Contributory mechanisms for the beneficial effects of myocyte preconditioning during cardioplegic arrest

Preconditioning protects the myocardium from ischemia and may be a potent means of endogenous cardioprotection during cardioplegic arrest and rewarming. However, fundamental mechanisms that potentially contribute to the beneficial effects of preconditioning during cardioplegic arrest and rewarming remain unclear. Accordingly, the overall goal of the present study was to examine the potential mechanisms by which preconditioning protects myocyte contractile function during simulated cardioplegic arrest and rewarming.

Left ventricular isolated porcine myocyte contractile function was examined with the use of videomicroscopy under three conditions: (1) normothermia, maintained in cell medium (37 degrees C) for 2 hours; (2) simulated cardioplegic arrest and rewarming, incubated in crystalloid cardioplegic solution (24 mEq/L K+, 4 degrees C) for 2 hours followed by normothermic reperfusion; and (3) preconditioning/cardioplegic arrest and rewarming, hypoxia (20 minutes) and reoxygenation (20 minutes) followed by simulated cardioplegic arrest and rewarming. Cardioplegic arrest and rewarming caused a decline in steady-state myocyte shortening velocity compared with normothermic controls (22.0 +/- 1.6 versus 57.2 +/- 2.6 microns/s, respectively, P < .05), which was significantly improved with preconditioning (36.1 1.7 microns/s, P < .05). In the next series of experiments, the influence of nonmyocyte cell populations with respect to preconditioning and cardioplegic arrest was examined. Endothelial or smooth muscle cell cultures were subjected to a period of hypoxia (20 minutes) and reoxygenation (20 minutes) and the eluent incubated with naive myocytes, which were then subjected to simulated cardioplegic arrest and rewarming. Pretreatment with the eluent from endothelial cultures followed by cardioplegic arrest and rewarming improved myocyte function compared with cardioplegia-alone values (31.7 +/- 2.2 versus 24.7 +/- 1.6 microns/s, respectively, P < .05), whereas smooth muscle culture eluent pretreatment resulted in no change (23.7 +/- 4.0 microns/s, P = .81). Molecular mechanisms for the protective effects of preconditioning on myocyte contractile processes with cardioplegic arrest and rewarming were examined in a final series of experiments. Adenosine-mediated pathways or ATP-sensitive potassium channels were activated by augmenting cardioplegic solutions with adenosine (200 mumol/L) or the potassium channel opener aprikalim (100 mumol/L), respectively. Both adenosine and aprikalim augmentation significantly improved myocyte function compared with cardioplegia-alone values (53.5 +/- 1.7, 57.6 +/- 2.0 versus 25.7 +/- 1.4 microns/s, respectively, P < .05).

The unique findings from the present study demonstrated that preconditioning provides protective effects on myocyte contractile processes independent of nonmyocyte cell populations and that these effects are mediated in part through the activation of adenosine pathways or ATP-sensitive potassium channels. Thus, preconditioning adjuvant to cardioplegia may provide a novel means of protecting myocardial function after cardioplegic arrest and rewarming.