Abstract

The cardioprotection afforded by ischemic preconditioning (IPC) and ischemic postconditioning (PC) are receptor mediated. In this review, we will focus on the major ligand classes and receptors that contribute to IPC and PC-induced cardioprotection. Ligand classes discussed include adenosine, bradykinin, opioids, erythropoietin, adrenergics and muscarinics. The cardioprotective therapeutic window of each ligand class will also be summarized, with particular focus as to whether ligands are protective when administered at or close to the time of reperfusion. Information will primarily be directed at studies in which infarct size reduction is the gold standard to assess the efficacy of IPC and PC. Myocardial stunning is a less robust endpoint for assessing cardioprotection and the use of this endpoint is only limited to studies with human tissue where infarct size assessment is not possible. Receptor cross-talk between ligands and the common signaling pathways involved for these ligands will also be briefly discussed.

1. General introduction

Brief intermittent periods of ischemia and reperfusion are protective, both at a time prior to ischemia, known as ischemic preconditioning (IPC), or immediately after reperfusion, known as ischemic postconditioning (PC) [1,2]. Although these stimuli are powerful means of protecting the ischemic myocardium from irreversible injury, their clinical applicability may be limited since 1) the mechanical intervention may require precise, timed pulsations of ischemia and reperfusion, 2) a reservation of physicians to purposely create an ischemic myocardium and 3) training of emergency medical professionals in this technique to provide timely intervention. Therefore, an alternative means of harnessing this protection by the use of specific receptor agonists or antagonists may provide a feasible means of effectively producing cardioprotection clinically.

Within the last 15–20 years, a number of cardioprotective ligands were identified in animal models, including adenosine, bradykinin, opioids, erythropoietin, adrenergic and muscarinic agonists. Of particular interest is the ability for these ligands to initiate cardioprotective salvage pathways in a timely manner when administered after ischemia; i.e., reperfusion injury. Unlike a mechanical stimulus, it would appear that some ligands may produce greater cardioprotection when given after the initiation of ischemia or may lose their cardioprotective efficacy if administered too late after the initiation of reperfusion. In lieu of our present findings with opioids, ligand-mediated cardioprotection may vary based on the time of administration and the specific receptor subtype an agonist targets [3,4]. Therefore, this review will focus on the ability of different receptor-mediated ligands to achieve acute cardioprotection, with particular focus on studies conducted with agents administered after the initiation of ischemia or at the time of reperfusion. This review will also briefly discuss ligand receptor cross-talk and the signaling mechanisms responsible for ligand-mediated cardioprotection.

Ligands that contribute to acute cardioprotection

In this section, we will discuss the different cardioprotective classes of ligands. For additional extensive and excellent reviews on ischemic and pharmacological preconditioning and postconditioning, please consult the following Refs. [5–7]. This section is by no means a complete review of studies concerning ligand-induced cardioprotection. This review focuses on studies where either a direct comparison of a ligand administered prior, during or after ischemia was conducted or in studies where ligands were administered during ischemia or during reperfusion. This section summarizes several factors for each ligand including 1) whether the endogenous ligand has been reported to be elevated during ischemia/reperfusion, 2) receptor subtypes identified, 3) transgenic or genetic knockout animal studies, 4) the cardioprotective efficacy of each ligand when administered during ischemia or reperfusion, 5) whether the ligand can mimic the effects of IPC or PC and 6) if an antagonist of the agent can block the effects of IPC or PC. A summary of 3) and 4) is presented in Table 1.

Table 1

Summary of ligand-induced parameters of cardioprotection

Ligand Genetic manipulation Agents initiated during ischemia Agents initiated during reperfusion Window of protection 
Agent Target Species Agent Target Species During ischemia/reperfusion 
Adenosine Trans: A1, A3 [11,12] KO: A3 [13] Adenosine NS Mouse [16] rabbit [19] canine [20–23] Adenosine NS Mouse [16] canine [17] rabbit [18,19] Isch to start of rep* 
  BN-063 A1 Rat [14] BN-063 A1 Rat [14] Isch to start of rep* 
  CPA A1 Rabbit [18]    Isch* 
     CHA A1 Mouse [16] Start of rep* 
  NECA A1/A2 Rabbit [31]    Isch* 
  AMP-579 A1/A2 Porcine [15] rabbit [30,31,32] AMP-579 A1/A2 Canine [17] rabbit [27,33] Isch to <10 min post-rep 
  CGS 21680 A2 Rabbit [18,35] CGS 21689 A2 Porcine [34] canine [23] Isch to start of rep* 
  CLIB-MECA A3 Canine [37] CLIB-MECA A3 Rats [36] porcine [34] Isch to start of rep* 
Bradykinin Trans: NT KO: B1, B2 [54,53] Bradykinin B1, B2 Swine [55,56] rabbit [31,57] Bradykinin B1, B2 Mice [58] Isch to start of rep* 
Opioids Trans: NT Morphine NS Rat [74]    Isch 
 KO: NT BW373U86 δ Rat [74] BW373U86 δ Rat [Fig. 1Isch to 10 sec post-rep 
     FIT δ Rat [4] Isch to 10 sec post-rep 
  U50,488 κ Rat [Fig. 1   Isch 
Erythropoietin Trans: NT KO: NT Erythropoietin EPOR Rabbit [85] rat [84,86] canine [87] Erythropoietin EPOR Rabbit [82,83,85] rat [84] Isch to 5 min post-rep 
Adrenergics Trans: β2 [92]    Metoprolol β1 Rabbit [94] Start of rep* 
 KO: β2 [91]    Bisprolol β1 Rabbit [95] Start of rep* 
     Carvedilol β1 Rabbit [94, 95] Start of rep* 
Ligand Genetic manipulation Agents initiated during ischemia Agents initiated during reperfusion Window of protection 
Agent Target Species Agent Target Species During ischemia/reperfusion 
Adenosine Trans: A1, A3 [11,12] KO: A3 [13] Adenosine NS Mouse [16] rabbit [19] canine [20–23] Adenosine NS Mouse [16] canine [17] rabbit [18,19] Isch to start of rep* 
  BN-063 A1 Rat [14] BN-063 A1 Rat [14] Isch to start of rep* 
  CPA A1 Rabbit [18]    Isch* 
     CHA A1 Mouse [16] Start of rep* 
  NECA A1/A2 Rabbit [31]    Isch* 
  AMP-579 A1/A2 Porcine [15] rabbit [30,31,32] AMP-579 A1/A2 Canine [17] rabbit [27,33] Isch to <10 min post-rep 
  CGS 21680 A2 Rabbit [18,35] CGS 21689 A2 Porcine [34] canine [23] Isch to start of rep* 
  CLIB-MECA A3 Canine [37] CLIB-MECA A3 Rats [36] porcine [34] Isch to start of rep* 
Bradykinin Trans: NT KO: B1, B2 [54,53] Bradykinin B1, B2 Swine [55,56] rabbit [31,57] Bradykinin B1, B2 Mice [58] Isch to start of rep* 
Opioids Trans: NT Morphine NS Rat [74]    Isch 
 KO: NT BW373U86 δ Rat [74] BW373U86 δ Rat [Fig. 1Isch to 10 sec post-rep 
     FIT δ Rat [4] Isch to 10 sec post-rep 
  U50,488 κ Rat [Fig. 1   Isch 
Erythropoietin Trans: NT KO: NT Erythropoietin EPOR Rabbit [85] rat [84,86] canine [87] Erythropoietin EPOR Rabbit [82,83,85] rat [84] Isch to 5 min post-rep 
Adrenergics Trans: β2 [92]    Metoprolol β1 Rabbit [94] Start of rep* 
 KO: β2 [91]    Bisprolol β1 Rabbit [95] Start of rep* 
     Carvedilol β1 Rabbit [94, 95] Start of rep* 

Trans: transgenic, KO: knockout, NT: not tested, NS: non-specific, EPOR: erythropoietin receptor, Isch: ischemia, rep: reperfusion, post-rep: after reperfusion, *: the complete window of protection for these agents has not been determined.

