Calcium

Calcium is used in many different signaling pathways, i.e. through G proteins, ion channel.

Every person starts out as a calcium wave. The wave triggered during fertilization it stops multiple sperm from fertilizing the ovum.
Calcium is used as an intracellular messenger as it is kept at low levels in resting cells; cells have many sensitive mechanisms that detect rises and falls in Calcium levels.

"OFF" mechanisms are important in the recovery of intracellular calcium levels, particularly in muscle cells, as the cell cannot contract again until calcium levels fall after the rise associated with contraction. "OFF" mechanisms include buffer proteins in the sarcoplasmic reticulum (e.g. Calsequestrin). These proteins have low affinity and high capacity for calcium, so that it is still easily released for signalling. Another "OFF" mechanism is Na⁺/Ca²⁺ exchanger pump in the plasma membrane, which operates at higher levels of Ca^2+. The differences in affinity of the "OFF" mechanisms for calcium allow the cell to respond effectively to different concentrations. PMCA and SERCA also keep cytosolic Ca²⁺ levels low by utilising ATP to pump Ca²⁺ out of the cytosol into the E.R., S.R., or outside the cell.

"ON" mechanisms respond to extracellular signals indicating to increase the levels of cytosolic Ca²⁺. These include InsP₃ and Ryanodine receptors, both which are activated by the binding of ATP, however, InsP₃ receptors also require binding of the coagonist InsP₃ to activate them.

References:

Heldin C. & Purton M,(1996) "Signal Transduction", Chapman & Hall, London