Shape change in human platelets: measurement with a channelyzer and visualisation by electron microscopy.

The increase in median platelet volume (MPV) that occurs following the addition of various agonists can be recorded by high-resolution channelyzers. However, this increase in MPV may not represent the platelet shape change (PSC), an early phase of platelet activation characterised by the formation of pseudopodia. We therefore compared the MPV after adding various agonists (serotonin, ADP or a thromboxane A2 analogue) with scanning electron micrographs (EM) of the platelets in the same sample. Saline alone did not increase the MPV or alter platelet morphology. In contrast, the selected agonists altered both the MPV and EM. Adrenaline (5.0 mumol/l) alone did not influence either of these indices of platelet activation. However, when a lower concentration of adrenaline (0.2 mumol/l) was added in combination with a dose of serotonin (0.02 mumol/l) that did not affect the MPV on its own, the measured MPV increased significantly and the EM showed pseudopodia formation. Because the channelyzer method is reproducible and suited to serial measurements of the MPV, it may become a useful tool for the investigation of platelet activation.