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The adverse impact of common diseases like diabetes mellitus and acute hyperglycemia on morbidity and mortality from myocardial infarction (MI) has been well documented over the past years of research. In the clinical setting, the relationship between blood glucose and mortality appears linear, with amplifying risk associated with increasing blood glucose levels. Further, this seems to be independent of a diagnosis of diabetes. In the experimental setting, various comorbidities seem to impact ischemic and pharmacological conditioning strategies, protecting the heart against ischemia and reperfusion injury. In this translational experimental approach from bedside to bench, we set out to determine whether acute and/or prolonged hyperglycemia have an influence on the protective effect of transferred human RIPC-plasma and, therefore, might obstruct translation into the clinical setting. Control and RIPC plasma of young healthy men were transferred to isolated hearts of young male Wistar rats in vitro. Plasma was administered before global ischemia under either short hyperglycemic (HGs Con, HGs RIPC) conditions, prolonged hyperglycemia (HGl Con, HGl RIPC), or under normoglycemia (Con, RIPC). Infarct sizes were determined by TTC staining. Control hearts showed an infarct size of 55 +/- 7%. Preconditioning with transferred RIPC plasma under normoglycemia significantly reduced infarct size to 25 +/- 4% (p < 0.05 vs. Con). Under acute hyperglycemia, control hearts showed an infarct size of 63 +/- 5%. Applying RIPC plasma under short hyperglycemic conditions led to a significant infarct size reduction of 41 +/- 4% (p < 0.05 vs. HGs Con). However, the cardioprotective effect of RIPC plasma under normoglycemia was significantly stronger compared with acute hyperglycemic conditions (RIPC vs. HGs RIPC; p < 0.05). Prolonged hyperglycemia (HGl RIPC) completely abolished the cardioprotective effect of RIPC plasma (infarct size 60 +/- 7%; p < 0.05 vs. HGl Con; HGl Con 59 +/- 5%).

Ramelteon is a Melatonin 1 (MT1)-and Melatonin 2 (MT2)-receptor agonist conferring cardioprotection by pharmacologic preconditioning. While activation of mitochondrial calcium-sensitive potassium (mK(Ca))-channels is involved in this protective mechanism, the specific upstream signaling pathway of Ramelteon-induced cardioprotection is unknown. In the present study, we (1) investigated whether Ramelteon-induced cardioprotection involves activation of protein kinase G (PKG) and/or protein kinase B (Akt) and (2) determined the precise sequence of PKG and Akt in the signal transduction pathway of Ramelteon-induced preconditioning. Hearts of male Wistar rats were randomized and placed on a Langendorff system, perfused with Krebs-Henseleit buffer at a constant pressure of 80 mmHg. All hearts were subjected to 33 min of global ischemia and 60 min of reperfusion. Before ischemia, hearts were perfused with Ramelteon (Ram) with or without the PKG or Akt inhibitor KT5823 and MK2206, respectively (KT5823 + Ram, KT5823, MK2206 + Ram, MK2206). To determine the precise signaling sequence, subsequent experiments were conducted with the guanylate cyclase activator BAY60-2770 and the mK(Ca)-channel activator NS1619. Infarct size was determined by 2,3,5-triphenyltetrazolium chloride (TTC) staining. Ramelteon-induced infarct size reduction was completely blocked by KT5823 (p = 0.0012) and MK2206 (p = 0.0005). MK2206 with Ramelteon combined with BAY60-2770 reduced infarct size significantly (p = 0.0014) indicating that PKG activation takes place after Akt. Ramelteon and KT5823 (p = 0.0063) or MK2206 (p = 0.006) respectively combined with NS1619 also significantly reduced infarct size, indicating that PKG and Akt are located upstream of mK(Ca)-channels. This study shows for the first time that Ramelteon-induced preconditioning (1) involves activation of PKG and Akt; (2) PKG is located downstream of Akt and (3) both enzymes are located upstream of mK(Ca)-channels in the signal transductio

Anesthetics, especially propofol, are discussed to influence ischemic preconditioning. We investigated whether cardioprotection by milrinone or levosimendan is influenced by the clinically used anesthetics propofol, sevoflurane or dexmedetomidine. Hearts of male Wistar rats were randomised, placed on a Langendorff system and perfused with Krebs-Henseleit buffer (KHB) at a constant pressure of 80 mmHg. All hearts underwent 33 min of global ischemia and 60 min of reperfusion. Three different anesthetic regimens were conducted throughout the experiments: propofol (11 mu M), sevoflurane (2.5 Vol%) and dexmedetomidine (1.5 nM). Under each anesthetic regimen, pharmacological preconditioning was induced by administration of milrinone (1 mu M) or levosimendan (0.3 mu M) 10 min before ischemia. Infarct size was determined by TTC staining. Infarct sizes in control groups were comparable (KHB-Con: 53 +/- 9%, Prop-Con: 56 +/- 9%, Sevo-Con: 56 +/- 8%, Dex-Con: 53 +/- 9%; ns). Propofol completely abolished preconditioning by milrinone and levosimendan (Prop-Mil: 52 +/- 8%, Prop-Lev: 52 +/- 8%; ns versus Prop-Con), while sevoflurane did not (Sevo-Mil: 31 +/- 9%, Sevo-Lev: 33 +/- 7%; p < 0.05 versus Sevo-Con). Under dexmedetomidine, results were inconsistent; levosimendan induced infarct size reduction (Dex-Lev: 36 +/- 6%; p < 0.05 versus Dex-Con) but not milrinone (Dex-Mil: 51 +/- 8%; ns versus Dex-Con). The choice of the anesthetic regimen has an impact on infarct size reduction by pharmacological preconditioning.

