Abstract
Digital imaging of calcium indicator signals (fura-2 fluorescence) from single cardiac cells has revealed different subcellular patterns of cytoplasmic calcium ion concentration ([Ca$^{2+}$]$_{\text{i}}$) 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 [Ca$^{2+}$]$_{\text{i}}$ is spatially uniform, constant over time, and relatively low; (ii) spontaneously contracting cells, which have an increased [Ca$^{2+}$]$_{\text{i}}$, but in which the spatial uniformity of [Ca$^{2+}$]$_{\text{i}}$ is interrupted periodically by spontaneous propagating waves of high [Ca$^{2+}$]$_{\text{i}}$; and (iii) cells that are hypercontracted (rounded up) and that have higher levels of [Ca$^{2+}$]$_{\text{i}}$ than the other two types. The observed cellular and subcellular heterogeneity of [Ca$^{2+}$]$_{\text{i}}$ in isolated cells indicates that experiments performed on suspensions of cells should be interpreted with caution. The spontaneous [Ca$^{2+}$]$_{\text{i}}$ fluctuations previously observed without spatial resolution in multicellular preparations may actually be inhomogeneous at the subcellular level.