Rapid cooling preserves the ischaemic myocardium against mitochondrial damage and left ventricular dysfunction

AIMS

We investigated whether rapid cooling instituted by total liquid ventilation (TLV) improves cardiac and mitochondrial function in rabbits submitted to ischaemia-reperfusion.

METHODS AND RESULTS

Rabbits were chronically instrumented with a coronary artery occluder and myocardial ultrasonic crystals for assessment of segment length-shortening. Two weeks later they were re-anaesthetized and underwent either a normothermic 30-min coronary artery occlusion (CAO) (Control group, n = 7) or a comparable CAO with cooling initiated by a 10-min hypothermic TLV and maintained by a cold blanket placed on the skin. Cooling was initiated after 5 or 15 min of CAO (Hypo-TLV and Hypo-TLV(15') groups, n = 6 and 5, respectively). A last group underwent normothermic TLV during CAO (Normo-TLV group, n = 6). Wall motion was measured in the conscious state over three days of reperfusion before infarct size evaluation and histology. Additional experiments were done for myocardial sampling in anaesthetized rabbits for mitochondrial studies. The Hypo-TLV procedure induced a rapid decrease in myocardial temperature to 32-34 degrees C. Throughout reperfusion, segment length-shortening was significantly increased in Hypo-TLV and Hypo-TLV(15') vs. Control and Normo-TLV (15.1 +/- 3.3%, 16.4 +/- 2.3%, 1.8 +/- 0.6%, and 1.1 +/- 0.8% at 72 h, respectively). Infarct sizes were also considerably attenuated in Hypo-TLV and Hypo-TLV(15') vs. Control and Normo-TLV (4 +/- 1%, 11 +/- 5%, 39 +/- 2%, and 42 +/- 5% infarction of risk zones, respectively). Mitochondrial function in myocardial samples obtained at the end of ischaemia or after 10 min of reperfusion was improved by Hypo-TLV with respect to ADP-stimulated respiration and calcium-induced opening of mitochondrial permeability transition pores (mPTP). Calcium concentration opening mPTP was, e.g., increased at the end of ischaemia in the risk zone in Hypo-TLV vs. Control (157 +/- 12 vs. 86 +/- 12 microM). Histology and electron microscopy also revealed better preservation of lungs and of cardiomyocyte ultrastructure in Hypo-TLV when compared with Control.

CONCLUSION

Institution of rapid cooling by TLV during ischaemia reduces infarct size as well as other sequelae of ischaemia, such as post-ischaemic contractile and mitochondrial dysfunction.

Mapping preconditioning's signaling pathways: an engineering approach

Preconditioning the heart by exposure to brief cycles of ischemia-reperfusion causes it to become very resistant to ischemia-induced infarction. This protection has been shown to depend on a large number of signal transduction components whose arrangements within the cardiomyocyte are unknown. To aid the translation of this phenomenon to the clinical setting, we have attempted to map the signal transduction pathways responsible for this protection. To resolve the signaling order we have injected a signal at an intermediate point in the system transduction pathway and monitored it at a downstream site. System analysis reveals both parallel and series signaling arrangements. Separate trigger and mediator phases could be identified. The trigger phase is now well mapped. During the preconditioning ischemia, autacoids--including adenosine, opioids, and bradykinin--are released from the heart. These substances occupy their respective Gi-coupled receptors. Opioid and bradykinin receptors activate phosphatidylinositol 3-kinase (PI3-kinase) which, through phosphoinositide-dependent protein kinase, causes activation of Akt. Opioid couples through transactivation of the epidermal growth factor receptor, while bradykinin's coupling to PI3-kinase is unknown. PI3-kinase causes extracellular signal regulated kinase (ERK)-dependent activation of endothelial nitric oxide synthase. The resulting nitric oxide activates soluble guanylyl cyclase resulting in cyclic C-GMP-dependent protein kinase (PKG) activation through production of cyclic guanosine monophosphate. PKG initiates opening of ATP-sensitive potassium channels on the inner membrane of the mitochondria. Potassium entry into mitochondria causes the generation of free radicals during reperfusion when oxygen is reintroduced. Through redox signaling, these radicals activate protein kinase C (PKC) and put the heart into the protected phenotype that persists for one to two hours. Although adenosine receptors activate PI3-kinase, they also have a second direct coupling to PKC and thus bypass the mitochondrial pathway. The mediator phase occurs during the first minutes of reperfusion following the lethal ischemic insult and is still poorly defined. Briefly, PKC somehow potentiates adenosine's ability to activate signaling from low-affinity A(2b) adenosine receptors. These receptors couple to the survival kinases, Akt and ERK, believed to inhibit the formation of deadly mitochondrial permeability transition pores through the phosphorylation of glycogen synthase kinase-3beta. The proposed signaling maps reveal many points at which drugs can trigger the protected phenotype.

