Preservation of myofilament calcium responsiveness underlies protection against myocardial stunning by ischemic preconditioning

Simulated ischemia-induced preconditioning of isolated ventricular myocytes from young adult and aged Fischer-344 rat hearts

The impact of ischemic preconditioning (IPC) on contraction, Ca(2+) homeostasis, and cell survival was compared in isolated ventricular myocytes from young adult ( approximately 3 mo) and aged ( approximately 24 mo) male Fischer-344 rats. Myocytes were field stimulated at 4 Hz (37 degrees C). Contraction (edge detector) and intracellular Ca(2+) (fura-2) were measured simultaneously. Viability was assessed with trypan blue. All cells were exposed to 30 min of simulated ischemia followed by reperfusion. Some cells were preconditioned by exposure to 5 min of simulated ischemia before prolonged ischemia. Pretreatment with IPC abolished postischemic contractile depression, inhibited diastolic contracture, and increased Ca(2+) transient amplitudes in reperfusion in young adult and aged cells. IPC did not affect the modest rise in diastolic Ca(2+) in ischemia in young adult myocytes. However, IPC abolished the marked rise in diastolic Ca(2+) observed in ischemia and early reperfusion in aged myocytes. IPC also suppressed mechanical alternans in ischemia in aged cells, but younger myocytes showed little evidence of mechanical alternans whether or not cells were preconditioned. IPC markedly improved cell viability in reperfusion in young adult but not aged cells. These results suggest that IPC augments the recovery of contractile function in reperfusion by increasing Ca(2+) transient amplitudes in ventricular myocytes from young adult and aged rats. IPC reduced diastolic Ca(2+) accumulation in ischemia in aged myocytes, which may diminish the severity of mechanical alternans in aged cells. Nonetheless, the efficacy of IPC is compromised in aging, as IPC did not improve survival of aged myocytes exposed to ischemia and reperfusion.

[Role of Levosimendan in intensive care treatment of myocardial insufficiency]

Levosimendan is a calcium sensitizer that is currently in the focus of intensive care medicine because it may be superior to standard inotropic agents in the treatment of acute myocardial insufficiency. The effects of levosimendan mainly depend on three predominant mechanisms: 1) positive inotropic effect by increasing the sensitivity of cardiac myofilaments to calcium ions, 2) vasodilatory effect by stimulation of adenosine triphosphate-sensitive potassium channels and 3) inhibition of phosphodiesterase-III. In a large number of experimental and clinical studies further possible indications for levosimendan have been described, e.g. cardioprotection during ischemia, cardiogenic shock, septic myocardial insufficiency and pulmonary hypertension. This review article critically summarizes the current scientific and clinical knowledge about levosimendan, its pharmacologic characteristics, mechanisms of action as well as indications and potential risks.

Hypoxia tolerance and preconditioning are not additive in the trout(Oncorhynchus mykiss) heart

Research has shown that the trout heart is normally hypoxia-sensitive, and that it can be preconditioned. However, we have identified a group of rainbow trout Oncorhynchus mykiss that shows a surprising degree of myocardial hypoxia tolerance. In this study, we used in situ hearts from these fish as a comparative model to examine whether the cardioprotective effects afforded by hypoxic adaptation and preconditioning are additive. In situ trout hearts were exposed to severe hypoxia (perfusate PO2 5–10 mmHg) in the absence and presence of a transient hypoxic pre-exposure (preconditioning). The four groups studied were: (1) control (no hypoxia); (2) 5 min of severe hypoxia;(3) 30 min of severe hypoxia; and (4) 5 min of severe hypoxia (hypoxic preconditioning) followed 20 min later by 30 min of severe hypoxia. 30 min of severe hypoxia significantly decreased maximum cardiac output and stroke volume by 15–30%. However, hypoxic preconditioning failed to confer any protection against post-hypoxic myocardial dysfunction. This work shows that the protection afforded by inherent myocardial hypoxia tolerance and preconditioning are not additive in this population of trout, and strongly suggests that the relationship between hypoxic adaptation and preconditioning in fishes resembles that of the neonatal/immature, not adult, mammalian heart. Further, our results (1) indicate that stretch (volume loading) and chronic exposure to low levels of adrenaline (15 nmol l-1) do not confer any protection against hypoxia-related myocardial dysfunction in this population, and (2) validate the use of the in situ trout heart as a comparative model for studying aspects of myocardial hypoxia tolerance and preconditioning in vertebrates.

