Stimulus-response coupling in human platelets. Changes evoked by platelet-activating factor in cytoplasmic free calcium monitored with the fluorescent calcium indicator quin2

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Abstract
The role of changes in cytoplasmic free Ca, [Ca2+]i, in the responses to platelet-activating factor (PAF) was studied in human platelets loaded with the fluorescent Ca indicator, quin2. In the presence of 1 mM external Ca, PAF raised [Ca2+]i 8-10-fold in a few s to peak near 1 .mu.M. [Ca2+]i then declined over several min towards the basal level. In the absence of external Ca there was a much smaller increase in [Ca2+]i of similar pattern. PAF apparently increases [Ca2+]i partly by discharge of internal Ca2+, but mainly by stimulated influx. Blockade of cyclooxygenase with aspirin only slightly reduced the [Ca2+]i changes, indicating that thromboxane A2 is not a major mediator of the Ca movements. In control conditions PAF could stimulate shape-change, aggregation and secretion. Aggregation and secretion were roughly halved by blockade of cyclooxygenase. Shape-change and secretion still occurred under conditions where the [Ca2+]i rise was small or suppressed, indicating a role for intracellular activators other than Ca2+. The possible involvement of products of phosphoinositide breakdown is discussed.

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