Akihito Tsuchida

Cardioprotective Mechanism of Ischemic Preconditioning Is Impaired by Postinfarct Ventricular Remodeling Through Angiotensin II Type 1 Receptor Activation

BACKGROUND Activation of protein kinase C-linked receptors and subsequent opening of the mitochondrial ATP-sensitive K(+) (mitoK(ATP)) channel are crucial in preconditioning (PC). This study examined whether postinfarct ventricular remodeling interferes with the PC mechanism. METHODS AND RESULTS Two weeks before isolation of hearts, rabbits underwent a sham operation or coronary ligation (COL) to induce remodeling. Isolated buffer-perfused hearts were subjected to 30-minute global ischemia/2-hour reperfusion, and infarct size was expressed as a percentage of the left ventricle (%I/LV), from which the scarred infarct by COL was excluded. Although %I/LV was similar in sham-operated and remodeled hearts (52.9+/-6.5% versus 45.8+/-5.2%), PC with 2 episodes of 5-minute ischemia protected sham-operated but not remodeled hearts (%I/LV=18.1+/-2.5% versus 54.8+/-2.9%, P<0.05). Infusion of valsartan (10 mg x kg(-1). d(-1), an angiotensin II type 1 (AT(1)) receptor blocker, for 2 weeks after COL prevented the ventricular remodeling and preserved the response to PC (%I/LV=27.4+/-3.8%), although valsartan alone did not change %I/LV. Diazoxide, a mitoK(ATP) channel opener, protected both sham-operated and remodeled hearts (%I/LV=14.1+/-3.1% and 8.3+/-3.6%). CONCLUSIONS The myocardium remodeled after infarction is refractory to PC, which is probably due to interruption of cellular signaling by PC upstream of mitoK(ATP) channels. An AT(1) receptor blocker is beneficial not only for suppression of ventricular remodeling but also for preservation of the PC mechanism.

Roles of tyrosine kinase and protein kinase C in infarct size limitation by repetitive ischemic preconditioning in the rat.

In this study, we examined the possibility that infarct-size limitation by repetitive preconditioning (PC) is achieved by activation of both protein kinase C (PKC) and tyrosine kinase. In addition, we assessed whether such kinase activation is triggered by angiotensin II type 1 (AT1) and alpha1-adrenergic receptors and whether sarcolemmal and mitochondrial adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels play roles as effectors of cardioprotection in the rat. Under pentobarbital anesthesia, myocardial infarction was induced by 20-min coronary occlusion and 3-h reperfusion in the rat. Infarct size was determined by tetrazolium and expressed as a percentage of area at risk (%IS/AR). PC with one cycle of 5-min ischemia/5-min reperfusion before 20-min ischemia significantly reduced %IS/AR from the control value of 49.4 +/- 2.0 to 35.4 +/- 2.8, and repetitive PC with two cycles of 5-min ischemia/5-min reperfusion further limited %IS/AR to 3.2 +/-0.9. Infarct-size limitation by single-cycle PC was completely abolished by a PKC inhibitor, staurosporine (100 microg/kg; %IS/ AR, 45.7 +/- 5.0). In contrast, the cardioprotection by repetitive PC was only partially blocked by staurosporine (%IS/AR, 19.8 +/- 2.4), another PKC inhibitor, polymyxin B (5 mg/kg; %IS/AR, 16.2 +/- 3.1), or a tyrosine kinase inhibitor, genistein (5 mg/kg; %IS/AR, 21.8 +/- 1.4). However, a combined injection of genistein and staurosporine additively inhibited protection of repetitive PC (%IS/AR, 36.4 +/- 1.7). Staurosporine, polymyxin B, or genistein alone did not modify %IS/AR in nonpreconditioned rat hearts. Infarct-size limitation by repetitive PC was not attenuated by pretreatment with a selective AT1-receptor blocker (CV11974, 10 mg/kg), prazosin (0.6 mg/kg; %IS/AR, 6.4 +/- 3.2 and 1.6 +/- 0.5, respectively). A selective blocker of mitochondrial K(ATP) channels, 5-hydroxydecanoate (3 mg/kg), completely abolished the cardioprotective effect (%IS/AR, 50.8 +/-3.5), but HMR1883 (3 mg/kg), a selective blocker of sarcolemmal K(ATP) channels, failed to inhibit the preconditioning effect (%IS/AR, 4.4 +/- 0.7). These findings suggest that repetition of PC provokes activation of both PKC and tyrosine kinase, leading to enhanced antiinfarct tolerance by opening of mitochondrial but not sarcolemmal K(ATP) channels. It is unlikely that activation of either AT1 or alpha1-adrenergic receptor alone is crucial to trigger preconditioning. Key Words: Tyrosine kinase-Genistein-Angiotensin II-alpha1-Adrenergic receptor-Sarcolemmal K(ATP) channel-Mitochondrial K(ATP) channel.

