Effect of adenosine A2 receptor stimulation on platelet activation-aggregation: differences between canine and human models

INTRODUCTION

Adenosine A(2) agonists improve arterial patency in experimental models of recurrent thrombosis, an effect purportedly triggered by stimulation of platelet A(2) receptors and subsequent down-regulation of platelet function. However: (i) there is no direct evidence to substantiate this premise; and (ii) given the recognized differences among species in platelet signaling, it is possible that the mechanisms of A(2) receptor stimulation may be model-dependent. Accordingly, we applied an integrated in vivo and in vitro approach, using both canine and human models, to test the hypothesis that the anti-thrombotic effects of A(2) agonist treatment are due in part to inhibition of platelet activation.

METHODS

In Protocol 1, recurrent coronary thrombosis was triggered in anesthetized dogs by application of a stenosis at a site of arterial injury. Coronary patency and flow cytometric indices of platelet activation (P-selectin expression; formation of heterotypic aggregates) were compared in dogs pre-treated with the A(2) agonist CGS 21680 versus controls. In Protocols 2 and 3, blood samples were obtained from dogs and human volunteers. In vitro aggregation and platelet activation (assessed by impedance aggregometry and flow cytometry, respectively) were quantified in paired aliquots pre-incubated with CGS versus vehicle.

RESULTS

In the canine models, CGS improved in vivo coronary patency and attenuated in vitro aggregation but, contrary to our hypothesis, did not evoke a down-regulation in platelet activation. In contrast, in human blood samples, CGS attenuated both in vitro aggregation and flow cytometric markers of platelet activation-aggregation.

CONCLUSION

The mechanisms contributing to the anti-thrombotic effect of A(2) agonist treatment are species-dependent: adenosine A(2) receptor stimulation inhibits platelet activation in human, but not canine, models.

Ischemic Preconditioning: Implications for the Geriatric Heart

Ischemic preconditioning is among the most consistent and powerful modes of reducing myocardial infarct size. Although several clinical studies have suggested that the human heart can be preconditioned, controversy exists in both the experimental and clinical literature as to whether the senescent heart can be preconditioned. The authors recently reported that older patients (> or = 60 years of age) in the Thrombolysis in Myocardial Infarction-4 study appeared to benefit from a history of angina prior to acute myocardial infarction. This observation may lead to a clinical counterpart to successful preconditioning in the older heart.

Adaptation of a photochemical method to initiate recurrent platelet-mediated thrombosis in small animals

Platelet-mediated thrombosis represents the initial precipitating event in the genesis of unstable angina, acute myocardial infarction, and stroke. As a result, there is considerable interest in the preclinical discovery and screening of new 'anti-platelet' therapies aimed at limiting the incidence and reoccurrence of arterial thrombosis-efforts that, to date, have largely required the use of large animal models of thrombotic occlusion. In the current report, we describe the successful development of a small-animal (rat) model of spontaneous and recurrent platelet-mediated arterial thrombosis achieved by the in vivo administration of a photoactive dye (rose bengal) followed by focal illumination with green laser light.

Indices of platelet activation and the stability of coronary artery disease

AIM

To determine whether indices of platelet activation are associated with the stability of coronary artery disease (CAD).

METHODS

Platelet function was examined in 677 consecutive aspirin-treated patients presenting for cardiac catheterization. Patients were grouped into recent myocardial infarction (MI), no MI but angiographically documented CAD (non-MI CAD) and no angiographically detectible CAD (no CAD), as well as additional subgroups.

RESULTS

Compared with non-MI CAD or no CAD patients, more patients with recent MI had a shortened platelet function analyzer (PFA)-100 collagen-epinephrine closure time (CT) and increased circulating monocyte-platelet aggregates, neutrophil-platelet aggregates, activated platelet surface GPIIb-IIIa and plasma soluble CD40 ligand (sCD40L). More patients with non-MI CAD had shortened PFA-100 CTs and increased monocyte-platelet aggregates compared with patients with no CAD. Platelet surface P-selectin did not differ among the groups. Subgroup analysis revealed that decreasing PFA-100 CT correlated with the stability of CAD.

