Adenosine prevents hyperkalemia-induced calcium loading in cardiac cells: relevance for cardioplegia

Cardioprotection in ischaemia-reperfusion injury: novel mechanisms and clinical translation

In recent decades, robust successes have been achieved in conquering the acutely lethal manifestations of heart disease. Nevertheless, the prevalence of heart disease, especially heart failure, continues to rise. Among the precipitating aetiologies, ischaemic disease is a leading cause of heart failure. In the context of ischaemia, the myocardium is deprived of oxygen and nutrients, which elicits a cascade of events that provokes cell death. This ischaemic insult is typically coupled with reperfusion, either spontaneous or therapeutically imposed, wherein blood supply is restored to the previously ischaemic tissue. While this intervention limits ischaemic injury, it triggers a new cascade of events that is also harmful, viz. reperfusion injury. In recent years, novel insights have emerged regarding mechanisms of ischaemia-reperfusion injury, and some hold promise as targets of therapeutic relevance. Here, we review a select number of these pathways, focusing on recent discoveries and highlighting prospects for therapeutic manipulation for clinical benefit.

Effects of potassium/lidocaine-induced cardiac standstill during cardiopulmonary resuscitation in a pig model of prolonged ventricular fibrillation

OBJECTIVES

Several studies in patients who underwent open heart surgery found that myocardial ischemic damage was reduced by potassium cardioplegia combined with lidocaine infusion. The authors evaluated the effects of potassium/lidocaine-induced cardiac standstill during conventional cardiopulmonary resuscitation (CPR) on myocardial injury and left ventricular dysfunction after resuscitation from prolonged ventricular fibrillation (VF) cardiac arrest in a pig model.

METHODS

Ventricular fibrillation was induced in 16 pigs, and circulatory arrest was maintained for 14 minutes. Animals were then resuscitated by standard CPR. Animals were randomized at the start of CPR to receive 20 mL of saline (control group) or 0.9 mEq/kg potassium chloride and 1.2 mg/kg lidocaine diluted to 20 mL (K-lido group).

RESULTS

Seven animals in each group achieved return of spontaneous circulation (ROSC; p=1.000). Four of the K-lido group animals (50%) achieved ROSC without countershock. Resuscitated animals in the K-lido group required fewer countershocks (p=0.004), smaller doses of epinephrine (p=0.009), and shorter durations of CPR (p=0.004) than did the control group. The uncorrected troponin-I at 4 hours after ROSC was lower in the K-lido group compared with the control group (2.82 ng/mL, 95% confidence interval [CI]=1.07 to 3.38 ng/mL vs. 6.55 ng/mL, 95% CI=4.84 to 13.30 ng/mL; p=0.025), although the difference was not significant after Bonferroni correction. The magnitude of reduction in left ventricular ejection fraction (LVEF) between baseline and 1 hour after ROSC was significantly lower in the K-lido group (26.5%, SD±6.1% vs. 39.1%, SD±6.8%; p=0.004).

CONCLUSIONS

In a pig model of untreated VF cardiac arrest for 14 minutes, resuscitation with potassium/lidocaine-induced cardiac standstill during conventional CPR tended to reduce myocardial injury and decreased the severity of postresuscitation myocardial dysfunction significantly.

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.

Myocardial preservation: controlled reperfusion

Reperfusion injury after reestablishing coronary flow by releasing the aortic cross clamp after cardiac surgery with cardioplegic arrest causes myocardial damage and even death. Attenuation of this reperfusion response by controlling the biochemical and physical environment can avoid morbidity and mortality. Use of a warm reperfusate with addition of drugs that are known to decrease reperfusion injury with manipulation of coronary vascular resistance and the physical parameters of the reperfusion environment helps the heart to reestablish coronary perfusion while decreasing the harm produced by the period of ischemia that occurs during cardiac surgery with intermittent cardioplegic arrest.

Modulation of mitochondrial bioenergetics in the isolated Guinea pig beating heart by potassium and lidocaine cardioplegia: implications for cardioprotection

Mitochondria are damaged by cardiac ischemia/reperfusion (I/R) injury but can contribute to cardioprotection. We tested if hyperkalemic cardioplegia (CP) and lidocaine (LID) differently modulate mitochondrial (m) bioenergetics and protect hearts against I/R injury. Guinea pig hearts (n = 71) were perfused with Krebs Ringer's solution before perfusion for 1 minute just before ischemia with either CP (16 mM K) or LID (1 mM) or Krebs Ringer's (control, 4 mM K). The 1-minute perfusion period assured treatment during ischemia but not on reperfusion. Cardiac function, NADH, FAD, m[Ca], and superoxide (reactive oxygen species) were assessed at baseline, during the 1-minute perfusion, and continuously during I/R. During the brief perfusion before ischemia, CP and LID decreased reactive oxygen species and increased NADH without changing m[Ca]. Additionally, CP decreased FAD. During ischemia, NADH was higher and reactive oxygen species was lower after CP and LID, whereas m[Ca] was lower only after LID. On reperfusion, NADH and FAD were more normalized, and m[Ca] and reactive oxygen species remained lower after CP and LID. Better functional recovery and smaller infarct size after CP and LID were accompanied by better mitochondrial function. These results suggest that mitochondria may be implicated, directly or indirectly, in protection by CP and LID against I/R injury.

