed at the end of REP. During REP, Ht increased more than P, reducing muscle economy (P/Ht) and the ATP content. CPG induced an increase in P recovery during REP (to 90% +/- 10% of preISCH) with respect to nonpretreated hearts (control, C, to 64% +/- 10%, p < 0.05). In contrast, CLZP reduced P recovery of CPG-hearts (50% +/- 6.4%, p < 0.05) and increased LVEDP in C hearts. To evaluate effects on sarcoplasmic reticulum (SR) function, ischemic hearts were reperfused with 10 mmol/L caffeine -36 mmol/L Na (C - caff - low Na). It increased LVEDP, which afterwards slowly relaxed, whereas Ht increased (by about 6.5 mW/g). CLZP sped up the relaxation with higher DeltaHt, C - caff - low Na produced higher contracture and lower Ht in perfused than in ischemic hearts. Values of DeltaHt were compared with reported fluxes of Ca2+-transporters, suggesting that mitochondria may be in part responsible for the DeltaHt during C - caff - low Na REP. Results suggest that ISCH-REP reduced the SR store for the recovery of contractility, but induced Ca2+ movement from the mitochondria to the SR stores. Also, mitochondria and SR are able to remove cytosolic Ca2+ during overloads (as under caffeine), through the mNCX and the uniporter. CPG increases Ca2+ cycling from mitochondria to the SR, which contributes to the higher recovery of P. In contrast, CLZP produces a deleterious effect on ISCH-REP associated with higher heat release and reduced resynthesis of high energy phosphates, which suggests the induction of mitochondrial Ca cycling and uncoupling.

Temperature preconditioning of isolated rat hearts--a potent cardioprotective mechanism involving a reduction in oxidative stress and inhibition of the mitochondrial permeability transition pore

We investigate whether temperature preconditioning (TP), induced by short-term hypothermic perfusion and rewarming, may protect hearts against ischaemic/reperfusion injury like ischaemic preconditioning (IP). Isolated rat hearts were perfused for 40 min, followed by 25 min global ischaemia and 60 min reperfusion (37 degrees C). During pre-ischaemia, IP hearts underwent three cycles of 2 min global ischaemia and 3 min reperfusion at 37 degrees C, whereas TP hearts received three cycles of 2 min hypothermic perfusion (26 degrees C) interspersed by 3 min normothermic perfusion. Other hearts received a single 6 min hypothermic perfusion (SHP) before ischaemia. Both IP and TP protocols increased levels of high energy phosphates in the pre-ischaemic heart. During reperfusion, TP improved haemodynamic recovery, decreased arrhythmias and reduced necrotic damage (lactate dehydrogenase release) more than IP or SHP. Measurements of tissue NAD+ levels and calcium-induced swelling of mitochondria isolated at 3 min reperfusion were consistent with greater inhibition of the mitochondrial permeability transition at reperfusion by TP than IP; this correlated with decreased protein carbonylation, a surrogate marker for oxidative stress. TP increased protein kinase Cepsilon (PKCepsilon) translocation to the particulate fraction and pretreatment with chelerythrine (PKC inhibitor) blocked the protective effect of TP. TP also increased phosphorylation of AMP-activated protein kinase (AMPK) after 5 min index ischaemia, but not before ischaemia. Compound C (AMPK inhibitor) partially blocked cardioprotection by TP, suggesting that both PKC and AMPK may mediate the effects of TP. The presence of N-(2-mercaptopropionyl) glycine during TP also abolished cardioprotection, indicating an involvement of free radicals in the signalling mechanism.

ROS scavenging before 27 degrees C ischemia protects hearts and reduces mitochondrial ROS, Ca2+ overload, and changes in redox state

We have shown that cold perfusion of hearts generates reactive oxygen and nitrogen species (ROS/RNS). In this study, we determined 1) whether ROS scavenging only during cold perfusion before global ischemia improves mitochondrial and myocardial function, and 2) which ROS leads to compromised cardiac function during ischemia and reperfusion (I/R) injury. Using fluorescence spectrophotometry, we monitored redox balance (NADH and FAD), O(2)(*-) levels and mitochondrial Ca(2+) (m[Ca(2+)]) at the left ventricular wall in 120 guinea pig isolated hearts divided into control (Con), MnTBAP (a superoxide dismutase 2 mimetic), MnTBAP (M) + catalase (C) + glutathione (G) (MCG), C+G (CG), and N(G)-nitro-L-arginine methyl ester (L-NAME; a nitric oxide synthase inhibitor) groups. After an initial period of warm perfusion, hearts were treated with drugs before and after at 27 degrees C. Drugs were washed out before 2 h at 27 degrees C ischemia and 2 h at 37 degrees C reperfusion. We found that on reperfusion the MnTBAP group had the worst functional recovery and largest infarction with the highest m[Ca(2+)], most oxidized redox state and increased ROS levels. The MCG group had the best recovery, the smallest infarction, the lowest ROS level, the lowest m[Ca(2+)], and the most reduced redox state. CG and L-NAME groups gave results intermediate to those of the MnTBAP and MCG groups. Our results indicate that the scavenging of cold-induced O(2)(*-) species to less toxic downstream products additionally protects during and after cold I/R by preserving mitochondrial function. Because MnTBAP treatment showed the worst functional return along with poor preservation of mitochondrial bioenergetics, accumulation of H(2)O(2) and/or hydroxyl radicals during cold perfusion may be involved in compromised function during subsequent cold I/R injury.