Infarct limitation by a protein kinase G activator at reperfusion in rabbit hearts is dependent on sensitizing the heart to A2b agonists by protein kinase C

PKG activator 8-(4-chlorophenylthio)-guanosine 3',5'-cyclic monophosphate (CPT) at reperfusion protects ischemic hearts, but the mechanism is unknown. We recently proposed that in preconditioned hearts PKC lowers the threshold for adenosine to initiate signaling from low-affinity A2b receptors during early reperfusion thus allowing endogenous adenosine to activate survival kinases phosphatidylinositol 3-kinase (PI3K) and ERK. We tested whether CPT might also sensitize A2b receptors to adenosine. CPT (10 microM) during the first minutes of reperfusion markedly reduced infarction in isolated rabbit hearts undergoing 30-min regional ischemia/2-h reperfusion, and salvage was blocked by MRS 1754, an A2b-selective antagonist. Coadministration of wortmannin (PI3K inhibitor) or PD-98059 (MEK1/2 and therefore ERK1/2 inhibitor) also blocked protection. In nonischemic hearts, 10-min infusion of CPT did not change phosphorylation of Akt or ERK1/2. Neither did a subthreshold dose (2.5 nM) of the nonselective but A2b-potent receptor agonist 5'-(N-ethylcarboxamido)adenosine (NECA). However, when 2.5 nM NECA was combined with 10 microM CPT, both phospho-Akt and phospho-ERK1/2 significantly increased, indicating CPT had lowered the threshold for A2b-dependent signaling. The PKC antagonist chelerythrine blocked this phosphorylation induced by CPT + NECA. Chelerythrine also blocked the anti-infarct effect of CPT as did nonselective (glibenclamide) and mitochondrial-selective (5-hydroxydecanoate) K(ATP) channel blockers. A free radical scavenger, N-(2-mercaptopropionyl)glycine, also blocked CPT protection. We propose CPT targets PKG, which activates PKC through mitochondrial K(ATP) channel (mitoKATP)-dependent redox signaling, a sequence mimicking that already documented in preconditioning. Activated PKC then augments sensitivity of normally low-affinity cardiac adenosine A2b receptors so endogenous adenosine can protect by activating Akt and ERK.

Acidosis, oxygen, and interference with mitochondrial permeability transition pore formation in the early minutes of reperfusion are critical to postconditioning's success

