Widely used as a second messenger in signal transduction, it can also bind to proteins such as troponin-C and calmodulin.  
This ion plays an important role in the structure of coccolithophores, and it is stored in oxalate crystals in plastids.  Its presence in living tissue can be detected due to its ability to bind with BAPTA. The uptake of this element is regulated by its namesake receptor in the parathyroid's chief cells, and it is used as a regulator in secreting renin by the kidneys' juxtaglomerular cells.  Widely used as a second messenger in signal transduction, it can also bind to proteins such as troponin-C and calmodulin.  Stored in the sarcoplasmic reticulum and important in muscle contraction, FTP, identify this ion found most commonly in teeth and bones.
This ion plays an important role in the structure of coccolithophores, and it is stored in oxalate crystals in plastids.  
calcium ions or Ca2+ (prompt on "calcium")