The local Ca concentration profile in the vicinity of a Ca channel.

Intracellular Ca signaling is governed by diffusion and buffering of Ca ions. Mobile endogenous buffers increase the redistribution of Ca ions and tend to restrict the regions of elevated Ca concentrations to the close vicinity of the channel. These Ca microdomains dominate fast Ca signaling, as observed, e.g., in synaptic transmission. The steady-state solution of the linearized differential equations of buffered Ca diffusion, as developed by Neher and Naraghi and Neher, will be reviewed and generalized to the case of more than two buffers. Immobile buffers do not enter the steady-state equations, but instead slow down Ca diffusion and prolong the time to reach the steady state. Based on this phenomenon, a quite different putative mechanism to localize Ca will be suggested that is likely to be realized in photoreceptors where Ca source, Ca sink, and Ca sensor form a complex, as was recently reported.