Cellular and subcellular heterogeneity of [Ca2+]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 (i) mechanically quiescent cells in which [Ca²⁺]_i is spatially uniform, constant over time, and relatively low; (ii) 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 (iii) 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 ofcells 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.