Mitochondrial ATP-sensitive K+ channels play a role in cardioprotection by Na+-H+ exchange inhibition against ischemia/reperfusion injury.

OBJECTIVES The possible role of the ATP-sensitive potassium (KATP) channel in cardioprotection by Na+-H+ exchange (NHE) inhibition was examined. BACKGROUND The KATP channel is suggested to be involved not only in ischemic preconditioning but also in some pharmacological cardioprotection. METHODS Infarction was induced by 30-min coronary occlusion in rabbit hearts in situ or by 30-min global ischemia in isolated hearts. Myocardial stunning was induced by five episodes of 5-min ischemia/5-min reperfusion in situ. In these models, the effects of NHE inhibitors (cariporide and ethylisopropyl-amiloride [EIPA]) and the changes caused by KATP channel blockers were assessed. In another series of experiments, the effects of EIPA on mitochondrial KATP (mito-KATP) and sarcolemmal KATP (sarc-KATP) channels were examined in isolated cardiomyocvtes. RESULTS Cariporide (0.6 mg/kg) reduced infarct size in situ by 40%, and this effect was abolished by glibenclamide (0.3 mg/kg), a nonselective KATP channel blocker. In vitro, 1 microM cariporide limited infarct size by 90%, and this effect was blocked by 5-hydroxydecanoate (5-HD), a mito-KATP channel blocker but not by HMR1098, a sarc-KATP channel blocker. Infarct size limitation by 1 microM EIPA was also prevented by 5-HD. Cariporide attenuated regional contractile dysfunction by stunning, and this protection was abolished by glibenclamide and 5-HD. Ethylisopropyl amiloride neither activated the mito-KATP channel nor enhanced activation of this channel by diazoxide, a KATP channel opener. CONCLUSIONS Opening of the mito-KATP channel contributes to cardioprotection by NHE inhibition, though the interaction between NHE and this KATP channel remains unclear.

Infarct size limitation by nicorandil ☆: Roles of mitochondrial KATP channels, sarcolemmal KATP channels, and protein kinase C

OBJECTIVES This study aimed to examine:1) whether nicorandil protects the ischemic myocardium by activating sarcolemmal adenosine triphosphate (ATP)-sensitive K(+) (sarcK(ATP)) channels or the mitochondrial K(ATP) (mitoK(ATP)) channels, and 2) whether protein kinase C (PKC) activity is necessary for cardioprotection afforded by nicorandil. BACKGROUND Nicorandil is a hybrid of nitrate and a K(ATP) channel opener that activates the sarcK(ATP) and mitoK(ATP) channels. Both of these K(ATP) channels are regulated by PKC, and this kinase may be activated by nitric oxide and also by oxygen free radicals (OFR) generated after mitoK(ATP) channel opening. METHODS In isolated rabbit hearts, infarction was induced by 30-min global ischemia/2-h reperfusion with monitoring of the activation recovery interval (ARI), an index of action potential duration. Protein kinase C translocation was assessed by Western blotting. RESULTS Nicorandil did not change ARI before ischemia, but it accelerated ARI shortening after the onset of ischemia and reduced infarct size by 90%. A sarcK(ATP) channel selective blocker, HMR1098, abolished acceleration of ischemia-induced ARI-shortening by nicorandil and eliminated 40% of nicorandil-induced infarct size limitation. A mitoK(ATP) channel selective blocker, 5-hydroxydecanoate, abolished the protection afforded by nicorandil without affecting ARI. Cardioprotection by nicorandil was inhibited neither by an OFR scavenger, N-2-mercaptopropionylglycine nor by a PKC inhibitor, calphostin C, at a dose that was capable of inhibiting PKC- epsilon translocation after preconditioning. CONCLUSIONS Both the sarcK(ATP) and mitoK(ATP) channels are involved in anti-infarct tolerance afforded by nicorandil, but PKC activation induced by nitric oxide or OFR generation, if any, does not play a crucial role.

