RESUMO
In Ca(2+)-overloaded ventricular myocytes, SERCA is crucial to steadily achieve the critical sarcoplasmic reticulum (SR) Ca(2+) level to trigger and sustain Ca(2+) waves, that propagate at constant rate (Êwave). High luminal Ca(2+) sensitizes RyR2, thereby increasing Ca(2+) sparks frequency, and the larger RyR2-mediated SR Ca(2+) flux (dF/dt) sequentially activates adjacent RyR2 clusters. Recently, it was proposed that rapid SERCA Ca(2+) reuptake, ahead of the wave front, further sensitizes RyR2, increasing Êwave. Nevertheless, this is controversial because rapid cytosolic Ca(2+) removal could instead impair RyR2 activation. We assessed whether rapid SR Ca(2+) uptake enhances Êwave by changing SERCA activity (Ò¡Decay) over a large range (â¼175%). We used normal (Ctrl) and hyperthyroid rat (HT; reduced phospholamban by â¼80%) myocytes treated with thapsigargin or isoproterenol (ISO). We found that Êwave and dF/dt had a non-linear dependency with Ò¡Decay, while Ca(2+) waves amplitude was largely unaffected. Furthermore, SR Ca(2+) also showed a non-linear dependency with Ò¡Decay, however, the relationships Êwave vs. SR Ca(2+) and Êwave vs. dF/dt were linear, suggesting that high steady state SR Ca(2+) determines Êwave, while rapid SERCA Ca(2+) uptake does not. Finally, ISO did not increase Êwave in HT cells, therefore, ISO-enhanced Êwave in Ctrl depended on high SR Ca(2+).
Assuntos
Cálcio/metabolismo , Ventrículos do Coração/metabolismo , Miócitos Cardíacos/metabolismo , ATPases Transportadoras de Cálcio do Retículo Sarcoplasmático/metabolismo , Retículo Sarcoplasmático/metabolismo , Animais , Sinalização do Cálcio , Proteínas de Ligação ao Cálcio/química , Citosol/metabolismo , Modelos Animais de Doenças , Hipertireoidismo/metabolismo , Isoproterenol/química , Masculino , Ratos , Ratos Wistar , Canal de Liberação de Cálcio do Receptor de Rianodina/metabolismo , Tapsigargina/químicaRESUMO
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.