Abstract
Ca 2+ overload is a cardinal feature of cardiomyocyte injury, and its progression to irreversible state leads to cell death. However, unknowns are the precise spatiotemporal changes in the myocyte Ca 2+ dynamics and the relevant cell morphology of irreversibly injured hearts. On the hypothesis that myocytes exhibit high-frequency Ca 2+ waves and contraction band necrosis in saponin-permeabilized injured heart, we observed changes in the Ca 2+ dynamics and the relevant morphological changes in the subepicardial myocardium of the Fluo4-loaded rat hearts (n = 14) by rapid-scanning confocal microscopy (100 frames/s) under Langendorff perfusion with 0.3 mM Ca 2+ -Tyrode solution including 0.4 % saponin at 30°C. Also performed was confocal imaging of tetramethylrhodamine methyl ester (TMRM) fluorescence of the myocardium. Under quasi-quiescence of the heart after dissection of the SA node, individual myocytes barely exhibited spontaneous Ca 2+ waves, whereas after commencement of saponin perfusion high-frequency (118 ± 9.7 /min/cell, mean ± SEM) Ca 2+ waves (hereafter, “agonal waves”) emerged within 1 min, showing asynchronous, oscillatory contractions in the individual myocytes with a V prop of 124 ± 2.5 μm/s (n = 60). Subsequently, the waves gradually decreased in frequency with concomitant slowing of its decay time course, and eventually, disappeared in 6 min; myocytes exhibited high, static Fluo4-fluorescence intensity. Along with the progression of Ca 2+ overload by saponin, the TMRM fluorescence intensity was discretely lost in individual myocytes. The myocytes showing the agonal waves exhibited contraction bands, i.e., band-like aggregations of the actin fibers. Under mechanical arrest of the heart by 2,3-butanedione monoxime (20 mM), saponin still induced the agonal waves with a frequency of 253 ± 10.6 /cell/min and V prop of 118 ± 2.1 μm/s (n = 60); however, contraction bands were barely seen.In conclusion, irreversible myocyte injury by saponin provoked agonal Ca 2+ waves and oscillatory contractions indicating progressive Ca 2+ overload and the following mitochondrial damage, which may provide deeper insights into understanding the mechanism of contraction band necrosis.
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