Myocardial stunning--are calcium antagonists useful?

Recent Advances in Pharmacological and Non-Pharmacological Strategies of Cardioprotection

Ischemic heart diseases (IHD) are the leading cause of death worldwide. Although the principal form of treatment of IHD is myocardial reperfusion, the recovery of coronary blood flow after ischemia can cause severe and fatal cardiac dysfunctions, mainly due to the abrupt entry of oxygen and ionic deregulation in cardiac cells. The ability of these cells to protect themselves against injury including ischemia and reperfusion (I/R), has been termed “cardioprotection”. This protective response can be stimulated by pharmacological agents (adenosine, catecholamines and others) and non-pharmacological procedures (conditioning, hypoxia and others). Several intracellular signaling pathways mediated by chemical messengers (enzymes, protein kinases, transcription factors and others) and cytoplasmic organelles (mitochondria, sarcoplasmic reticulum, nucleus and sarcolemma) are involved in cardioprotective responses. Therefore, advancement in understanding the cellular and molecular mechanisms involved in the cardioprotective response can lead to the development of new pharmacological and non-pharmacological strategies for cardioprotection, thus contributing to increasing the efficacy of IHD treatment. In this work, we analyze the recent advances in pharmacological and non-pharmacological strategies of cardioprotection.

Dihydropyridines' metabolites-induced early apoptosis after myocardial infarction in rats; new outlook on preclinical study with M-2 and M-3

Our previous studies established cardio-protective effects of furnidipine and its active metabolites called M-2 and M-3. The aim of current research was to compare the effects of single oral pretreatment with 20 mg kg(-1) of M-2 and M-3 on mortality, different forms of arrhythmias, blood pressures parameters and ST-segment changes during occlusion (for 90 min) and reperfusion in the model of myocardial infarction in rats evoked by left anterior descending coronary artery occlusion. Additionally, the development of programmed cell death and biochemical parameters in blood serum were studied at 4th day after infarction. Furnidipines' metabolites effectively reduced mortality index while did not markedly influence on blood pressures parameters, arrhythmias, ST-segment changes as well as biochemical parameters. Intriguingly, programmed cell death study (TUNEL) showed distinct increase in the amount of apoptotic nuclei in post-infarcted myocardium, granulation tissue and what is more in arteriolar walls after M-2 and M-3 application. Moreover, M-2 turned out to be more powerful in stimulation of apoptosis in granulation tissue surrounding infarcted area whereas M-3 presented balanced profile in this matter. Taking into account that programmed cell death plays positive role in post-infarcted heart healing, M-2 presents itself as more attractive agent for oral pretreatment in early stages of ischemia by non-stable individuals due to its more specific action in stimulation repairing processes in granulation tissue as well as in arteriolar walls. While M-2 and M-3 are common metabolites present in degradation pathways of many widely used dihydropyridines in clinic, this key fact put the new outlook on understanding additional mechanism and effects of not only furnidipines' metabolites but also other dihydropyridines.

Anti-arrhythmic and cardio-protective effects of furnidipine in a rat model: A dose response study

Protective effects of acute oral or intravenous doses of furnidipine against ischemia and re-perfusion-induced arrhythmias and creatine kinase release were studied in a rat model for cardiac ischemia and re-perfusion. Transient cardiac ischemia was induced by occluding the left coronary descending artery of anaesthetized rats for 7 min, and re-perfusion period studied was 15 min. Pre-treatment period for oral doses (1, 5 or 10 mg/kg) was 1 h, whereas that for the intravenous ones (1.25, 2.5, 5 or 10 microg/kg) was 10 min. After both routes of administration, significant protective effects of furnidipine on creatine kinase release were observed after the two lowest doses only. In contrast, its higher dosages were more effective in preventing re-perfusion-induced mortality, arrhythmias and hypotensive episodes, and for transiently lowering arterial blood pressure before initiation of ischemia. These observations suggest potential uses of furnidipine for preventing re-perfusion triggered lethal arrhythmias. Efforts to evaluate therapeutic potential of low dose furnidipine as a cardio-protective agent seem warrantable.

