Differences in features of calcium transients between the nucleus and the cytosol in cultured heart muscle cells: analyzed by confocal microscopy.

We analyzed spatio-temporal characteristics of Ca2+ transients in the cytosol and the nucleus of cultured neonatal rat heart cells using confocal imaging with Indo-1 and Fluo-3. In resting heart muscle cells, nuclear [Ca2+] was maintained lower than the cytosolic level. The rise in nuclear [Ca2+], during either E-C coupling or propagation of the Ca2+ wave, began at the edge of the nucleus in the immediate vicinity of the rise in global or localized cytosolic [Ca2+], and spread inwardly. The rise in [Ca2+] was slower and smaller in the nucleus than in the cytosol. The decay in [Ca2+] was also slower in the nucleus than the cytosol, thereby reversing the initial [Ca2+] gradient between them. Caffeine markedly enhanced the rise in nuclear [Ca2+] while maintaining inward spreading. The heterogeneity of nuclear Ca2+ transients during cellular contractilities suggests that influx of Ca2+ from perinuclear stores into the nucleus plays a predominant role in the nuclear [Ca2+] rise. The results also indicated that spatio-temporal characteristics of Ca2+ transients are quite different between the nucleus and the cytosol, thereby suggesting that they are differentially regulated in the nucleus and the cytosol.