Ischaemic preconditioning by partial occlusion without intermittent reperfusion

Remote ischemic conditioning: evolution of the concept, mechanisms, and clinical application

Remote ischemic conditioning is a novel concept of protection against ischemia-reperfusion injury. Brief controlled episodes of intermittent ischemia of the arm or leg may confer a powerful systemic protection against prolonged ischemia in a distant organ. This conditioning phenomenon is clinically applicable and can be performed before--preconditioning, during--perconditioning, or after--postconditioning prolonged distant organ ischemia. The remote ischemic conditioning may have an immense impact on clinical practice in the near future.

Interaction of cardiovascular risk factors with myocardial ischemia/reperfusion injury, preconditioning, and postconditioning

Therapeutic strategies to protect the ischemic myocardium have been studied extensively. Reperfusion is the definitive treatment for acute coronary syndromes, especially acute myocardial infarction; however, reperfusion has the potential to exacerbate lethal tissue injury, a process termed "reperfusion injury." Ischemia/reperfusion injury may lead to myocardial infarction, cardiac arrhythmias, and contractile dysfunction. Ischemic preconditioning of myocardium is a well described adaptive response in which brief exposure to ischemia/reperfusion before sustained ischemia markedly enhances the ability of the heart to withstand a subsequent ischemic insult. Additionally, the application of brief repetitive episodes of ischemia/reperfusion at the immediate onset of reperfusion, which has been termed "postconditioning," reduces the extent of reperfusion injury. Ischemic pre- and postconditioning share some but not all parts of the proposed signal transduction cascade, including the activation of survival protein kinase pathways. Most experimental studies on cardioprotection have been undertaken in animal models, in which ischemia/reperfusion is imposed in the absence of other disease processes. However, ischemic heart disease in humans is a complex disorder caused by or associated with known cardiovascular risk factors including hypertension, hyperlipidemia, diabetes, insulin resistance, atherosclerosis, and heart failure; additionally, aging is an important modifying condition. In these diseases and aging, the pathological processes are associated with fundamental molecular alterations that can potentially affect the development of ischemia/reperfusion injury per se and responses to cardioprotective interventions. Among many other possible mechanisms, for example, in hyperlipidemia and diabetes, the pathological increase in reactive oxygen and nitrogen species and the use of the ATP-sensitive potassium channel inhibitor insulin secretagogue antidiabetic drugs and, in aging, the reduced expression of connexin-43 and signal transducer and activator of transcription 3 may disrupt major cytoprotective signaling pathways thereby significantly interfering with the cardioprotective effect of pre- and postconditioning. The aim of this review is to show the potential for developing cardioprotective drugs on the basis of endogenous cardioprotection by pre- and postconditioning (i.e., drug applied as trigger or to activate signaling pathways associated with endogenous cardioprotection) and to review the evidence that comorbidities and aging accompanying coronary disease modify responses to ischemia/reperfusion and the cardioprotection conferred by preconditioning and postconditioning. We emphasize the critical need for more detailed and mechanistic preclinical studies that examine car-dioprotection specifically in relation to complicating disease states. These are now essential to maximize the likelihood of successful development of rational approaches to therapeutic protection for the majority of patients with ischemic heart disease who are aged and/or have modifying comorbid conditions.

Intravenous adenosine protects the myocardium primarily by activation of a neurogenic pathway

Endogenous adenosine is a trigger for ischemic myocardial preconditioning (IPC). Although intravascular administration of adenosine has been used to further unravel the mechanism of protection by IPC, it is questionable whether adenosine and IPC employ the same signaling pathways to exert cardioprotection. We therefore investigated whether the active metabolic barrier of the endothelium prevents an increase in myocardial interstitial adenosine concentrations by intravenous adenosine, using microdialysis, and also the role of NO and activation of a neurogenic pathway in the cardioprotection by adenosine. In pentobarbital-anesthetized rats, area at risk and infarct size (IS) were determined 120 min after a 60-min coronary artery occlusion (CAO), using trypan blue and nitro-blue-tetrazolium staining, respectively. IPC with a single 15-min CAO and a 15-min adenosine infusion (ADO, 200 microg min(-1) i.v.) limited IS to the same extent (IS = 41 +/- 6% and IS = 40 +/- 4%, respectively) compared to control rats (IS = 63 +/- 3%, both P < 0.05). However, IPC increased myocardial interstitial adenosine levels seven-fold from 4.3 +/- 0.7 to 27.1 +/- 10.0 microM (P < 0.05), while ADO had no effect on interstitial adenosine (4.1 +/- 1.2 microM), or any of the other purines. The NO synthase inhibitor N(omega)-nitro-L-arginine (LNNA), which did not affect IS (IS = 62 +/- 3%), attenuated the protection by ADO (IS = 56 +/- 3%; P < 0.05 vs ADO, P = NS vs LNNA). The ganglion blocker hexamethonium, which had also no effect on IS (IS = 66 +/- 3%), blunted the protection by ADO (IS = 55 +/- 4%; P < 0.05 vs ADO and vs hexamethonium). These observations demonstrate that cardioprotection by ADO is dependent on NO, and is primarily mediated by activation of a neurogenic pathway.