CONCLUSIONS

Indices of platelet activation, especially the PFA-100 CT, are associated with the stability of CAD, and may reflect plaque instability, an ongoing thrombotic state and/or reduced responsiveness to aspirin.

’Postconditioning’ the human heart: Multiple balloon inflations during primary angioplasty may confer cardioprotection

Growing evidence from experimental models suggests that relief of myocardial ischemia in a stuttering manner (i.e., 'postconditioning' [PostC] with brief cycles of reperfusion-reocclusion) limits infarct size. However, the potential clinical efficacy of PostC has, to date,been largely unexplored. Using a retrospective study design, our aim was to test the hypothesis that creatine kinase release (CK: clinical surrogate of infarct size) would be attenuated in ST-segment elevation myocardial infarction (STEMI) patients requiring multiple balloon inflations-deflations during primary angioplasty versus STEMI patients who received minimal balloon inflations and/or direct stenting. To investigate this concept, we reviewed the records of all STEMI patients with single vessel occlusion who presented to our institution from November 2004 - April 2006 for primary angioplasty. Exclusion criteria were: previous MI, cardiogenic shock, patients resuscitated from cardiac arrest, or pre-infarct angina. Patients were prospectively divided into two subsets: those receiving 1-3 balloon inflations (considered the minimum range to achieve patency and stent placement) versus those in whom 4 or more inflations were applied. Peak CK release was significantly lower in patients requiring > or =4 versus 1-3 inflations (1655 versus 2272 IU/L; p<0.05), an outcome consistent with the concept that relief of sustained ischemia in a stuttered manner (analogous to postconditioning) may evoke cardioprotection in the clinical setting.

Preconditioning ischemia attenuates molecular indices of platelet activation-aggregation

BACKGROUND

Previous studies have shown that ischemic preconditioning (PC) not only limits infarct size, but also improves arterial patency in models of recurrent thrombosis. We hypothesize that this enhanced patency is presumably because of a PC-induced attenuation of platelet-mediated thrombosis. However, there is, at present, no direct evidence that PC acts on the platelets per se and favorably down-regulates platelet reactivity.

OBJECTIVES

Our goal was to test the concept that PC ischemia attenuates molecular indices of platelet activation-aggregation.

METHODS

Anesthetized dogs were randomly assigned to receive 10 min of PC ischemia followed by 10 min of reperfusion or a time-matched control period. Spontaneous recurrent coronary thrombosis was then initiated in all dogs by injury + stenosis of the left anterior descending coronary artery. Coronary flow was monitored for 3 h poststenosis, and molecular indices of platelet activation-aggregation were quantified by whole blood flow cytometry.

RESULTS

Coronary patency was, as expected, better-maintained following injury + stenosis in the PC group vs. controls (53% +/- 5%* vs. 23% +/- 5% of baseline flow, respectively; *P < 0.05). Moreover, PC was accompanied by: (i) a significant down-regulation of platelet-fibrinogen binding and formation of neutrophil-platelet aggregates (112% +/- 14%* vs. 177% +/- 21% and 107% +/- 8%* vs. 155% +/- 19% of baseline values in PC vs. control groups); and (ii) a trend towards a reduction in platelet P-selectin expression (148% +/- 12% vs. 190% +/- 21% of baseline; *P < 0.05 and P = 0.09 vs. control).

CONCLUSION

These data provide novel, direct evidence in support of the concept that ischemic PC attenuates molecular indices of platelet activation-aggregation.