Ischemia reperfusion dysfunction changes model-estimated kinetics of myofilament interaction due to inotropic drugs in isolated hearts

BACKGROUND

The phase-space relationship between simultaneously measured myoplasmic [Ca2+] and isovolumetric left ventricular pressure (LVP) in guinea pig intact hearts is altered by ischemic and inotropic interventions. Our objective was to mathematically model this phase-space relationship between [Ca2+] and LVP with a focus on the changes in cross-bridge kinetics and myofilament Ca2+ sensitivity responsible for alterations in Ca2+-contraction coupling due to inotropic drugs in the presence and absence of ischemia reperfusion (IR) injury.

METHODS

We used a four state computational model to predict LVP using experimentally measured, averaged myoplasmic [Ca2+] transients from unpaced, isolated guinea pig hearts as the model input. Values of model parameters were estimated by minimizing the error between experimentally measured LVP and model-predicted LVP.

RESULTS

We found that IR injury resulted in reduced myofilament Ca2+ sensitivity, and decreased cross-bridge association and dissociation rates. Dopamine (8 microM) reduced myofilament Ca2+ sensitivity before, but enhanced it after ischemia while improving cross-bridge kinetics before and after IR injury. Dobutamine (4 microM) reduced myofilament Ca2+ sensitivity while improving cross-bridge kinetics before and after ischemia. Digoxin (1 microM) increased myofilament Ca2+ sensitivity and cross-bridge kinetics after but not before ischemia. Levosimendan (1 microM) enhanced myofilament Ca2+ affinity and cross-bridge kinetics only after ischemia.

CONCLUSION

Estimated model parameters reveal mechanistic changes in Ca2+-contraction coupling due to IR injury, specifically the inefficient utilization of Ca2+ for contractile function with diastolic contracture (increase in resting diastolic LVP). The model parameters also reveal drug-induced improvements in Ca2+-contraction coupling before and after IR injury.

Improved mitochondrial bioenergetics by anesthetic preconditioning during and after 2 hours of 27 degrees C ischemia in isolated hearts

We examined if sevoflurane given before cold ischemia of intact hearts (anesthetic preconditioning, APC) affords additional protection by further improving mitochondrial energy balance and if this is abolished by a mitochondrial KATP blocker. NADH and FAD fluorescence was measured within the left ventricular wall of 5 groups of isolated guinea pig hearts: (1) hypothermia alone; (2) hypothermia+ischemia; (3) APC (4.1% sevoflurane)+cold ischemia; (4) 5-HD+cold ischemia, and (5) APC+5-HD+cold ischemia. Hearts were exposed to sevoflurane for 15 minutes followed by 15 minutes of washout at 37 degrees C before cooling, 2 hours of 27 degrees C ischemia, and 2 hours of 37 degrees C reperfusion. The KATP channel inhibitor 5-HD was perfused before and after sevoflurane. Ischemia caused a rapid increase in NADH and a decrease in FAD that waned over 2 hours. Warm reperfusion led to a decrease in NADH and an increase in FAD. APC attenuated the changes in NADH and FAD and further improved postischemic function and reduced infarct size. 5-HD blocked the cardioprotective effects of APC but not APC-induced alterations of NADH and FAD. Thus, APC improves redox balance and has additive cardioprotective effects with mild hypothermic ischemia. 5-HD blocks APC-induced cardioprotective effects but not improvements in mitochondrial bioenergetics. This suggests that mediation of protection by KATP channel opening during cold ischemia and reperfusion is downstream from the APC-induced improvement in redox state or that these changes in redox state are not attenuated by KATP channel antagonism.