The tyrosine phosphatase inhibitor bis(maltolato)oxovanadium attenuates myocardial reperfusion injury by opening ATP-sensitive potassium channels

Vanadate has been shown to inhibit tyrosine phosphatase, leading to an increased tyrosine phosphorylation state. The latter has been demonstrated to be involved in the signal transduction pathway of ischemic preconditioning, the most potent endogenous mechanism to limit myocardial infarct size. Furthermore, there is evidence that phosphatase inhibition may be cardioprotective when given late after the onset of ischemia, but the mechanism of protection is unknown. We tested the hypothesis that the organic vanadate compound bis(maltolato)oxovanadium (BMOV) limits myocardial infarct size by attenuating reperfusion injury and investigated the underlying mechanism. Myocardial infarction was produced in 112 anesthetized rats by a 60-min coronary artery occlusion, and infarct size was determined histochemically after 180 min of reperfusion. Intravenous infusion of BMOV in doses of 3.3, 7.5, and 15 mg/kg i.v. decreased infarct size dose-dependently from 70 +/- 2% of the area at risk in vehicle-treated rats down to 41 +/- 5% (P < 0.05 versus control), when administered before occlusion. Administration of the low dose just before reperfusion was ineffective, but administration of the higher doses was equally cardioprotective as compared with administration before occlusion. The cardioprotection by BMOV was abolished by the tyrosine kinase inhibitor genistein and by the ATP-sensitive potassium (K(+)(ATP)) channel blocker glibenclamide but was not affected by the ganglion blocker hexamethonium. We conclude that BMOV afforded significant cardioprotection principally by limiting reperfusion injury. The mode of action appears to be by opening of cardiac K(+)(ATP) channels via increased tyrosine phosphorylation.

Cardioprotection in pigs by exogenous norepinephrine but not by cerebral ischemia-induced release of endogenous norepinephrine

BACKGROUND AND PURPOSE

Endogenous norepinephrine release induced by cerebral ischemia may lead to small areas of necrosis in normal hearts. Conversely, norepinephrine may be one of the mediators that limit myocardial infarct size by ischemic preconditioning. Because brief ischemia in kidneys or skeletal muscle limits infarct size produced by coronary artery occlusion, we investigated whether cardiac norepinephrine release during transient cerebral ischemia also elicits remote myocardial preconditioning.

METHODS

Forty-one crossbred pigs of either sex were assigned to 1 of 7 experimental groups, of which in 6 groups myocardial infarct size was determined after a 60-minute coronary occlusion and 120 minutes of reperfusion. One group served as control (no pretreatment), while the other groups were pretreated with either cerebral ischemia or an intracoronary infusion of norepinephrine.

RESULTS

In 10 anesthetized control pigs, infarct size was 84+/-3% (mean+/-SEM) of the area at risk after a 60-minute coronary occlusion and 120 minutes of reperfusion. Intracoronary infusion of 0.03 nmol/kg. min(-)(1) norepinephrine for 10 minutes before coronary occlusion did not affect infarct size (80+/-3%; n=6), whereas infusion of 0.12 nmol/kg. min(-)(1) limited infarct size (65+/-2%; n=7; P:<0.05). Neither 10-minute (n=5) nor 30-minute (n=6) cerebral ischemia produced by elevation of intracranial pressure before coronary occlusion affected infarct size (83+/-4% and 82+/-3%, respectively). Myocardial interstitial norepinephrine levels tripled during cerebral ischemia and during low-dose norepinephrine but increased 10-fold during high-dose norepinephrine. Norepinephrine levels increased progressively up to 500-fold in the area at risk during the 60-minute coronary occlusion, independent of the pretreatment, while norepinephrine levels remained unchanged in adjacent nonischemic myocardium and arterial plasma.