Myosin binding protein C is differentially phosphorylated upon myocardial stunning in canine and rat hearts — Evidence for novel phosphorylation sites

Myocardial stunning is the transient cardiac dysfunction that follows brief episodes of ischemia and reperfusion without associated myocardial necrosis. Currently, there is limited knowledge about its cellular and biochemical mechanisms. In order to better understand the underlying mechanisms of contractile dysfunction associated with the stunning, comprehensive proteomic studies using 2-D DIGE were performed using a regional stunning model in canine heart. Cardiac myosin binding protein C (cMyBP-C), a regulatory myofilament protein associated with the thick filament, and nebulette, a thin filament associated protein, were differentially expressed. Phosphoprotein specific staining indicated both protein changes were due to phosphorylation. Subsequent phosphorylation mapping of canine cMyBP-C using IMAC and MS/MS identified five phosphorylation sites, including three novel sites. In order to further evaluate this finding in a different model, cMyBP-C phosphorylation was examined in a rat model of global stunning. In the rat model, stunning was associated with increased phosphorylation of cMyBP-C at a critical calcium/calmodulin-dependent kinase II site, and the increased phosphorylation was largely inhibited when stunning was prevented by either ischemic preconditioning or reperfusion in the presence of low-calcium buffer. These data indicate cMyBP-C phosphorylation plays an important role in myocardial stunning.

First molecular evidence that inositol trisphosphate signaling contributes to infarct size reduction with preconditioning

Considerable attention has focused on the role of protein kinase C (PKC) in triggering the profound infarct-sparing effect of ischemic preconditioning (PC). In contrast, the involvement of inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)], the second messenger generated in parallel with the diacylglycerol-PKC pathway, remains poorly understood. We hypothesized that, if Ins(1,4,5)P(3) signaling [i.e., release of Ins(1,4,5)P(3) and subsequent binding to Ins(1,4,5)P(3) receptors] contributes to PC-induced cardioprotection, then the reduction of infarct size achieved with PC would be attenuated in mice that are deficient in Ins(1,4,5)P(3) receptor protein. To test this concept, hearts were harvested from 1) B6C3Fe-a/a-Itpr-1(opt+/-)/J mutants displaying reduced expression of Ins(1,4,5)P(3) receptor-1 protein, 2) Itpr-1(opt+/+) wild types from the colony, and 3) C57BL/6J mice. All hearts were buffer-perfused and randomized to receive two 5-min episodes of PC ischemia, pretreatment with d-myo-Ins(1,4,5)P(3) [sodium salt of native Ins(1,4,5)P(3)], the mitochondrial ATP-sensitive K(+) channel opener diazoxide, or no intervention (controls). After the treatment phase, all hearts underwent 30-min global ischemia followed by 2 h of reperfusion, and infarct size was delineated by tetrazolium staining. In both wild-type and C57BL/6J cohorts, area of necrosis in hearts that received PC, d-myo-Ins(1,4,5)P(3), and diazoxide averaged 28-35% of the total left ventricle (LV), significantly smaller than the values of 52-53% seen in controls (P < 0.05). In contrast, in Itpr-1(opt+/-) mutants, protection was only seen with diazoxide: neither PC nor d-myo-Ins(1,4,5)P(3) limited infarct size (52-58% vs. 56% of the LV in mutant controls). These data provide novel evidence that Ins(1,4,5)P(3) signaling contributes to infarct size reduction with PC.

Reduction of infarct size with d-myo-inositol trisphosphate: role of PI3-kinase and mitochondrial KATP channels

