BackgroundMitochondrial large-conductance Ca2+-sensitive potassium (mBKCa) channels are involved in myocardial ischemic preconditioning. Their role in sildenafil-induced cardioprotection is unknown. We investigated whether sildenafil-induced acute cardioprotection is mediated by activation of mBKCa channels in the rat heart in vitro.MethodsMale Wistar rats (n = 8 per group) were randomized and anesthetized with pentobarbital (90 mg/kg). Hearts were isolated, mounted on a Langendorff system and perfused with Krebs-Henseleit buffer at a constant pressure of 80 mmHg. Hearts underwent 30 min of global ischemia followed by 60 min of reperfusion. At the end of the experiments infarct size was determined by TTC staining. In the control group rats were not further treated. Sildenafil (3 μM) was administered over 10 min before the beginning of ischemia. The mBKCa channel inhibitor paxilline (1 μM) was administered with and without sildenafil before the onset of ischemia. The pathway underlying sildenafil-induced cardioprotection was further investigated with the protein kinase G blocker KT5823 (1 μM). Myocardial cGMP concentration was measured by ELISA. Data (mean±SD) were analysed with a one and two-way analysis of variance as appropriate.ResultsIn control animals infarct size was 52±8%. Sildenafil increased cGMP concentration and reduced infarct size to 35±6% (P<0.05 vs. control). Paxilline and KT5823 completely blocked sildenafil-induced cardioprotection (paxilline+sildenafil: 50±8%, KT5823+sildenafil: 45±8%; both P<0.05 vs. sildenafil). Functional heart parameters and coronary flow were not different between the study groups.ConclusionThis study shows that in male rats protein kinase G-dependent opening of mBKCa channels plays a pivotal role in sildenafil-induced cardioprotection.

BackgroundMorphine induces myocardial preconditioning (M-PC) via activation of mitochondrial large conductance Ca2+-sensitive potassium (mKCa) channels. An upstream regulator of mKCa channels is protein kinase A (PKA). Furthermore, mKCa channel activation regulates mitochondrial bioenergetics and thereby prevents opening of the mitochondrial permeability transition pore (mPTP). Here, we investigated in the rat heart in vivo whether 1) M-PC is mediated by activation of PKA, and 2) pharmacological opening of the mPTP abolishes the cardioprotective effect of M-PC and 3) M-PC is critically dependent on STAT3 activation, which is located upstream of mPTP within the signalling pathway.MethodsMale Wistar rats were randomised to six groups (each n = 6). All animals underwent 25 minutes of regional myocardial ischemia and 120 minutes of reperfusion. Control animals (Con) were not further treated. Morphine preconditioning was initiated by intravenous administration of 0.3 mg/kg morphine (M-PC). The PKA blocker H-89 (10 μg/kg) was investigated with and without morphine (H-89+M-PC, H-89). We determined the effect of mPTP opening with atractyloside (5 mg/kg) with and without morphine (Atr+M-PC, Atr). Furthermore, the effect of morphine on PKA activity was tested in isolated adult rat cardiomyocytes. In further experiments in isolated hearts we tested the protective properties of morphine in the presence of STAT3 inhibition, and whether pharmacological prevention of the mPTP-opening by cyclosporine A (CsA) is cardioprotective in the presence of STAT3 inhibition.ResultsMorphine reduced infarct size from 64±5% to 39±9% (P<0.05 vs. Con). H-89 completely blocked preconditioning by morphine (64±9%; P<0.05 vs. M-PC), but H-89 itself had not effect on infarct size (61±10%; P>0.05 vs. Con). Also, atractyloside abolished infarct size reduction of morphine completely (65±9%; P<0.05 vs. M-PC) but had no influence on infarct size itself (64±5%; P>0.05 vs. Con). In isolated hearts STAT3 inhibitor Stattic completely abolished morphine-induced preconditioning. Administration of Stattic and mPTP inhibitor cyclosporine A reduced infarct size to 31±6% (Stat+CsA, P<0.05 vs. Con). Cyclosporine A alone reduced infarct size to 26±7% (CsA P<0.05 vs. Con). In cardiomyocytes, PKA activity was increased by morphine.ConclusionOur data suggest that morphine-induced cardioprotection is mediated by STAT3-activation and inhibition of mPTP, with STA3 located upstream of mPTP. There is some evidence that protein kinase A is involved within the signalling pathway.

No abstract