CONCLUSIONS

Cerebral ischemia preceding a coronary occlusion did not modify infarct size, which is likely related to the modest increase in myocardial norepinephrine levels during cerebral ischemia. The infarct size limitation by high-dose exogenous norepinephrine is not associated with blunting of the ischemia-induced increase in myocardial interstitial norepinephrine levels.

Microembolization in pigs: effects on coronary blood flow and myocardial ischemic tolerance

Coronary microembolization has been reported to increase coronary blood flow (CBF) through adenosine release. Because adenosine may increase ischemic tolerance against infarction, we tested the hypothesis that myocardial microembolization, a common finding in patients with ischemic heart disease, induces cardioprotection. Additionally, because the use of microspheres is a common tool to measure tissue perfusion, the effects of small amounts of microspheres on CBF were examined. Using anesthetized pigs, we measured CBF with a transit time flow probe on the left anterior descending coronary artery (LAD). In six pigs the relationship between the amount of injected microspheres (0-40 x 10(6), 15 micrometer in diameter, left atrial injections) and the effect on CBF was examined. Coronary hyperemia occurred, which was linearly related to the amount of microspheres injected: maximal increase in CBF (%) = 2.8 +/- 1.5 (SE) + (5.8 +/- 0.7 x 10(-7) x number of injected microspheres). Because injection of 40 x 10(6) microspheres induced a long-lasting hyperemic response, which could be blocked by 8-p-sulfophenyl theophylline, ischemic tolerance was examined in five other pigs after two injections, each of 40 x 10(6) microspheres, at a 30-min interval. Six control pigs had no injections. Ischemic tolerance was evaluated by measuring infarct size (tetrazolium stain) as the percentage of area at risk (fluorescent particles) after 45 min of LAD occlusion followed by 2 h of reperfusion. Pretreatment by microspheres increased infarct size from 60 +/- 3% of area at risk in control animals to 84 +/- 6% (P < 0.05). The injection of microspheres induced a significant hyperemic flow response without causing necrosis by itself. We conclude that microembolization, evoking coronary hyperemia, does not improve but reduces myocardial ischemic tolerance against infarction in pigs.

New insights into cardioprotection by ischemic preconditioning and other forms of stress

Ischemic preconditioning has not only received wide attention in heart research, but has also been a topic of extensive studies involving other organs. In several of these studies, it has been shown that in spite of differences in the endpoints used to assess protection, the same mediators as in myocardial ischemic preconditioning may be involved. However, several of the putative mediators do not require ischemia to become activated. This has guided us and others to investigate whether the myocardium can also be protected by brief ischemia in other organs and whether other non-pharmacological forms of stress, which do not produce ischemia but are capable of activating these potential mediators, are also cardioprotective.

Ischemic preconditioning: exploring the paradox

Brief transient episodes of nonlethal myocardial ischemia protect or "precondition" the heart and render the myocardium resistant to a subsequent more sustained ischemic insult. The hallmark of this phenomenon--documented in virtually all species and experimental models evaluated to date in countless laboratories worldwide--is the profound reduction in infarct size seen in preconditioned groups versus time-matched controls. Efforts to identify the cellular mechanisms responsible for this paradoxical ischemia-induced cardioprotection, to expand the definition of ischemic preconditioning beyond infarct size reduction, and, perhaps most importantly, to evaluate the efficacy of preconditioning in disease models and in the clinical setting, are all topics of intensive ongoing investigation.

Lack of adenosine causes myocardial refractoriness

OBJECTIVES

This study sought to investigate the myocardial mechanisms causing refractoriness to ischemic preconditioning in pigs.

BACKGROUND

Ischemic preconditioning in the pig vanishes after 60 min and cannot be reinstated by a second cycle of brief coronary occlusions at this time point. Ischemic preconditioning has been shown to be mediated by adenosine A1-receptors. Because myocardial adenosine production during ischemia ceases as the number of repeated brief ischemic episodes increases, we hypothesized that this lack of adenosine may cause this myocardial refractoriness.

