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Platelet morphology has been classified in our laboratory by phase-contrast microscopy into discocyte (D), disco- echinocyte (DE) and sphero-echinocyte (SE). Following the addition of 1-20 μM ADP to citrated platelet-rich plasma (PRP), platelet morphology changes as D→DE→SE with the maximum in DE occurring at 6-10 sec and that for SE at 1-3 min, and then slowly returns to D over the course of ∼30 min. Here we compare the time courses for early platelet aggregation (PA) and the onset of refractoriness, to the time course for platelet shape change, the latter measured in initially mixed unstirred PRP diluted with plasma to N≤50,000μl-1 to minimize aggregation. PA was measured microscopically as the % decrease in single platelets and refractoriness was measured by aggregometry from ADP dose-response curves determined as a function of time after initial stimulation. The time course for PA was similar to that for the appearance of DE. The DE at the time of maximal PA had pseudopods, but a main body geometry which differed only slightly from that of a D. The time course for onset of refractoriness at 2 μM ADP resembled that for the appearance of SE, indicating that the lifetime for single platelets able to participate in PA is &lt;60 sec. In contrast to ADP, PA induced by 10 μM adrenalin was maximal at ∼ 30 sec and during this time there was little shape change (&lt;20% decrease in D). Thus although PA can occur without shape change, the rate is 3-4 fold faster if shape change occurs. The similarity of the time course and ADP concentration dependence for PA and DE formation suggest an intimate relationship between these processes. Moreover, the above results emphasize the transient nature of single DE participation in aggregation, and in particular point out that the relatively slowly formed ‘sphered’ platelets (SE) are not sticky as single platelets and do not participate in early aggregation.

Comparison Of The Time Course For Adp-Induced Shape Change, Aggregation And Onset Of Refractoriness