Prophylactic treatment with d- myo-inositol 1,4,5-trisphosphate hexasodium [d- myo-Ins(1,4,5)P3], the sodium salt of the endogenous second messenger Ins(1,4,5)P3, triggers a reduction of infarct size comparable in magnitude to that seen with ischemic preconditioning (PC). However, the mechanisms underlying d- myo-Ins(1,4,5)P3-induced protection are unknown. Accordingly, our aim was to investigate the role of four archetypal mediators implicated in PC and other cardioprotective strategies (i.e., PKC, PI3-kinase/Akt, and mitochondrial and/or sarcolemmal KATP channels) in the infarct-sparing effect of d- myo-Ins(1,4,5)P3. Fifteen groups of isolated buffer-perfused rabbit hearts [5 treated with d- myo-Ins(1,4,5)P3, 5 treated with PC, and 5 control cohorts] underwent 30 min of coronary artery occlusion and 2 h of reflow. One set of control, d- myo-Ins(1,4,5)P3, and PC groups received no additional treatment, whereas the remaining sets were infused with chelerythrine, LY-294002, 5-hydroxydecanoate (5-HD), or HMR-1098 [inhibitors of PKC, PI3-kinase, and mitochondrial and sarcolemmal ATP-sensitive K+ (KATP) channels, respectively]. Infarct size (delineated by tetrazolium staining) was, as expected, significantly reduced in both d- myo-Ins(1,4,5)P3- and PC-treated hearts versus controls. d- myo-Ins(1,4,5)P3-induced cardioprotection was blocked by 5-HD but not HMR-1098, thereby implicating the involvement of mitochondrial, but not sarcolemmal, KATP channels. Moreover, the benefits of d- myo-Ins(1,4,5)P3 were abrogated by LY-294002, whereas, in contrast, chelerythrine had no effect. These latter pharmacological data were corroborated by immunoblotting: d- myo-Ins(1,4,5)P3 evoked a significant increase in expression of phospho-Akt but had no effect on the activation/translocation of the cardioprotective ε-isoform of PKC. Thus PI3-kinase/Akt signaling and mitochondrial KATP channels participate in the reduction of infarct size afforded by prophylactic administration of d- myo-Ins(1,4,5)P3.

Cardioprotection with adenosine: 'a riddle wrapped in a mystery'

Review of the published literature on adenosine and cardioprotection could lead one to paraphrase the famous words of Sir Winston Churchill (Radio broadcast, 1 October 1939 (in reference to Russia)) and conclude: 'I cannot forecast to you the action of adenosine. It is a riddle wrapped in a mystery inside an enigma'. That is, although it is well-established that adenosine can render cardiomyocytes resistant to lethal ischemia/reperfusion-induced injury, new and intriguing insights continue to emerge as to the mechanisms by which adenosine might limit myocardial infarct size.

Postconditioning via stuttering reperfusion limits myocardial infarct size in rabbit hearts: role of ERK1/2

Emerging evidence suggests that restoration of blood flow in a stuttering manner may limit lethal myocardial ischemia-reperfusion injury. However, the mechanisms contributing to this phenomenon, termed postconditioning (post-C), remain poorly defined. Our aim was to test the hypothesis that activation of classic “survival kinases,” phosphatidylinositol 3-kinase (PI3-kinase) and/or extracellular signal-regulated kinase (ERK)1/2, may play a role in post-C-induced cardioprotection. In protocol 1, isolated buffer-perfused rabbit hearts underwent 30 min of sustained coronary artery occlusion and were randomized to receive abrupt reperfusion (controls) or four cycles of 30 s of reperfusion and 30 s of reocclusion before full restoration of flow (post-C). Protocol 2 was identical except control and postconditioned hearts received the PI3-kinase inhibitor LY-294002 ( protocol 2A) or the ERK1/2 antagonist PD-98059 ( protocol 2B) throughout the first 25 min of reperfusion, whereas in protocol 3, myocardial samples were obtained during the early minutes of reflow from additional control, postconditioned, and nonischemic sham hearts for the assessment, by standard immunoblotting, of phospho-Akt (downstream target of PI3-kinase) and phospho-ERK. Protocols 1 and 2 corroborated that infarct size (delineated by tetrazolium staining and expressed as a percent of risk region) was reduced in postconditioned hearts vs. control hearts and also revealed that post-C-induced cardioprotection was maintained despite LY-294002 treatment but was abrogated by PD-98059. These pharmacological data were supported by protocol 3, which showed increased immunoreactivity of phospho-ERK but not phospho-Akt with post-C. Thus our results implicate the involvement of ERK1/2 rather than PI3-kinase/Akt in the reduction of infarct size achieved with post-C.