METHODS

In open chest pigs, ischemic preconditioning was achieved by repeated brief coronary occlusions. Myocardial adenosine content was assessed by high performance liquid chromatographic analysis of serial myocardial biopsy samples; infarct size (percent infarcted area of the area at risk) was determined using tetrazolium salts.

RESULTS

Ischemic preconditioning by two cycles of occlusion of the left anterior descending coronary artery (10 min) and reperfusion (30 min) decreased infarct size ([mean +/- SEM] 40.4+/-2.9%; control: 76.9+/-1.8%, p < 0.001). Prolonging the second reperfusion period to 60 min caused ischemic preconditioning to vanish (79.0+/-0.5%) and caused refractoriness to a second cycle of preconditioning (70.0+/-2.0%). Myocardial adenosine content increased only during the first coronary occlusion (baseline: 110.9+/-42.0 nmol/g dry weight; first coronary occlusion: 1,686.2+/-244.1, p < 0.001) but not during subsequent coronary occlusions. In refractory myocardium, intramyocardial microinfusion of the adenosine A1-receptor agonist N6-cyclohexyladenosine (CHA [0.3 mmol/liter]) again decreased infarct size (27.4+/-7.0%, p < 0.001 vs. control).

CONCLUSIONS

Myocardial refractoriness may be caused by the inability to produce adenosine endogenously. In refractory myocardium, application of CHA reinduces cardioprotection.

Ischemic preconditioning at a distance: reduction of myocardial infarct size by partial reduction of blood supply combined with rapid stimulation of the gastrocnemius muscle in the rabbit

BACKGROUND

Limitation of myocardial infarct size by an earlier brief complete occlusion of a coronary artery is defined as ischemic preconditioning. However, myocardial protection also can be achieved by partial reduction of coronary flow, rapid cardiac pacing, or brief ischemia-reperfusion of a remote region of the heart. Our study assesses the effect on myocardial infarct size of preconditioning at a distance induced by partial reduction of blood flow to a hind limb with or without increase of demand by electrical stimulation of a skeletal muscle.

METHODS AND RESULTS

Anesthetized rabbits were randomized to 30 minutes of waiting period (controls), 55% to 65% reduction of femoral artery blood flow (stenosis), electrical stimulation of the gastrocnemius muscle at a rate of one per second (stimulation), or stenosis+stimulation. Thereafter, rabbits underwent 30 minutes of coronary artery occlusion and 4 hours of reperfusion. Each group included 8 rabbits. Risk zones were comparable among groups. However, the ratio of infarct size to risk zone was smaller in the stenosis+stimulation group (0.09+/-0.02) compared with the control (0.26+/-0.03), stenosis (0.36+/-0.05), and stimulation (0.30+/-0.05) groups (P=.0006). ANCOVA performed on the fraction of infarction (infarct size/left ventricular weight) and the fraction of risk zone revealed a significant group effect (P=.0004).

CONCLUSIONS

Remote ischemia of a skeletal muscle induced by muscle stimulation combined with restriction of blood flow preconditioned the myocardium. The combination of muscle stimulation with reduction of femoral arterial blood flow but not muscle stimulation without blood flow restriction or of flow restriction without muscle stimulation reduced myocardial infarct size considerably.

Presence of a critical coronary artery stenosis does not abolish the protective effect of ischemic preconditioning

BACKGROUND

Episodic, severe coronary artery flow restriction preconditions the myocardium much like brief occlusions. The necessity for full reperfusion after a preconditioning intervention to elicit the preconditioning response is unclear. This study investigated in closed-chest swine the effect of a persistent critical coronary stenosis with moderate flow reduction on ischemic preconditioning.

METHODS AND RESULTS

Farm pigs (n = 23) assigned to one of four groups--(1) control, (2) stenosis, (3) preconditioned (PC), or (4) preconditioned plus stenosis (PC/S)--underwent percutaneous instrumentation with a percutaneous transluminal coronary angioplasty catheter advanced to the mid-left anterior descending coronary artery. An artificial coronary stenosis (82% diameter reduction) was mounted on the catheter just proximal to the balloon in the two stenosis groups. Preconditioning stimulus consisted of two 10-minute balloon occlusions followed by 15 minutes of reperfusion. All groups subsequently underwent 45 minutes of occlusion followed by 120 minutes of reperfusion. Baseline regional myocardial blood flow in the area at risk (AAR), measured with colored microspheres, was lowest in the stenosis groups, with flow expressed as a percentage of normal zone flow. Infarct size (percent of AAR), determined by staining slices of the heart with triphenyltetrazolium, was significantly reduced in PC compared with control pigs (15.1 +/- 5.9% versus 66.8 +/- 6.4%, respectively; P < .001). Infarct size in PC/S pigs was also significantly reduced (29.7 +/- 7.1%, P = .004 versus control) but was not different in degree from PC pigs (P = .6). The stenosis by itself conferred no preconditioning benefit (percent of AAR = 69.0 +/- 5.4%).

