The cytoplasmic concentration of free calcium in platelets is controlled by stimulators of cyclic AMP production (PGD2, PGE1, forskolin).

Maximal stimulation of platelets with thrombin results in a rapid increase in cytoplasmic Ca2+ (from 0.1 microM to 1-3 microM), as measured with the fluorescent intracellular Ca2+ indicator Quin-2. Prior addition of the adenylate cyclase stimulators PGD2, PGE1 or forskolin inhibited the rise in cytoplasmic Ca2+. When added after the maximal response to thrombin was attained adenylate cyclase stimulators caused a rapid fall of cytoplasmic Ca2+ back to the original "resting" level. This effect coincides with the reversal of thrombin-induced, Ca2+-dependent protein phosphorylation, and cytoskeleton assembly. It is suggested that cAMP-dependent reactions maintain low levels of cytoplasmic Ca2+ by promoting transport and/or binding of Ca2+.