Repetitive cycles of reflow/reocclusion in the initial 2 min following release of a prolonged coronary occlusion, i.e., ischemic postconditioning (IPoC), salvages ischemic myocardium. We have proposed that the intermittent ischemia prevents formation of mitochondrial permeability transition pores (MPTP) by maintaining an acidic myocardial pH for several minutes until survival kinases can be activated. To determine other requisites of IPoC, isolated rabbit hearts were subjected to 30 min of regional myocardial ischemia and 120 min of reperfusion. Infarct size was determined by staining with triphenyltetrazolium chloride. During the first 2 min of reperfusion the perfusate was either at pH 7.4 following equilibration with 95% O(2)/5% CO(2), pH 6.9 following equilibration with 80% N(2)/20% CO(2), or pH 7.8 following equilibration with 100% O(2). Whereas acidic, oxygenated perfusate for the first 2 min of reperfusion was cardioprotective, protection was lost when acidic perfusate was hypoxic. However, the acidic, hypoxic hearts could be rescued by addition of phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, to the perfusate. Therefore, both low pH and restoration of oxygenation are necessary for protection, and the signaling step requiring combined oxygen and H(+) must be upstream of PKC. To gain further insight into the mechanism of IPoC, the latter was effected with 6 cycles of 10-s reperfusion/10-s reocclusion. Its protective effect was abrogated by either making the oxygenated perfusate alkaline during the reperfusion phases or making the reperfusion buffer hypoxic. Presumably the repeated coronary occlusions during IPoC keep myocardial pH low while the resupply of oxygen during the intermittent reperfusion provides fuel for the redox signaling that acts to prevent MPTP formation even after restoration of normal myocardial pH. Hearts treated simultaneously with IPoC and alkaline perfusate could not be rescued by addition to the perfusate of either PMA or SB216763 which inhibits GSK-3beta, the putative last cytoplasmic signaling step in the signal transduction cascade leading to MPTP inhibition. Yet cyclosporin A which also inhibits MPTP formation does rescue hearts made alkaline during IPoC. In view of prior studies in which the ROS scavenger N-2-mercaptopropionyl glycine aborts IPoC's protection, our data reveal that IPoC's reperfusion periods are needed to support redox signaling rather than improve metabolism. The low pH, on the other hand, is equally necessary and seems to suppress MPTP directly rather than through upstream signaling.

Redox signaling triggers protection during the reperfusion rather than the ischemic phase of preconditioning

In ischemic preconditioning (IPC) brief ischemia/reperfusion renders the heart resistant to infarction from any subsequent ischemic insult. Protection results from binding of surface receptors by ligands released during the preconditioning ischemia. The downstream pathway involves redox signaling as IPC will not protect in the presence of a free radical scavenger. To determine when in the IPC protocol the redox signaling occurs, seven groups of isolated rabbit hearts were studied. All hearts underwent 30 min of coronary branch occlusion and 2 h of reperfusion. IPC groups were subjected to 5 min of regional ischemia followed by 10 min of reperfusion prior to the 30-min coronary occlusion. The Control group had only the 30-min occlusion and 2-h reperfusion. In the second group IPC preceded the index coronary occlusion. The third group was also preconditioned, but the free radical scavenger N-2-mercaptopropionyl glycine (MPG 300 microM) was infused during the 10-min reperfusion and therefore was present in the myocardium in the distribution of the snared coronary artery during the entire reperfusion phase and also during the subsequent 30-min ischemia. In another preconditioned group MPG was added to the perfusate before the preconditioning ischemia and therefore was present in the tissue only during the preconditioning ischemia and then was washed out during reperfusion. In the fifth group MPG was added to the perfusate for only the last 5 min of the preconditioning reperfusion and therefore was present in the tissue during the last minutes of the reperfusion phase and the 30 min of ischemia. In an additional group of IPC hearts MPG was infused for only the initial 5 min of the preconditioning reperfusion and then allowed to wash out so that the scavenger was present for only the first half of the reperfusion phase. Infarct and risk zone sizes were measured by triphenyltetrazolium staining and fluorescent microspheres, resp. IPC reduced infarct size from 31.3 +/- 2.7% of the ischemic zone in control hearts to only 8.4 +/- 1.9%. MPG completely blocked IPC's protection in the third (39.4 +/- 2.8%) and sixth (36.1 +/- 7.7%) groups but did not affect its protection in groups 4 (8.1 +/- 1.5%) or 5 (7.8 +/- 1.1%). When deoxygenated buffer was used during IPC's reperfusion phase in the seventh group of hearts, protection was lost and infarct size was increased over that seen in control hearts (74.5 +/- 9.0%). Hence redox signaling occurs during the reperfusion phase of IPC, and the critical component in that reperfusion phase appears to be molecular oxygen.

