title: Cytoplasmic free Ca2+ in human platelets: Ca2+ thresholds and Ca-independent activation for shape-change and secretion.
abstract: Cytoplasmic free [Ca2+], [Ca2+]i, was measured in human platelets using the intracellularly-trapped, fluorescent indicator quin2. Basal [Ca2+]i with the Ca2+ -ionophore ionomycin revealed apparent thresholds for shape-change, 5-HT release and aggregation of approx. 0.5 microM, 0.8 microM and 2 microM. Thrombin raised [Ca2+]i to 3 microM fast enough for the Ca2+ to have triggered the cell activation. However, thrombin released more 5-HT than ionomycin could, and in Ca2+ -free medium thrombin evoked shape-change and secretion even when [Ca2+]i remained near basal levels throughout, suggesting the existence of alternative triggers for shape-change and secretory exocytosis.
mesh_terms: D001792:Blood Platelets / Q000166:cytology / Q000502:physiology*; D002118:Calcium / Q000502:physiology*; D004533:Egtazic Acid / Q000494:pharmacology; D004987:Ethers / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D015759:Ionomycin; D010974:Platelet Aggregation; D012701:Serotonin / Q000378:metabolism; D013917:Thrombin / Q000494:pharmacology
chemical_list: D004987:Ethers; D012701:Serotonin; D004533:Egtazic Acid; D015759:Ionomycin; D013917:Thrombin; D002118:Calcium