Role of reactive oxygen species in ischemic preconditioning of subcellular organelles in the heart

Ischemic preconditioning (IPC) is an endogenous adaptive mechanism and is manifested by early and delayed phases of cardioprotection. Brief episodes of ischemia-reperfusion during IPC cause some subtle functional and structural alterations in sarcolemma, mitochondria, sarcoplasmic reticulum, myofibrils, glycocalyx, as well as nucleus, which render these subcellular organelles resistant to subsequent sustained ischemia-reperfusion insult. These changes occur in functional groups of various receptors, cation transporters, cation channels, and contractile and other proteins, and may explain the initial effects of IPC. On the other hand, induction of various transcriptional factors occurs to alter gene expression and structural changes in subcellular organelles and may be responsible for the delayed effects of IPC. Reactive oxygen species (ROS), which are formed during the IPC period, may cause these changes directly and indirectly and act as a trigger of IPC-induced cardioprotection. As ROS may be one of the several triggers proposed for IPC, this discussion is focused on the current knowledge of both ROS-dependent and ROS-independent mechanisms of IPC. Furthermore, some events, which are related to functional preservation of subcellular organelles, are described for a better understanding of the IPC phenomenon.

Reactive oxygen species as mediators of signal transduction in ischemic preconditioning

Ischemic preconditioning (IPC) is a most powerful endogenous mechanism for myocardial protection against ischemia/reperfusion injury. It is now apparent that reactive oxygen species (ROS) generated in the mitochondrial respiratory chain act as a trigger of IPC. ROS mediate signal transduction in the early phase of IPC through the posttranslational modification of redox-sensitive proteins. ROS-mediated activation of Src tyrosine kinases serves a scaffold for interaction of proteins recruited by G protein-coupled receptors and growth factor receptors that is necessary for amplification of cardioprotective signal transduction. Protein kinase C (PKC) plays a central role in this signaling cascade. A crucial target of PKC is the mitochondrial ATP-sensitive potassium channel, which acts as a trigger and a mediator of IPC. Mitogen-activated protein (MAP) kinases (extracellular signal-regulated kinase, p38 MAP kinase, and c-Jun NH(2)-terminal kinase) are thought to exist downstream of the Src-PKC signaling module, although the role of MAP kinases in IPC remains undetermined. The late phase of IPC is mediated by cardioprotective gene expression. This mechanism involves redox-sensitive activation of transcription factors through PKC and tyrosine kinase signal transduction pathways that are in common with the early phase of IPC. The effector proteins then act against myocardial necrosis and stunning presumably through alleviation of oxidative stress and Ca(2+) overload. Elucidation of IPC-mediated complex signaling processes will help in the development of more effective pharmacological approaches for prevention of myocardial ischemia/reperfusion injury.

Preconditioning: myocardial function and energetics during coronary hypoperfusion and reperfusion

BACKGROUND

Ischemic preconditioning (IP) is gaining more acceptance as a protective method in beating heart surgery. Yet it remains controversial whether preconditioning can attenuate myocardial dysfunction during reperfusion after severe coronary hypoperfusion. We examined this issue and also the issue of whether this protection is mediated by adenosine A1 receptors.

METHODS

In isolated, blood-perfused rabbit hearts, the effects of IP (3 minutes of no flow ischemia and 8 minutes of reperfusion) during 30 minutes of coronary hypoperfusion and 60 minutes of reperfusion were investigated. In two groups (n = 8 each) with and without (control group) preconditioning, ventricular function was assessed by load-insensitive measures: slope of the end-systolic pressure-volume relation (Emax), slope of the stroke work/end-diastolic volume relation (Mw), and end-diastolic pressure-volume relation. External efficiency was calculated, and contractile efficiency was assessed using the reciprocal of the myocardial oxygen consumption-pressure-volume area relationship. To investigate the possible role of adenosine, the adenosine A1 receptor antagonist DPCPX (2.5 micromol/L) was administered before preconditioning in a third group (n = 7).