Ischemia-reperfusion injury changes the dynamics of Ca2+-contraction coupling due to inotropic drugs in isolated hearts

Positive inotropic drugs may attenuate or exacerbate the deleterious effects of ischemia and reperfusion (IR) injury on excitation-contraction coupling in hearts. We 1) quantified the phase-space relationship between simultaneously measured myoplasmic Ca2+ concentration ([Ca2+]) and isovolumetric left ventricular pressure (LVP) using indexes of loop area, orientation, and position; and 2) quantified cooperativity by linearly modeling the phase-space relationship between [Ca2+] and rate of LVP development in intact hearts during administration of positive inotropic drugs before and after global IR injury. Unpaced, isolated guinea pig hearts were perfused at a constant pressure with Krebs-Ringer solution (37 degrees C, 1.25 mM CaCl2). [Ca2+] was measured ratiometrically by indo 1 fluorescence by using a fiber-optic probe placed at the left ventricular free wall. LVP was measured by using a saline-filled latex balloon and transducer. Drugs were infused for 2 min, 30 min before, and for 2 min, 30 min after 30-min global ischemia. IR injury worsened Ca2+-contraction coupling, as seen from decreased orientation and repositioning of the loop rightward and downward and reduced cooperativity of contraction and relaxation with or without drugs. Dobutamine (4 microM) worsened, whereas dopamine (8 microM) improved Ca2+-contraction coupling before and after IR injury. Dobutamine and dopamine improved cooperativity of contraction and relaxation after IR injury, whereas only dopamine increased cooperativity of relaxation before IR injury. Digoxin (1 microM) improved Ca2+-contraction coupling and cooperativity of contraction after but not before ischemia. Levosimendan (1 microM) did not alter Ca2+-contraction coupling or cooperativity, despite producing concomitant increases in contractility, relaxation, and Ca2+ flux before and after ischemia. Dynamic indexes based on LVP-[Ca2+] diagrams (area, shape, position) can be used to identify and measure alterations in Ca2+-contraction coupling during administration of positive inotropic drugs in isolated hearts before and after IR injury.

Warm ischemic preconditioning improves mitochondrial redox balance during and after mild hypothermic ischemia in guinea pig isolated hearts

Ischemic preconditioning (IPC) induces distinctive changes in mitochondrial bioenergetics during warm (37 degrees C) ischemia and improves function and tissue viability on reperfusion. We examined whether IPC before 2 h of hypothermic (27 degrees C) ischemia affords additive cardioprotection and improves mitochondrial redox balance assessed by mitochondrial NADH and flavin adenine dinucleotide (FAD) autofluorescence in intact hearts. A mediating role of ATP-sensitive K(+) (K(ATP)) channel opening was investigated. NADH and FAD fluorescence was measured in the left ventricular wall of guinea pig isolated hearts assigned to five groups of eight animals each: hypothermia alone, hypothermia with ischemia, IPC with cold ischemia, 5-hydroxydecanoic acid (5-HD) alone, and 5-HD with IPC and cold ischemia. IPC consisted of two 5-min periods of warm global ischemia spaced 5 min apart and 15 min of reperfusion before 2 h of ischemia at 27 degrees C and 2 h of warm reperfusion. The K(ATP) channel inhibitor 5-HD was perfused from 5 min before until 5 min after IPC. IPC before 2 h of ischemia at 27 degrees C led to better recovery of function and less tissue damage on reperfusion than did 27 degrees C ischemia alone. These improvements were preceded by attenuated increases in NADH and decreases in FAD during cold ischemia and the reverse changes during warm reperfusion. 5-HD blocked each of these changes induced by IPC. This study indicates that IPC induces additive cardioprotection with mild hypothermic ischemia by improving mitochondrial bioenergetics during and after ischemia. Because effects of IPC on subsequent changes in NADH and FAD were inhibited by 5-HD, this suggests that mitochondrial K(ATP) channel opening plays a substantial role in improving mitochondrial bioenergetics throughout mild hypothermic ischemia and reperfusion.

Hypothermia augments reactive oxygen species detected in the guinea pig isolated perfused heart