Adenosine instead of supranormal potassium in cardioplegic solution improves cardioprotection

OBJECTIVE

To determine whether adenosine instead of supranormal potassium in cold crystalloid cardioplegia gives satisfactory cardiac arrest and improved cardioprotection. Cold crystalloid cardioplegia with adenosine, procaine and magnesium (A) was compared with standard cold crystalloid hyperkalemic cardioplegia (K).

METHODS

Sixteen pigs were randomized to receive either cold K (n=8) or A (n=8), where hyperkalemia was substituted with 1.2 mM adenosine. The cold (6 degrees C) cardioplegia was given intermittently and antegradely, with an aortic cross-clamp time of 1 h. Hemodynamic data was continuously measured and pressure-volume conductance catheters were used to determine global left ventricular systolic and diastolic function. Coronary flow and O2 content differences allowed determination of left ventricular energetics. Blood samples, and left ventricular microdialysis were used to measure parameters of ischemia. Measurements were done at 1 and 2 h after cross-clamp release.

RESULTS

Mean arterial pressure was reduced with 55 mmHg (standard deviation, SD: 19) in the K group versus 30 mmHg (SD: 14) in the A group 2 h after cross-clamp release (p=0.030). Left ventricular contractility expressed as slope of the preload recruitable stroke work index (Mw) was reduced to 53% (SD: 14) in the K group versus 78% (SD: 23) in the A group 2h after cross-clamp release (p=0.046). Reduction of maximum of first derivate of pressure with respect to time (dP/dtmax) was 804 mmHg/s (SD: 189) in the K group versus 538 mmHg/s (SD: 184) in the A group (p=0.033). The slope of the myocardial oxygen consumption-pressure volume area was at 2 h reperfusion increased from 1.37 (SD: 0.64) to 2.86 (SD: 1.27) in the K group, whereas no shift was detected in the A group (p=0.019). Cardiac troponin T measured in the coronary sinus 1 h after cross-clamp release was 1.25 microg/l (SD: 0.64) in the K group versus 0.73 microg/l (SD: 0.31) in the A group (p=0.046).

CONCLUSION

Adenosine instead of supranormal potassium in cold crystalloid cardioplegia gives satisfactory cardiac arrest, improves post cardioplegic left ventricular systolic function and efficiency, and attenuates myocardial cell damage.

Left ventricular mechanoenergetics after hyperpolarized cardioplegic arrest by nicorandil and after depolarized cardioplegic arrest by KCl

We hypothesized that there are no differences in left ventricular (LV) mechanoenergetics between after hyperpolarized cardioplegic arrest by nicorandil (nicorandil arrest) and after depolarized one by high potassium chloride (KCl arrest). The aim of the present study was to test this hypothesis using LV curved end-systolic pressure-volume relation (ESPVR) and linear pressure-volume area (PVA)-myocardial oxygen consumption per beat (Vo2) relation. All hearts underwent 30 min global ischemia (30°C) after infusion of 5 ml of cardioplegia. Cardioplegia consisted of either 30 mmol/l KCl (7 hearts) or nicorandil (100 μmol/l) in Tyrode solution (6 hearts). After a 30-min blood reperfusion, ESPVR and Vo2-PVA relation were assessed again. Mean end-systolic pressure (ESPmLVV) and mean PVA at midrange LV volume (PVAmLVV) significantly ( P < 0.05) decreased to 79.1 ± 13.4% and 85.4 ± 17.1% of control after KCl arrest and to 85.3 ± 14.8% and 86.4 ± 16.9% of control after nicorandil arrest. There were no significant differences in both decreases of mean ESPmLVV and PVAmLVV between each arrest. The slopes of Vo2-PVA relations were also unchanged after each arrest. There was a significant ( P < 0.005) difference in the decreases of mean Vo2 intercepts of Vo2-PVA relations between post-KCl arrest (73.9 ± 8.2% of control) and post-nicorandil arrest (99.2 ± 10.1% of control), however. Proteolysis of α-fodrin due to Ca2+ overload was significantly marked after KCl arrest. The present results indicate that the total calcium handling in excitation-contraction coupling is transiently impaired after KCl arrest, whereas it is unchanged after nicorandil arrest. This suggests the possibility that nicorandil is a better cardioplegia than KCl.

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.

Mechanisms and alternative methods of achieving cardiac arrest

Elective cardiac arrest during surgery can be achieved by inducing depolarization, polarization, or influencing calcium mechanisms. Depolarized arrest, induced by elevating the extracellular potassium concentration, is currently the most commonly used technique. However, injury associated with ionic imbalance involving sodium and calcium overload, together with maintained metabolic processes aimed at correcting these imbalances, have lead to alternatives being sought. "Polarized" arrest, induced by sodium-channel blockers or by agents that activate potassium channels, has been shown to exert equal or superior protection. Similarly, agents that induce calcium desensitization may also prove to enhance protection. These alternative techniques, however, require extensive characterization before introduction into routine clinical use can be recommended.

