Stimulus-response coupling in human platelets. Changes evoked by platelet-activating factor in cytoplasmic free calcium monitored with the fluorescent calcium indicator quin2.
The role of changes in cytoplasmic free calcium, [Ca2+]i, in the responses to platelet-activating factor (PAF) was studied in human platelets loaded with the fluorescent calcium indicator, quin2. In the presence of 1 mM external calcium, PAF raised [Ca2+]i 8-10-fold in a few seconds to peak near 1 microM. [Ca2+]i then declined over several minutes towards the basal level. In the absence of external calcium there was a much smaller increase in [Ca2+]i of similar pattern. These findings suggest that PAF increases [Ca2+]i partly by discharge of internal Ca2+, but mainly by stimulated influx. Blockade of cyclo-oxygenase with aspirin only slightly reduced the [Ca2+]i changes, indicating that thromboxane A2 is not a major mediator of the calcium movements. In control conditions PAF could stimulate shape-change, aggregation and secretion. Aggregation and secretion were roughly halved by blockade of cyclo-oxygenase. 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.
D000634:Aminoquinolines; D001241:Aspirin / Q000494:pharmacology; D001792:Blood Platelets / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D002118:Calcium / Q000097:blood*; D016861:Cyclooxygenase Inhibitors; D006801:Humans; D066298:In Vitro Techniques; D010972:Platelet Activating Factor / Q000494:pharmacology*; D010974:Platelet Aggregation / Q000187:drug effects; D013050:Spectrometry, Fluorescence
D000634:Aminoquinolines; D016861:Cyclooxygenase Inhibitors; D010972:Platelet Activating Factor; C035738:Quin2-acetoxymethyl ester; D001241:Aspirin; D002118:Calcium