The activation of mitochondrial calcium-sensitive potassium (mBK(Ca)) channels is crucially involved in cardioprotection induced by preconditioning. For milrinone (Mil)-induced preconditioning, the involvement of mBK(Ca)-channels and further mitochondrial signaling is unknown. We hypothesize that (1) Mil-induced preconditioning is concentration-dependent and (2) that the activation of mBK(Ca)-channels, release of reactive oxygen species (ROS), and the mitochondrial permeability transition pore (mPTP) could be involved. Isolated hearts of male Wistar rats were perfused with Krebs-Henseleit buffer and underwent 33 min of ischemia followed by 60 min of reperfusion. For determination of a concentration-dependent effect of Mil, hearts were perfused with different concentrations of Mil (0.3-10 mu M) over 10 min before ischemia. In a second set of experiments, in addition to controls, hearts were pretreated with the lowest protective concentration of 1 mu M Mil either alone or combined with the mBK(Ca)-channel blocker paxilline (Pax + Mil), or paxilline alone (Pax). In additional groups, Mil was administered with and without the ROS scavenger N-2-mercaptopropionylglycine (MPG + Mil, MPG) or the mPTP inhibitor cyclosporine A (MPG + Mil + CsA, CsA + Mil), respectively. Infarct sizes were determined by triphenyltetrazolium chloride (TTC) staining. The lowest and most cardioprotective concentration was 1 mu M Mil (Mil 1: 32 +/- 6%; p < 0.05 vs. Con: 63 +/- 8% and Mil 0.3: 49 +/- 6%). Pax and MPG blocked the infarct size reduction of Mil (Pax + Mil: 53 +/- 6%, MPG + Mil: 59 +/- 7%; p < 0.05 vs. Mil: 34 +/- 6%) without having an effect on infarct size when administered alone (Pax: 53 +/- 7%, MPG: 58 +/- 5%; ns vs. Con). The combined administration of CsA completely restored the MPG-inhibited cardioprotection of Mil (MPG + Mil + CsA: 35 +/- 7%, p < 0.05 vs. MPG + Mil). Milrinone concentration-dependently induces preconditioning. Cardioprotection is mediated by the activation of m

Purpose Small and big conductance Ca2+-sensitive potassium (K-Ca) channels are involved in cardioprotective measures aiming at reducing myocardial reperfusion injury. For levosimendan, infarct size-reducing effects were shown. Whether activation of these channels is involved in levosimendan-induced postconditioning is unknown. We hypothesized that levosimendan exerts a concentration-dependent cardioprotective effect and that both types of Ca2+-sensitive potassium channels are involved. Methods In a prospective blinded experimental laboratory investigation, hearts of male Wistar rats were randomized and placed on a Langendorff system, perfused with Krebs-Henseleit buffer at a constant pressure of 80 mmHg. All hearts were subjected to 33 min of global ischemia and 60 min of reperfusion. At the onset of reperfusion, hearts were perfused with various concentrations of levosimendan (0.03-1 mu M) in order to determine a concentration-response relationship. To elucidate the involvement of K-Ca-channels for the observed cardioprotection, in the second set of experiments, 0.3 mu M levosimendan was administered in combination with the subtype-specific K-Ca-channel inhibitors paxilline (1 mu M, big K-Ca-channel) and NS8593 (0.1 mu M, small K-Ca-channel) respectively. Infarct size was determined by tetrazolium chloride (TTC) staining. Results Infarct size in controls was 60 +/- 7% and 59 +/- 6% respectively. Levosimendan at a concentration of 0.3 mu M reduced infarct size to 30 +/- 5% (P < 0.0001 vs. control). Higher concentrations of levosimendan did not induce a stronger effect. Paxilline but not NS8593 completely abolished levosimendan-induced cardioprotection while both substances alone had no effect on infarct size. Conclusions Cardioprotection by levosimendan-induced postconditioning shows a binary phenomenon, either ineffective or with maximal effect. The cardioprotective effect requires activation of big but not small K-Ca channels.