The spatial features of intracellular calcium increases differ between DADs (heterogeneous waves) and EADs (synchronous changes), suggesting different underlying ionic mechanisms.
Although changes in intracellular Ca2+ concentration (Ca2+i) are spatially heterogeneous during spontaneous contraction in mammalian cardiac muscle, it has not yet been observed how Ca2+i changes spatially within cardiac myocytes during delayed (DADs) and early (EADs) afterdepolarizations. The aim of this study is to characterize the spatial features of the increase in Ca2+i during such afterdepolarizations and to understand the ionic mechanisms responsible for them. Myocytes were enzymatically isolated from guinea pig ventricles and loaded with fura 2-acetoxymethylester, the Ca2+ fluorescence indicator dye. Membrane potential was recorded with a conventional microelectrode technique, and spatiotemporal changes in fura 2 fluorescence and cell length were recorded using a digital television system. After superfusion with potassium-free Tyrode solution, DADs and EADs were induced. During DADs, fluorescence transients were heterogeneous within myocytes (n = 11). Furthermore, they often propagated within myocytes as if they were "waves." In contrast, during EADs, fluorescence transients showed no waves within myocytes but rather showed synchronous changes throughout the myocytes (n = 15). The results of this study suggest that the spatial features of the increase in Ca2+i differ between the DADs and EADs. We concluded from these differing features that the ionic mechanisms responsible for the two triggered activities are different.
Miura et al. (Sun,) studied this question.