Effect of protein kinase C inhibitors on cardioprotection by ischemic preconditioning depends on the number of preconditioning episodes

OBJECTIVES This study examined the possibility that the role of PKC in [corrected] PC, and thus the response to PFC inhibitors, may differ depending on how many ischemic episodes are employed to precondition the heart. METHODS In the first series of experiments, myocardial infarct was induced by 30 min of coronary occlusion and 3 h of reperfusion in the rabbit. Infarct size was determined by tetrazolium staining and expressed as a percentage of area at risk (%IS/AR). Prior to the 30-min ischemia, rabbits were subjected to no PC, single PC (i.e., PC with an episode of 5 min ischemia/5 min reperfusion), and repetitive PC (2 cycles of 5 min ischemia/5 min reperfusion) with or without one of three treatments: polymyxin B (PolyB), staurosporine (Stauro), and 8-sulfophenylthephylline (SPT). In the second series of experiments, the rabbits received 5 min of coronary occlusion after repetitive PC with or without PolyB or Stauro treatment. Then, myocardial tissue in the ischemic region was sampled for assay of PKC activity. Untreated rabbits served as controls. RESULTS Single and repetitive PC limited %IS/AR to the same extent (%IS/AR = 9.8 +/- 1.9 and 10.4 +/- 2.3, both p < 0.05, vs. the control value of 44.5 +/- 3.4), and single PC was blocked by PolyB (%IS/AR = 43.9 +/- 2.7) and Stauro (%IS/AR = 31.5 +/- 3.2). Although the protocol of PolyB injection maintained the plasma PolyB level during sustained ischemia well above its Ki for PKC, this agent and also Stauro failed to abolish the protection by repetitive PC (%IS/AR = 21.6 +/- 3.0 and 11.4 +/- 4.3, respectively). SPT, an adenosine receptor antagonist, not only blocked single PC (%IS/AR = 44.4 +/- 4.4) but also attenuated protection by repetitive PC (%IS/AR = 28.3 +/- 3.6). Infarct sizes in non-preconditioned hearts were not modified by PolyB, Stauro, or SPT. The ratio of membrane fraction PKC activity to cytosolic fraction PKC activity was elevated by repetitive PC plus 5 min ischemia, and this change in PKC was inhibited in hearts given PolyB and Stauro. CONCLUSIONS In contrast to single PC, repetitive PC protects the heart against infarction even when PolyB and Stauro are administered to inhibit PKC during ischemic insult. This difference may be attributable to a PKC-independent mechanism, in which the adenosine receptor may be partly involved.

ADENOSINE AND PRECONDITIONING REVISITED

1. Myocardial tolerance against infarction is substantially increased by exposing myocytes to 3–10 min transient ischaemia. In this phenomenon, termed ‘preconditioning’, the adenosine receptor is one of the redundant triggers and the best characterized factor in the cardioprotective mechanism.