CONCLUSIONS

A moderate flow-limiting stenosis did not prevent preconditioning but may have attenuated the effect. This may be analogous to the clinical scenario in which intermittent coronary occlusion and reperfusion superimposed on a critical stenosis precede a prolonged occlusion treated with thrombolysis.

Does protein kinase C play a pivotal role in the mechanisms of ischemic preconditioning?

This communication reviews the evidence for the pivotal role of protein kinase C in ischemic myocardial preconditioning. It is believed that several intracellular signalling pathways via receptor-coupled phospholipase C and its "cross-talk" with phospholipase D converge to activation of protein kinase C isotypes which is followed by phosphorylation of until now (a number of) unknown target proteins which produce the protective state of ischemic preconditioning. After briefly introducing the general biochemical properties of protein kinase C, its isotypes and the limitations of the methodology used to investigate the role of protein kinase C, studies are discussed in which pharmacological inhibition and activation and (immunore) activity and/or isotypes measurements of protein kinase C isotypes were applied to assess the role of activation of protein kinase C in ischemic myocardial preconditioning. It is concluded that definitive proof for the involvement of protein kinase C in preconditioning requires future studies which must focus on the isotype(s) of protein kinase C that are activated, the duration of action, cellular translocation sites and the identity and stability (of covalently bound phosphate) of phosphorylated substrate proteins.

Metabolic adaptation during a sequence of no-flow and low-flow ischemia. A possible trigger for hibernation

BACKGROUND

Myocardial hibernation is an adaptive phenomenon occurring in patients with a history of acute ischemia followed by prolonged hypoperfusion.

METHODS AND RESULTS

We investigated, in isolated rabbit heart, whether a brief episode of global ischemia followed by hypoperfusion maintains viability. Four groups were studied; group 1,300 minutes of aerobia; group 2,240 minutes of total ischemia and 60 minutes of reperfusion; group 3, 10 minutes of total ischemia, 230 minutes of hypoperfusion (90% coronary flow reduction), and 60 minutes of reperfusion; and group 4, 240 minutes of hypoperfusion followed by reperfusion. In group 3, viability was maintained. Ten minutes of ischemia caused quiescence, a fall in interstitial pH (from 7.2 +/- 0.01 to 6.1 +/- 0.8), creatine phosphate (CP), and ATP (from 54.5 +/- 5.0 and 25.0 +/- 1.9 to 5.0 +/- 1.1 and 15.3 +/- 2.5 mumol/g dry wt, P < .01). Subsequent hypoperfusion failed to restore contraction and pH but improved CP (from 5.0 +/- 1.1 to 20.1 +/- 3.4, P < .01). Reperfusion restored pH, developed pressure (to 92.3%), and NAD/NADH and caused a washout of lactate and creatine phosphokinase with no alterations of mitochondrial function or oxidative stress. In group 4, hypoperfusion resulted in progressive damage. pH fell to 6.2 +/- 0.7, diastolic pressure increased to 34 +/- 5.6 mm Hg, CP and ATP became depressed, and oxidative stress occurred. Reperfusion partially restored cardiac metabolism and function (47%).

CONCLUSIONS

A brief episode of total ischemia without intermittent reperfusion maintains viability despite prolonged hypoperfusion. This could be mediated by metabolic adaptation, preconditioning, or both.

