  We study the dynamics of intracellular calcium oscillations in the presence
of proteins that bind calcium on multiple sites and that are generally believed
to act as passive calcium buffers in cells. We find that multisite
calcium-binding proteins set a sharp threshold for calcium oscillations. Even
with high concentrations of calcium-binding proteins, internal noise, which
shows up spontaneously in cells in the process of calcium wave formation, can
lead to self-oscillations. This produces oscillatory behaviors strikingly
similar to those observed in real cells. In addition, for given intracellular
concentrations of both calcium and calcium-binding proteins the regularity of
these oscillations changes and reaches a maximum as a function noise variance,
and the overall system dynamics displays stochastic coherence. We conclude that
calcium-binding proteins may have an important and active role in cellular
communication.
