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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.