Adenosine

Adenosine is released during ischemia and reperfusion, with blood and interstitial concentrations elevated after ischemic insults [8–10]. Four adenosine receptor subtypes exist, which include A1, A2A, A2B and A3. In genetically modified mice, evidence suggests the A1 and A3 receptors may contribute to cardioprotection, since overexpression of either receptor improved functional recovery from ischemia in transgenic mice [11,12]. However, adenosine A3 receptor knockout mice have also shown protection from ischemic insults [13]. If the degree of transgene A3 overexpression is too excessive, the mice develop hypertrophy, bradycardia, hypotension and systolic dysfunction [11]. Hence, additional genetic studies are warranted with careful monitoring of receptor level expression and furthermore, whether the genetic manipulation alters additional receptor subtypes that may contribute to generating a cardioprotective phenotype.

Fig. 1

Infarct size as a percent of area at risk (%IF) for rats (n=6/group) receiving either U50,488, BW373U86 or DMSO control (Panels A and B). Agents were administered either 10 min prior to ischemia (Pre-I), 5 min prior to reperfusion (Pre-R) or 10 s after reperfusion (Post-R, 10 s). BW373U86 was also administered 5 min after reperfusion (Post-R, 5 m). Significance is indicated by *(P<0.01).

Fig. 1

Infarct size as a percent of area at risk (%IF) for rats (n=6/group) receiving either U50,488, BW373U86 or DMSO control (Panels A and B). Agents were administered either 10 min prior to ischemia (Pre-I), 5 min prior to reperfusion (Pre-R) or 10 s after reperfusion (Post-R, 10 s). BW373U86 was also administered 5 min after reperfusion (Post-R, 5 m). Significance is indicated by *(P<0.01).

Alternatively, the role for adenosine in cardioprotection has also been studied by pharmacological manipulation. Specific adenosine receptor agonists reduce infarct size just as effectively when administered either prior to ischemia or just prior to reperfusion, implying that the cardioprotective effects of adenosine receptor agonists occur at the time of reperfusion [14,15]. In the mouse, it even appears that adenosine administration enhances cardioprotection when administered at reperfusion, even more so than administration just prior to ischemia [16]. Adenosine given prior to or at the start of reperfusion in rabbit or canine models is cardioprotective in some studies [17–23] while others report adenosine having no effect [24–26]. The acute administrative window is less than 10 min after reperfusion for adenosine or adenosine receptor agonists to induce cardioprotection [27,28].

The selective A1 receptor agonist, (N-[1S, (trans)-2-hydroxycyclopentyl] adenosine, GR79236, is cardioprotective in swine when administered either prior to ischemia or reperfusion [28]. Furthermore, the protective effect of GR79236 was abolished by prior administration of the selective A1 antagonist, DPCPX [28]. Alternatively, activating the A1 receptor just prior to reperfusion had no protective effect when GR79236 was given 10 min before reperfusion in rabbits [29], or when initiated 10 min prior to reperfusion and continued for 70 min in swine [15]. In rabbits, the A1 receptor agonist CPA protected when administered for 65 min, when the dose was initiated 5 min prior to reperfusion [18]. Improved functional recovery was also reported with the A1 agonist CHA [16].

Mixed A1 and A2A receptor agonists, AMP 579 and NECA, are cardioprotective when administered just prior to reperfusion [30,31]. However, AMP 579 administered 30 min prior to ischemia and continued through the first hour of reperfusion was markedly more effective in reducing infarct size as compared to administering AMP 579 10 min prior to reperfusion for 70 min in swine [15]. In rabbits, AMP 579 reduced infarct size equally when administered either prior to ischemia or prior to reperfusion, suggesting that the mechanism in rabbits may differ from swine [32]. A further study in rabbit hearts suggests that AMP 579, initiated at reperfusion, is protective only if AMP 579 is administered for longer than 40 min following reperfusion [27]. AMP 579 infusion started at reperfusion was also protective in canine hearts [17]. The protective effect of AMP 579 is suggested to be A2A receptor mediated, since the putative A2A receptor antagonist, ZM 241385 blocked the cardioprotective effect of AMP 579 [33].

The adenosine A2A receptor agonist, CGS 21680, is also cardioprotective when administered at the time of reperfusion. A reduction in infarct size has been reported with CGS 21680 when administered 5 min before reperfusion and continued for 65 min in rabbits [18]. CGS 21680 administered at reperfusion for 60 min in swine was also reported to be cardioprotective [34]. Protection also occurred in canine hearts following administration of CGS 21680 both prior to and continued into reperfusion or at reperfusion [23,35].

Adenosine A3 receptor activation at reperfusion is also cardioprotective. Administration of 2-chloro-IB-MECA (CLIB-MECA) in rats reduced infarct size when administered at reperfusion in a dose-dependent inverse bell-shaped curve. The protection was abolished by the adenosine A3 receptor antagonist MRS 1191 [36]. However the effects of CLIB-MECA in rats and swine also appear to be mediated by the adenosine A2A receptor, since inhibition of the A2A receptor abrogates CLIB-MECA-induced cardioprotection [34,36]. In canine hearts, CLIB-MECA was as effective in reducing infarct size when administered 5 min before reperfusion as compared to administration prior to ischemia [37].

Adenosine administration mimics the effects of IPC in all animal models tested, including human atrial trabeculae [38–40]. Specific A1 and A3 receptor agonists also mimic the effects of IPC [39,40]. Inhibition of adenosine receptors with the nonspecific adenosine antagonist, 8-phenyltheophylline (SPT) blocked the ability of IPC to reduce infarct size when given either prior to IPC or following IPC [40,41]. The adenosine receptor antagonist PD 115,199 also abolished IPC-induced cardioprotection when given before IPC [40]. Receptor specific adenosine antagonists, targeting the A1 receptor, including DPCPX, BG 9719, or BG 9928, did not abolish the protective effects of 4 cycles of IPC in the dog, while the adenosine A1 receptor antagonist, DPCPX, abolished the cardioprotective effect of 2 cycles of IPC in pigs [28,42]. Alternatively, selective inhibition of the A3 receptor by BW A1433 blocked the cardioprotection afforded by IPC in rabbits [39].

PC is abrogated with prior administration of the adenosine antagonist, SPT, in rabbit and rat hearts [43–45]. The specific adenosine receptors that mediate PC appear to involve both A2A and A3 receptors, since the putative A2A receptor antagonist, ZM241385 and the putative A3 receptor antagonist, MRS1523, blocked PC-induced infarct size reduction [45].

Although adenosine and adenosine receptor agonists are the most extensively studied cardioprotective ligands, there is no definitive consensus as to which adenosine receptor subtype contributes to cardioprotection during ischemia or reperfusion phases. Most likely the species of animal, dose, timing and receptor subtypes activated by agents all contribute to the variations between studies. It is also apparent that the IPC or PC cycle number may affect the outcome of cardioprotective blockade via adenosine receptor antagonists by affecting the release of adenosine, the adenosine receptor affinity or some other as yet to be defined mechanism.

Bradykinin

Bradykinin is elevated during and after an ischemic insult [46–48]. Two bradykinin receptors exist in cardiomyoctes, a constitutive B2 receptor and a B1 receptor that is induced after stress [49,50]. In rats, the induction of the B1 receptor in the left ventricle occurs 6 h after reperfusion, with B1 receptor expression increasing to four fold higher after 24 h, with similar trends reported for the B2 receptor [51,52]. IPC-induced cardioprotection is abolished in B2 receptor knockout mice [53]. Knockout of the B1 receptor in female mice suggest B1 receptors have no effect on remodeling after a myocardial infarction, however, the role of the B1 receptor concerning cardioprotection is unknown [54].

Bradykinin administered 15 min after the start of 45 min of ischemia and continued into reperfusion reduced creatine kinase release and elevated catecholamine and renin levels in swine [55]. Additionally in swine, bradykinin administered 15 min before and continued into reperfusion was beneficial, based on an observed reduction in creatine kinase and an improved electrocardiogram. However, this study did not show any differences in mortality rate between the bradykinin and saline-treated groups after two weeks [56]. Bradykinin administered in rabbits or mice, starting 5 min before reperfusion also reduced infarct size [31,57,58]. Collectively, these data suggest that the protective effect of bradykinin occurs at reperfusion and mimics PC. However, it is yet to be determined whether bradykinin is protective when administered after reperfusion.