Cardioprotection by ischemic and nonischemic myocardial stress and ischemia in remote organs. Implications for the concept of ischemic preconditioning

Ischemic preconditioning studies employ one or more brief total coronary artery occlusions separated by complete reperfusion to limit infarct size during a subsequent prolonged coronary artery occlusion. We now present evidence that in anesthetized pigs a partial coronary artery occlusion without intervening reperfusion between the partial and prolonged total occlusions can also precondition the myocardium provided that the reduction in coronary blood flow is sufficiently severe. Thus infarct size was reduced after a 60 min total coronary artery occlusion when the total occlusion was preceded by a partial coronary occlusion that reduced coronary blood flow by 70% but not when the flow reduction was only 30%. In this two-stage coronary occlusion model the degree of protection appears greater in the epicardial than in the endocardial half. In view of evidence that brief occlusions of a coronary artery also protect myocardium outside its perfusion territory, we subsequently investigated whether ischemia in remote organs can protect myocardium. Because of reports that development of infarct size may be temperature dependent, we also investigated whether the cardioprotection by remote organ ischemia was temperature dependent. In anesthetized rats a 15 min coronary artery occlusion was more effective in reducing infarct size produced by a subsequent 60 min total coronary artery occlusion when the experiments were performed at a body core temperature of 30-31 degrees C than at 36-37 degrees C, while infarct size of animals which were subjected to only the 60 min total coronary artery occlusion was the same for the two body core temperatures. In rats with a body core temperature of 36-37 degrees C a 15 min mesenteric artery occlusion, but not a 15 min renal artery occlusion, reduced infarct size produced by a subsequent 60 min coronary artery occlusion. When the experiments were performed at 30-31 degrees C both the mesenteric and renal artery occlusions were protective. These observations indicate the local myocardial ischemia is not required to protect the myocardium during a prolonged coronary occlusion. We further investigated whether myocardium could also be protected by a cardiac stimulus which does not produce ischemia at all. For this purpose we electrically paced the left ventricle of anesthetized pigs to produce heart rates of 200 bpm (which did not lead to ischemia as assessed by a number of functional and biochemical variables) and found that 30 min of ventricular pacing reduced myocardial infarct size produced by a subsequent 60 min coronary artery occlusion. The protection by ventricular pacing involved activation of K+ATP channels as pretreatment with glibenclamide abolished the protection by ventricular pacing. We conclude that a number of distinctly different stimuli can protect the myocardium suggesting that ischemic myocardial preconditioning could be just one feature of a more general protection phenomenon.

Glyburide enhancement of lactate production in ischemic heart is modified by preconditioning: an in vivo experimental study in pigs by microdialysis technique

The concentrations of lactate, pyruvate, and adenosine, together with some of their derivatives, were determined in microdialysates from 12 pig hearts, 6 of which were subjected to preconditioning and 40 min of ischemia (index ischemia) and 6 of which were subjected to only 40 min of index ischemia. Two microdialysis probes were inserted in ischemic myocardium. Glyburide (10 mu M) in a modified isotonic Krebs-Ringer phosphate buffer was administered through one of the probes and plain isotonic phosphate buffer was administered through the other. Accordingly, the experimental setup permitted us to study the metabolic effects of glyburide on ischemic myocardium constituting two groups that were either preconditioned or unpreconditioned. The preconditioning effect was validated with area at risk and infarction area measurements in 12 other pigs. We noted no functional differences between the groups. In the unpreconditioned group glyburide infusion resulted in enhanced 60% lactate production during index ischemia. However, preconditioning attenuated the enhancing effect of glyburide on lactate production. The interplay between the effects of glyburide and preconditioning on ischemic myocardium is suggested to be dependent on the different modes of action on the K(+)(ATP) channel.

Rapid ventricular pacing produces myocardial protection by nonischemic activation of KATP+ channels

BACKGROUND

Rapid ventricular pacing reduces the incidence of ventricular arrhythmias during a subsequent sustained period of ischemia and reperfusion. We investigated whether rapid ventricular pacing also limits myocardial infarction and determined the role of KATP+ channels in the protection afforded by ventricular pacing.

