Ca2+ Sparks and Ca2+ Waves are the subcellular events underlying Ca2+ overload during ischemia and reperfusion in perfused intact hearts

VALVERDE CA; ESCOBAR AL; MATTIAZZI A

New London

Congreso; Gordon Research Seminar and Conference; 2014

Gordon Research Conference

Abnormal intracellular Ca2+ cycling plays a key role in cardiac dysfunction, particularly during the setting of ischemia/reperfusion (I/R). During ischemia there is an increase in cytosolic and sarcoplasmic reticulum (SR) Ca2+. At the onset of reperfusion there is a transient and abrupt increase in cytosolic Ca2+ usually associated to reperfusion arrhythmias. However, little is known about the dynamics of subcellular events underlying Ca2+ increase during I/R. The aim is to test two hypotheses: 1. An increase in the frequency of diastolic Ca2+ sparks (cspf) constitutes a mayor substrate for diastolic Ca2+  increase during ischemia; 2. The diastolic Ca2+ rise during reperfusion is mediated by an increase in cytosolic Ca2+ pro-arrhythmogenic events (Ca2+ waves). Methods and Results: We used confocal microscopy on mouse intact hearts loaded with Fluo- 4. Hearts were submitted to global I/R (12/30 min) to assess epicardial sparks in the whole heart. Intact heart sparks were faster than in isolated myocytes whereas cspf was not different. During ischemia, cspf significantly increased relative to preischemia (2.07±0.33 vs. 1.13±0.20 sp/sec/100µm, n=29/34, 7 hearts). Reperfusion significantly changed Ca2+ sparks kinetics, by prolonging Ca2+ sparks rise time and decreased cspf. However it significantly increased Ca2+ wave frequency relative to ischemia (0.71±0.14 vs. 0.38±0.06 w/sec/100µm, n=32/33, 7 hearts). Moreover, perfusion of the hearts with the CaMKII inhibitor KN93, significantly diminished the increase in spark frequency during ischemia or the triggering of Ca2+ waves during reperfusion. The results show for the first time the morphological characteristics of intact perfused heart Ca2+ sparks and provides direct evidence of increased Ca2+ sparks in ischemia that transform into Ca2+  waves during reperfusion. These waves may constitute a main trigger of reperfusion arrhythmias. Furthermore, the fact that CaMKII inhibition prevents this anomalous Ca2+ cycling indicates that CaMKII?dependent phosphorylations, may be involved in sensitizing the heart during an ischemic insult.