Calcium and drugs
Calcium is an essential element, having many functions in the body. The ion radius of calcium is 0.99 Angström, closely related to that of sodium. Its atomic mass is 40.
The extracellular concentration of calcium is very high, 1000 to 10 000 times higher than its intracellular concentration. This concentration is maintained within a narrow range by parathyroid hormone and, to a less degree, by calcitonin and vitamin D.
The intracytoplasmic calcium concentration is low because its penetration into the cell is limited and because the cell has the means of expelling it and trapping it into certain intracellular organelles. Moreover, inside the cytoplasm, there are proteins likely to fix it in a passive way: calsequestrine, calreticuline, calbindine, calretinine and parvalbumine. The intracytoplasmic concentration of free or ionized calcium, Ca 2+ , is thus low but instantaneous variations resulting from transfers between the external medium and the cell and between intracellular organelles exist.