The effects of tetrandrine on the contractile function and microvascular permeability in the stunned myocardium of rats

The effects of tetrandrine (Tet) on the contractile function and microvascular permeability in stunned rat myocardium in vivo were studied. Stunned myocardium was induced by 15 (MS(15) group) or 20 (MS(20) group) min of myocardial ischemia plus 60 min of reperfusion. The following was shown. (1) FITC-BSA concentration was 166.0 +/- 7. 9 microg/g myocardium in the control group. The concentrations in ischemic myocardium increased by 35.4 and 45.6% in MS(15) and MS(20) groups respectively (p<0.05). (2) Administration of Tet (64.2 and 96. 3 micromol/kg, I.P.) 20 min before ischemia not only ameliorated the contractile function, but also reduced the FITC-BSA concentrations in ischemic myocardium. At 60 min after reperfusion, the contractile function parameters in Tet-treated groups were significantly superior to those in corresponding stunning groups. FITC-BSA concentrations in Tet-treated groups were lower than those in stunning groups. Then, there was already no significant difference in FITC-BSA concentrations between Tet-treated groups and the control group. The FITC-BSA concentrations at the end of experiments were correlated negatively with dp/dt(max) (r = -0.83, p<0.01). (3) Tet inhibited KCl-induced calcium influx in isolated cardiomyocytes. The results suggest that Tet given before ischemia may be involved in the reduction of microvascular permeability in stunned myocardium, which might be associated with its calcium channel blocking effect.

Effects of verapamil and ryanodine on activity of the embryonic chick heart during anoxia and reoxygenation

Perturbations of the trans-sarcolemmal and sarcoplasmic Ca2+ transport contribute to the abnormal myocardial activity provoked by anoxia and reoxygenation. Whether Ca2+ pools of the extracellular compartment and sarcoplasmic reticulum (SR) are involved to the same extent in the dysfunction of the anoxic-reoxygenated immature heart has not been investigated. Spontaneously contracting hearts isolated from 4-day-old chick embryos were submitted to repeated anoxia (1 min) followed by reoxygenation (5 min). Heart rate, atrioventricular propagation velocity, ventricular shortening, velocities of contraction and relaxation, and incidence of arrhythmias were studied, recorded continuously. Addition of verapamil (10 nM), which blocks selectively sarcolemmal L-type Ca2+ channels, was expected to protect against excessive entry of extracellular Ca2+, whereas addition of ryanodine (10 nM), which opens the SR Ca2+ release channel, was expected to increase cytosolic Ca2+ concentration. Verapamil (a) had no dromotropic effect by contrast to adult heart, (b) attenuated ventricular contracture induced by repeated anoxia, (c) shortened cardioplegia induced by reoxygenation, and (d) had remarkable antiarrhythmic properties during reoxygenation specially. On the other hand, ryanodine potentiated markedly arrhythmias both during anoxia and at reoxygenation. Thus despite its immaturity, the SR seems to be functional early in the developing chick heart and involved in the reversible dysfunction induced by anoxia-reoxygenation. Moreover, Ca2+ entry through L-type channels appears to worsen arrhythmias especially during reoxygenation. These findings show that the Ca2+-handling systems involved in irregular activity in immature heart, such as the embryonic chick heart, may differ from those in the adult.

Injury to the Ca2+ ATPase of the sarcoplasmic reticulum in anesthetized dogs contributes to myocardial reperfusion injury

OBJECTIVE

Sarcoplasmic reticulum dysfunction may contribute to calcium (Ca2+) overload during myocardial reperfusion. The aim of this study was to investigate its role in reperfusion injury.

METHODS

Open chest dogs undergoing 15 min of left anterior descending coronary artery occlusion and 3 h of reperfusion were randomized to intracoronary infusions of 0.9% saline, vehicle, or the Ca2+ channel antagonist, nifedipine (50 micrograms/min from 2 minutes before to 5 minutes after reperfusion). After each experiment, transmural myocardial biopsies were removed from ischemic/reperfused and nonischemic myocardium in the beating state and analyzed for (i) sarcoplasmic reticulum protein content (Ca2+ ATPase, phospholamban, and calsequestrin) by immunoblotting and (ii) Ca2+ uptake by sarcoplasmic reticulum vesicles with and without 300 micromolar ryanodine or the Ca2+ ATPase activator, antiphospholamban (2D12) antibody.