Pretreatment with D-myo-inositol trisphosphate reduces infarct size in rabbit hearts: role of inositol trisphosphate receptors and gap junctions in triggering protection

Pretreatment with D-myo-inositol-1,4,5-trisphosphate hexasodium (D-myo-IP(3)), the sodium salt of the second messenger inositol 1,4,5-trisphosphate (IP(3)), is cardioprotective and triggers a reduction of infarct size comparable in magnitude to that obtained with ischemic preconditioning. However, this observation is enigmatic; whereas IP(3) signaling is conventionally initiated by receptor binding, IP(3) receptors are typically considered to be intracellular, and D-myo-IP(3) is membrane-impermeable. We propose that this paradox is explained by the presence of poorly characterized external IP(3) receptors and hypothesize that: 1) infarct size reduction with D-myo-IP(3) is receptor-mediated; and 2) communication via gap junctions and/or hemichannels is required to initiate this protection. To investigate the role of receptor binding, isolated buffer-perfused rabbit hearts underwent 30 min of coronary occlusion (CO) and 2 h of reflow. Prior to CO, hearts received no treatment (controls), D-myo-IP(3), L-myo-IP(3) (enantiomer not recognized by the IP(3) receptor), D-myo-IP(3) + the IP(3) receptor inhibitor xestospongin C (XeC), or XeC alone. Infarct size, assessed by tetrazolium staining, was reduced with D-myo-IP(3) treatment, whereas hearts that received L-myo-IP(3) or D-myo-IP(3) + XeC showed no protection. To evaluate the contribution of gap junctions/hemichannels, additional control and D-myo-IP(3)-treated cohorts received a 5-min infusion of heptanol or Gap 27, two structurally distinct gap junction inhibitors, administered at doses confirmed to attenuate intercellular transmission of a gap junction-permeable fluorescent dye. There was no infarct-sparing effect of D-myo-IP(3) in inhibitor-treated hearts. These data support the concepts that infarct size reduction with D-myo-IP(3) is triggered by receptor binding and that communication via gap junctions/hemichannels is involved in initiating this protection.

Application of flow cytometry to platelet disorders

Flow cytometry is a powerful and versatile tool that can be used to yield definitive information regarding the phenotypic status of platelets. The method provides a quantitative assessment of the physical and antigenic properties of platelets (e.g., surface expression of receptors, bound ligands, components of granules, or interactions of platelets with other platelets, other blood cells, or components of the plasma coagulation system), thereby facilitating the diagnosis of inherited or acquired platelet disorders (e.g., Bernard-Soulier syndrome, Glanzmann thrombasthenia, storage pool disease), the pathological activation of platelets (e.g., in the setting of acute coronary syndromes, cerebrovascular ischemia, peripheral vascular disease, cardiopulmonary bypass), and changes in the ability of platelets to activate via specific stimuli (e.g., efficacy of antiplatelet therapies). Accordingly, this review summarizes the key technical and methodologic components of flow cytometric analysis of platelets, as well as specific examples of its application to diagnosis and patient care.

In Vitro Platelet Responsiveness to Adenosine-Mediated ‘Preconditioning’ is Age-Dependent

BACKGROUND

Brief preconditioning (PC) ischemia, in addition to its well-described cardioprotective effects, has been shown in some studies to act on circulating platelets and attenuate platelet adhesion and aggregation in models of unstable angina and acute myocardial infarction. This "anti-platelet" effect of PC may be triggered by release of adenosine from ischemic/reperfused myocardium and activation of adenosine A(2) receptors on the platelets' surface. However: (1) all current data on the platelet inhibitory effects of PC ischemia and A(2) receptor stimulation have been obtained in adult populations; and (2) there is evidence of age-associated alterations in myocardial adenosine release, receptor responsiveness and post-receptor signaling.

OBJECTIVE

Our aim was to evaluate, using an established in vitro model of platelet aggregation and exogenous administration of an adenosine A(2) agonist, whether the favorable effects of adenosine A(2) receptor stimulation on platelet responsiveness are compromised in aging populations.