Bradykinin mimics IPC and the selective bradykinin B2 receptor antagonist, HOE-140 (incatibant), abolished IPC-induced cardioprotection [59]. Bradykinin use in humans mimics the effects of IPC in patients undergoing percutaneous transluminal coronary angioplasty (PCTA) [60]. It is unknown whether bradykinin receptor inhibitors can abrogate the effects of PC.

One alternative strategy is to target the inhibition of enzymes responsible for the degradation of kinins. These are a family of kinin peptidases, which include angiotensin converting enzyme (ACE), neutral endopeptidase (NEP), kininase I, carboxypeptidase M, and aminopeptidase P [61]. Addition of the ACE inhibitor ramiprilat increased bradykinin concentrations in the perfusate of isolated rat hearts [46]. Inhibition of ACE or NEP prior to reperfusion was also effective in reducing infarct size [57,62], with combined inhibition of ACE or NEP enhancing infarct size reduction [62]. Prior administration of HOE-140 abolished the infarct size sparing effect of the ACE inhibitor ramiprilat [63,64] or the NEP inhibitors [57,62]. These effects were not attributed to angiotensin II [65], since angiotensin II or the angiotensin II antagonist, losartan, did not effect infarct size [64].

Opioids

Myocardial ischemia results in the synthesis and release of endogenous opioid peptides including both Met- and Leu-enkephalin and dynorphins [66]. The highest level of preproenkephalin mRNA is in rat ventricular tissue compared to the other rat organ systems, indicating that the heart may have a very significant endogenous opiate system [67]. Three opioid receptor subtypes, μ,κ and δ have been cloned. In adult ventricular cardiomyocytes, only the κ and δ receptor subtypes were reported [68–71]. Traditionally, μ receptors were reported to be absent in the heart [69], however, a more recent study suggests that this receptor is present within human atrial trabeculae [72]. Previously, administration of Met5-enkephalin or Leu5-enkephalin reduced the incidence of myocardial cell death, which did not occur with administration of β-endorphins that bind primarily to the μ opioid receptor [73]. Collectively, these studies support an important role for enkephalins, and perhaps dynorphins, as an endogenous opioid system responsible for cardioprotection.

Morphine is cardioprotective when administered just prior to reperfusion, with the efficacy of protection equivalent to that observed when morphine is administered prior to ischemia in rats [74]. The protective effect of morphine appears to only be beneficial when administered prior to reperfusion, since morphine administered only 10 s after reperfusion failed to reduce infarct size in rats [3]. In contrast, the selective irreversible δ agonist, fentanyl isothiocyanate (FIT), reduced infarct size equally when given prior to ischemia or reperfusion and this protection, assessed by infarct size reduction, was extended to 10 s after reperfusion [4]. BW373U86, a δ selective opioid agonist, also reduced infarct size when administered 5 min prior to reperfusion [74]. Taken together, these data suggest that opioids elicit or mimic PC in rat hearts.

To further investigate these findings, our laboratory subjected intact rats to 30 min of ischemia and 2 h of reperfusion, and rats were treated with either the selective κ opioid agonist, U50,488, or the selective reversible δ opioid agonist, BW373U86, administered as a single bolus during time points either prior to ischemia, prior to reperfusion or after reperfusion. As shown in Fig. 1, both opioids were able to reduce infarct size, as assessed by TTC staining 2 h after reperfusion, as effectively as when administered either just prior to ischemia or reperfusion. However, the infarct size sparing effects only occurred after reperfusion following administration of the selective δ opioid agonist, BW373U86. These data, in addition to our previous findings with morphine and FIT, would suggest that selective δ opioid agonists have a more extensive therapeutic window to produce PC than κ opioid agonists [3,4,74]. These findings will also need to be confirmed in additional animal species.

Administration of the non-selective opioid agonist morphine [77], the κ selective agonist U50,488 or the selective δ-receptor agonists DADLE, TAN-67 or BW373U86 mimicked the effects of IPC, while μ specific opioid agonists failed to produce cardioprotection [75,76,78]. The δ opioid agonist, D-Ala2-Leu-enkephalin, (DADLE), mimicked the effects of IPC in human atrial trabeculae [79]. Alternatively, the opioid receptor antagonist, naloxone, abrogated IPC-induced cardioprotection in rat and rabbit hearts [76,80]. Both κ and δ selective receptor antagonists also partially abrogated IPC-induced infarct size reduction [78]. Recent preliminary evidence also suggests that PC is mediated through endogenous opioid receptors, since both naloxone and the peripheral acting naloxone derivative, naloxone methiodide, both abrogated the effects of PC [81]. Further studies will be needed to determine the opioid receptor subtypes important in mediating PC.

Erythropoietin

Erythropoietin administered in mice at the time of reperfusion produced a beneficial effect of normalizing LVEDP 1 week after infarction as well as normalizing ventricular wall stress [82], and reduced infarct size in rabbits [83]. Erythropoietin administered 5 min after reperfusion also reduced infarct size equally compared to erythropoietin administered either 2 h before ischemia or at the start of ischemia [84] indicating a PG-like effect. Comparative analysis of erythropoietin administered at either 1000 or 5000 U/kg also showed erythropoietin reduced infarct size when administered either at the time of ischemia or reperfusion [85]. Erythropoietin reduced infarct size when administered just prior to reperfusion in isolated rat hearts and dog hearts [86,87]. Most erythropoietin doses used for these studies ranged between 1000 and 5000 U/kg, however, a lower and perhaps more clinically relevant erythropoietin dose of 100 U/kg reduced infarct size when administered just prior to reperfusion in canine hearts [87].

The erythropoietin receptor is expressed in cardiac myocytes [88,89], however, no studies have examined whether receptor blockade can abrogate the cardioprotective effects of IPC or PC. The ability for erythropoietin to play a direct role in IPC or PC-induced cardioprotection is also unknown.

Adrenergic agents

The contribution of adrenergic receptors in cardioprotection was recently revisited. Adrenergic receptors include both α and β subtypes with α1, β1, β2 and β3 found to be present in cardiomyocytes [90]. Knockout mice deficient in the β2 receptor lacked the ability to be preconditioned by IPC [91]. Over-expression of the β2 receptor in transgenic mice worsened ischemic injury, suggesting chronic upregulation of the β2 receptor may also deleteriously alter cardioprotective signaling pathways [92]. Administration of isoproterenol at the time of reperfusion improved both regional and global cardiac function in canine hearts, however, it failed to reduce infarct size, perhaps due to the fairly long 2 h ischemic period used [93].

Adrenergic receptor blocker administration at reperfusion yielded promising results. The selective β1 receptor antagonists, bisoprolol and metoprolol, were found to produce a significant reduction of infarct size [94,95]. Carvedilol, a nonselective β receptor antagonist, α1 receptor antagonist and free radical scavenger, produced more substantial infarct size reduction compared to other selective β1 receptor antagonists, perhaps due to the free radical scavenging properties of carvedilol.

Cardioprotection also occurs via α1 adrenergic receptor activation, since norepinephrine reduced infarct size to an extent that mimicked IPC [96]. The cardioprotective effects of norepinephrine were also abolished by selective α1 receptor antagonists in rabbits and rats [96,97]. Isoproterenol also mimicked the effects of IPC [91], however, contradictory findings suggest that inhibition of β receptors by atenolol or esmolol abrogate IPC-induced infarct size reduction [98]. Collectively, the importance for the availability of adrenergic receptors during ischemia and reperfusion is still in its infancy, and additional studies are required to investigate the role of adrenergic agents to effectively mimic or block IPC and PC.

