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. 0.5μM, 0.8μM and 2 μM. Thrombin raised [Ca2+]i to 3 μM 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.
