Cellular and Subcellular Heterogeneity of Ca 2+ i in Single Heart Cells Revealed by Fura-2

Digital imaging of calcium indicator signals (fura-2 fluorescence) from single cardiac cells has revealed different subcellular patterns of cytoplasmic calcium ion concentration (Ca2+i) that are associated with different types of cellular appearance and behavior. In any population of enzymatically isolated rat heart cells, there are mechanically quiescent cells in which Ca2+i is spatially uniform, constant over time, and relatively low; spontaneously contracting cells, which have an increased Ca2+i, but in which the spatial uniformity of Ca2+i is interrupted periodically by spontaneous propagating waves of high Ca2+i; and cells that are hypercontracted (rounded up) and that have higher levels of Ca2+i than the other two types. The observed cellular and subcellular heterogeneity of Ca2+i in isolated cells indicates that experiments performed on suspensions of cells should be interpreted with caution. The spontaneous Ca2+i fluctuations previously observed without spatial resolution in multicellular preparations may actually be inhomogeneous at the subcellular level.