Time Window for the Contribution of the δ-opioid Receptor to Cardioprotection by Ischemic Preconditioning in the Rat Heart

The present study aimed to examine (1) whether the role of the opioid receptor in ischemic preconditioning (PC) is consistent regardless of the duration of ischemic insult and (2) which opioid receptor subtype contributes to PC. In the first series of experiments, the effects of PC, a nonselective opioid receptor antagonist (naloxone), and their combination on the infarct size after various durations of ischemia were assessed. In anesthetized, open-chest rats, the coronary artery was occluded for 20, 30, or 40 minutes to induce infarction and was reperfused for 3 hours, PC was performed with two cycles of 5-minute ischemia followed by 5-minute reperfusion before the sustained ischemia. At 25 minutes before the ischemia, naloxone was injected alone or in combination with subsequent PC. Infarct size was determined by tetrazolium staining and was expressed as a percentage of the risk area size (%IS/RA). In the second series of experiments, the effects of a δ-receptor selective antagonist, naltrindole (NTI), and a κ-receptor selective antagonist, nor-binaltrophimine (nor-BNI), on PC before 30-minute coronary occlusion were assessed. In untreated controls, %IS/RA was 53.1 ± 3.2 after 20 minutes, 67.9 ± 3.9 after 30 minutes, and 87.8 ± 2.0 after 40 minutes of ischemia, respectively. PC significantly reduced %IS/RA after 20, 30, and 40 minutes of ischemia to 3.1 ± 0.8, 12.8 ± 1.1, and 42.1 ± 4.3, respectively (P < 0.05 vs. each control). Naloxone (6 mg/kg) partially attenuated the protection afforded by PC when the sustained ischemia was 30 minutes (%IS/RA = 27.4 ± 4.5; P < 0.05 vs. PC), but this inhibitory effect of naloxone was not detected when the duration of the ischemia was 20 or 40 minutes. NTI (10 mg/kg) also attenuated infarct size limitation by PC after 30 minutes of ischemia (%IS/RA = 25.6 ± 3.7), but nor-BNI (10 mg/kg) failed to modify infarct size limitation by PC (%IS/RA = 13.3 ± 3.2). The present results suggest that activation of the opioid δ-receptor partly contributes to preconditioning against infarction in the rat and that there may be a time window (at around 30 minutes after the onset of ischemia) for this opioid receptor–mediated protective mechanism.

Suppression of reperfusion arrhythmias by preconditioning is inhibited by an ATP-sensitive potassium channel blocker, 5-hydroxydecanoate, but not by protein kinase C blockers in the rat.

It has been suggested that preconditioning (PC) against infarction in the rat heart is mediated by opening of ATP-sensitive potassium (KATP) channels, which may be induced by receptor-triggered activation of protein kinase C (PKC). However, the mechanism of suppression of reperfusion arrhythmias by PC remains unclear. This study first examined whether suppression of reperfusion arrhythmias by PC requires the activation of KATP channels, PKC, and opioid receptors. In anesthetized rats, reperfusion arrhythmias were induced by occluding the left main coronary artery for 5 min and subsequent reperfusion. In untreated control rats, the incidence of reperfusion ventricular tachycardia (VT) was 100%, and 80% of the VT deteriorated to ventricular fibrillation (VF). PC with 2-min ischemia/5-min reperfusion reduced the incidences of VT and VF to 30% and 0, respectively. Although a KATP channel blocker, 5-hydroxydecanoate (5-HD), alone caused no significant effect on the incidence of reperfusion VT, this agent blocked the suppression of reperfusion VT by PC (VT incidence, 91%). The incidence of reperfusion VF in the 5-HD-treated rats tended to be lower than that in the untreated controls and was not different between preconditioned and nonpreconditioned groups (30 vs. 27%). PKC inhibitors staurosporine and calphostin C modified neither reperfusion arrhythmias nor the antiarrhythmic effect of PC on reperfusion VT and VF. Naloxone (6 mg/kg, i.v.) did not alter the incidence or duration of VT in the nonpreconditioned heart. However, suppression of reperfusion VT by PC was prevented by naloxone (VT incidence, 70%). The incidence of reperfusion VF was similarly low in the naloxone-treated rats in both nonpreconditioned and preconditioned groups (20% vs. 0). In the second series of experiments, the effect of 5-HD on repetitive PC was assessed. Repetitive PC was performed with three cycles of 2-min ischemia/5-min reperfusion, which totally abolished reperfusion VT and VF. This antiarrhythmic effect of repetitive PC was not inhibited by 5-HD. These results suggest that KATP channels and opioid receptors may be partly involved in suppression of reperfusion arrhythmias, although their roles may be compensated for by other antiarrhythmic mechanisms in repetitive PC. In contrast with PC against infarction, PKC is unlikely to play a major role in PC against reperfusion arrhythmias in the rat.

