  It is known that curvature relation plays a key role in the propagation of
two-dimensional waves in an excitable model. Such a relation is believed to
obey the eikonal equation for typical excitable models (e.g., the
FitzHugh-Nagumo (FHN) model), which states that the relation between the normal
velocity and the local curvature is approximately linear. In this paper, we
show that for a simplified model of intracellular calcium dynamics, although
its temporal dynamics can be investigated by analogy with the FHN model, the
curvature relation does not obey the eikonal equation. Further, the
inconsistency with the eikonal equation for the calcium model is because of the
dispersion relation between wave speed $s$ and volume-ratio parameter $\gamma$
in the closed-cell version of the model, not because of the separation of the
fast and the slow variables as in the FHN model. Hence this simplified calcium
model may be an unexpected excitable system, whose wave propagation properties
cannot be always understood by analogy with the FHN model.