RESULTS

The effects of hypoperfusion on systolic function, diastolic function (dP/dtmin, end-diastolic pressure-volume relation), external efficiency, and contractile efficiency were similar in both the IP and control groups. Lactate efflux was significantly reduced after preconditioning (p = 0.02). During reperfusion, recovery of systolic function and coronary flow were significantly improved in the IP group compared with controls: aortic flow, 85% versus 63% (p = 0.01); dP/dtmax, 91% versus 67% (p = 0.001); pressure-volume area, 97% versus 68% (p = 0.01); Emax, 74% versus 62% (p = 0.03); and Mw, 94% versus 84% (p = 0.04). Release of creatine kinase was reduced in the IP group, 9.6 +/- 1.3 U x 5 min(-1) x 100 g(-1) wet weight, versus controls, 12.7 +/- 2.7 U x 5 min(-1) x 100 g(-1) wet weight (p = 0.04). During reperfusion, contractile efficiency (p = 0.03) and external efficiency (p = 0.02) recovered better in preconditioned than in untreated hearts. Recovery was less pronounced in the DPCPX group compared with the IP group (p, not significant).

CONCLUSIONS

The results, derived from load-insensitive measures, confirm that IP provides protection after episodes of severe hypoperfusion by attenuating systolic dysfunction without improving diastolic dysfunction and reduces the severity of anaerobic metabolism as well as ischemic injury. Contractile efficiency and external efficiency both indicate improved energetics after IP (oxygen utilization by the contractile apparatus). The protective effect, at least in part, is mediated by adenosine A1 receptors.

Recovery of trout myocardial function following anoxia: preconditioning in a non-mammalian model

Studies with mammals and birds clearly demonstrate that brief preexposure to oxygen deprivation can protect the myocardium from damage normally associated with a subsequent prolonged hypoxic/ischemic episode. However, is not known whether this potent mechanism of myocardial protection, termed preconditioning, exists in other vertebrates including fishes. In this study, we used an in situ trout (Oncorhynchus mykiss) working heart preparation at 10 degrees C to examine whether prior exposure to 5 min of anoxia (PO(2) < or = 5 mmHg) could reduce or eliminate the myocardial dysfunction that normally follows 15 min of anoxic exposure. Hearts were exposed either to a control treatment (oxygenated perfusion) or to one of three anoxic treatments: 1) anoxia with low P(out) [15 min of anoxia at an output pressure (P(out)) of 10 cmH(2)O]; 2) anoxia with high P(out) [10 min of anoxia at a P(out) of 10 cmH(2)O, followed by 5 min of anoxia at P(out) = 50 cmH(2)O]; and 3) preconditioning [5 min of anoxia at P(out) = 10 cmH(2)O, followed after 20 min of oxygenated perfusion by the protocol described for the anoxia with high P(out) group]. Changes in maximum cardiac function, measured before and after anoxic exposure, were used to assess myocardial damage. Maximum cardiac performance of the control group was unaffected by the experimental protocol, whereas 15 min of anoxia at low P(out) decreased maximum stroke volume (V(s max)) by 15% and maximum cardiac output (Q(max)) by 23%. When the anoxic workload was increased by raising P(out) to 50 cmH(2)O, these parameters were decreased further (by 23 and 38%, respectively). Preconditioning with anoxia completely prevented the reductions in V(s max) and Q(max) that were observed in the anoxia with high P(out) group and any anoxia-related increases in the input pressure (P(in)) required to maintain resting Q (16 ml. min(-1). kg(-1)). Myocardial levels of glycogen and lactate were not affected by any of the experimental treatments; however, lactate efflux was sevenfold higher in the preconditioned hearts. These data strongly suggest that 1) a preconditioning-like mechanism exists in the rainbow trout heart, 2) increased anaerobic glycolysis, fueled by exogenous glucose, was associated with anoxic preconditioning, and 3) preconditioning represents a fundamental mechanism of cardioprotection that appeared early in the evolution of vertebrates.

