CONICET | Buscador de Institutos y Recursos Humanos

AIMS: Myocardial stunning is a contractile dysfunction that occurs after a brief ischemic insult. Substantial evidence supports that this dysfunction is triggered by Ca(2+) overload during reperfusion. The aim of the present manuscript is to define the origin of this Ca(2+) increase in the intact heart. Methods and Results To address this issue, Langendorff-perfused mouse hearts positioned on a pulsed local field fluorescence microscope and loaded with fluorescent dyes Rhod-2, Mag-fluo-4 and Di-8-ANEPPS, to assess cytosolic Ca(2+), sarcoplasmic reticulum (SR) Ca(2+), and transmembrane action potentials (AP), respectively, in the epicardial layer of the hearts, were submitted to 12 min of global ischemia followed by reperfusion. Ischemia increased cytosolic Ca(2+) in association with a decrease in intracellular Ca(2+) transients and a depression of Ca(2+) transient kinetics; i.e., the rise time and decay time constant of Ca(2+) transients were significantly prolonged. Reperfusion produced a transient increase in cytosolic Ca(2+) (Ca(2+) bump), that was temporally associated with a decrease in SR Ca(2+) content, as a mirror-like image. Caffeine pulses (20 mM) confirmed that SR Ca(2+) content was greatly diminished at the onset of reflow. The SR Ca(2+) decrease was associated with a decrease in Ca(2+) transient amplitude and a shortening of AP duration mainly due to a decrease in the phase 2. CONCLUSIONS: To the best of our knowledge, this is the first study in which SR Ca(2+) transients are recorded in the intact heart, revealing a previously unknown participation of SR on cytosolic Ca(2+) overload upon reperfusion in the intact beating heart. Additionally, the associated shortening of phase 2 of the AP may provide a clue to explain early reperfusion arrhythmias.