METHODS AND RESULTS

Myocardial infarction was produced by a 60-minute coronary artery occlusion in open chest pigs. Infarct size of pigs subjected to 10 minutes of ventricular pacing at 200 beats per minute followed by 15 minutes of normal sinus rhythm before the occlusion (79 +/- 3% of the area at risk, mean +/- SEM) was not different from control infarct size (84 +/- 2%). Thirty-minute pacing followed by 15-minute sinus rhythm resulted in modest reductions in infarct size (71 +/- 2%, P<.05 versus control). Thirty minutes of pacing immediately preceding the occlusion without intervening sinus rhythm resulted in considerable limitation of infarct size (63 +/- 4%, P<.05), which was abolished by pretreatment with the KATP+ channel blocker glibenclamide (78 +/- 4%, P=NS). KATP+ channel activation did not appear to involve ischemia: (1) myocardial endocardial/epicardial blood flow ratio was 1.07 +/- 0.08, (2) phosphocreatine and ATP levels and arterial-coronary venous differences in pH and PCO2 were unchanged, (3) end-systolic segment length did not increase and postsystolic shortening was not observed during pacing, and (4) systolic shortening recovered immediately to baseline levels and coronary reactive hyperemia was absent after cessation of pacing. Administration of glibenclamide after 30 minutes of pacing at the onset of 15 minutes of normal sinus rhythm did not attenuate the protection (73 +/- 3%, P<.05 versus control), suggesting the KATP+ channels did not contribute to the moderate degree of protection that was still present 15 minutes after cessation of pacing.

CONCLUSIONS

Rapid ventricular pacing protects the myocardium against infarction via nonischemic KATP+ channel activation. Continued activation of KATP+ channels does not appear mandatory for the protection that is still present 15 minutes after cessation of pacing.

Stenosis severity and the occurrence of ventricular ectopic activity during acute coronary occlusion during balloon angioplasty

To elucidate the incidence and determinants of early ventricular arrhythmias (VA) during acute coronary occlusion, continuous electrocardiographic, heart rate, and blood pressure recordings were performed in 152 patients during standardized balloon occlusions of significant (50% to 95%) coronary artery stenoses. A control group of 13 patients with chronic total occlusion of a coronary artery was also studied. None of them developed VA during balloon inflation in the preexisting total occlusion of the artery. Balloon occlusion of a coronary artery was associated with occurrence of ventricular ectopy in 18 patients (VA group, 12%). The VA group had milder stenosis severity (72% vs 81%, p < 0.001) than the rest of the patients, and none of them had visible collaterals to the occluded vessel. The VA group also had ST-segment deviations more often (p < 0.05) during occlusion than patients with no VA. Occlusion of the left anterior descending artery caused VA more often (p < 0.05) than occlusion of the left circumflex or right coronary artery. No clinical or hemodynamic variable or medication was associated with the occurrence of VA. In stepwise logistic regression analysis, the only significant predictors of ventricular ectopic activity were the stenosis severity and the anterior site of coronary occlusion. Even a nonstenotic plaque can be so fragile that it is prone to rupture. The present findings suggest that such an occlusion may result in electrical instability more easily than occlusion of a more advanced coronary lesion.

Intraischemic preconditioning. Increased tolerance to sustained low-flow ischemia by a brief episode of no-flow ischemia without intermittent reperfusion

Ischemic preconditioning (IP) and myocardial hibernation (MH) are both adaptive phenomena during acute myocardial ischemia, characterized by preserved myocardial viability and attenuated alterations of energy metabolism. Recent data from isolated buffer-perfused rabbit hearts pointed to a further link between IP and MH, in that an initial stimulus of no-flow ischemia was required to permit the development of MH during subsequent sustained low-flow ischemia. In the present study, we therefore investigated in the in situ pig heart whether a brief episode of no-flow ischemia enhances the myocardial tolerance to subsequent sustained low-flow ischemia. By blocking ATP-dependent potassium channels, we attempted to further determine whether such increased tolerance to ischemia is related to IP or MH, since blockade of ATP-dependent potassium channels abolishes the cardioprotection achieved by IP but not by MH. In 8 enflurane-anesthetized pigs serving as controls (group 1), the inflow into the cannulated left anterior descending coronary artery was reduced to achieve a 90% reduction in the anterior myocardial work index (sonomicrometry) for 90 minutes. In 15 pigs (group 2), a 10-minute no-flow ischemic episode preceded 80 minutes of sustained ischemia at a blood flow reduction identical to that in pigs of group 1. In 8 additional pigs (group 3), glibenclamide was administered before the 10-minute no-flow ischemic episode. In all pigs after 120 minutes of reperfusion, infarct size (IS, percentage of area at risk) was determined by triphenyltetrazolium chloride staining. In group 2, IS was reduced (6.8 +/- 6.0% [mean +/- SD], P < .05) when compared with groups 1 (13.2 +/- 9.8%) and 3 (16.7 +/- 8.3%).(ABSTRACT TRUNCATED AT 250 WORDS)