Internal calcium: How might a cell detect fluctuations of internal ca2+???

Hi there!

A question worth thinking about:
As surely all of you know the activites of many enzymes show a
dependency on intracellular concentrations of ions e.g. calcium
(ca2+). So, regarding the enzymatic activity, one can obtain
infomation about the level of intracellular ca2+. Or from a more
mechanistic point of view: with a molecule like for example calmodulin
a cell may "measure" the internal ca2+.

That brings me to the point i'm breeding over:
The internal ca2+ naturally is not a constant quantity but fluctuates
around a constant mean. Now consider the amplitude of these
fluctuations as variance of the internal ca2+. And here is my
question:
Is there a enzyme (or more generally spoken: a biomolecule) that
allows you to gain infomation, not only about the internal
concentration of ca2+, but about the variance of the deviations of
ca2+ from its mean.

I am searching for a way how a cell might use this parameter (the
ca2+-variance) for cellular biophysical computations.

I'm grateful for any hint or idea. Maybe someone knows some literature
about this stuff .

Best regards,
Thomas