  In this letter we present the results of a simple model for intercellular
communication via calcium oscillations, motivated in part by a recent
experimental study. The model describes two cells (a "donor" and "sensor")
whose intracellular dynamics involve a calcium-induced, calcium release
process. The cells are coupled by assuming that the the input of the sensor
cell is proportional to the output of the donor cell. As one varies the
frequency of calcium oscillations of the donor cell, the sensor cell passes
through a sequence of N:M phase locked regimes and exhibits a "Devil's
staircase" behavior. Such a phase locked response has been seen experimentally
in pulsatile stimulation of single cells. We also study a stochastic version of
the coupled two cell model. We find that phase locking holds for realistic
choices for the cell volume.