Muscarinics

Myocardial interstitial levels of acetylcholine during IPC and during prolonged ischemia in felines were shown to be significantly elevated compared to baseline [99]. The addition of acetylcholine prior to ischemia also mimicked the effects of IPC in canine and rabbit hearts [100–102]. In rats, acetylcholine administered prior to ischemia and continued throughout ischemia and reperfusion reduced infarct size, but not as substantially as IPC. This study also showed that the muscarinic antagonist atropine did not abrogate IPC [103]. However, acetylcholine given 5 min prior to reperfusion for 60 min in isolated perfused rabbit hearts did not reduce infarct size [31]. Hence, the contributions of muscarinics in cardioprotection need further investigation.

Timing and dosing considerations of ligand administration

A direct comparison of the cardioprotective effects at each time of intervention in different animal species, similar to the study shown in Fig. 1, will be needed for each agent to discern whether the maximal efficacy is similar at different time points of administration. Additionally, the ceiling of cardioprotection produced by IPC and PC should warrant further investigation, since it would appear that the ability for ligands to salvage myocardium may be dependent upon the length of index ischemia experienced prior to ligand administration [104]. Characterization of these parameters in animal models would be fruitful in order to further design more effective clinical trials and maximize the efficacy of these ligands in humans. Once these parameters are established, whether a combination of ligands can act in synergy to more effectively reduce infarct size as compared to a single agent administered alone should also be determined.

Secondly, the dosing effects of ligands need more extensive investigation, since a number of ligands, including opioids and adenosine generate an inverse bell shaped dose response curves in relation to infarct size reduction. This would indicate that ligands have a finite dosing window that induces cardioprotection. The dose response curves also suggest that endogenous feedback systems are initiated when an agent is administered, and further studies should target ways to inactivate feedback systems in order to achieve greater ligand efficacy when ligands are given at higher doses.

Receptor cross-talk and importance in ligand-mediated protection

At the receptor level, receptors are somewhat promiscuous, since they can both homodimerize and heterodimerize with different receptor subtypes or receptor classes [105]. With this in mind, a number of studies suggest cross-talk occurs between receptors. For example, antagonism of δ opioid receptors blocks the protective effect of the adenosine A1 receptor agonist, CCPA and in addition, morphine or fentanyl-induced infarct size reduction was abolished by an A1 receptor antagonist [106,107]. The infarct size sparing effects of carvedilol were abrogated by an adenosine receptor antagonist [108]. It also appears that the κ opioid receptor subtypes and β adrenergic receptors share a cross-talk phenomenon [109]. The importance of receptor cross-talk in cardioprotection, particularly at reperfusion, will need further investigation, and is likely to be consistent with the “threshold hypothesis” of IPC previously hypothesized by Downey's laboratory [110].

Ligand-mediated signaling pathways initiated at the time of reperfusion

The molecular pathways involved in acute cardioprotection have been reviewed extensively [7,111–113]. Evidently, the means by which ligands induce protection are unclear and further consideration has to be made as to the cellular components initiated or inhibited during ischemia/reperfusion as well as their contribution to reducing necrosis, apoptosis, endothelial injury and/or microvascular and macrovascular injury.

Presently, evidence indicates that there are at least two molecular signaling pathways, the phosphatidylinositol-3 kinase (PI3k) and mitogen activated kinase pathways, responsible at reperfusion for relaying the ligand-induced cardioprotective effect from the receptor in the myocardium to a downstream end effector, as shown in Fig. 2. These pathways converge to cause inhibition of glycogen synthase kinase-3β (GSK3β) [111,112]. In the myocardium, PI3k has classically been shown to regulate GSK3β inhibition by activation of protein kinase B (Akt). Non-myocardial cell lines have also shown that extracellular regulated kinase (ERK) primes GSK3β to allow for phosphorylation and inhibition at the Ser9 site [114]. GSK3β inhibition then leads to inhibition of the mitochondrial permeability transition pore (MPTP) [115]. Agonists including opioids, adenosine and bradykinin, in addition to IPC, are suggested to initiate cardioprotection via GSK3β and MPTP inhibition, suggesting this pathway may be a common mechanism for how ligands mediate their cardioprotective effect [74,115,116]. Both GSK3β inhibition and MPTP inhibition at the time of reperfusion reduce infarct size and may perhaps be the end effectors of cardioprotection [74,117].

Fig. 2

Proposed signaling components of ligand-induced cardioprotection at the time of reperfusion.

Fig. 2

Proposed signaling components of ligand-induced cardioprotection at the time of reperfusion.

One paradigm that presently seems to exist in cardioprotective signaling is the benefit of reactive oxygen species (ROS) generation. ROS generation was previously found to trigger cardioprotection when acetylcholine, bradykinin, opioids and phenylepherine are administered prior to ischemia, which in turn caused distal signaling events [118,119]. However, since agents such as bradykinin and opioids are cardioprotective when administered at the time of reperfusion, one would question whether these agents generate an initial burst of ROS that activate cardioprotective pathways and protect against the substantial ROS generation that occurs during reperfusion that leads to injury. For δ opioids, the infarct size sparing effect, at least in rats, is equally effective when administered at the time of reperfusion as compared to administration prior to or during ischemia. Therefore, agents that were previously shown to induce protection by an initial ROS burst prior to ischemia/reperfusion need to be re-examined in different animal species that have different free radical scavenging mechanisms, to determine whether ROS can trigger cardioprotective signaling events. Furthermore, whether the kinases altered at reperfusion, such as GSK3β inhibition, reduce the ROS generated at reperfusion that leads to injury needs further investigation.

Summary/conclusion

The resurgence of interest in effectively giving drugs at the time of reperfusion has suggested a promising avenue of research. The different cardioprotective agents discussed have potential as therapeutics to reduce the extent of myocardial infarction. Of importance is the characterization of the therapeutic window for each cardioprotective ligand, dosing efficacy, species dependent effects and whether synergism occurs with a cocktail of agents administered. With this in mind, many questions are unanswered in ligand-mediated cardioprotection and future directions, such as those in Table 2, should be pursued. By these means, we will be one step closer in harnessing the maximal cardioprotective efficacy of ligands that could one day be used as standard intervention for patients presenting with an acute myocardial infarction by using IPC and/or PC.

Table 2

Future directions for ligand-induced cardioprotection research

• What receptor subtypes are important to post-conditioning? 
– Are these receptors important consistent with those required for IPC? 
– Are there different receptor subtypes important in ischemia compared to reperfusion? 
• What are the most efficacious exogenous ligands? 
– Can an optimal cardioprotective cocktail be created and perhaps include IPC or PC? 
– Is the order of ligand administration important? 
• What experimental factors are important for a ligand to achieve maximal efficacy? 
– Dose? 
– Window of administration? 
– Ceiling (duration of ischemia)? 
• Does receptor cross-talk occur between ligands at reperfusion? 
• Is GSK3β/MPTP inhibition common signaling mediators or cardioprotective ligands? 
– Is inhibition connected to modulation of ROS burst at reperfusion? 
– Is this mechanism species dependent? 
• Are these pathways and agents as effective in female species? 
• How do diseases alter the efficacy of ligand-induced protection? 
• What is the role for genetic background in ligand-induced myocardial protection? 
• What receptor subtypes are important to post-conditioning? 
– Are these receptors important consistent with those required for IPC? 
– Are there different receptor subtypes important in ischemia compared to reperfusion? 
• What are the most efficacious exogenous ligands? 
– Can an optimal cardioprotective cocktail be created and perhaps include IPC or PC? 
– Is the order of ligand administration important? 
• What experimental factors are important for a ligand to achieve maximal efficacy? 
– Dose? 
– Window of administration? 
– Ceiling (duration of ischemia)? 
• Does receptor cross-talk occur between ligands at reperfusion? 
• Is GSK3β/MPTP inhibition common signaling mediators or cardioprotective ligands? 
– Is inhibition connected to modulation of ROS burst at reperfusion? 
– Is this mechanism species dependent? 
• Are these pathways and agents as effective in female species? 
• How do diseases alter the efficacy of ligand-induced protection? 
• What is the role for genetic background in ligand-induced myocardial protection? 

Acknowledgements

This work was supported by NIH grants HL 08311 and HL 074314.