METHODS

Arterial blood samples were obtained from young adult versus old rabbits (6 months versus 4 years of age) and young adult versus senescent rats (4 months versus 2 years of age). Matched aliquots from each animal were randomly assigned to receive exogenous treatment with either the A(2) agonist CGS 21680 or vehicle. Maximum platelet aggregation was quantified by whole blood impedance aggregometry, using collagen as the aggregatory stimulus.

RESULTS

In young adult rabbits, maximum platelet aggregation was, as expected, reduced by 30 +/- 4% in CGS-treated aliquots versus vehicle-controls. In contrast, blood samples from 4 year old rabbits were refractory to A(2) receptor stimulation: in the old cohort, treatment with CGS evoked no change in platelet aggregation (decrease of 2 +/- 3% versus age-matched vehicle controls; p < .01 versus the decrease of 30% seen in young adults). Data obtained in the rat model were analogous to those seen in the rabbit: maximum platelet aggregation decreased by 18 +/- 5% versus 1 +/- 7% with CGS treatment in young adult versus senescent animals.

CONCLUSION

Our results provide novel in vitro evidence of an age-associated loss in platelet responsiveness to adenosine-mediated "preconditioning".

Stunned peri-infarct canine myocardium is characterized by degradation of troponin T, not troponin I

OBJECTIVE

Degradation of cardiac troponin I (cTnI) has been proposed to represent the underlying molecular mechanism responsible for post-ischemic contractile dysfunction of viable but 'stunned' myocardium. However, this concept is largely derived from models of brief, sublethal ischemia essentially devoid of necrosis, and there is speculation that defects in cTnI may be model-dependent. Accordingly, our primary aim was to evaluate the integrity of cardiac troponins-i.e., cTnI, as well as cTnT and cTnC-in viable but stunned peri-infarct tissue. In addition, we addressed the as-yet unexplored issue of whether the profound reduction of infarct size evoked by brief preconditioning ischemia (PC) was accompanied by a favorable attenuation in ischemia/reperfusion-induced degradation of cTnI, cTnT or cTnC in the remaining viable subepicardium.

METHODS

Anesthetized open-chest dogs received 10 min of PC ischemia or a comparable control period, followed by 1 h of sustained coronary occlusion and 3 h of reperfusion. Subepicardial biopsies from the center of the soon-to-be ischemic territory were obtained at baseline and at 30 min and 3 h post-reflow, and myofilament protein integrity (intact cTnI, cTnT and cTnC, as well as degradation bands and covalent complexes) were assessed by Western immunoblotting. In addition, in all dogs, wall thickening was measured by echocardiography, collateral blood flow was assessed during sustained occlusion by injection of radiolabeled microspheres, and infarct size was delineated by tetrazolium staining.

RESULTS

Although PC was, as expected, cardioprotective (infarct size of 2 +/- 1% of the risk region vs. 17 +/- 6% in controls; p < 0.05), both control and PC groups exhibited profound and comparable contractile dysfunction following reflow (mean wall thickening reduced to 20-22% of baseline values). There was, however, no significant degradation of cTnI in the viable but stunned, peri-infarct tissue. We did observe degradation of cTnT in the stunned subepicardium, an effect that was attenuated in dogs that received antecedent PC ischemia. However, there was no correlation between post-ischemic wall thickening and the immunoreactivity of the intact cTnT band, or wall thickening and the intensity of the cTnT degradation products.

CONCLUSIONS

Our results suggest cTnI degradation is not a universal determinant of post-ischemic myocardial stunning. Moreover, the dissociation between cTnT degradation and wall thickening argue against a direct 'cause-and-effect' relationship between proteolysis of cTnT and acute, post-ischemic contractile dysfunction of stunned peri-infarct myocardium.