Redox signaling at reperfusion is required for protection from ischemic preconditioning but not from a direct PKC activator

Redox signaling prior to a lethal ischemic insult is an important step in triggering the protected state in ischemic preconditioning. When the preconditioned heart is reperfused a second sequence of signal transduction events, the mediator pathway, occurs which is believed to inhibit mitochondrial permeability transition pore formation that normally destroys mitochondria in much of the reperfused tissue. Prominent among the mediator pathway's events is activation of phosphatidylinositol 3-kinase and extracellular signal-regulated kinase. Recently it was found that both activation of PKC and generation of reactive oxygen species (ROS) at the time of reperfusion are required for protection in preconditioned hearts. To establish their relative order we tested whether ROS formation at reperfusion is required in hearts protected by direct activation of PKC at reperfusion. Isolated rabbit hearts were exposed to 30 min of regional ischemia and 2 h of reperfusion. Preconditioned hearts received 5 min of global ischemia and 10 min of reperfusion prior to the index ischemia. Another group of preconditioned hearts was exposed to 300 microM of the ROS scavenger N-(2-mercaptopropionyl) glycine (MPG) for 20 min starting 5 min prior to reperfusion. Infarct size was measured by triphenyltetrazolium staining. Preconditioning reduced infarct size from 36% +/- 2% of the ischemic zone in control hearts to only 18 +/- 2%. MPG during early reperfusion completely blocked preconditioning's protection (33 +/- 3% infarction). MPG given in the same dose and schedule to non-preconditioned hearts had no effect on infarct size. In the last group phorbol 12-myristate 13-acetate (PMA) (0.05 nM) was given to non-preconditioned hearts from 1 min before to 5 min after reperfusion in addition to MPG administered as in the other groups. MPG did not block protection from an infusion of PMA as infarct size was only 9 +/- 2% of the risk zone. We conclude that while redox signaling during the first few minutes of reperfusion is an essential component of preconditioning's protective mechanism, this step occurs upstream of PKC activation.

Making the heart resistant to infarction: how can we further decrease infarct size?

Acute myocardial infarction (AMI) following coronary artery occlusion is a common cause of mortality and morbidity world-wide. Patients currently receive reperfusion therapy as the only anti-infarct intervention. A number of agents have been evaluated to further improve myocardial salvage, but until recently, none has demonstrated clear efficacy in clinical trials. A new target of cardioprotection, the Reperfusion Injury Salvage Kinase (RISK) pathway, has been proposed. These kinases are involved in mediating the cardioprotection of myocardial preconditioning and postconditioning induced by short non-lethal cycles of ischemia/reperfusion performed before (preconditioning) or just after (postconditioning) a lethal ischemic insult. Many pharmacological interventions are now available that protect the heart by activating the RISK pathway at the time of reperfusion. The present review will examine the efficacy of several strategies that have been proposed to protect the acutely ischemic myocardium including (1) those intended to directly alter adverse reperfusion events (e.g., calcium overload and free radical attack), (2) those based on activation of the RISK pathway including postconditioning, and (3) myocardial cooling.

Signaling pathways in ischemic preconditioning

Ischemic preconditioning renders the heart resistant to infarction from ischemia/reperfusion. Over the past two decades a great deal has been learned about preconditioning's mechanism. Adenosine, bradykinin, and opioids act in parallel to trigger the preconditioned state and do so by activating PKC. While adenosine couples directly to PKC through the phospholipases, bradykinin and opioids do so through a complex pathway that includes in order: phosphatidylinositol 3-kinase (PI3-kinase), Akt, nitric oxide synthase, guanylyl cyclase, PKG, opening of mitochondrial K(ATP) channels, and activation of PKC by redox signaling. There are even differences between the opioid and bradykinin coupling as the former activates PI3-kinase through transactivation of the epidermal growth factor receptor while the latter has an unknown coupling mechanism. Protection stems from inhibition of formation of mitochondrial permeability transition pores early in reperfusion through activation of the survival kinases, Akt and ERK. These kinases are activated as a result of PKC somehow promoting signaling from adenosine A(2) receptors early in reperfusion. The survival kinases are thought to inhibit pore formation by phosphorylating GSK-3beta. The reperfused heart requires the support of the protective signals for only about an hour after which the ischemic injury is repaired and the signals are no longer needed.