Gender-specific difference in cardiac ATP-sensitive K(+) channels

OBJECTIVES

The main objective of this study was to establish whether gender regulates expression and/or properties of cardiac ATP-sensitive K(+) (K(ATP)) channels.

BACKGROUND

Recently, evidence has been provided that differing cardiac responses in males and females to metabolic stress may result from gender-specific difference(s) in the efficiency of endogenous cardioprotective mechanism(s) such as K(ATP) channels.

METHODS

A reverse transcription polymerase chain reaction (RT-PCR) using primers specific for Kir6.2, Kir6.1 and SUR2A subunits was performed on total RNA from guinea pig ventricular tissue. Western blotting using anti-Kir6.2 and anti-SUR2A antibodies was performed on cardiac membrane fraction. Whole-cell, single-channel electrophysiology and digital epifluorescent Ca(2+) imaging were performed on isolated guinea pig ventricular cardiomyocytes.

RESULTS

The RT-PCR revealed higher levels of SUR2A, but not Kir6.1 and Kir6.2, messenger RNA in female tissue relative to male tissue, while much higher levels of both Kir6.2 and SUR2A proteins in cardiac membrane fraction in female tissue compared with male tissue were found. In both male and female tissue, pinacidil (100 microM), a K(ATP) channel opener, induced outward whole-cell currents. The current density of the pinacidil-sensitive component was significantly higher in female tissue than it was in male tissue, while no differences in single K(ATP) channel properties between genders were observed. Ischemia-reperfusion challenge induced significant intracellular Ca(2+) loading in male, but not female, cardiomyocytes. To test the hypothesis that SUR2A expression is the limiting factor in K(ATP) channel formation, we took different volumes of Kir6.2 and SUR2A complementary DNA (cDNA) from the same cDNA pool and subjected them to PCR. In order to obtain a band having 50% of the maximal intensity, a volume of SUR2a cDNA approximately 20 times the volume of Kir6.2 cDNA was required.

CONCLUSIONS

This study has demonstrated that female tissue expresses higher levels of functional cardiac K(ATP) channels than male tissue due to the higher expression of the SUR2A subunit, which has an impact on cardiac response to ischemia-reperfusion challenge.

Effect of estrogen on global myocardial ischemia-reperfusion injury in female rats

We investigated the effects of estrogen on global myocardial ischemia-reperfusion injury in rats that were ovariectomized (Ovx), sham-operated, or ovariectomized and then given 17beta-estradiol (E(2)beta) supplementation (Ovx+E(2)beta). Hearts were excised, cannulated, perfused with and then immersed in chilled (4 degrees C) cardioplegia solution for 30 min, and then retrogradely perfused with warm (37 degrees C), oxygenated Krebs-Henseleit bicarbonate buffer for 120 min. The coronary flow rate, first derivative of left ventricular pressure, and nitrite production were all significantly lower in Ovx than in sham-operated or Ovx+E(2)beta hearts. However, coronary flow rates or nitrate production were not consistently different throughout the entire reperfusion period. Ca(2+) accumulated more in Ovx rat hearts than in sham-operated or Ovx+E(2)beta hearts, and mitochondrial respiratory function was lower in Ovx hearts than in hearts from the other two groups. Marked interstitial edema and contraction bands were seen in hematoxylin-eosin-stained sections of Ovx rat hearts but not in hearts from either of the other groups. Hematoxylin-basic fuchsin-picric acid-stained sections revealed fewer viable myocytes in hearts from the Ovx group than from the sham or Ovx+E(2)beta group. Transmission electron microscopy demonstrated more severely damaged mitochondria and ultrastructural damage to myocytes in Ovx rat hearts. Our results indicate that estrogen plays a cardioprotective role in global myocardial ischemia-reperfusion injury in female rats.

Low concentrations of 17beta-estradiol protect single cardiac cells against metabolic stress-induced Ca2+ loading

OBJECTIVES

The main objective of the present study was to determine whether low physiological levels of estrogen directly protect cardiac cells against metabolic stress.

BACKGROUND

The beneficial effect of estrogens on the cardiovascular system has been traditionally ascribed to decrease in peripheral vascular resistance and to an antiatherogenic action. Whether physiological concentrations of 17beta-estradiol (E2) are also able to protect cardiomyocytes against metabolic insult directly is unknown.

METHODS

Isolated ventricular cardiomyocytes were loaded with the Ca2+-sensitive fluorescent dye Fluo-3 and imaged by a digital epifluorescence imaging system. In cardiac cells preincubated with hormones and/or drugs for 8 h, metabolic stress was induced by addition and removal of 2,4-dinitrophenol (DNP).