References

[1]
Murry
C.E.
Jennings
R.B.
Reimer
K.A.
Preconditioning with ischemia: a delay of lethal cell injury in ischemic myocardium
Circulation
 
1986
74
1124
1136
[2]
Zhao
Z.Q.
Corvera
J.S.
Halkos
M.E.
Kerendi
F.
Wang
N.P.
Guyton
R.A.
et al
Inhibition of myocardial injury by ischemic postconditioning during reperfusion: comparison with ischemic preconditioning
Am J Physiol Heart Circ Physiol
 
2003
285
H579
H588
[3]
Gross
E.R.
Hsu
A.K.
Gross
G.J.
Acute aspirin treatment abolishes, whereas acute ibuprofen treatment enhances morphine-induced cardioprotection: role of 12-lipoxygenase
J Pharmacol Exp Ther
 
2004
310
185
191
[4]
Gross
E.R.
Peart
J.N.
Hsu
A.K.
Auchampach
J.A.
Gross
G.J.
Extending the cardioprotective window using a novel delta-opioid agonist fentanyl isothiocyanate via the PI3-kinase pathway
Am J Physiol Heart Circ Physiol
 
2005
288
H2744
H2749
[5]
Piper
H.M.
Abdallah
Y.
Schafer
C.
The first minutes of reperfusion: a window of opportunity for cardioprotection
Cardiovasc Res
 
2004
61
365
371
[6]
Vinten-Johansen
J.
Zhao
Z.Q.
Zatta
A.J.
Kin
H.
Halkos
M.E.
Kerendi
F.
Postconditioning a new link in nature's armor against myocardial ischemia–reperfusion injury
Basic Res Cardiol
 
2005
100
295
310
[7]
Yellon
D.M.
Downey
J.M.
Preconditioning the myocardium: from cellular physiology to clinical cardiology
Physiol Rev
 
2003
83
1113
1151
[8]
Headrick
J.P.
Ischemic preconditioning: bioenergetic and metabolic changes and the role of endogenous adenosine
J Mol Cell Cardiol
 
1996
28
1227
1240
[9]
Van Wylen
D.G.
Effect of ischemic preconditioning on interstitial purine metabolite and lactate accumulation during myocardial ischemia
Circulation
 
1994
89
2283
2289
[10]
Van Wylen
D.G.
Schmit
T.J.
Lasley
R.D.
Gingell
R.L.
Mentzer
R.M.
Jr.
Cardiac microdialysis in isolated rat hearts: interstitial purine metabolites during ischemia
Am J Physiol
 
1992
262
H1934
H1938
[11]
Black
R.G.
Jr.
Guo
Y.
Ge
Z.D.
Murphree
S.S.
Prabhu
S.D.
Jones
W.K.
et al
Gene dosage-dependent effects of cardiac-specific overexpression of the A3 adenosine receptor
Circ Res
 
2002
91
165
172
[12]
Reichelt
M.E.
Willems
L.
Molina
J.G.
Sun
C.X.
Noble
J.C.
Ashton
K.J.
et al
Genetic deletion of the A1 adenosine receptor limits myocardial ischemic tolerance
Circ Res
 
2005
96
363
367
[13]
Guo
Y.
Bolli
R.
Bao
W.
Wu
W.J.
Black
R.G.
Jr.
Murphree
S.S.
et al
Targeted deletion of the A3 adenosine receptor confers resistance to myocardial ischemic injury and does not prevent early preconditioning
J Mol Cell Cardiol
 
2001
33
825
830
[14]
Lee
Y.M.
Sheu
J.R.
Yen
M.H.
BN-063, a newly synthesized adenosine A1 receptor agonist, attenuates myocardial reperfusion injury in rats
Eur J Pharmacol
 
1995
279
251
256
[15]
Smits
G.J.
McVey
M.
Cox
B.F.
Perrone
M.H.
Clark
K.L.
Cardioprotective effects of the novel adenosine A1/A2 receptor agonist AMP 579 in a porcine model of myocardial infarction
J Pharmacol Exp Ther
 
1998
286
611
618
[16]
Peart
J.
Headrick
J.P.
Intrinsic, A(1) adenosine receptor activation during ischemia or reperfusion improves recovery in mouse hearts
Am J Physiol Heart Circ Physiol
 
2000
279
H2166
H2175
[17]
Budde
J.M.
Velez
D.A.
Zhao
Z.
Clark
K.L.
Morris
C.D.
Muraki
S.
et al
Comparative study of AMP 579 and adenosine in inhibition of neutrophil-mediated vascular and myocardial injury during 24 h of reperfusion
Cardiovasc Res
 
2000
47
294
305
[18]
Norton
E.D.
Jackson
E.K.
Turner
M.B.
Virmani
R.
Forman
M.B.
The effects of intravenous infusions of selective adenosine A1-receptor and A2-receptor agonists on myocardial reperfusion injury
Am Heart J
 
1992
123
332
338
[19]
Norton
E.D.
Jackson
E.K.
Virmani
R.
Forman
M.B.
Effect of intravenous adenosine on myocardial reperfusion injury in a model with low myocardial collateral blood flow
Am Heart J
 
1991
122
1283
1291
[20]
Olafsson
B.
Forman
M.B.
Puett
D.W.
Pou
A.
Cates
C.U.
Friesinger
G.C.
et al
Reduction of reperfusion injury in the canine preparation by intracoronary adenosine: importance of the endothelium and the no-reflow phenomenon
Circulation
 
1987
76
1135
1145
[21]
Pitarys
C.J.
Virmani
R.
Vildibill
H.D.
Jr.
Jackson
E.K.
Forman
M.B.
Reduction of myocardial reperfusion injury by intravenous adenosine administered during the early reperfusion period
Circulation
 
1991
83
237
247
[22]
Velasco
C.E.
Turner
M.
Cobb
M.A.
Virmani
R.
Forman
M.B.
Myocardial reperfusion injury in the canine model after 40 minutes of ischemia: effect of intracoronary adenosine
Am Heart J
 
1991
122
1561
1570
[23]
Jordan
J.E.
Zhao
Z.Q.
Sato
H.
Taft
S.
Vinten-Johansen
J.
Adenosine A2 receptor activation attenuates reperfusion injury by inhibiting neutrophil accumulation, superoxide generation and coronary endothelial adherence
J Pharmacol Exp Ther
 
1997
280
301
309
[24]
Goto
M.
Miura
T.
Iliodoromitis
E.K.
Leary
O'E.L.
Ishimoto
R.
Yellon
D.M.
et al
Adenosine infusion during early reperfusion failed to limit myocardial infarct size in a collateral deficient species
Cardiovasc Res
 
1991
25
943
949
[25]
Homeister
J.W.
Hoff
P.T.
Fletcher
D.D.
Lucchesi
B.R.
Combined adenosine and lidocaine administration limits myocardial reperfusion injury
Circulation
 
1990
82
595
608
[26]
Heide
Vander R.S.
Reimer
K.A.
Effect of adenosine therapy at reperfusion on myocardial infarct size in dogs
Cardiovasc Res
 
1996
31
711
718
[27]
Xu
Z.
Downey
J.M.
Cohen
M.V.
Timing and duration of administration are crucial for antiinfarct effect of AMP 579 infused at reperfusion in rabbit heart
Heart Dis
 
2003
5
368
371
[28]
Louttit
J.B.
Hunt
A.A.
Maxwell
M.P.
Drew
G.M.
The time course of cardioprotection induced by GR79236, a selective adenosine A1-receptor agonist, in myocardial ischaemia–reperfusion injury in the pig
J Cardiovasc Pharmacol
 
1999
33
285
291
[29]
Baxter
G.F.
Hale
S.L.
Miki
T.
Kloner
R.A.
Cohen
M.V.
Downey
J.M.
et al
Adenosine A1 agonist at reperfusion trial (AART): results of a three-center, blinded, randomized, controlled experimental infarct study
Cardiovasc Drugs Ther
 
2000
14
607
614
[30]
Xu
Z.
Downey
J.M.
Cohen
M.V.
Amp 579 reduces contracture and limits infarction in rabbit heart by activating adenosine A2 receptors
J Cardiovasc Pharmacol
 
