ADP-induced refractory state of platelets in vitro. I. Methodological studies on aggregation in platelet rich plasma.

ADP-induced aggregation was determined at various times of incubation with ADP in unstirred human platelet rich plasma (PRP) to which adenosine deaminase was added. In the early stages of incubation the shape change response was absent, the aggregation response was poor, and not reversible. These three parameters returned slowly towards normal as the incubation proceeded. Pronounced impairment of the aggregation response was present after all added ADP had been degraded in plasma. Log dose-response (rate of aggregation) curves for the platelets incubated with ADP had shifted to higher ADP concentrations and had also a small decrease in maximal height and in the slope compared to those obtained with control platelets. However, the shift in log dose response to higher ADP concentrations was far more striking, so the increase in log dose (R) necessary to obtain the same rate of aggregation as with control PRP was taken as a measure of refractoriness of aggregation towards ADP. R increased with the time of incubation to an optimal value and then decreased. The magnitude of the optimal value and the time at which optimum was reached increased with the concentration of the ADP incubated with the platelets. The variations in R during incubation did not correlate with the breakdown of added ADP. The platelets began to recover their ability to aggregate with ADP while there was still ADP in the system.