Human blood platelet secretion: optical multichannel analyzer measurements using acriflavine as a release indicator

Blood platelets preloaded with the fluorescent amine acriflavine release the trapped fluorophore after stimulation with thrombin or the divalent cation ionophore A23187. Release was detected by an increase in acriflavine fluorescence, which is otherwise strongly quenched in the platelet, by using an optical multichannel analyzer to monitor the spectral and temporal reaction parameters. The secretion of [14C]serotonin and acriflavine is well correlated, suggesting that acriflavine, like serotonin and the closely related fluorescent drugs mepacrine and acridine orange, is accumulated in and released from platelet dense bodies. Acriflavine secretion at 37 degrees C in the absence of external calcium is characterized by a short delay, followed by a rapid biphasic increase in fluorescence that implies at least a three-stage secretory process. For saturating levels of thrombin the delay was 1.5 s and release was 90% complete within 6-7 s. The delay could not be shortened by prestimulation under conditions that induce shape changes but not release, i.e., with ADP, arachidonic acid, or low levels of thrombin or A23187. Acriflavine secretion induced by A23187 was similar but less effective; the reaction was slower, the yield was smaller, and, in contrast to thrombin, the longer lag period could be significantly shortened by prestimulation.