RESULTS

In cardiomyocytes, a 3-min-long exposure to chemical hypoxia, followed by reoxygenation, produced intracellular Ca2+ loading independently of gender (female: 729 +/- 88 nmol/liter; male: 778 +/- 97 nmol/liter). Pretreatment with E2 (10 nmol/liter) significantly reduced the magnitude of hypoxia/reoxygenation-induced Ca2+ loading in female (E2-treated: 298 +/- 39 nmol/liter; untreated: 729 +/- 88 nmol/liter), but not in male (E2-treated: 1029 +/- 177 nmol/liter; untreated: 778 +/- 97 nmol/liter) cardiac cells. The protective action of E2 was not mimicked by the inactive estrogen stereoisomer, 10 nmol/liter 17alpha estradiol (17alpha estradiol-treated: 886 +/- 122 nmol/liter; untreated: 729 +/- 88 nmol/liter), and was abolished by tamoxifen (1 micromol/liter), which acts as an antagonist of E2 on estrogen receptors (E2 plus tamoxifen-treated: 702 +/- 98 nmol/liter; untreated: 729 +/- 88 nmol/liter).

CONCLUSIONS

In a gender-dependent manner, E2 directly protects cardiac cells against hypoxia-reoxygenation injury through an estrogen receptor-mediated mechanism. Such property of E2 may contribute to cardioprotection in the female gender.

Mechanisms responsible for cell volume regulation during hyperkalemic cardioplegic arrest

BACKGROUND

Cardioplegia has been shown to induce significant cell swelling. This study tested the hypothesis that (1) the [K+][Cl-] product of the cardioplegia solution is the main determinant of myocyte swelling, and (2) reperfusion myocyte shrinkage results from a rectifying Cl- conductance.

METHODS

Rabbit ventricular myocytes were superfused with 37 degrees C Krebs-Henseleit solution for 10 minutes. Then cells underwent 20 minutes of superfusion with standard St. Thomas' solution ([K+][Cl-] product = 2566 mmol/L2) and two solutions with lower [K+][Cl-] product (1500 and 700 mmol/L2) at 9 degrees C. Cells were then resuperfused with 37 degrees C Krebs-Henseleit solution for 30 minutes. Cell volume was measured by videomicroscopy.

RESULTS

Cells superfused with St. Thomas' having [K+][Cl-] products of 2,566, 1,500, and 700 mmol/L2 swelled by 9.18%+/-3.57%, 5.51%+/-1.08%, and 1.49%+/-1.56%, respectively. Reexposure to Krebs-Henseleit solution caused these cells to shrink by 5.79%+/-1.41%, 8.72% +/-3.68%, and 13.46%+/-5.60%, respectively. This shrinkage was blocked by Cl- channel blockers given at the onset of superfusion.

CONCLUSIONS

Lowering the [K+][Cl-] product of St. Thomas' solution attenuated myocyte edema. Myocyte shrinkage during reexposure to Krebs-Henseleit solution resulted from the volume-activated Cl- channel.

Activation of mitogen-activated protein kinases in human heart during cardiopulmonary bypass

Mitogen-activated protein kinases (MAPKs) have been shown to be activated in both in vitro and in vivo models of cardiac tissue in response to ischemia/reperfusion injury. We investigated whether MAPKs are activated in human heart during coronary artery bypass grafting (CABG) surgery. During elective CABG surgery of 8 patients, 3 right atrial appendage biopsies were obtained at baseline, at the end of cross-clamping, and after coronary reperfusion. The expression of the p38-MAPK, c-Jun N-terminal kinase (JNK), and extracellular signal-regulated kinases (ERK1/2) MAPKs was not altered during CABG. The phosphorylation and activation of both ERK1/2 and p38-MAPK were increased approximately 2-fold by ischemia and even more (8- and 4-fold, respectively) by reperfusion. Although the ischemic period did not result in a significant activation of JNK, an approximately 6-fold increase in JNK activity could be observed after reperfusion. In conclusion, distinct activation patterns of ERK1/2, p38, and JNK MAPKs can be observed in human heart during CABG.

Kappa-opioid receptor stimulation increases the expression of Na+-H+ exchange gene in the heart

Kappa-opioid receptor (OR) stimulation increases intracellular pH (pHi) via activating the Na+-H+ exchange (NHE). In the present study, we determined the expression of the gene of NHE1, the predominant NHE isoform in the heart, and intracellular pH (pHi) upon kappa-OR stimulation in the rat heart. We found that 1 microM U50,488H (trans-3,4-dichloro-N-methyl-N-(2-(1 pyrrolidinyl)cyclohexyl)benzeneacetamide), a selective kappa-OR agonist, increased the expression of the NHE1 gene. We also found that U50,488H dose-dependently increased pHi in the heart. The effects were abolished by 1 microM nor-binaltorphimine (nor-BNI), a selective kappa-OR antagonist, indicating that the events were kappa-OR mediated. The effects on both NHE1 gene expression and pHi were also abolished by 5 microM chelerythrine and 5 microM BSM (bisyndolylmaleimide), protein kinase C (PKC) inhibitors, indicating that PKC mediated the actions. In addition, the effect of U50,488H on pHi was blocked by 10 microM EIPA (ethylisopropyl amiloride), a NHE1 inhibitor, indicating that NHE1 also mediated the action of U50,488H. The present study provides evidence for the first time that kappa-OR stimulation increased the NHE1 gene expression in the heart via a PKC dependent pathway. Kappa-OR stimulation also increases pHi via PKC and NHE in the heart.

