Classes and mechanisms of calcium waves

Abstract

The best known calcium waves move at about 5–30 μm/s (at 20°C) and will be called fast waves to distinguish them from slow (contractile) ones which move at 0.1-1 μm/s as well as electrically propagated, ultrafast ones. Fast waves move deep within cells and seem to underlie most calcium signals. Their velocity and hence mechanism has been remarkably conserved among all or almost all eukaryotic cells. In fully active (but not overstimulated) cells of all sorts, their mean speeds lie between about 15–30 μm/s at 20°C. Their amplitudes usually lie between 3–30 μM and their frequencies from one per 10–300 s. They are propagated by a reaction diffusion mechanism governed by the Luther equation in which Ca 2+ ions are the only diffusing propagators, and calcium induced calcium release, or CICR, the only reaction; although this reaction traverses various channels which are generally modulated by IP 3 or cADPR. However, they may be generally initiated by a second, lumenal mode of CICR which occurs within the ER. Moreover, they are propagated between cells by a variety of mechanisms. Slow intracellular waves, on the other hand, may be mechanically propagated via stretch sensitive calcium channels.