cGMP signalling in pre- and post-conditioning: the role of mitochondria

Much of cell death from ischaemia/reperfusion in heart and other tissues is generally thought to arise from mitochondrial permeability transition (MPT) in the first minutes of reperfusion. In ischaemic pre-conditioning, agonist binding to G(i) protein-coupled receptors prior to ischaemia triggers a signalling cascade that protects the heart from MPT. We believe that the cytosolic component of this trigger pathway terminates in activation of guanylyl cyclase resulting in increased production of cGMP and subsequent activation of protein kinase G (PKG). PKG phosphorylates a protein on the mitochondrial outer membrane (MOM), which then causes the mitochondrial K(ATP) channel (mitoK(ATP)) on the mitochondrial inner membrane to open, leading to increased production of reactive oxygen species (ROS) by the mitochondria. This implies that the protective signal is somehow transmitted from the MOM to its inner membrane. This is accomplished by a series of intermembrane signalling steps that includes protein kinase C (PKCepsilon) activation. The resulting ROS then activate a second PKC pool which, through another signal transduction pathway termed the mediator pathway, causes inhibition of MPT and reduction in cell death.

Cardioprotection: spotlight on PKG

Classical ischaemic preconditioning, delayed or second window preconditioning and postconditioning are forms of cardioprotection that are dependent on cell surface receptors, intracellular signalling molecules and kinases that ultimately block formation of the mitochondrial permeability transition. The latter is presumed to cause myocardial necrosis as well as apoptosis, so prevention of its formation upon resumption of perfusion after a prolonged coronary occlusion should be cardioprotective. In all of these forms of cardioprotection, formation of cGMP and activation of protein kinase G (PKG) are recognized to be key steps in the signal transduction pathway. Burley et al. highlight the roles of cGMP and PKG in their comprehensive review. They describe the basic biology of PKG and emphasize its compartmentalization, which may be responsible for the frustration induced by assays for PKG in whole cell lysates and for the spurious conclusions about the role of PKG in cardioprotection. This review will be useful to both the novice and the seasoned investigator.

Protecting the acutely ischemic myocardium beyond reperfusion therapies: are we any closer to realizing the dream of infarct size elimination?

Patients currently treated for acute myocardial infarction receive reperfusion therapy as their only anti-infarct intervention. Although pharmacologic agents have been evaluated in the past for their ability to salvage ischemic myocardium when administered at reperfusion, until very recently none has demonstrated clear efficacy in clinical trials. However, a new generation of interventions has emerged which protects the heart by activating the reperfusion-induced salvage kinase (RISK) pathway. Unlike the disappointing results documented with previously touted putative cardioprotective agents, the preclinical experience with these newer interventions is very consistent indicating that there is a high likelihood that they will be effective clinically. Ischemic postconditioning, which also acts by activating the RISK pathway, has shown marked reduction in infarct size in small-scale trials. Finally, if a strategy for rapidly cooling the heart can be devised so that the in-hospital normothermic ischemic time can be significantly reduced, then infarct size can be even further decreased. In our opinion it is well within our reach using existing technologies to see the day when infarction can be virtually eliminated in the patient with acute coronary occlusion.

Protein kinase C protects preconditioned rabbit hearts by increasing sensitivity of adenosine A2b-dependent signaling during early reperfusion

