The path of calcium in cytosolic calcium oscillations: a unifying hypothesis

Data from 42 systems have been assembled in which the overall spatial course of relatively natural, intracellular calcium pulses has been or can be determined. These include 21 cases of solitary pulses in activating eggs and 21 cases of periodic (as well as solitary) pulses in various fully active cells. In all cases, these pulses prove to be waves of elevated calcium that travel from one pole of a cell to the other or from the periphery inward. The velocities of these waves are remarkably conserved--at approximately 10 microns/sec in activating eggs and approximately 25 microns/sec in other cells at room temperature. Moreover, in three cases, the data suffice to show that these velocities fit the Luther equation for a reaction/diffusion wave of calcium through the cytosol. It is proposed that (i) natural intracellular calcium pulses quite generally take the form of cytosolic calcium waves and (ii) cytoplasmically controlled calcium waves are triggered and then propagated by the successive action of two distinct modes of calcium-induced calcium release. First, in the lumenal mode, a slow increase of calcium within the lumen of the endoplasmic reticulum reaches a level that triggers fast lumenal release as well as fast localized release into the cytosol. Then, the well-known cytosolic mode drives a reaction/diffusion wave across or into the cell.