2001
38
474
481
[31]
Yang
X.M.
Krieg
T.
Cui
L.
Downey
J.M.
Cohen
M.V.
NECA and bradykinin at reperfusion reduce infarction in rabbit hearts by signaling through PI3K, ERK, and NO
J Mol Cell Cardiol
 
2004
36
411
421
[32]
Xu
Z.
Yang
X.M.
Cohen
M.V.
Neumann
T.
Heusch
G.
Downey
J.M.
Limitation of infarct size in rabbit hearts by the novel adenosine receptor agonist AMP 579 administered at reperfusion
J Mol Cell Cardiol
 
2000
32
2339
2347
[33]
Kis
A.
Baxter
G.F.
Yellon
D.M.
Limitation of myocardial reperfusion injury by AMP579, an adenosine A1/A2A receptor agonist: role of A2A receptor and Erk1/2
Cardiovasc Drugs Ther
 
2003
17
415
425
[34]
Lasley
R.D.
Jahania
M.S.
Mentzer
R.M.
Jr.
Beneficial effects of adenosine A(2a) agonist CGS-21680 in infarcted and stunned porcine myocardium
Am J Physiol Heart Circ Physiol
 
2001
280
H1660
H1666
[35]
Schlack
W.
Schafer
M.
Uebing
A.
Schafer
S.
Borchard
U.
Thamer
V.
Adenosine A2-receptor activation at reperfusion reduces infarct size and improves myocardial wall function in dog heart
J Cardiovasc Pharmacol
 
1993
22
89
96
[36]
Maddock
H.L.
Mocanu
M.M.
Yellon
D.M.
Adenosine A(3) receptor activation protects the myocardium from reperfusion/reoxygenation injury
Am J Physiol Heart Circ Physiol
 
2002
283
H1307
H1313
[37]
Auchampach
J.A.
Ge
Z.D.
Wan
T.C.
Moore
J.
Gross
G.J.
A3 adenosine receptor agonist IB-MECA reduces myocardial ischemia–reperfusion injury in dogs
Am J Physiol Heart Circ Physiol
 
2003
285
H607
H613
[38]
Cleveland
J.C.
Jr.
Meldrum
D.R.
Rowland
R.T.
Banerjee
A.
Harken
A.H.
Adenosine preconditioning of human myocardium is dependent upon the ATP-sensitive K+ channel
J Mol Cell Cardiol
 
1997
29
175
182
[39]
Liu
G.S.
Richards
S.C.
Olsson
R.A.
Mullane
K.
Walsh
R.S.
Downey
J.M.
Evidence that the adenosine A3 receptor may mediate the protection afforded by preconditioning in the isolated rabbit heart
Cardiovasc Res
 
1994
28
1057
1061
[40]
Liu
G.S.
Thornton
J.
Van Winkle
D.M.
Stanley
A.W.
Olsson
R.A.
Downey
J.M.
Protection against infarction afforded by preconditioning is mediated by A1 adenosine receptors in rabbit heart
Circulation
 
1991
84
350
356
[41]
Tsuchida
A.
Miura
T.
Miki
T.
Shimamoto
K.
Iimura
O.
Role of adenosine receptor activation in myocardial infarct size limitation by ischaemic preconditioning
Cardiovasc Res
 
1992
26
456
461
[42]
Auchampach
J.A.
Jin
X.
Moore
J.
Wan
T.C.
Kreckler
L.M.
Ge
Z.D.
et al
Comparison of three different A1 adenosine receptor antagonists on infarct size and multiple cycle ischemic preconditioning in anesthetized dogs
J Pharmacol Exp Ther
 
2004
308
846
856
[43]
Solenkova
N.V.
Cohen
M.V.
Downey
J.M.
Ischemic preconditioning protects the heart through adenosine receptor activation during reperfusion
J Mol Cell Cardiol
 
2005
848
[44]
Yang
X.M.
Philipp
S.
Downey
J.M.
Cohen
M.V.
Postconditioning's protection is not dependent on circulating blood factors or cells but involves adenosine receptors and requires PI3-kinase and guanylyl cyclase activation
Basic Res Cardiol
 
2005
100
57
63
[45]
Kin
H.
Zatta
A.J.
Lofye
M.T.
Amerson
B.S.
Halkos
M.E.
Kerendi
F.
et al
Postconditioning reduces infarct size via adenosine receptor activation by endogenous adenosine
Cardiovasc Res
 
2005
67
124
133
[46]
Baumgarten
C.R.
Linz
W.
Kunkel
G.
Scholkens
B.A.
Wiemer
G.
Ramiprilat increases bradykinin outflow from isolated hearts of rat
Br J Pharmacol
 
1993
108
293
295
[47]
Pan
H.L.
Chen
S.R.
Scicli
G.M.
Carretero
O.A.
Cardiac interstitial bradykinin release during ischemia is enhanced by ischemic preconditioning
Am J Physiol Heart Circ Physiol
 
2000
279
H116
H121
[48]
Schulz
R.
Post
H.
Vahlhaus
C.
Heusch
G.
Ischemic preconditioning in pigs: a graded phenomenon: its relation to adenosine and bradykinin
Circulation
 
1998
98
1022
1029
[49]
Bhoola
K.D.
Elson
C.J.
Dieppe
P.A.
Kinins-key mediators in inflammatory arthritis?
Br J Rheumatol
 
1992
31
509
518
[50]
Marceau
F.
Hess
J.F.
Bachvarov
D.R.
The B1 receptors for kinins
Pharmacol Rev
 
1998
50
357
386
[51]
Tschope
C.
Heringer-Walther
S.
Koch
M.
Spillmann
F.
Wendorf
M.
Hauke
D.
et al
Myocardial bradykinin B2-receptor expression at different time points after induction of myocardial infarction
J Hyperten
 
2000
18
223
228
[52]
Tschope
C.
Koch
M.
Spillmann
F.
Heringer-Walther
S.
Mochmann
H.C.
Stauss
H.
et al
Upregulation of the cardiac bradykinin B2 receptors after myocardial infarction
Immunopharmacology
 
1999
44
111
117
[53]
Yang
X.P.
Liu
Y.H.
Scicli
G.M.
Webb
C.R.
Carretero
O.A.
Role of kinins in the cardioprotective effect of preconditioning: study of myocardial ischemia/reperfusion injury in B2 kinin receptor knockout mice and kininogen-deficient rats
Hypertension
 
1997
30
735
740
[54]
Xu
J.
Carretero
O.A.
Sun
Y.
Shesely
E.G.
Rhaleb
N.E.
Liu
Y.H.
et al
Role of the B1 kinin receptor in the regulation of cardiac function and remodeling after myocardial infarction
Hypertension
 
2005
45
747
753
[55]
Tio
R.A.
Tobe
T.J.
Bel
K.J.
Langen
de C.D.
van Gilst
W.H.
Wesseling
H.
Beneficial effects of bradykinin on porcine ischemic myocardium
Basic Res Cardiol
 
1991
86
107
116
[56]
Tobe
T.J.
de Langen
C.D.
Tio
R.A.
Bel
K.J.
Mook
P.H.
Wesseling
H.
In vivo effect of bradykinin during ischemia and reperfusion: improved electrical stability two weeks after myocardial infarction in the pig
J Cardiovasc Pharmacol
 
1991
17
600
607
[57]
Yang
X.P.
Liu
Y.H.
Peterson
E.
Carretero
O.A.
Effect of neutral endopeptidase 24.11 inhibition on myocardial ischemia/reperfusion injury: the role of kinins
J Cardiovasc Pharmacol
 
1997
29
250
256
[58]
Bell
R.M.
Yellon
D.M.
Bradykinin limits infarction when administered as an adjunct to reperfusion in mouse heart: the role of PI3K, Akt and eNOS
J Mol Cell Cardiol
 
2003
35
185
193
[59]
Wall
T.M.
Sheehy
R.
Hartman
J.C.
Role of bradykinin in myocardial preconditioning
J Pharmacol Exp Ther
 