Hypothermic perfusion of the heart decreases oxidative phosphorylation and increases NADH. Because O(2) and substrates remain available and respiration (electron transport system, ETS) may become impaired, we examined whether reactive oxygen species (ROS) exist in excess during hypothermic perfusion. A fiberoptic probe was placed on the left ventricular free wall of isolated guinea pig hearts to record intracellular ROS, principally superoxide (O(2)(-).), and an extracellular reactive nitrogen reactant, principally peroxynitrite (ONOO(-)), a product of nitric oxide (NO.) + O(2)(-). Hearts were loaded with dihydroethidium (DHE), which is oxidized by O(2)(-). to ethidium, or were perfused with l-tyrosine, which is oxidized by ONOO(-) to dityrosine (diTyr). Shifts in fluorescence were measured online; diTyr fluorescence was also measured in the coronary effluent. To validate our methods and to examine the source and identity of ROS during cold perfusion, we examined the effects of a superoxide dismutase mimetic Mn(III) tetrakis(4-benzoic acid)porphyrin chloride (MnTBAP), the nitric oxide synthase inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME), and several agents that impair electron flux through the ETS: menadione, sodium azide (NaN(3)), and 2,3-butanedione monoxime (BDM). Drugs were given before or during cold perfusion. ROS measured by DHE was inversely proportional to the temperature between 37 degrees C and 3 degrees C. We found that perfusion at 17 degrees C increased DHE threefold versus perfusion at 37 degrees C; this was reversed by MnTBAP, but not by l-NAME or BDM, and was markedly augmented by menadione and NaN(3). Perfusion at 17 degrees C also increased myocardial and effluent diTyr (ONOO(-)) by twofold. l-NAME, MnTBAP, or BDM perfused at 37 degrees C before cooling or during 17 degrees C perfusion abrogated, whereas menadione and NaN(3) again enhanced the cold-induced increase in ROS. Our results suggest that hypothermia moderately enhances O(2)(-). generation by mitochondria, whereas O(2)(-). dismutation is markedly slowed. Also, the increase in O(2)(-). during hypothermia reacts with available NO. to produce ONOO(-), and drug-induced O(2)(-). dismutation eliminates the hypothermia-induced increase in O(2)(-).

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.

Na+/H+ exchange inhibition with cardioplegia reduces cytosolic [Ca2+] and myocardial damage after cold ischemia

Cold cardioplegia protects against reperfusion damage. Blocking Na+/H+ exchange may be as protective as cardioplegia by improving the left ventricular pressure (LVP)-[Ca2+] relationship after cold ischemia. In guinea pig isolated hearts subjected to cold ischemia (4 h, 17 degrees C) and reperfusion, the cardioprotective effects of a Krebs-Ringer (KR) solution, a cardioplegia solution, a KR solution containing the Na+/H+ exchange inhibitor eniporide (1 microM), and a cardioplegia solution containing eniporide were compared. Treatments were given before and initially after cold ischemia. Systolic and diastolic [Ca2+] were calculated from indo-1 fluorescence transients recorded at the LV free wall. During ischemia, diastolic [Ca2+] increased in each group but more so in the KR group. Peak systolic and diastolic [Ca2+] on initial reperfusion were highest after KR and smallest after cardioplegia + eniporide. After reperfusion, systolic-diastolic LVP (% of baseline) and infarct size (%), respectively, were KR, 47 +/- 3%, 37 +/- 4%; cardioplegia, 71 +/- 5%*, 20 +/- 2.2%*; KR + eniporide, 73 +/- 5%*, 11 +/- 3%* dagger; and cardioplegia + eniporide 77 +/- 3%*, 10 +/- 1.4%* dagger (*P </= 0.05 vs KR; dagger P </= 0.05 vs cardioplegia). Ca2+ overload was reduced in each treated group, and most in the cardioplegia + eniporide group, and was associated with the improved function. Inhibition of Na+/H+ exchange was as effective as cardioplegia in restoring function and better than cardioplegia in reducing infarct size after hypothermic ischemia. The combination of cardioplegia and Na+/H+ exchange inhibition did not produce additive protective effects but caused a larger decrease in Ca2+ loading.

Cross-bridge kinetics modeled from myoplasmic [Ca2+] and LV pressure at 17 degrees C and after 37 degrees C and 17 degrees C ischemia

We modeled changes in contractile element kinetics derived from the cyclic relationship between myoplasmic [Ca(2+)], measured by indo 1 fluorescence, and left ventricular pressure (LVP). We estimated model rate constants of the Ca(2+) affinity for troponin C (TnC) on actin (A) filament (TnCA) and actin and myosin (M) cross-bridge (A x M) cycling in intact guinea pig hearts during baseline 37 degrees C perfusion and evaluated changes at 1) 20 min 17 degrees C pressure, 2) 30-min reperfusion (RP) after 30-min 37 degrees C global ischemia during 37 degrees C RP, and 3) 30-min RP after 240-min 17 degrees C global ischemia during 37 degrees C RP. At 17 degrees C perfusion versus 37 degrees C perfusion, the model predicted: A x M binding was less sensitive; A x M dissociation was slower; Ca(2+) was less likely to bind to TnCA with A x M present; and Ca(2+) and TnCA binding was less sensitive in the absence of A x M. Model results were consistent with a cold-induced fall in heart rate from 260 beats/min (37 degrees C) to 33 beats/min (17 degrees C), increased diastolic LVP, and increased phasic Ca(2+). On RP after 37 degrees C ischemia vs. 37 degrees C perfusion, the model predicted the following: A x M binding was less sensitive; A x M dissociation was slower; and Ca(2+) was less likely to bind to TnCA in the absence of A. M. Model results were consistent with reduced myofilament responsiveness to [Ca(2+)] and diastolic contracture on 37 degrees C RP. In contrast, after cold ischemia versus 37 degrees C perfusion, A x M association and dissociation rates, and Ca(2+) and TnCA association rates, returned to preischemic values, whereas the dissociation rate of Ca(2+) from A x M was ninefold faster. This cardiac muscle kinetic model predicted a better-restored relationship between Ca(2+) and cross-bridge function on RP after an eightfold longer period of 17 degrees C than 37 degrees C ischemia.

