Failure of magnesium to protect isolated cardiomyocytes from effects of hypoxia or metabolic poisoning

Retention of Nanoparticles-Labeled Bone Marrow Mononuclear Cells in the Isolated Ex Vivo Perfused Heart After Myocardial Infarction in Animal Model

Cell therapy of myocardial infarction (MI) is under clinical investigation, yet little is known about its underlying mechanism of function. Our aims were to induce a sub-lethal myocardial infarction in a rabbit, to evaluate the abilities of labeled bone marrow mononuclear cells to migrate from the vessel bed into extracellular space of the myocardium, and to evaluate the short-term distribution of cells in the damaged left ventricle. Sub-lethal myocardial infarction was induced in rabbits by ligation of the left coronary vessel branch ( in vivo). The Langendorff heart perfusion model ( ex vivo) was used in the next phase. The hearts subjected to MI induction were divided into 3 groups (P1–P3), and hearts without MI formed a control group (C). Nanoparticles-labeled bone marrow mononuclear cells were injected into coronary arteries via the aorta. Perfusion after application lasted 2 minutes in the P1 group, 10 minutes in the P2 and C groups, and 25 minutes in the P3 group. The myocardium of the left ventricle was examined histologically, and the numbers of labeled cells in vessels, myocardium, and combined were determined. The numbers of detected cells in the P1 and C groups were significantly lower than in the P2 and P3 groups. In the P2 and P3 groups, the numbers of cells found distally from the ligation were significantly higher than proximally from the ligation site. Bone marrow mononuclear cells labeled with iron oxide nanoparticles proved the ability to migrate in the myocardium interstitium with significantly higher affinity for the tissue damaged by infarction.

Mg2+ protects adult beating cardiomyocytes against ischaemia

Although Mg2+ reduces infarct size in whole heart models of ischaemia/reperfusion, the cardioprotective effect of Mg2+ at the cellular level is still a controversial issue. Therefore, we tested whether Mg2+ protects cardiomyocytes against ischaemia. To accomplish this aim we used an experimental model of ischaemia that utilises single beating adult cardiomyocytes in which oxygen tension is tightly regulated without the use of oxygen scavengers or metabolic inhibitors. Taking all these into consideration, this model is probably closer to in vivo conditions than the majority of previously published cellular models of ischaemia. We found that the addition of extracellular Mg2+ (8 mM) increased the survival of cells exposed to ischaemia. As sarcolemma and mitochondria are end-effectors of cardioprotective signalling, we examined whether Mg2+ regulates sarcolemmal and mitochondrial events. Mg2+ (8 mM) did not affect the whole cell K+ current as revealed by patch clamp electrophysiology. Experiments with laser confocal microscopy and the mitochondrial membrane potential-sensitive dye, JC-1, showed that Mg2+ (8 mM) did not affect ischaemia-induced mitochondrial membrane depolarisation. However, a significantly lower JC-1 ratio was required to kill cells under control conditions than cells treated with Mg2+ (8 mM). Based on the obtained data, we conclude that Mg2+ protects single beating cardiomyocytes against ischaemia by increasing cellular resistance to the consequences of mitochondrial membrane depolarisation in the cytosol.

Anti-infarct effect of magnesium is not mediated by adenosine A1 receptors in rat globally ischaemic isolated hearts

1. The aim of present study was to investigate the effects of magnesium (Mg) on cardiac function and infarct size and to compare it effects with those of adenosine. The mechanism of Mg-mediated cardioprotection was explored by combined use of Mg and a selective adenosine A(1) receptor antagonist. 2. Rat isolated hearts were used for Langendorff perfusion. Hearts were either non-preconditioned or preconditioned with Mg (6 mmol/L) or adenosine (1 mmol/L) before 30 min sustained ischaemia followed by 120 min reperfusion. Within each of these protocols, hearts were divided into two groups; one group was exposed to the A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 200 nmol/L). Infarct size was measured by the triphenyltetrazolium chloride method. Left ventricular function was assessed by left ventricular developed pressure (LVDP), the product of heart rate x LVDP and coronary flow (CF). 3. The administration of Mg had an anti-infarct effect independent of its effect on postischaemic functional recovery in rats. Both Mg and adenosine equipotently reduced infarct size, but this effect of Mg was not blocked by the simultaneous administration of DPCPX. Cardiac function was improved by both adenosine and Mg and blockade of adenosine A(1) receptors attenuated these effects for both agents. 4. In conclusion, the results of the present study indicate that stimulation of adenosine A(1) receptors is not responsible for the anti-infarct effect of Mg in ischaemic myocardium in rats, but that the Mg-mediated protection of postischaemic functional recovery in rats is mediated by these receptors.

Hypoxia-induced preconditioning in adult stimulated cardiomyocytes is mediated by the opening and trafficking of sarcolemmal KATP channels

The opening of sarcolemmal and mitochondrial ATP-sensitive K(+) (KATP) channels in the heart is believed to mediate ischemic preconditioning, a phenomenon whereby brief periods of ischemia/reperfusion protect the heart against myocardial infarction. Here, we have applied digital epifluorescent microscopy, immunoprecipitation and Western blotting, perforated patch clamp electrophysiology, and immunofluorescence/laser confocal microscopy to examine the involvement of KATP channels in cardioprotection afforded by preconditioning. We have shown that adult, stimulated-to-beat, guinea-pig cardiomyocytes survived in sustained hypoxia for approximately 17 min. An episode of 5-min-long hypoxia/5-min-long reoxygenation before sustained hypoxia dramatically increased the duration of cellular survival. Experiments with different antagonists of KATP channels, applied at different times during the experimental protocol, suggested that the opening of sarcolemmal KATP channels at the beginning of sustained hypoxia mediate preconditioning. This conclusion was supported by perforated patch clamp experiments that revealed activation of sarcolemmal KATP channels by preconditioning. Immunoprecipitation and Western blotting as well as immunofluorescence and laser confocal microscopy showed that the preconditioning is associated with the increase in KATP channel proteins in sarcolemma. Inhibition of trafficking of KATP channel subunits prevented preconditioning without affecting sensitivity of cardiomyocytes to hypoxia in the absence of preconditioning. We conclude that the preconditioning is mediated by the activation and trafficking of sarcolemmal KATP channels.