title: Synergism of platelet-aggregating agents. Role of elevation of cytoplasmic calcium.
abstract: When a pair of platelet agonists, each in subthreshold concentration, is added together or in sequence to a platelet suspension, the platelet response is enhanced. Addition of two agonists to platelets loaded with aequorin also enhanced the observed rise in cytoplasmic ionized calcium ([Ca2+]i) in response to the second agonist if the agonists were added within 20 s of each other. Enhancement of aggregation and secretion required that an increase in [Ca2+]i (as indicated by aequorin but not necessarily indo-1) followed the first agonist, but not that the [Ca2+]i remain elevated until addition of the second agonist. Enhancement was not prevented by aspirin, ADP scavengers, or chelators of extracellular Ca2+. We conclude that a rise in [Ca2+]i induced by a first agonist "primes" platelets for an augmented functional response to a second agonist, which is not, however, determined by the [Ca2+]i at the time of addition of the second agonist.
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D000331:Aequorin; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000097:blood*; D003593:Cytoplasm / Q000378:metabolism; D004357:Drug Synergism; D004533:Egtazic Acid / Q000494:pharmacology; D004837:Epinephrine / Q000494:pharmacology; D006801:Humans; D010974:Platelet Aggregation / Q000187:drug effects*; D011450:Prostaglandin Endoperoxides, Synthetic / Q000494:pharmacology; D013917:Thrombin / Q000494:pharmacology
chemical_list: D011450:Prostaglandin Endoperoxides, Synthetic; D000001:Calcimycin; D000331:Aequorin; D004533:Egtazic Acid; D000244:Adenosine Diphosphate; D013917:Thrombin; D002118:Calcium; D004837:Epinephrine