Ischemic preconditioning alters real-time measure of O2 radicals in intact hearts with ischemia and reperfusion

Reactive oxygen species (ROS) are believed to be involved in triggering cardiac ischemic preconditioning (IPC). Decreased formation of ROS on reperfusion after prolonged ischemia may in part underlie protection by IPC. In heart models, these contentions have been based either on the effect of ROS scavengers to abrogate IPC-induced preservation or on a measurement of oxidation products on reperfusion. Using spectrophotofluorometry at the left ventricular wall and the fluorescent probe dihydroethidium (DHE), we measured intracellular ROS superoxide (O(2)(-).) continuously in isolated guinea pig heart and tested the effect of IPC and the O(2)(-). scavenger manganese(III) tetrakis (4-benzoic acid) porphyrin chloride (MnTBAP) on O(2)(-). formation throughout the phases of preconditioning (PC), 30-min ischemia and 60-min reperfusion (I/R). IPC was evidenced by improved contractile function and reduced infarction; MnTBAP abrogated these effects. Brief PC pulses increased O(2)(-). during the ischemic but not the reperfusion phase. O(2)(-). increased by 35% within 1 min of ischemia, increased further to 95% after 20 min of ischemia, and decreased slowly on reperfusion. In the IPC group, O(2)(-). was not elevated over 35% during index ischemia and was not increased at all on reperfusion; these effects were abrogated by MnTBAP. Our results directly demonstrate how intracellular ROS increase in intact hearts during IPC and I/R and clarify the role of ROS in triggering and mediating IPC.

A comparative study of the most widely used solutions for cardiac graft preservation during hypothermia

BACKGROUND

Reports conflict on the benefits of preservative solutions. We investigated the efficacy of the most widely used cardioplegic solutions by comparing extracellular solutions such as Celsior solution, St. Thomas Hospital solutions 1 and 2 (STH-1, STH-2), the modified University of Wisconsin solution (UW-1), Lyon Preservation solution (LYPS) from our laboratory, and intracellular solutions such as standard University of Wisconsin solution (UW), Bretschneider solution (HTK), Stanford solution (STF), and Euro-Collins solution (EC).

METHODS

Male rats (n = 110) were randomized into 11 groups: LYPS, Celsior, STH-1, STH-2, UW-1, UW, HTK, STF, EC, and normal saline solution groups, and a control group. All hearts, except controls, were preserved by cold storage (8 hours at 4 degrees C) in the various solutions. We used an isolated non-working-heart model and biopsy specimens to assess heart preservation (n = 5/group).

RESULTS

Hearts stored in the EC and saline solutions had poor left ventricular developed pressure (LVDP) x heart rate (HR) (1,407.5 +/- 154 and 1,390 +/- 439 mm Hg/mn, respectively). In contrast, hearts stored in LYPS and Celsior had a LVDP x HR close to control hearts (31,349 +/- 1,847, 27,620 +/- 1,207, and 36,627 +/- 1,322 mm Hg/mn, respectively), whereas hearts stored in STH-1, STH-2, UW-1, UW, HTK, and STF had intermediate functional response (14,278 +/- 2,176, 12,402 +/- 1,571, 11,428 +/- 1,629, 11,603 +/- 2,521, 7,045 +/- 537, and 7,086 +/- 1,206 mm Hg/mn, respectively). Hearts preserved with STH-2, UW, HTK, STF, EC, and saline solution showed significantly increased release of creatine kinase and lactate dehydrogenase than did control hearts or hearts preserved in Celsior, LYPS, STH-1, and UW-1. The energetic charge (EC = [(0.5 adenosine diphosphate + adenosine triphosphate) / (adenosine triphosphate + adenosine diphosphate + adenosine monophosphate)]) in STH-2, UW, HTK, STF, EC, and saline groups was significantly lower (p < 0.05) than in the other groups.