1994
270
681
689
[60]
Leesar
M.A.
Stoddard
M.F.
Manchikalapudi
S.
Bolli
R.
Bradykinin-induced preconditioning in patients undergoing coronary angioplasty
J Am Coll Cardiol
 
1999
34
639
650
[61]
Baxter
G.F.
Ebrahim
Z.
Role of bradykinin in preconditioning and protection of the ischaemic myocardium
Br J Pharmacol
 
2002
135
843
854
[62]
Schriefer
J.A.
Broudy
E.P.
Hassen
A.H.
Endopeptidase inhibitors decrease myocardial ischemia/reperfusion injury in an in vivo rabbit model
J Pharmacol Exp Ther
 
1996
278
1034
1039
[63]
Hartman
J.C.
Wall
T.M.
Hullinger
T.G.
Shebuski
R.J.
Reduction of myocardial infarct size in rabbits by ramiprilat: reversal by the bradykinin antagonist HOE 140
J Cardiovasc Pharmacol
 
1993
21
996
1003
[64]
Liu
Y.H.
Yang
X.P.
Sharov
V.G.
Sigmon
D.H.
Sabbath
H.N.
Carretero
O.A.
Paracrine systems in the cardioprotective effect of angiotensin-converting enzyme inhibitors on myocardial ischemia/reperfusion injury in rats
Hypertension
 
1996
27
7
13
[65]
Hartman
J.C.
Hullinger
T.G.
Wall
T.M.
Shebuski
R.J.
Reduction of myocardial infarct size by ramiprilat is independent of angiotensin II synthesis inhibition
Eur J Pharmacol
 
1993
234
229
236
[66]
Romano
M.A.
Seymour
E.M.
Berry
J.A.
Nish
McR.A.
Bolling
S.F.
Relative contribution of endogenous opioids to myocardial ischemic tolerance
J Surg Res
 
2004
118
32
37
[67]
Howells
R.D.
Kilpatrick
D.L.
Bailey
L.C.
Noe
M.
Udenfriend
S.
Proenkephalin mRNA in rat heart
Proc Natl Acad Sci U S A
 
1986
83
1960
1963
[68]
Krumins
S.A.
Faden
A.I.
Feuerstein
G.
Opiate binding in rat hearts: modulation of binding after hemorrhagic shock
Biochem Biophys Res Commun
 
1985
127
120
128
[69]
Ventura
C.
Bastagli
L.
Bernardi
P.
Caldarera
C.M.
Guarnieri
C.
Opioid receptors in rat cardiac sarcolemma: effect of phenylephrine and isoproterenol
Biochim Biophys Acta
 
1989
987
69
74
[70]
Wittert
G.
Hope
P.
Pyle
D.
Tissue distribution of opioid receptor gene expression in the rat
Biochem Biophys Res Commun
 
1996
218
877
881
[71]
Zhang
W.M.
Jin
W.Q.
Wong
T.M.
Multiplicity of kappa opioid receptor binding in the rat cardiac sarcolemma
J Mol Cell Cardiol
 
1996
28
1547
1554
[72]
Bell
S.P.
Sack
M.N.
Patel
A.
Opie
L.H.
Yellon
D.M.
Delta opioid receptor stimulation mimics ischemic preconditioning in human heart muscle
J Am Coll Cardiol
 
2000
36
2296
2302
[73]
Takasaki
Y.
Wolff
R.A.
Chien
G.L.
van Winkle
D.M.
Met5-enkephalin protects isolated adult rabbit cardiomyocytes via delta-opioid receptors
Am J Physiol
 
1999
277
H2442
H2450
[74]
Gross
E.R.
Hsu
A.K.
Gross
G.J.
Opioid-induced cardioprotection occurs via glycogen synthase kinase beta inhibition during reperfusion in intact rat hearts
Circ Res
 
2004
94
960
966
[75]
Peart
J.N.
Patel
H.H.
Gross
G.J.
Delta-opioid receptor activation mimics ischemic preconditioning in the canine heart
J Cardiovasc Pharmacol
 
2003
42
78
81
[76]
Schultz
J.E.
Hsu
A.K.
Gross
G.J.
Morphine mimics the cardioprotective effect of ischemic preconditioning via a glibenclamide-sensitive mechanism in the rat heart
Circ Res
 
1996
78
1100
1104
[77]
Schultz
J.J.
Hsu
A.K.
Gross
G.J.
Ischemic preconditioning and morphine-induced cardioprotection involve the delta (delta)-opioid receptor in the intact rat heart
J Mol Cell Cardiol
 
1997
29
2187
2195
[78]
Wang
G.Y.
Wu
S.
Pei
J.M.
Yu
X.C.
Wong
T.M.
Kappa- but not delta-opioid receptors mediate effects of ischemic preconditioning on both infarct and arrhythmia in rats
Am J Physiol Heart Circ Physiol
 
2001
280
H384
H391
[79]
Tomai
F.
Crea
F.
Gaspardone
A.
Versaci
F.
Ghini
A.S.
Ferri
C.
et al
Effects of naloxone on myocardial ischemic preconditioning in humans
J Am Coll Cardiol
 
1999
33
1863
1869
[80]
Chien
G.L.
Van Winkle
D.M.
Naloxone blockade of myocardial ischemic preconditioning is stereoselective
J Mol Cell Cardiol
 
1996
28
1895
1900
[81]
Kin
H.
Zatta
A.J.
Jiang
R.
Reeves
J.G.
Mykytenko
J.
Sorescu
G.
et al
Activation of opioid receptors mediates the infarct size reduction by postconditioning
J Mol Cell Cardiol
 
2005
38
827
[82]
Namiuchi
S.
Kagaya
Y.
Ohta
J.
Shiba
N.
Sugi
M.
Oikawa
M.
et al
High serum erythropoietin level is associated with smaller infarct size in patients with acute myocardial infarction who undergo successful primary percutaneous coronary intervention
J Am Coll Cardiol
 
2005
45
1406
1412
[83]
Calvillo
L.
Latini
R.
Kajstura
J.
Leri
A.
Anversa
P.
Ghezzi
P.
et al
Recombinant human erythropoietin protects the myocardium from ischemia–reperfusion injury and promotes beneficial remodeling
Proc Natl Acad Sci U S A
 
2003
100
4802
4806
[84]
Parsa
C.J.
Matsumoto
A.
Kim
J.
Riel
R.U.
Pascal
L.S.
Walton
G.B.
et al
A novel protective effect of erythropoietin in the infarcted heart
J Clin Invest
 
2003
112
999
1007
[85]
Lipsic
E.
van der Meer
P.
Henning
R.H.
Suurmeijer
A.J.
Boddeus
K.M.
vanVeldhuisen
D.J.
et al
Timing of erythropoietin treatment for cardioprotection in ischemia/reperfusion
J Cardiovasc Pharmacol
 
2004
44
473
479
[86]
Parsa
C.J.
Kim
J.
Riel
R.U.
Pascal
L.S.
Thompson
R.B.
Petrofski
J.A.
et al
Cardioprotective effects of erythropoietin in the reperfused ischemic heart: a potential role for cardiac fibroblasts
J Biol Chem
 
2004
279
20655
20662
[87]
Bullard
A.J.
Govewalla
P.
Yellon
D.M.
Erythropoietin protects the myocardium against reperfusion injury in vitro and in vivo
Basic Res Cardiol
 
2005
[88]
Hirata
A.
Minamino
T.
Asanuma
H.
Sanada
S.
Fujita
M.
Tsukamoto
O.
et al
Erythropoietin just before reperfusion reduces both lethal arrhythmias and infarct size via the phosphatidylinositol-3 kinase-dependent pathway in canine hearts
Cardiovasc Drugs Ther
 
2005
19
33
40
[89]
Tramontano
A.F.
Muniyappa
R.
Black
A.D.
Blendea
M.C.
Cohen
I.
Deng
L.
et al
Erythropoietin protects cardiac myocytes from hypoxia-induced apoptosis through an Akt-dependent pathway
Biochem Biophys Res Commun
 
