Measurement and Manipulation of [Ca2+]i in Suspensions of Platelets and Cell Cultures

The importance of cytosolic calcium (Ca²⁺ ) elevation in the regulation of platelet functions (shape change, aggregation, and secretion) has been widely acknowledged. Therefore its concentration must be tightly regulated, with most of it being sequestrated in intracellular organelles such as the dense tubular system (the releasable intracellular Ca²⁺ store) and mitochondria, or bound to membranes and cytoplasmic proteins. Only a small fraction is freely available in the ionized form. The maintenance of low cytosolic calcium concentration (around 100 nM), necessary to keep platelets in a resting state or to reestablish a resting state after activation, is achieved by a combination of Ca²⁺ /ATPases and a Na⁺ /Ca²⁺ exchanger. Platelets contain two types of Ca²⁺ /ATPases: the plasma membrane Ca²⁺ /ATPase (PMCA types 1–4) (1 ) and the sarco-endoplasmic reticulum Ca²⁺ /ATPase (SERCA) present on intracellular stores (2 ). Two types of Na⁺ /Ca²⁺ exchangers have been identified; the platelets have been suggested to contain the retinal type, which is K⁺ -dependent and located in the plasma membrane (3 ,4 ).