CONCLUSION

Extracellular-type solutions provided better preservation than did intracellular-type solutions. However, UW and UW-1 (intracellular- and extracellular-type solutions) provided equivalent preservation of cardiac function. Preservation quality may be attributed to calcium, often added to extracellular solutions. Among extracellular solutions, Celsior and LYPS solution showed comparable efficacy on left ventricular function and seemed to offer better preservation than the other solutions tested in this study.

Cardiac preconditioning with 4-h, 17 degrees C ischemia reduces [Ca(2+)](i) load and damage in part via K(ATP) channel opening

Brief ischemia before normothermic ischemia protects hearts against reperfusion injury (ischemic preconditioning, IPC), but it is unclear whether it protects against long-term moderate hypothermic ischemia. We explored in isolated guinea pig hearts 1) the influence of two 2-min periods of normothermic ischemia before 4 h, 17 degrees C hypothermic ischemia on cardiac cytosolic [Ca(2+)], mechanical and metabolic function, and infarct size, and 2) the potential role of K(ATP) channels in eliciting cardioprotection. We found that IPC before 4 h moderate hypothermia improved myocardial perfusion, contractility, and relaxation during normothermic reperfusion. Protection was associated with markedly reduced diastolic [Ca(2+)] loading throughout both hypothermic storage and reperfusion. Global infarct size was markedly reduced from 36 +/- 2 (SE)% to 15 +/- 1% with IPC. Bracketing ischemic pulses with 200 microM 5-hydroxydecanoic acid or 10 microM glibenclamide increased infarct size to 28 +/- 3% and 26 +/- 4%, respectively. These results suggest that brief ischemia before long-term hypothermic storage adds to the cardioprotective effects of hypothermia and that this is associated with decreased cytosolic [Ca(2+)] loading and enhanced ATP-sensitive K channel opening.

Inhibition of Na(+)/H(+) isoform-1 exchange protects hearts perfused after 6-hour cardioplegic cold storage

OBJECTIVES

Cardiac ischemia-reperfusion activates Na(+)/H(+) exchange; excess Na(+) and the resulting Ca(2+) overload, through reverse Na(+)/Ca(2+) exchange, cause cellular injury and cardiac dysfunction. We postulated that inhibiting the Na(+)/H(+) isoform-1 exchanger would add to the protection of hearts after long-term cold storage in acidic cardioplegic solution.

METHODS

Guinea pig hearts were isolated and perfused at 37 degrees C with Krebs-Ringer's solution (KRS) and then switched to an acidic St. Thomas solution (STS) at 25 degrees C. Perfusion was stopped at 10 degrees C, and hearts were stored for 6 hours in STS at 3.4 degrees C. On reperfusion to 25 degrees C, hearts were perfused with KRS for 60 minutes. Hearts were divided into 4 groups: sham control (SHAM); eniporide (EPR, EMD96785) IV, 1 mg/kg given IV over 15 minutes before heart isolation; EPR intracoronary, 1 micromol/liter in STS given intracoronary after heart isolation; and EPR IV and intracoronary.

RESULTS

Values at 60 minutes reperfusion (the percentage of control [100%] before cold storage) are given, respectively, for EPR IV, EPR intracoronary, and EPR IV and intracoronary vs drug-free SHAM (SEM, *p < 0.05 vs SHAM): 72% +/- 3%*, 65% +/- 3%*, and 81% +/- 2%* vs 55% +/- 3% for left ventricular pressure; 94% +/- 3%*, 96% +/- 5%*, and 102% +/- 2%* vs 81% +/- 3% for coronary flow; 60% +/- 2%, 58% +/- 3%, and 74%* +/- 3% vs 58% +/- 4% for cardiac efficiency; 106% +/- 2%*, 108% +/- 3%*, and 107% +/- 2%* vs 116% +/- 4% for percentage of O(2) extraction. Infarct size as percentage of ventricular weight was 20% +/- 3%*, 31% +/- 3%, and 6% +/- 2%* vs 35% +/- 3% (SHAM) after 60 minutes of reperfusion.

CONCLUSIONS

Na(+)/H(+) isoform-1 exchanger inhibition, particularly if given IV before storage and intracoronary during cooling and rewarming, adds to the protection of cardioplegic solutions.

Acute ischemic syndromes. Early response

The optimal treatment of patients with AIS depends on a well-run, integrated system of care involving patients and teams of health care professionals. It begins with patient education and extends to a method for accessing an efficient and effective EMS system. Medics must be well equipped and well trained to evaluate and begin initial treatment during prompt transport to an appropriate hospital. The role of out-of-hospital 12-lead ECGs and thrombolysis is reviewed and may be appropriate for some EMS systems. The initial evaluation and treatment in the ED goes on simultaneously and is a dynamic process. Prompt treatment with oxygen, nitroglycerin, morphine, and aspirin is indicated. Initial risk stratification is based on the first ECG, cardiac biomarkers, and the clinical history and physical exam. Disposition and further evaluation is individualized according to the initial work-up and risk assessment.

