Time dependency in the protection from myocardial injury after myocardial ischemia and reperfusion : New insights from experimental studies with the ultrashort-acting calcium antagonist clevidipine

Abstract: Interruption of the coronary blood flow causes myocardial ischemia and, after a critical time period, myocardial cell death. Accordingly, early reinstitution of arterial blood flow is necessary to minimize the damage. There is, however, ample evidence that restoration of arterial blood flow per se will increase functional and structural injury (ischemialreperfusion injury) in the jeopardized myocardium. Cytosolic calcium overloading in the myocytes is supposed to be one of the major mechanisms and calcium antagonists have been reported to have beneficial effects. However, the site of action and myocardial protective mechanisms of this class of drugs are far from understood. The development of an ultrashort-acting calcium antagonist, clevidipine recently opened new possibilities for investigating the site of action and mechanisms behind the cardioprotective effects of calcium antagonists. In Study I, using Langendorff-perfused rat hearts, some basic cardiac pharmacology of clevidipine was examined and compared with that of isradipine and nifedipine. Spontaneous heart rate and atrio-ventricular conduction were not affected by clevidipine, while nifedipine and isradipine caused a decrease in heart rate and atrio-ventricular block at high concentrations. In Study II pigs were subjected to 45 min of LAD ligation followed by 240 min of reperfusion. Either clevidipine or the vehicle was administered to the ischemic myocardium by retrograde coronary venous infusion in association with reperfusion. The infarct size was determined with triphenyl tetrazolium chloride staining. The results demonstrated that clevidipine exerted cardioprotective effects against ischemialreperfusion injury. The protection was probably generated by local mechanisms within the ischemic tissue. In Study III pigs were subjected to 45 min of myocardial ischemia followed by 120 min of reperfusion via the left internal thoracic artery using an off-pump coronary arterial bypass grafting model. The animals were divided into 4 groups: one control group, one group given clevidipine (Cle), one group subjected to ischemic preconditioning (IP) and the last group received both Cle and IP. In the groups subjected to IP, the LAD was occluded for 5 min and reperfused for 15 min before the 45 min of ischemia. In all groups, clevidipine or vehicle was administered for 3 min via an intracoronary arterial infusion to the ischemic area, starting 5 min after induction of ischemia. Both clevidipine and ischemic preconditioning were myocardioprotective and the combination of clevidipine and ischemic preconditioning caused additive protection compared with ischemic preconditioning only. In Study IV pigs were subjected to ischemia through LAD ligation for 15, 35, or 45 min while one group was left without LAD occlusion. Clevidipine was infused for 5 min through a catheter distal to the LAD ligation in the ischemic pig hearts, or at the corresponding site of the non-ligated LAD. The pigs were subjected to 60 min of reperfusion or in the non-ligated group following the drug infusion. Blood samples were obtained from the femoral artery and the coronary vein for analysis of clevidipine. The results show that in pigs clevidipine has an ultrashort blood half-life of less than 0.5 min. Very low levels of clevidipine were detected in the coronary venous blood only during the first two min of reperfusion and there were no detectable levels in the arterial blood at any time. In Study V pigs were subjected to 45 min of LAD occlusion followed by 240 min of reperfusion. Clevidipine was infused over 5 min into the LAD starting at 5, 35 or 44 min after the onset of ischemia. The infarct size was determined as in Study II. The study revealed that blockade of calcium influx by clevidipine either at the early phase of ischemia or just prior to reperfusion limited infarct size. However, when the drug was given late during the ischemia it did not salvage myocardial tissue at risk. This indicates that calcium influx is of variable pathophysiological importance during different phases of myocardial ischemia and reperfusion. In conclusion clevidipine has pharmacokinetic properties that makes it a relevant pharmacological tool for studying the cardioprotective mechanisms of calcium antagonists. It was found that clevidipine reduces myocardial damage caused by ischemia followed by reperfusion in pigs. Moreover, it is likely that blockade of calcium influx during the early phase of ischemia and at the time of reperfusion are protective, while administration of the drug during a late phase of ischemia does not influence the final infarct size. Finally, when given together with ischemic preconditioning clevidipine enhances myocardioprotection compared to ischemic preconditioning alone.