Adenosine with cold blood cardioplegia during coronary revascularization

OBJECTIVE

To investigate whether adenosine in association with blood cardioplegia results in more rapid cardiac arrest or improved myocardial protection.

DESIGN

A prospective, randomized, placebo-controlled double-blind clinical study.

SETTING

Operative and intensive care units in a university hospital, Finland.

PARTICIPANTS

Forty patients undergoing primary, elective coronary revascularization.

INTERVENTION

Adenosine as a bolus dose, 12 mg intravenously, was given immediately before the induction of blood cardioplegia.

MEASUREMENTS AND MAIN RESULTS

There were nonsignificantly higher serial serum values of CK (MB) (p = 0.33), troponin-T (p = 0.23), and troponin-I (p = 0.10) in the adenosine group. There were no differences between the groups in arrest time, blood pressure decrease, or lactate extraction.

CONCLUSIONS

The adenosine regimen used in this study did not cause more rapid arrest with blood cardioplegia. The effect on cardioprotection was insignificant.

Polarization and myocardial protection

Heart surgery or transplantation generally involve global ischemia, and techniques have been developed to protect the myocardium from ischemic and reperfusion injury. Hyperkalemic cardioplegia has been the gold standard for myocardial protection for years, but patients undergoing surgery almost invariably have some postoperative dysfunction. One factor may be the depolarizing nature of hyperkalemia, which results in continuing transmembrane fluxes and metabolism, even during hypothermic ischemia. A potentially beneficial alternative to hyperkalemic (depolarizing) cardioplegia is arrest in a "hyperpolarized" or "polarized" state, which maintains the myocardial membrane potential at or near the resting potential. This should minimize transmembrane fluxes and metabolic demand and improve myocardial protection. These alternative concepts have recently been investigated by using adenosine and potassium-channel openers (which are thought to induce hyperpolarized arrest) or the sodium-channel blocker tetrodotoxin (which induces polarized arrest), and results have been beneficial compared with the results of hyperkalemia. Additional studies are needed before experimental promise can become clinical reality.

Protection of cardiomyocytes by pinacidil during metabolic inhibition and hyperkalemia

The objective of this study is to understand the mechanism underlying the cardioprotective effects of pinacidil, an ATP-sensitive K+ channel (K(ATP)) opener. We examined the effects of 10 microM pinacidil in cultured chicken cardiomyocytes. Pinacidil caused a concentration-dependent delay in metabolic inhibition-induced increase in intracellular calcium concentration ([Ca2+]i) and creatine phosphokinase release, and this action was antagonized by glyburide, a K(ATP) blocker. Neither verapamil, an L-type Ca2+ channel blocker, nor bepridil, a Na+-Ca2+ exchange inhibitor, affected the time course of increase in [Ca2+]i induced by metabolic inhibition. Pinacidil did not have an effect on the amplitude of K+-induced increase in [Ca2+]i, but accelerated the rate of decline following peak stimulation. In contrast, glyburide reduced the amplitude of K+-induced increase in [Ca2+]i and prolonged the rate of decline. These results provide direct evidence that pinacidil protects cardiomyocytes from metabolic inhibition-induced injury by cyanide (CN) through a delay in the onset of increase in [Ca2+]i, rather than by inhibition of the L-type Ca2+-channels or by alteration of Na+-Ca2+ exchange.

Domoic Acid Attenuates the Adenosine-5'-Triphosphate-Induced Increase in

BACKGROUND: Although domoic acid (DA), a shellfish neurotoxin, carries a negative surface charge at physiological pH like that of adenosine-5'-triphosphate (ATP), very little is known about its cellular effects. In view of the potentially significant role of extracellular ATP as a signaling molecule for increasing the intracellular concentration of Ca(2+) ([Ca(2+)](i)), we examined the possibility that DA may interfere with this signal transduction mechanism in the myocardium. METHODS AND RESULTS: Cardiomyocytes were isolated from rat heart and loaded with Fura-2 to measure the [Ca(2+)](i). ATP produced a gradual rise in [Ca(2+)](i), reaching a peak level in 25-30 seconds and declining thereafter. DA did not affect the [Ca(2+)](i) in cardiomyocytes; however, it diminished the ATP-induced elevation in [Ca(2+)](i) in the concentration-dependent manner. Kainic acid, an analogue of DA, had a similar effect but at a 25-fold higher concentration, whereas glutamate and aspartate did not modify the action of ATP. Well-known inhibitors of L-type voltage-sensitive Ca(2+) channels, nifedipine and nicardipine, depressed the ATP-induced increase in [Ca(2+)](i), but DA did not produce additive effects with either of these agents. On the other hand, DA potentiated the KCl-induced increase in [Ca(2+)](i) in quiescent cardiomyocytes and augmented the nicardipine-sensitive Ca(2+) transients in electrically stimulated cardiomyocytes. CONCLUSIONS: These results suggest that DA may diminish the ATP-induced increase in [Ca(2+)](i) by inhibiting the ATP interaction with cardiomyocytes in a specific manner.