Blocking Na(+)/H(+) exchange reduces [Na(+)](i) and [Ca(2+)](i) load after ischemia and improves function in intact hearts

We determined in intact hearts whether inhibition of Na(+)/H(+) exchange (NHE) decreases intracellular Na(+) and Ca(2+) during ischemia and reperfusion, improves function during reperfusion, and reduces infarct size. Guinea pig isolated hearts were perfused with Krebs-Ringer solution at 37 degrees C. Left ventricular (LV) free wall intracellular Na(+) concentration ([Na(+)](i)) and intracellular Ca(2+) concentration ([Ca(2+)](i)) were measured using fluorescence dyes. Hearts were exposed to 30 min of ischemia with or without 10 microM of benzamide (BIIB-513), a selective NHE-1 inhibitor, infused for 10 min just before ischemia or for 10 min immediately on reperfusion. At 2 min of reperfusion, BIIB-513 given before ischemia decreased peak increases in [Na(+)](i) and [Ca(2+)](i), respectively, from 2.5 and 2.3 times (controls) to 1.6 and 1.3 times pre-ischemia values. At 30 min of reperfusion, BIIB-513 increased systolic-diastolic LV pressure (LVP) from 49 +/- 2% (controls) to 80 +/- 2% of pre-ischemia values. BIIB-513 reduced ventricular fibrillation by 54% and reduced infarct size from 64 +/- 1% to 20 +/- 3%. First derivative of the LVP, O(2) consumption, and cardiac efficiency were also improved by BIIB-513. Similar results were obtained with BIIB-513 given on reperfusion. These data show that Na(+) loading is a marker of reperfusion injury in intact hearts in that inhibiting NHE reduces Na(+) and Ca(2+) loading during reperfusion while improving function. These results clearly implicate the ionic basis by which inhibiting NHE protects the guinea pig intact heart from ischemia-reperfusion injury.

Ischemic and anesthetic preconditioning reduces cytosolic [Ca2+] and improves Ca(2+) responses in intact hearts

Ca(+) loading during reperfusion after myocardial ischemia is linked to reduced cardiac function. Like ischemic preconditioning (IPC), a volatile anesthetic given briefly before ischemia can reduce reperfusion injury. We determined whether IPC and sevoflurane preconditioning (SPC) before ischemia equivalently improve mechanical and metabolic function, reduce cytosolic Ca(2+) loading, and improve myocardial Ca(2+) responsiveness. Four groups of guinea pig isolated hearts were perfused: no ischemia, no treatment before 30-min global ischemia and 60-min reperfusion (control), IPC (two 2-min occlusions) before ischemia, and SPC (3.5 vol%, two 2-min exposures) before ischemia. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured at the left ventricular (LV) free wall with the fluorescent probe indo 1. Ca(2+) responsiveness was assessed by changing extracellular [Ca(2+)]. In control hearts, initial reperfusion increased diastolic [Ca(2+)] and diastolic LV pressure (LVP), and the maximal and minimal derivatives of LVP (dLVP/dt(max) and dLVP/dt(min), respectively), O(2) consumption, and cardiac efficiency (CE). Throughout reperfusion, IPC and SPC similarly reduced ischemic contracture, ventricular fibrillation, and enzyme release, attenuated rises in systolic and diastolic [Ca(2+)], improved contractile and relaxation indexes, O(2) consumption, and CE, and reduced infarct size. Diastolic [Ca(2+)] at 50% dLVP/dt(min) was right shifted by 32-53 +/- 8 nM after 30-min reperfusion for all groups. Phasic [Ca(2+)] at 50% dLVP/dt(max) was not altered in control but was left shifted by -235 +/- 40 nM [Ca(2+)] after IPC and by -135 +/- 20 nM [Ca(2+)] after SPC. Both SPC and IPC similarly reduce Ca(2+) loading, while augmenting contractile responsiveness to Ca(2+), improving postischemia cardiac function and attenuating permanent damage.

