SPATIAL FEATURES OF CALCIUM TRANSIENTS DURING EARLY AND DELAYED AFTERDEPOLARIZATIONS

Although changes in intracellular Ca2+ concentration ([Ca2+]i) are spa tially heterogeneous during spontaneous contraction in mammalian cardi ac muscle, it has not yet been observed how [Ca2+]i changes spatially within cardiac myocytes during delayed (DADs) and early (EADs) afterde polarizations. The aim of this study is to characterize the spatial fe atures of the increase in [Ca2+]i during such afterdepolarizations and to understand the ionic mechanisms responsible for them. Myocytes wer e enzymatically isolated from guinea pig ventricles and loaded with fu ra 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 po tassium-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 ''w aves.'' In contrast, during EADs, fluorescence transients showed no wa ves within myocytes but rather showed synchronous changes throughout t he myocytes (n = 15). The results of this study suggest that the spati al features of the increase in [Ca2+]i differ between the DADs and EAD s. We concluded from these differing features that the ionic mechanism s responsible for the two triggered activities are different.