SIGNIFICANCE OF TESTING PLATELET FUNCTIONS IN-VITRO

Platelets respond through discrete receptors to a number of physiologi cal agonists and foreign surfaces with a sequence of measurable respon ses: shape change, aggregation, secretion and arachidonate liberation. Three secretory responses are distinguished: exocytosis of substances from (1) dense granules, (2) alpha-granules and (3) lysosomes. Free a rachidonate, liberated from phospholipids by phospholipase A(2), is ra pidly converted (by oxygenation) to prostaglandins and thromboxanes wh ich, together with secreted ADP and close cell contact, will cause fur ther platelet activation through 'positive feedback' (autocrine stimul ation). Some agonists are classified as 'weak' (ADP, vasopressin, plat elet-activating factor [PAF], serotonin) because they depend on autocr ine stimulation to promote the full sequence of responses, while other s are 'strong' agonists (thrombin, collagen) and activate all response s directly without autocrine stimulation. Adrenaline, long thought to be a platelet agonist per se, most probably acts by amplifying the act ivation brought about by other, proper, agonists. Such synergistic int eraction among agonists is very typical for platelet activation and mo st likely takes place in vivo. Shape change, aggregation and secretion (s) may be tested by flow cytometry or electron microscopy in vitro un der conditions that probably reflect the in vivo situation. However, t he aggregation response to weak agonists in vitro is dependent on the extracellular [Ca2+], with biphasic aggregation at the low [Ca2+] pres ent when citrate is used as anticoagulant (or in suspension of washed platelets) but not at the physiological [Ca2+] present in platelet-ric h plasma from heparinized blood.