Platelet/Surface Interactions

The development of nonthrombogenic artificial surfaces for extracorporeal circulation (ECC) is ongoing. Ex vivo animal studies are labor intensive and expensive, thus the development of a reliable in vivo screening method is essential. The following model utilizing test tubes incubated with 111In labeled platelets demonstrates such a method. The following surfaces were chosen because of their known platelet surface interactions: glass & PVC (activate platelets), polypropelene (PP) (nonactivator). All studies were carried out using rabbit platelets. The harvested platelets were labeled using a modification to a technique first described by Thakur in 1976. For this model the harvested platelets were incubated with 30 μCi 111In: 2.0x106/mm3 platelets for 10 minutes to obtain a 60–80% labeling efficiency. The plasma harvested was retained; the Ca2+ was readjusted and the radiolabeled platelets were then resuspended in the plasma to produce a count of 1.6x105/mm3. Two mL aliquots of the above were placed in each tube and incubated for 2 hr at 37®C. Free indium levels, platelet counts, and aggregometry were performed pre and post incubation. Post incubation, test tubes were gently rinsed and placed in a gamma counter to determine platelet adherence. Results of PVC and PP were compared against glass. Validation of the method was demonstrated via platelet metabolic inhibition studies and plasma free indium testing. For all studies performed, platelets demonstrated functionality to collagen, γ thrombin, and ADP pre and post labeling and incubation, except for the metabolic inhibition studies where there was no response to any agonist as expected. Platelet counts and pre and post free indium levels were similar within each test tube across all 3 groups. Platelet adhesion was greatest on glass (n=12) with PVC (n=16) at 67±8% and PP (n=12) at 43±5% when compared to glass. Metabolic inhibition reduced active platelet adhesion significantly in the glass and PVC groups (p < 0.0001) but did not change the passive adherence in the PP group (p=0.19). Free indium within the plasma did not significantly affect resultant gamma counts (p>0.1). With these results, the model can now be used as a screening test to select those nonthrombogenic surfaces that warrant further ex vivo animal and clinical testing.