Cardioprotection of preconditioning by metabolic inhibition in the rat ventricular myocyte. Involvement of kappa-opioid receptor

To determine whether opioid receptors (ORs) are involved in the delayed cardioprotection of ischemic preconditioning (IP), the effect of severe metabolic inhibition (MI) with a glucose-free buffer that contained sodium cyanide and 2-deoxy-D-glucose on the viability of isolated rat ventricular myocytes was first determined 20 hours after preconditioning with a sublethal metabolic inhibition (MIP) with a glucose-free buffer that contained 2-deoxy-D-glucose and lactate for 30 minutes in the presence of OR antagonists. With the use of trypan blue exclusion as an index of cell viability, severe MI killed >60% of the cells and the value increased significantly after MIP. In the presence of 5x10(-6) mol/L nor-binaltorphimine (nor-BNI), a selective kappa-OR antagonist, but not 5x10(-6) mol/L CTOP, a selective mu-OR antagonist, or 5x10(-6) mol/L naltrindole, a selective delta-OR antagonist, the cardioprotection of MIP was significantly attenuated. To verify the role of kappa-OR, we studied the effects of severe MI after pretreatment with the kappa-OR agonist U50,488H (UP) for 30 minutes. U50,488H at 3x10(-6) to 1x10(-4) mol/L increased cell viability concentration-dependently with an EC50 of 3.311x10(-6) mol/L. In the presence of 5x10(-6) nor-BNI, the cardioprotection of UP (3x10(-5) mol/L) was blocked. A time course study showed that UP-induced cardioprotection occurred in 2 windows: the first occurred approximately 1 hour later and the other occurred 16 to 20 hours later. Additional studies on cell contraction and intracellular Ca2+ ([Ca2+]i) revealed that both UP and MIP attenuated the inhibitory effects of severe MI on contractility and electrically induced [Ca2+]i transient in single ventricular myocytes. On blockade of protein kinase C, the delayed cardioprotections of UP and MIP were significantly attenuated. In conclusion, the results of the present study have provided evidence that kappa-OR mediates the cardioprotection of MIP, which may involve protein kinase C and [Ca2+]i.

Gene delivery of Kir6.2/SUR2A in conjunction with pinacidil handles intracellular Ca2+ homeostasis under metabolic stress

Metabolic injury is a complex process affecting various tissues, with intracellular Ca2+ loading recognized as a common precipitating event leading to cell death. We have recently observed that cells overexpressing recombinant ATP-sensitive K+ (KATP) channel subunits may acquire resistance against metabolic stress. To examine whether, under metabolic challenge, intracellular Ca2+ homeostasis can be maintained by an activator of channel proteins, we delivered Kir6.2 and SUR2A genes, which encode KATP channel subunits, into a somatic cell line lacking native KATP channels. Hypoxia-reoxygenation was simulated by application and removal of the mitochondrial poison 2,4 dinitrophenol. Under such metabolic stress, Ca2+ loading was induced by Ca2+ influx during hypoxia and release of Ca2+ from intracellular stores during reoxygenation. Delivery of Kir6.2/SUR2A genes, in conjunction with the KATP channel activator pinacidil, prevented intracellular Ca2+ loading irrespective of whether the channel opener was applied throughout the duration of hypoxia-reoxygenation or transiently during the hypoxic or reoxygenation stage. In all stages of injury, the effect of pinacidil was inhibited by the selective antagonist of KATP channel, 5-hydroxydecanoate. The present study provides evidence that combined use of gene delivery and pharmacological targeting of recombinant proteins can handle intracellular Ca2+ homeostasis under hypoxia-reoxygenation irrespective of the stage of the metabolic insult.

Protective action of 17beta-estradiol in cardiac cells: implications for hyperkalemic cardioplegia

BACKGROUND

Hyperkalemic cardioplegic solutions effectively arrest the heart, but may also induce intracellular Ca2+ loading and cellular hypercontracture, which could contribute to ventricular dysfunction associated with global surgical ischemia. Recently, it has been proposed that 17beta-estradiol may possess protective properties in the ischemic myocardium. The purpose of the present study was to examine the action of 17beta-estradiol on cardiac cells exposed to hyperkalemic stress.

METHODS

Single ventricular cardiomyocytes, a preparation devoid of vascular and neuronal elements, were isolated from guinea pig hearts, loaded with a Ca2+-sensitive fluorescent probe, and imaged by digital epifluorescent microscopy. The emitted fluorescence of the probe, a measure of intracellular Ca2+ concentration, and cell length were simultaneously recorded during hyperkalemic challenge, in the absence or presence of 17beta-estradiol.