RESULTS

Contractile function did not recover in controls and vehicle-treated dogs after ischemia and reperfusion (mean systolic shortening, -2 +/- 2%), but completely recovered in nifedipine-treated dogs (17 +/- 2%, p = NS vs. baseline, p < 0.01 vs. control). Ventricular fibrillation occurred in 50% of controls and vehicle dogs and 0% of nifedipine-treated dogs (p < 0.01). Ca2+ uptake by the sarcoplasmic reticulum vesicles was severely reduced in ischemic/reperfused myocardium of controls and vehicle dogs (p < 0.01 vs. nonischemic). Ryanodine and the 2D12 antibody improved, but did not reverse the low Ca2+ uptake. Protein content was similar in ischemic/reperfused and nonischemic myocardium. In contrast, Ca2+ uptake and the responses to ryanodine and 2D12 antibody were normal in ischemic/reperfused myocardium from nifedipine-treated dogs.

CONCLUSION

Dysfunction of the sarcoplasmic reticulum Ca2+ ATPase pump correlates with reperfusion injury. Reactivation of Ca2+ channels at reperfusion contributed to Ca2+ pump dysfunction. Ca2+ pump injury may be a critical event in myocardial reperfusion injury.

The role of magnesium therapy in acute myocardial infarction

Magnesium possesses numerous salutary effects for the treatment of acute myocardial infarction (AMI). It is a coronary vasodilator, calcium antagonist, afterload reducer, antiarrhythmic, and antiplatelet drug that modulates autonomic function and limits reperfusion injury when administered early in infarction. Various clinical trials of magnesium therapy for AMI have proffered conflicting results as to the efficacy of magnesium therapy because of significant differences in the timing of magnesium administration. Additional clinical trials that focus on early administration of magnesium are warranted to delineate the role of magnesium therapy for AMI.

The scientific basis for hypocalcemic cardioplegia and reperfusion in cardiac surgery

The scientific rationale for avoiding the use of calcium-enriched cardioplegic solutions and calcium supplementation during cardioplegic induction and the early phase of reperfusion in open heart surgical procedures is reviewed. The role of the extracellular and intracellular free ionized calcium concentrations during ischemia and reperfusion is explored and the biochemical effects of ischemia on calcium fluxes, adenosine triphosphate levels, and mitochondrial function are discussed. The role of calcium in causing myocardial stunning and the biochemical basis of reperfusion injury are also addressed. Both prolonged ischemia and an increased concentration of Ca2+ during reperfusion have proved to be deleterious. I conclude on the basis of my review that there is no justification for the use of calcium chloride before and during the early phase of reperfusion and that hypocalcemic perfusion is an effective and easily controllable means of myocardial protection.

Mega-trials and management of acute myocardial infarction

Clinical management of acute myocardial infarction has been strongly influenced by large, simple trials (mega-trials) with unrestrictive protocols and limited data collection. The design has been adopted to increase statistical power to a maximum. Its validity rests on an effective randomisation procedure and intention-to-treat analysis of deaths. Experience has shown that mega-trials tend to generate effect-estimates nearer the null than those from conventional trials or meta-analyses. When a small or absent observed treatment effect (or subgroup effect) in a mega-trial contrasts with the results of conventionally designed trials, it is necessary to assess both null bias and failure to increase the true treatment effect to a maximum in the mega-trial. Null bias will arise when the contrast between treatment and no-treatment, or between subgroups, is blunted either by non-protocol therapy or by inaccuracy of data, including misclassification between subgroups. Each is more likely with an unrestrictive design. To increase the true treatment effect to a maximum, trial conditions must be specified with insight into mechanism, dose-dependence, and time-dependence. The mega-trial design is therefore unsuited to an exploratory role. These issues are illustrated by the examples of nitrates, angiotensin-converting-enzyme inhibitors, and magnesium in acute myocardial infarction but have general relevance to the validity and generalisability of simple trials.

Calcium antagonists and left ventricular dysfunction

Calcium antagonists are used in the management of a variety of cardiovascular disorders. Ischemia leads to left ventricular dysfunction, which is the clinical entity on which the calcium antagonists are expected to have their effect as a result of their anti-ischemic action. This article reviews the efficacy of calcium antagonists in several different settings of left ventricular dysfunction due to ischemia and reperfusion.