Differential effect of preconditioning on post-ischaemic myocardial performance in the absence of substantial infarction and in extensively infarcted rat hearts

OBJECTIVES

There is controversy concerning the beneficial effects of ischaemic preconditioning during short periods of ischaemia (stunning). The aim of the study was to investigate post-ischaemic myocardial performance after various periods of ischaemia in both non-preconditioned and preconditioned hearts and to compare these results with infarct volume estimation.

METHODS

Isolated perfused rat hearts were subjected to various periods of sustained ischaemia (15, 20, 30, and 45 min). Haemodynamic parameters, infarct size and lactate dehydrogenase (LDH) leakage were recorded in both preconditioned and non-preconditioned hearts.

RESULTS

After 15 min of ischaemia, preconditioned hearts revealed significantly lower developed pressure than non-preconditioned hearts (80+/-4.1 vs. 95+/-0.3%, P=0.02). In the 20 min ischaemia group, preconditioning resulted in non-significantly lower developed pressure (76+/-3.1% in preconditioned hearts vs. 87+/-5.3% in non-preconditioned hearts, P=0.11). In these groups infarct volume was small and not different between non-preconditioned and preconditioned hearts. After 30 min of ischaemia, preconditioning significantly improved developed pressure (66+/-3.1% in preconditioned and 44+/-5% in non-preconditioned hearts, P=0.002). LDH leakage was significantly higher in non-preconditioned hearts compared with preconditioned hearts (16+/-2.3 vs. 9.0+/-1.3, P=0.04), whereas infarct volume was not (12.5+/-0.8 and 9.8+/-1.5, respectively, P=0.1). Non-preconditioned hearts of this group, subjected to inotropic stimulation at the end of reperfusion, responded poorly. Significantly higher developed pressure was attained by preconditioned hearts (150+/-3.1 vs. 123+/-7.5%, P=0.01). After 45 min of ischaemia, preconditioning resulted in 69% limitation of infarct volume (P<0.0001) and 53% reduction in LDH release (P=0.009). Developed pressure was 57+/-8.5% in preconditioned hearts and 32+/-4.5% in non-preconditioned hearts (P=0.02).

CONCLUSIONS

When ischaemic insult results in minimally lethal injuries, preconditioned hearts do not have the advantage of not being prone to stunning rather than non-preconditioned. If ischaemic insult is potentially able to produce extensive infarction, improvement in post-ischaemic myocardial function is mainly due to infarct size limitation evoked by preconditioning.

Is a functional sarcoplasmic reticulum necessary for preconditioning?

Several studies have shown that the protective effect of ischemic preconditioning (PC) is associated with decreased calcium release from the sarcoplasmic reticulum (SR). However, no study has yet demonstrated whether these changes are essential in the mechanism of PC. In order to investigate whether a functional SR was necessary for PC, we manipulated SR calcium handling using (i) 0.1microM ryanodine (RY), a concentration known to lock the SR calcium release channel in the open state and (ii) 50microM cyclopiazonic acid (CPA), a specific inhibitor of the SR calcium ATPase. Initial experiments confirmed that both RY and CPA eliminated the ability of the SR to accumulate calcium. Isolated rat hearts (n=6-7/group) were perfused normoxically for 30 min prior to either a further 40 min of perfusion [control (C)] or 4x[5 min ischemia (I) + 5 min reperfusion (R)] (PC). All hearts were then subjected to a further 40 min I + 40 min R. The C and PC protocols were then repeated in the presence of RY or CPA, introduced after 10 min of perfusion.(31)P-NMR was used to measure ATP, PCr, P(i)and intracellular pH. RY and CPA decreased developed pressure (DP) by 75% and 59%, respectively. Percentage recovery of LVDP was significantly higher in PC (72+/-8%), PC+RY (72+/-7%) and PC+CPA (49+/-7%) groups compared with their respective controls (43+/-7%, 47+/-7% and 10+/-4%) (P<0.05). Thus, PC remains protective in the presence of a SR unable to accumulate calcium, suggesting that the changes in SR calcium release are not essential in the mechanism of preconditioning.