Although protein kinase C (PKC) plays a key role in ischemic preconditioning (IPC), the actual mechanism of that protection is unknown. We recently found that protection from IPC requires activation of adenosine receptors during early reperfusion. We, therefore, hypothesized that PKC might act to increase the heart's sensitivity to adenosine. IPC limited infarct size in isolated rabbit hearts subjected to 30-min regional ischemia/2-h reperfusion and IPC's protection was blocked by the PKC inhibitor chelerythrine given during early reperfusion revealing involvement of PKC at reperfusion. Similarly chelerythrine infused in the early reperfusion period blocked the increased phosphorylation of the protective kinases Akt and ERK1/2 observed after IPC. Infusing phorbol 12-myristate 13-acetate (PMA), a PKC activator, during early reperfusion mimicked IPC's protection. As expected, the protection triggered by PMA at reperfusion was blocked by chelerythrine, but surprisingly it was also blocked by MRS1754, an adenosine A(2b) receptor-selective antagonist, suggesting that PKC was somehow facilitating signaling from the A(2b) receptors. NECA [5'-(N-ethylcarboxamido) adenosine], a potent but not selective A(2b) receptor agonist, increased phosphorylation of Akt and ERK1/2 in a dose-dependent manner. Pretreating hearts with PMA or brief preconditioning ischemia had no effect on phosphorylation of Akt or ERK1/2 per se but markedly lowered the threshold for NECA to induce their phosphorylation. BAY 60-6583, a highly selective A(2b) agonist, also caused phosphorylation of ERK1/2 and Akt. MRS1754 prevented phosphorylation induced by BAY 60-6583. BAY 60-6583 limited infarct size when given to ischemic hearts at reperfusion. These results suggest that activation of cardiac A(2b) receptors at reperfusion is protective, but because of the very low affinity of the receptors endogenous cardiac adenosine is unable to trigger their signaling. We propose that the key protective event in IPC occurs when PKC increases the heart's sensitivity to adenosine so that endogenous adenosine can activate A(2b)-dependent signaling.

The pH hypothesis of postconditioning: staccato reperfusion reintroduces oxygen and perpetuates myocardial acidosis

BACKGROUND

It is unclear how reperfusion of infarcting hearts with alternating cycles of coronary reperfusion/occlusion attenuates infarction, but prevention of mitochondrial permeability transition pore (MPTP) formation is crucial. Acidosis also suppresses MPTP formation. We tested whether postconditioning protects by maintaining acidosis during early reoxygenation.

METHODS AND RESULTS

After 30-minute regional ischemia in isolated rabbit hearts, reperfusion with buffer (pH 7.4) caused 34.4+/-2.2% of the risk zone to infarct, whereas 2 minutes of postconditioning (6 cycles of 10-second reperfusion/10-second occlusion) at reperfusion resulted in 10.7+/-2.9% infarction. One minute (3 cycles) of postconditioning was not protective. Hypercapnic buffer (pH 6.9) for the first 2 minutes of reperfusion in lieu of postconditioning caused equivalent cardioprotection (15.0+/-2.6% infarction), whereas 1 minute of acidosis did not protect. Delaying postconditioning (6 cycles) or 2 minutes of acidosis for 1 minute aborted protection. Reperfusion with buffer (pH 7.7) blocked postconditioning protection, but addition of the MPTP closer cyclosporin A restored protection. Reactive oxygen species scavenger N-2-mercaptopropionyl glycine, protein kinase C antagonist chelerythrine, and mitochondrial K(ATP) channel closer 5-hydroxydecanoate each blocked protection from 2 minutes of acidosis as they did for postconditioning.

CONCLUSIONS

Thus, postconditioning prevents MPTP formation by maintaining acidosis during the first minutes of reperfusion as reoxygenated myocardium produces reactive oxygen species that activate protective signaling to inhibit MPTP formation after pH normalization.

Total liquid ventilation provides ultra-fast cardioprotective cooling

OBJECTIVES

We tested whether total liquid ventilation (TLV) can be used to rapidly cool and protect the infarcting heart.

BACKGROUND

Decreasing myocardial temperature during ischemia is a powerful cardioprotective strategy, but clinical application has been impaired by lack of practical methodology to quickly cool the heart.

METHODS

We performed 30-min coronary artery occlusion/3-h reperfusion in rabbits. Upon occlusion, rabbits underwent either oxygen (Gas), normothermic liquid (Liquid Warm), or cold liquid (Liquid Cool) ventilation.

RESULTS

Left atrial chamber temperature decreased to 32.4 degrees +/- 0.2 degrees C within 5 min of onset of cold TLV. Blood gases were within acceptable limits during TLV. In the Liquid Warm group, perfluorocarbon inhalation did not alter infarct size compared with Gas (37.7 +/- 1.3% and 42.5 +/- 4.9% of risk zone, respectively). However, infarction was significantly reduced in the Liquid Cool group (4.0 +/- 0.5%). Cooling only during the initial 30 min of reperfusion did not reduce infarction.