RESULTS

In control cardiomyocytes, the cytosolic concentration of Ca2+ was 138+/-11 nmol/L and cell length 93+/-11 microm. Exposure to high K+ (+16 mmol/L KCl) significantly increased cytosolic Ca2+ to 2,191+/-87 nmol/L (p < 0.001), and produced cell shortening (length at 39+/-5 microm; p < 0.001). 17beta-Estradiol (10 micromol/L) acutely prevented high K+ to induce either intracellular Ca2+ loading (144+/-13 nmol/L, p < 0.001) or hypercontracture (91+/-10 microm, p < 0.001). Tamoxifen (10 micromol/L), an antiestrogen, abolished the protective effect of 17beta-estradiol.

CONCLUSIONS

We conclude that 17beta-estradiol prevents hyperkalemia-induced Ca2+ loading and hypercontracture through a direct and tamoxifen-sensitive action in cardiomyocytes. This study raises the possibility that 17beta-estradiol should be considered as a cardioprotective adjunct toward a safer hyperkalemic cardioplegia.

Recombinant cardiac ATP-sensitive K+ channel subunits confer resistance to chemical hypoxia-reoxygenation injury

BACKGROUND

Opening of cardiac ATP-sensitive K+ (KATP) channels has emerged as a promising but still controversial cardioprotective mechanism. Defining KATP channel function at the level of recombinant channel proteins is a necessary step toward further evaluation of the cardioprotective significance of this ion conductance.

METHODS AND RESULTS

KATP channel deficient COS-7 cells were found to be vulnerable to chemical hypoxia-reoxygenation injury that induced significant cytosolic Ca2+ loading (from 97+/-3 to 236+/-11 nmol/L). In these cells, the potassium channel opener pinacidil (10 micromol/L) did not prevent Ca2+ loading (from 96+/-3 nmol/L before to 233+/-12 nmol/L after reoxygenation) or evoked membrane current. Cotransfection with Kir6.2/SUR2A genes, which encode cardiac KATP channel subunits, resulted in a cellular phenotype that, in the presence of pinacidil (10 micromol/L), expressed K+ current and gained resistance to hypoxia-reoxygenation (Ca2+ concentration from 99+/-7 to 127+/-11 nmol/L; P>0.05). Both properties were abolished by the KATP channel blocker glyburide (1 micromol/L). In COS-7 cells transfected with individual channel subunits Kir6.2 or SUR2A, which alone do not form functional cardiac KATP channels, pinacidil did not protect against hypoxia-reoxygenation.

CONCLUSIONS

The fact that transfer of cardiac KATP channel subunits protected natively KATP channel deficient cells provides direct evidence that the cardiac KATP channel protein complex harbors intrinsic cytoprotective properties. These findings validate the concept that targeting cardiac KATP channels should be considered a valuable approach to protect the myocardium against injury.

Inhibition of the pacemaker current: a bradycardic therapy for off-pump coronary operations

BACKGROUND

The accurate performance of coronary anastomoses on the beating heart requires some form of myocardial immobilization that can be achieved pharmacologically. Different classes of drugs can be used to induce bradycardia, but the most effective in this setting of off-pump operation has not yet been determined.

METHODS

Fifty-six isolated buffer-perfused rabbit hearts were divided into seven equal groups. Control hearts were continuously perfused throughout the experimental time course. A second group of hearts underwent 60 minutes of potassium arrest (at 37 degrees C) followed by 1 hour of reperfusion. The following pharmacologic approaches were tested in the remaining five groups: short-acting beta-blockade (esmolol, 6 x 10(-3) mol/L and 3 x 10(-4) mol/L), opening of adenosine triphosphate-dependent potassium channels (nicorandil, 10(-3) mol/L and 10(-5) mol/L), and inhibition of the pacemaker current, which largely accounts for the diastolic depolarization of sinoatrial node cells (S 16257-2, 3 x 10(-6) mol/L). Each drug was infused at a constant rate for 60 minutes, after which hearts were perfused for 1 additional hour with drug-free buffer. Heart rate and isovolumic measurements of function and coronary flow were serially taken during and after drug infusion.

RESULTS

The worst recovery of systolic and, moreover, diastolic function was yielded by potassium arrest. Neither esmolol nor nicorandil was able to induce a significant bradycardia. However, nicorandil did not impair function which, conversely, was markedly depressed after esmolol therapy. Significant bradycardia (p < 0.0001 versus corresponding baseline values and versus all other groups) was only achieved with pacemaker current inhibition, which was otherwise associated with an excellent preservation of contractility, diastolic function, and coronary flow.

CONCLUSIONS

Inhibition of the pacemaker current seems to be an effective approach for inducing intraoperative bradycardia without compromising left ventricular function or flow.

