Answer:
Cell uses Ca2+ for intra cellular signaling but not Na+ because Na+ is a potent regulator of plasma osmolarity as well as blood pressure.
Explanation:
If cell uses Na+ ion for intra cellular signalling then the normal plasma level of Na+ ion wil fluctuate during the occurrence of cell signaling.This fluctuation ultimately result in the disturbance of plasma osmolarity which will be very much harmful for the cell because disturbance in the plasma osmolarity can hamper the normal osmotic pressure that can either swells or shrinks the cell.