The cytoplasmic concentration of free calcium in platelets is controlled by stimulators of cyclic AMP production (PGD 2, PGE 1, forskolin)

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