Myocardial preconditioning using adenosine: review and clinical experience

Adenosine is an endogenous nucleotide and a breakdown product of adenosine triphosphate. Adenosine has been proposed as a mediator of the ischaemic preconditioning phenomenon. Ischaemic reperfusion injury incurred during and following cardiopulmonary bypass contributes to depressed myocardial function after cardiac surgery. It is believed that administering adenosine via the aortic root, immediately following aortic crossclamping as well as just prior to removal of the aortic crossclamp, provides myocardial preconditioning resulting in improved cardiac protection during ischaemic arrest and retarding ischaemic reperfusion injury. A retrospective analysis was done utilizing consecutive patients undergoing coronary artery bypass grafting performed by the same surgeon. Some of the patients received myocardial preconditioning with adenosine. A comparison was made in postoperative cardiac function between patients who underwent myocardial preconditioning and those who did not receive adenosine. Results demonstrate a greater improvement in postoperative cardiac function, when compared to preoperative values, in those patients receiving myocardial preconditioning with adenosine.

Diadenosine 5',5"-P1,P5-pentaphosphate harbors the properties of a signaling molecule in the heart

Dinucleotide polyphosphates (ApnA) have emerged as signaling molecules in rapidly dividing cells. The presence and role of Ap5A in the heart remain unknown. Here, we report that the myocardium contains abundant amounts of diadenosine 5',5"-P1,P5-pentaphosphate (Ap5A), a member of the ApnA family. Ischemia induced 10-fold decrease in the myocardial concentration of Ap5A. A target of Ap5A action was identified to be the cardiac ATP-sensitive K+ (K(ATP)) channel, a metabolism-sensitive ion conductance activated in ischemia. At levels found in hearts prior to ischemia, Ap5A maintained a low probability of K(ATP) channel opening, but at levels found in hearts following ischemia, Ap5A allowed a high probability of K(ATP) channel opening. Taken together, the present data suggest that Ap5A harbors the properties of a signaling molecule involved in the cardiac response to metabolic stress.

Adenosine prevents K+-induced Ca2+ loading: insight into cardioprotection during cardioplegia

In clinical practice, hyperkalemic cardioplegia induces sarcolemmic depolarization, and therefore is used to arrest the heart during open heart operations. However, the elevated concentration of K+ that is present in cardioplegic solutions promotes intracellular Ca2+ loading, which could aggravate ventricular dysfunction after cardiac operations. This review highlights recent findings that have established, at the single cell level, the protective action of adenosine against hyperkalemia-induced Ca2+ loading. When it was added to hyperkalemic cardioplegic solutions, adenosine, at millimolar concentrations and through a direct action on ventricular cardiomyocytes, prevented K+-induced Ca2+ loading. This action of adenosine required the activation of protein kinase C, and it was effective only in cardiomyocytes with low diastolic Ca2+ levels. Of importance, adenosine did not diminish the magnitude of K+-induced membrane depolarization, allowing unimpeded cardiac arrest. Taken together, these findings provide direct support for the idea that adenosine is valuable when used as an adjunct to hyperkalemic cardioplegia. This idea has emerged from previous clinical studies that have shown improvement of the clinical outcome after cardiac operations when adenosine or related substances were used to supplement cardioplegic solutions. Further studies are required to define more precisely the mechanism of action of adenosine, and the conditions that may determine the efficacy of adenosine as a cytoprotective supplement to cardioplegia.

Adenosine decreases post-ischaemic cardiac TNF-alpha production: anti-inflammatory implications for preconditioning and transplantation

Tumour necrosis factor-alpha (TNF-alpha) is an autocrine contributor to myocardial dysfunction and cardiomyocyte death in ischaemia-reperfusion injury (I/R), sepsis, chronic heart failure and cardiac allograft rejection. Cardiac resident macrophages, infiltrating leucocytes, and cardiomyocytes themselves produce TNF-alpha. Although adenosine reduces macrophage TNF-alpha production and protects myocardium against I/R, it remains unknown whether I/R induces an increase in cardiac TNF-alpha in a crystalloid-perfused model (in the absence of blood), and, whether adenosine decreases cardiac TNF-alpha and protects function after I/R. To study this, isolated rat hearts were crystalloid-perfused using the Langendorff method and subjected to I/R, with or without adenosine pretreatment. Post-ischaemic cardiac TNF-alpha (enzyme-linked immunosorbent assay and bioassay) and function were determined (Langendorff). I/R increased cardiac TNF-alpha and impaired myocardial function. Adenosine decreased cardiac TNF-alpha and improved post-ischaemic functional recovery. This study demonstrates that: first, I/R induces an increase in cardiac tissue TNF-alpha in a crystalloid-perfused model: second, adenosine decreases cardiac TNF-alpha and improves post-ischaemic myocardial function; third, decreased cardiac TNF-alpha may represent a mechanism by which adenosine protects myocardium; and fourth, adenosine-induced suppression of cardiac TNF-alpha may provide an anti-inflammatory link to preconditioning and have implications for cardiac allograft preservation.