Brief apnea induces myocardial ischemic tolerance by an opioid-insensitive mechanism

BACKGROUND

Intermittent brief "preconditioning" (PC) ischemia has been shown to render the heart resistant to a subsequent sustained ischemic insult, in part through an opioid-dependent mechanism. Using the rabbit model, we tested the hypothesis that intermittent in vivo apnea elicits a cardioprotective response similar to that achieved with conventional PC ischemia. In addition, we sought to determine if infarct size reduction seen in this model was stimulated via opioid receptor activation.

METHODS

Anesthetized, intubated rabbits (n=35) were randomized to receive three 4.5-min bouts of apnea interspersed with 5 min normal ventilation or time-matched standard ventilation (controls). Upon completion of the in vivo PC/control period, the hearts were excised and assessed for ischemic tolerance on a modified Langendorff apparatus (40 min global ischemia+2h reperfusion). To assess the contribution of opioid receptor stimulation, two additional control and PC groups received the nonspecific opioid antagonist naloxone (10 mg/kg) prior to the in vivo intervention phase. Infarct size (delineated by tetrazoliam staining and expressed as a percentage of the left ventricle [LV]) was compared among the four groups by ANOVA.

RESULTS

Infarct size was significantly reduced in hearts that received antecedent apneic PC when compared with controls (63+/-5% vs. 34+/-8%) of the LV, respectively; P<.05). Pretreatment with naloxone had no significant effect on infarct size in nonpreconditioned hearts (80+/-6%) and did not inhibit the protective effects of apnea-induced PC (52+/-10% in naloxone+PC group).

CONCLUSIONS

Intermittent apnea evokes significant myocardial ischemic tolerance through an opioid-insensitive mechanism.

Mechanisms of myocardial ischemic preconditioning are age related: PKC-epsilon does not play a requisite role in old rabbits

Data obtained from adult cohorts have implicated activation/translocation of protein kinase C (PKC)-epsilon as an important cellular mediator of myocardial infarct size reduction with ischemic preconditioning (PC). Age-related alterations in cellular signaling may, however, confound the extrapolation of mechanistic insight derived from adults to the aging population, the specific subset in which cardioprotection is undoubtedly most relevant. Accordingly, our aim was to investigate the role of PKC-epsilon as a mediator of infarct size reduction with PC in old vs. adult rabbits. In protocol 1, we assessed the effect of PKC-epsilon translocation inhibitor peptide (PKC-epsilon-TIP) and the pan-PKC inhibitor chelerythrine on infarct size reduction with PC in adult and approximately 4-yr-old rabbits, a population previously shown to exhibit definitive hallmarks of cardiovascular aging. Rabbits received 5 min of PC ischemia or a matched control period followed by 30 min of coronary artery occlusion and 3 h of reperfusion, with infarct size (delineated by tetrazolium staining) serving as the primary endpoint. In protocol 2, we obtained insight (by Western immunoblotting) into the subcellular redistribution of PKC-epsilon in response to the 5-min PC stimulus in adult and old rabbits. In adults, infarct size reduction with PC was abrogated by both PKC-epsilon-TIP and chelerythrine. However, in old rabbits, 1). PC-induced cardioprotection was maintained despite inhibitor treatment and 2). brief PC ischemia was not associated with activation/translocation of PKC-epsilon. Thus the mechanisms responsible for PC are age related in the rabbit heart, with no apparent, requisite role of PKC-epsilon in aging animals.

Cardioprotection 'outside the box'--the evolving paradigm of remote preconditioning

Conventional ischemic preconditioning is the phenomenon whereby brief episodes of myocardial ischemia render the ischemic territory resistant to a subsequent, sustained ischemic insult. A growing body of evidence further indicates that brief ischemia applied in distant organs and tissues can also protect naïve, virgin myocardium from ischemic injury. In this review, we describe the initial observations that provided the impetus for the study of 'remote preconditioning', and summarize our current knowledge of the three facets of 'preconditioning at a distance' --intra-cardiac, inter-organ and transferred inter-cardiac preconditioning.