Platelet aggregation in laminar flow. I. Adenosine diphosphate concentration, time and shear rate dependence

The relative contributions of physiologic activators and shear rates (G) on the formation and stability of platelet aggregates for in vitro laminar flow need to be characterized for conditions of low G believed to be important in vivo. Platelet aggregation was therefore studied in human citrated platelet-rich plasma (PRP) prepared at 37 degrees C for G = 0-190 sec-1 in laminar flow in a cone-in-plate device as a function of adenosine diphosphate (ADP) activation. Macroaggregation (TA) was assessed by visual observation and light transmission (%T), as routinely done in aggregometry, while actual percent aggregation (PA) was determined by microscopy of sub-samples from the decrease in single platelet concentration. Unactivated PRP never yielded shear-induced aggregation while samples pre-mixed with low [ADP] (less than or approximately 0.7 microM) led to large visible aggregate formation stable only for G less than or approximately 30-60 sec-1, apparently arising from pre-existing microaggregates. Direct ADP injections into the flow device showed that at low [ADP] (less than 1 microM), maximal values for both PA and TA were reached when step-wise increases in G reached less than or approximately 30 sec-1, with TA decreasing at higher G; these values were only some 10-20% of maximal values achieved with high (ADP] (greater than or approximately 4 microM), when PA was again maximal at G greater than or approximately 30 sec-1, but TA expressed as a rate of aggregation continued to increase with G up to 150 sec-1. The results are discussed in terms of the temporal shear-rate and ADP-dependent dynamics of a platelet stickiness factor, 's'.