CONCLUSIONS

Total liquid ventilation can elicit rapid cardioprotective cooling during ischemia.

Preconditioning-mimetics bradykinin and DADLE activate PI3-kinase through divergent pathways

We previously reported that pharmacological preconditioning of rabbit hearts with acetylcholine involves activation of phosphatidylinositol 3-kinase (PI3-K) through transactivation of the epidermal growth factor receptor (EGFR). Transactivation is thought to be initiated by cleavage of membrane-bound pro-heparin-binding EGF-like growth factor (HB-EGF) by a membrane metalloproteinase thus releasing HB-EGF which binds to the EGFR. This pathway leads to redox signaling with the generation of reactive oxygen species (ROS) by mitochondria. We tested whether preconditioning's physiological triggers, bradykinin and opioid, also signal through the EGFR. Both bradykinin and the synthetic delta-opioid agonist DADLE increased ROS production in isolated cardiomyocytes by approximately 50%. DADLE's effect was abrogated by either metalloproteinase inhibitor III (MPI) or the diphtheria toxin mutant CRM-197 which blocks heparin-binding EGF shedding indicating that DADLE signals through EGFR transactivation. MPI also blocked DADLE's infarct-sparing effect in whole hearts. Additionally, blocking Src kinase (a component of the EGFR's signaling complex) with PP2 or PI3-K with wortmannin blocked DADLE's effect on cardiomyocyte ROS production and PP2 blocked DADLE's salvage of ischemic myocardium. Finally, DADLE increased phosphorylation of Akt and extracellular signal-regulated protein kinases (ERK) 1/2 in left ventricular myocardium, and this increase was blocked by the EGFR antagonist AG1478. On the other hand, neither MPI nor CRM-197 prevented bradykinin from increasing ROS production, and MPI did not affect bradykinin's infarct-sparing effect in intact hearts. Conversely, both PP2 and wortmannin blocked bradykinin's effect on ROS generation and also aborted bradykinin's cardioprotective effect in intact hearts. While bradykinin also increased phosphorylation of Akt and ERK in myocardium, that increase was not affected by AG1478. Hence bradykinin, unlike acetylcholine or opioid, does not transactivate EGFR, although all 3 agonists do signal through Src and PI3-K.

Nicorandil opens mitochondrial K(ATP) channels not only directly but also through a NO-PKG-dependent pathway

Nicorandil, a hybrid of nitrate generator and potassium channel opener, protects ischemic myocardium by opening mitochondrial ATP sensitive potassium (mitoK(ATP)) channels. We recently found that nitric oxide (NO) opened K(ATP) channels in rabbit hearts by a protein kinase G (PKG) mechanism. This study examined whether the NO-donor property of nicorandil also contributes to opening of mitoK(ATP) channels through PKG. MitoK(ATP) channel opening was monitored in adult rabbit cardiomyocytes by measuring reactive oxygen species (ROS) production, an established marker of channel opening. Nicorandil increased ROS production in a dose-dependent manner. The selective mitoK(ATP) channel inhibitor 5-hydroxydecanoate (200 microM) completely blocked ROS production by nicorandil at all doses. The PKG inhibitor 8-bromoguanosine-3',5'-cyclic monophosphorothioate, Rpisomer (Rp-8-Br-cGMPs, 50 microM) shifted the dose-ROS production curve to the right with an increase of the EC(50) from 2.4 x 10(-5) M to 6.9 x 10(-5) M. Rp- 8-Br-cGMPs did not affect the increase in ROS production by the selective mitoK(ATP) channel opener diazoxide while it completely blocked increased ROS production from the NO donor S-nitroso-N-acetylpenicillamine (1 microM). Furthermore ODQ, an antagonist of soluble guanylyl cyclase, blocked nicorandil's ability to increase ROS generation. These results indicate that nicorandil, in addition to its direct effect on the channels, opens mitoK(ATP) channels indirectly via a NO-PKG signaling pathway.