Impaired oxidative metabolism and calcium mishandling underlie cardiac dysfunction in a rat model of post-acute isoproterenol-induced cardiomyopathy.

Stress-induced cardiomyopathy, triggered by acute catecholamine discharge, is a syndrome characterized by transient, apical ballooning linked to acute heart failure and ventricular arrhythmias. Rats receiving an acute isoproterenol (ISO) overdose (OV) suffer cardiac apex ischemia-reperfusion damage and arrhythmia, and then undergo cardiac remodeling and dysfunction. Nevertheless, the subcellular mechanisms underlying cardiac dysfunction after acute damage subsides are not thoroughly understood. To address this question, Wistar rats received a single ISO injection (67 mg/kg). We found in vivo moderate systolic and diastolic dysfunction at 2 wk post-ISO-OV; however, systolic dysfunction recovered after 4 wk, while diastolic dysfunction worsened. At 2 wk post-ISO-OV, cardiac function was assessed ex vivo, while mitochondrial oxidative metabolism and stress were assessed in vitro, and Ca(2+) handling in ventricular myocytes. These were complemented with sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), phospholamban (PLB), and RyR2 expression studies. Ex vivo, basal mechanical performance index (MPI) and oxygen consumption rate (MVO2) were unchanged. Nevertheless, upon increase of metabolic demand, by ß-adrenergic stimulation (1-100 nM ISO), the MPI versus MVO2 relation decreased and shifted to the right, suggesting MPI and mitochondrial energy production uncoupling. Mitochondria showed decreased oxidative metabolism, membrane fragility, and enhanced oxidative stress. Myocytes presented systolic and diastolic Ca(2+) mishandling, and blunted response to ISO (100 nM), and all these without apparent changes in SERCA, PLB, or RyR2 expression. We suggest that post-ISO-OV mitochondrial dysfunction may underlie decreased cardiac contractility, mainly by depletion of ATP needed for myofilaments and Ca(2+) transport by SERCA, while exacerbated oxidative stress may enhance diastolic RyR2 activity.

Abnormal mitochondrial function during ischemia reperfusion provides targets for pharmacological therapy.

The concept of reperfusion injury has been a subject of intense debate. Some researchers believe that the entire injury develops during the ischemic period, whereas others argue that blood reflow extends tissue injury due to the release of oxygen-derived free radicals, an inflammatory reaction involving influx of various populations of immune cell, and dysregulation of intracellular and particularly mitochondrial calcium concentration. Mitochondrial calcium overload in the presence of oxygen-derived free radicals can result in the opening of the mitochondrial permeability transition pore (mPTP), which further compromises cellular energetics. The resultant low ATP and altered ion homeostasis lead to a rupture of the plasma membrane and cell death. Mitochondria have long been proposed as one of the main players in cell death, since the mitochondria are central to synthesis of both ATP and the formation of oxygen-derived free radicals. These mechanisms are centered on mitochondrial calcium overload as a key component of cell death. Pharmacological strategies that are cardioprotective attempt to reduce mitochondrial calcium overload to decrease the likelihood of arrhythmias and cardiac dysfunction elicited by reperfusion.