2003
308
990
994
[90]
Wright
G.L.
Hanlon
P.
Amin
K.
Steenbergen
C.
Murphy
E.
Arcasoy
M.O.
Erythropoietin receptor expression in adult rat cardiomyocytes is associated with an acute cardioprotective effect for recombinant erythropoietin during ischemia–reperfusion injury
FASEB J
 
2004
18
1031
1033
[91]
Broadley
K.J.
Penson
P.E.
The roles of alpha- and beta-adrenoceptor stimulation in myocardial ischaemia
Auton Autacoid Pharmacol
 
2004
24
87
93
[92]
Tong
H.
Bernstein
D.
Murphy
E.
Steenbergen
C.
The role of beta-adrenergic receptor signaling in cardioprotection
FASEB J
 
2005
19
983
985
[93]
Cross
H.R.
Steenbergen
C.
Lefkowitz
R.J.
Koch
W.J.
Murphy
E.
Overexpression of the cardiac beta(2)-adrenergic receptor and expression of a beta-adrenergic receptor kinase-1 (betaARK1) inhibitor both increase myocardial contractility but have differential effects on susceptibility to ischemic injury
Circ Res
 
1999
85
1077
1184
[94]
LaBruno
S.
Naim
K.L.
Li
J.K.
Drzewiecki
G.
Kedem
J.
Beta-adrenergic stimulation of reperfused myocardium after 2-hour ischemia
J Cardiovasc Pharmacol
 
1998
32
535
542
[95]
Feuerstein
G.
Liu
G.L.
Yue
T.L.
Cheng
H.Y.
Hieble
J.P.
Arch
J.R.
et al
Comparison of metoprolol and carvedilol pharmacology and cardioprotection in rabbit ischemia and reperfusion model
Eur J Pharmacol
 
1998
351
341
350
[96]
Gao
F.
Chen
J.
Lopez
B.L.
Christopher
T.A.
Gu
J.
Lysko
P.
et al
Comparison of bisoprolol and carvedilol cardioprotection in a rabbit ischemia and reperfusion model
Eur J Pharmacol
 
2000
406
109
116
[97]
Bankwala
Z.
Hale
S.L.
Kloner
R.A.
Alpha-adrenoceptor stimulation with exogenous norepinephrine or release of endogenous catecholamines mimics ischemic preconditioning
Circulation
 
1994
90
1023
1028
[98]
Banerjee
A.
Locke-Winter
C.
Rogers
C.
Mitchell
K.B.
Brew
M.B.
Cairns
E.C.
et al
Preconditioning against myocardial dysfunction after ischemia and reperfusion by an alpha 1-adrenergic mechanism
Circ Res
 
1993
73
656
670
[99]
Iliodromitis
E.K.
Tasouli
A.
Andreadou
I.
Bofilis
E.
Zoga
A.
Cokkinos
P.
et al
Intravenous atenolol and esmolol maintain the protective effect of ischemic preconditioning in vivo
Eur J Pharmacol
 
2004
499
163
169
[100]
Kawada
T.
Yamazaki
T.
Akiyama
T.
Mori
H.
Inagaki
M.
Shishido
T.
et al
Effects of brief ischaemia on myocardial acetylcholine and noradrenaline levels in anaesthetized cats
Auton Neurosci
 
2002
95
37
42
[101]
Yao
Z.
Gross
G.J.
Acetylcholine mimics ischemic preconditioning via a glibenclamide-sensitive mechanism in dogs
Am J Physiol
 
1993
264
H2221
H2225
[102]
Yao
Z.
Gross
G.J.
Role of nitric oxide, muscarinic receptors, and the ATP-sensitive K+ channel in mediating the effects of acetylcholine to mimic preconditioning in dogs
Circ Res
 
1993
73
1193
1201
[103]
Thornton
J.D.
Thornton
C.S.
Downey
J.M.
Effect of adenosine receptor blockade: preventing protective preconditioning depends on time of initiation
Am J Physiol
 
1993
265
H504
H508
[104]
Richard
V.
Blanc
T.
Kaeffer
N.
Tron
C.
Thuillez
C.
Myocardial and coronary endothelial protective effects of acetylcholine after myocardial ischaemia and reperfusion in rats: role of nitric oxide
Br J Pharmacol
 
1995
115
1532
1538
[105]
Gumina
R.J.
Buerger
E.
Eickmeier
C.
Moore
J.
Daemmgen
J.
Gross
G.J.
Inhibition of the Na(+)/H(+) exchanger confers greater cardioprotection against 90 minutes of myocardial ischemia than ischemic preconditioning in dogs
Circulation
 
1999
100
2519
2526
[discussion 2469-72]
[106]
Breitwieser
G.E.
G protein-coupled receptor oligomerization: implications for G protein activation and cell signaling
Circ Res
 
2004
94
17
27
[107]
Kato
R.
Foex
P.
Fentanyl reduces infarction but not stunning via delta-opioid receptors and protein kinase C in rats
Br J Anaesth
 
2000
84
608
614
[108]
Peart
J.N.
Gross
G.J.
Adenosine and opioid receptor-mediated cardioprotection in the rat: evidence for cross-talk between receptors
Am J Physiol Heart Circ Physiol
 
2003
285
H81
H89
[109]
Asanuma
H.
Minamino
T.
Sanada
S.
Takashima
S.
Ogita
H.
Ogai
A.
et al
Beta-adrenoceptor blocker carvedilol provides cardioprotection via an adenosine-dependent mechanism in ischemic canine hearts
Circulation
 
2004
109
2773
2779
[110]
Shan
J.
Yu
X.C.
Fung
M.L.
Wong
T.M.
Attenuated “cross talk” between kappa-opioid receptors and beta-adrenoceptors in the heart of chronically hypoxic rats
Pflugers Arch
 
2002
444
126
132
[111]
Cohen
M.V.
Baines
C.P.
Downey
J.M.
Ischemic preconditioning: from adenosine receptor to KATP channel
Annu Rev Physiol
 
2000
62
79
109
[112]
Hausenloy
D.J.
Tsang
A.
Yellon
D.M.
The reperfusion injury salvage kinase pathway: a common target for both ischemic preconditioning and postconditioning
Trends Cardiovasc Med
 
2005
15
69
75
[113]
Hausenloy
D.J.
Yellon
D.M.
New directions for protecting the heart against ischaemia–reperfusion injury: targeting the Reperfusion Injury Salvage Kinase (RISK)-pathway
Cardiovasc Res
 
2004
61
448
460
[114]
Murphy
E.
Steenbergen
C.
Inhibition of GSK-3beta as a target for cardioprotection: the importance of timing, location, duration and degree of inhibition
Expert Opin Ther Targets
 
2005
9
447
456
[115]
Ding
Q.
Xia
W.
Liu
J.C.
Yang
J.Y.
Lee
D.F.
Xia
J.
et al
Erk associates with and primes GSK-3beta for its inactivation resulting upregulation of beta-catenin
Mol Cell
 
2005
19
159
170
[116]
Juhaszova
M.
Zorov
D.B.
Kim
S.H.
Pepe
S.
Fu
Q.
Fishbein
K.W.
et al
Glycogen synthase kinase-3beta mediates convergence of protection signaling to inhibit the mitochondrial permeability transition pore
J Clin Invest
 
2004
113
1535
1549
[117]
Tong
H.
Imahashi
K.
Steenbergen
C.
Murphy
E.
Phosphorylation of glycogen synthase kinase-3beta during preconditioning through a phosphatidylinositol-3-kinase-dependent pathway is cardioprotective
Circ Res
 
2002
90
377
379
[118]
Hausenloy
D.J.
Duchen
M.R.
Yellon
D.M.
Inhibiting mitochondrial permeability transition pore opening at reperfusion protects against ischaemia–reperfusion injury
Cardiovasc Res
 
2003
60
617
625
[119]
Cohen
M.V.
Yang
X.M.
Liu
G.S.
Heusch
G.
Downey
J.M.
Acetylcholine, bradykinin, opioids, and phenylephrine, but not adenosine, trigger preconditioning by generating free radicals and opening mitochondrial K(ATP) channels
Circ Res
 
2001
89
273
278

Author notes

Time for primary review 18 days