Movements of Calcium Ions and Their Role in the Activation of Platelets

The increase of the cytoplasmic Ca-concentration plays a central role in the initiation of platelet activation. Four kinds of movements of Ca-ions are presumed to occur during this process: (a) Ca-ions liberated from membranes induce the rapid shape change. (b) Vesicular organelles release Ca-ions into the cytoplasm which initiate the release reaction, (c) The storage organelles, called dense bodies, secrete their contents including Ca-ions to the outside during the release reaction, (d) At the same time a rearrangement of the plasma membrane occurs, resulting in an increase in its permeability for Ca-ions as well as in an increase in the number of Ca-binding sites.

Since most processes occurring during platelet activation are reversible the platelet must be equipped with a mechanism which removes Ca-ions from the cytoplasm. A vesicular fraction of platelet homogenate indeed accumulates Ca actively. This Ca-pump is stimulated by cyclic AMP and protein kinase; it might be involved in the recovery of platelets after activation.