In cardiomyocytes of rats, inhibition of calcineurin by CysA or the selective calcineurin inhibitor FK506 increased the phosphorylation of protein kinase B (AKT) [35]

In cardiomyocytes of rats, inhibition of calcineurin by CysA or the selective calcineurin inhibitor FK506 increased the phosphorylation of protein kinase B (AKT) [35]. I respiration was similar between CysA (116 11 nmol O2/min/mg protein) and IFR (117 8), but better preserved with PoCo (160 9;p< 0.05). RS 8359 Calcium retention capacity was greater with both PoCo and CysA (1096 45 and 1287 128 nmol Ca2+/mg protein) than with IFR (756 103;p< 0.05). == Conclusions == Cyclosporine A's protection is not associated with improved mitochondrial respiration. Protection is unlikely related to an upstream calcineurin inhibition, but is indeed secondary to mPTP inhibition. Keywords:cyclosporine A, infarct size, ischemic postconditioning, mitochondrion, myocardial ischemia/reperfusion == Introduction == Timely restoration of blood flow is mandatory to salvage ischemic myocardium from irreversible damage; however, reperfusion induces additional damage, i.e. reperfusion injury, and contributes to final infarct size [13]. Ischemic postconditioning (PoCo) i.e. brief episodes of intermittent coronary re-occlusion during early reperfusion and gentle reperfusion i.e. slow restoration of coronary blood flow protect myocardium from reperfusion injury [4,5]. Attenuation of reperfusion injury and thus reduction of final infarct size can also be achieved pharmacologically by cyclosporine A (CysA) given prior to reperfusion [6,7]. Infarct size reduction by PoCo and by CysA treatment has been confirmed experimentally in all species tested so far [811] and the cardioprotective effects of both PoCo and CysA are also operative in humans. In patients with acute myocardial RS 8359 infarction, PoCo induced by intermittent re-inflation of the balloon-catheter used for coronary angioplasty decreased myocardial injury, as measured by the release of marker enzymes [1214]. A protective effect of similar magnitude was also seen Sav1 in patients with acute myocardial infarction who were treated with CysA prior to reperfusion [15]. Both types of cardioprotection may thus share a common step in their signal transduction. The underlying mechanisms of cardioprotection during early reperfusion are not completely understood, but mitochondria are potential end-effectors of cardioprotection. The preservation of mitochondrial function after ischemia/reperfusion is decisive for survival of cardiomyocytes and thus salvage of myocardium [16]. The mitochondrial permeability transition pore (mPTP) plays an important role in cell death. Opening of the mPTP results in collapse of the mitochondrial membrane potential, uncoupling of the respiratory chain, and efflux of cytochrome c and other pro-apoptotic factors which finally induce apoptosis and necrosis [17]. Thus, the inhibition of mPTP opening appears to be decisive for cardiomyocyte survival at early reperfusion [1719], in particular when the duration of ischemia is longer than 30 min [20]. Cyclosporine A inhibits the opening of mPTP by binding to cyclophilin D at the inner mitochondrial membrane, and CysA’s protection against myocardial infarction is usually attributed to this mechanism [2123] (Figure 1; path A). However, CysA also binds to cyclophilin A, which inhibits the calcium-dependent serine-threonine phosphatase calcineurin [24]. The inhibition of de-phosphorylating properties of calcineurin could therefore increase/ maintain the phosphorylation of proteins (Figure 1; path B). Cardioprotective signaling relies indeed on increased phosphorylation of cardioprotective proteins, and increased phosphorylation of one or more cardioprotective proteins results in better mitochondrial function, which becomes apparent as better preserved mitochondrial respiration after ischemia/ reperfusion. Indeed, with cardioprotection by PoCo, the better preserved mitochondrial complex I respiration is causally linked to increased phosphorylation of signal transducer and activator of transcription 3 (STAT3) in pig cardiomyocyte mitochondria [25]. == Figure 1. == Potential mechanisms of protection by cyclosporine A (CysA) through mitochondrial permeability transition pore (mPTP) inhibition (path A) or calcineurin inhibition after ischemia/reperfusion (path B). Calcineurin inhibition can increase the phosphorylation of cardioprotective proteins. Such an increase in protein phosphorylation may contribute to better preserved mitochondrial respiration and calcium retention capacity (CRC) and thus ultimately to infarct size reduction Calcineurin inhibition might also RS 8359 be associated with protection of mitochondria. The regulatory subunit calcineurin B is present in mitochondria from rat kidneys [26], and calcineurin dephosphorylates the pro-apoptotic protein Bad [27] and enables its translocation into cardiomyocyte mitochondria [28]. Cell death induced by stimulation of rat cardiomyocytes with the -adrenergic agonist isoproterenol is secondary to dephosphorylation of Bad and is inhibited by CysA or the selective calcineurin inhibitor FK506 [29]. In cardiomyocytes isolated from dogs with heart failure, CysA treatment improves mitochondrial respiration [30]. To elucidate whether.