Human Platelet Secretion and Aggregation Induced by Calcium Ionophores Inhibition by PGE and Dibutyryl Cyclic AMP

Ca 2+, Mg2+-ionophores X537A and A23,187 (10-r-10 -6 M) induced the release of adenine nucleotides adenosine diphosphate (ADP, adenosine triphosphate (ATP), serotonin, /3-glucuronidase, Ca 2+, and Mg 2+ from washed human platelets. Enzymes present in the cytoplasm or mitochondria, and Zn 2+ were not released. The rate of ATP and Ca 2+ release measured by firefly lantern extract and murexide dye, respectively, was equivalent to that produced by the physiological stimulant thrombin. Ionophore-induced release of ADP, and serotonin was substantially (~60 %) but not completely inhibited by EGTA, EDTA, and high extracellular Mg 2+, without significant reduction of Ca 2+ release. The ionophore-induced release reaction is therefore partly dependent upon uptake of extracellular Ca 2+ (demonstrated using ~Ca), but also occurs to a significant extent due to release into the cytoplasm of intracellular Ca 2+. The ionophore-induced release reaction and aggregation of platelets could be blocked by prostaglandin El (PGE1) or dibutyryl cyclic AMP. The effects of PGEx, and N 6, O2-dibutyryl adenosine 3': 5'-cyclic monophosphoric acid (dibutyryl cAMP) were synergistically potentiated by the phosphodiesterase inhibitor theophylline. It is proposed that Ca ~+ is the physiological trigger for platelet secretion and aggregation and that its intraeellular effects are strongly modulated by adenosine 3':5'-cyclic monophosphoric acid (cyclic AMP). I N T R O D U C T I O N As an essential feature of their role in hemostasis platelets undergo a physiological secretory process known as the "release reaction" in which the contents of certain intracellular granules are expelled into the surrounding medium. This occurs without damage or destruction to platelets since cytoplasmic constituents are not released (Holmsen and Day, 1970). Platelet secretion and aggregation are induced physiologically by low concentrations of the bloodclotting enzyme, thrombin, or by platelet interaction with subendothelial connective tissue components, especially collagen. Platelet aggregation may THE JOURNAL OF GENERAL PHYSIOLOGY • VOLUME 66, 1975 • p a g e s 5 6 1 5 8 t 56I on Jne 8, 2017 D ow nladed fom Published November 1, 1975