Changes in [Na(+)](i), compartmental [Ca(2+)], and NADH with dysfunction after global ischemia in intact hearts

We measured the effects of global ischemia and reperfusion on intracellular Na(+), NADH, cytosolic and mitochondrial (subscript mito) Ca(2+), relaxation, metabolism, contractility, and Ca(2+) sensitivity in the intact heart. Langendorff-prepared guinea pig hearts were crystalloid perfused, and the left ventricular (LV) pressure (LVP), first derivative of LVP (LV dP/dt), coronary flow, and O(2) extraction and consumption were measured before, during, and after 30-min global ischemia and 60-min reperfusion. Ca(2+), Na(+), and NADH were measured by luminescence spectrophotometry at the LV free wall using indo 1 and sodium benzofuran isophthalate, respectively, after subtracting changes in tissue autofluorescence (NADH). Mitochondrial Ca(2+) was assessed by quenching cytosolic indo 1 with MnCl(2). Mechanical responses to changes in cytosolic-systolic (subscript sys), diastolic (subscript dia), and mitochondrial Ca(2+) were tested over a range of extracellular [Ca(2+)] before and after ischemia-reperfusion. Both [Ca(2+)](sys) and [Ca(2+)](dia) doubled at 1-min reperfusion but returned to preischemia values within 10 min, whereas [Ca(2+)](mito) was elevated over 60-min reperfusion. Reperfusion dissociated [Ca(2+)](dia) and [Ca(2+)](sys) from contractile function as LVP(sys-dia) and the rise in LV dP/dt (LV dP/dt(max)) were depressed by one-third and the fall in LV dP/dt (LV dP/dt(min)) was depressed by one-half at 30-min reperfusion, whereas LVP(dia) remained markedly elevated. [Ca(2+)](sys-dia) sensitivity at 100% LV dP/dt(max) was not altered after reperfusion, but [Ca(2+)](dia) at 100% LV dP/dt(min) and [Ca(2+)](mito) at 100% LV dP/dt(max) were markedly shifted right on reperfusion (ED(50) +36 and +125 nM [Ca(2+)], respectively) with no change in slope. NADH doubled during ischemia but returned to normal on initial reperfusion. The intracellular [Na(+)] ([Na(+)](i)) increased minimally during ischemia but doubled on reperfusion and remained elevated at 60-min reperfusion. Thus Na(+) and Ca(2+) temporally accumulate during initial reperfusion, and cytosolic Ca(2+) returns toward normal, whereas [Na(+)](i) and [Ca(2+)](mito) remain elevated on later reperfusion. Na(+) loading likely contributes to Ca(2+) overload and contractile dysfunction during reperfusion.

Enhanced contractile responsiveness to cytosolic Ca(2+) by delta-2 opioid agonist deltorphin in intact guinea pig hearts

Opioid receptor subtypes, delta and kappa, are found in cardiac tissue and may play a role in cardiac function. We explored if the synthetic opioid delta(2)[D-Ala(2)]-deltorphin (DTP) and mu peptide agonist [D-Ala(2)]-enkephalin (DAMGO) alter the left ventricular pressure (LVP) [Ca(2+)](i) relationship in isolated guinea pig hearts. LV phasic [Ca(2+)](i) was measured from dual fluorescence signals using indo 1. Ca(2+) transients were corrected and calibrated to nM [Ca(2+)](i). Diastolic (d), systolic (s) [Ca(2+)](i), and s-d[Ca(2+)](i) were plotted v LVP at 0.3 to 6.8 mM [CaCl(2)](e)to assess the association of contractility to Ca(2+). Also given were naltriben (NTB) and CTOP, delta(2) and mu antagonists, and nifedipine (NIF) and thapsigargin (THAP). From a control of 880+/-95 nM (SEM), DTP decreased s-d[Ca(2+)](max) to 525+/-82 nM after DTP and to 405+/-84 nM after NIF, whereas THAP increased s-d[Ca(2+)](max)to 1605+/-275 nM. NTB, 795+/-33 nM, NTB+DTP, 820+/-98 nM, DAMGO, 970+/-82 nM, and DAMGO+CTOP, 830+/-93 nM, gave values similar to controls. From a control value of 61+/-4 mm Hg, LVP(max)was increased by DTP to 73+/-3 mmHg and by THAP to 77+/-2 mmHg, was unchanged by DAMGO at 48+/-6 mmHg, and was decreased by NIF to 24+/-2 mmHg. Compared to the control value of 594+/-18 nM, less s-d[Ca(2+)](i) was required to attain 50% s-dLVP(max)(curve left shift) with increasing [CaCl(2)](e) for DTP, 407+/-17 nM, and more was required for THAP, 737+/-35 nM. DTP raised the slope max of s-dLVP(max)(100%) v. s-d[Ca(2+)](i)by 2.7-fold. This indicates DTP enhances cardiac performance by enhancing responsiveness to cytosolic Ca(2+)rather than by raising diastolic Ca(2+) and subsequently released Ca(2+), as does THAP.