Infarct-size limitation by preconditioning is enhanced by dipyridamole administered before but not after preconditioning: evidence for the role of interstitial adenosine level during preconditioning as a primary determinant of cardioprotection.

Although the importance of adenosine (Ado)-receptor activation in preconditioning (PC) has been established, it is unclear whether cardioprotection afforded by PC is determined by the Ado level during PC ischemia or by that during sustained ischemia. Accordingly, we tested whether the PC effect is modified by augmenting the increase in the interstitial Ado level during PC or by that during sustained ischemia. In the first series of experiments, the effect of 0.25 mg/kg dipyridamole (DIP) on the interstitial Ado level was assessed by in vivo microdialysis in the rabbit heart. Dialysate Ado during 2-min ischemia was 70% higher in the heart pretreated with 0.25 mg/kg of DIP than in the untreated controls, indicating that DIP was capable of enhancing an ischemia-induced increase of interstitial Ado. In the second series of experiments, myocardial infarction was induced in the rabbit by 30-min coronary artery occlusion and 3-h reperfusion. Infarct size was determined by tetrazolium staining and expressed as percentage of area at risk (%IS/AR). Rabbits were subjected to one of nine treatments before the 30-min ischemia: no treatment, DIP (0.25 mg/kg, i.v.), PC with 2-min ischemia, DIP before 2-min PC, DIP after 2-min PC, PC with 3-min ischemia, DIP before 3-min PC, DIP after 3-min PC, or 8-sulfophenyltheophylline (SPT) after 3-min PC. DIP alone did not modify %IS/AR (38.8 +/- 5.8% vs. 41.2 +/- 4.7%), but administration of DIP before 2-min PC significantly enhanced the infarct size-limiting effect (14.6 +/- 2.1% with DIP vs. 32.1 +/- 4.7% without DIP). Although the 3-min PC per se could achieve significant infarct limitation, the effect of DIP on 3-min PC was not significant (14.7 +/- 1.9% with DIP vs. 20.5 +/- 1.8% without DIP). On the other hand, the effect of DIP administered after PC was very slight (only 7% reduction of %IS/AR) and statistically insignificant, regardless of the duration of PC ischemia. However, infarct limitation by 3-min PC was inhibited by SPT given after the PC (%IS/AR = 34.5 +/- 3.2), as reported previously. These results suggest that the interstitial Ado level during PC ischemia, not the level during sustained ischemia, is a primary determinant of the extent of cardioprotection by PC and that the threshold for Ado-receptor activation required during sustained ischemia is much lower than that for triggering PC.

Suppression of reperfusion arrhythmia by ischemic preconditioning in the rat : is it mediated by the adenosine receptor, prostaglandin, or bradykinin receptor ?

The mechanism for the suppression of reperfusion arrhythmia by preconditioning (PC) remains unknown. This study aimed to examine the roles of the adenosine receptor, prostaglandin (PG), and bradykinin (BK) receptor in PC. Under pentobarbital anesthesia, the coronary artery of the rat was occluded for 5 min and then reperfused. In untreated controls, this protocol induced ventricular tachycardia (VT) in 100% of the rats and ventricular fibrillation (VF) in 60%. PC with 2 min ischemia/5 min reperfusion prior to the 5 min coronary occlusion significantly reduced the incidence of reperfusion VT and VF to 30% and 0%, respectively. This antiarrhythmic effect of the PC was not blocked when rats were pretreated with 8-phenyltheophylline (8-PT, 10 mg/kg), aspirin-DL-lysin (18 mg/kg), or a specific BK receptor antagonist, Hoe 140 (20 nmol/kg). None of these agents alone significantly modified the incidence of reperfusion VT or VF. These results suggest that neither the adenosine receptor, endogenous PG, nor BK receptor play a major role in the mechanism of suppression of perfusion arrhythmias by PC in the rat heart.