The present study examined the influence of activation on platelet deformability. Aspiration of cells after exposure to thrombin, adenosine 5' -diphosphate, or the calcium ionophore A23187 at concentrations producing shape change and stickiness revealed significant changes from control cells. At the lowest negative pressure, 4 X 10(-2) dynes/cm (-1 cm H2O), there were no differences in lengths of membrane segments aspirated from control and activated platelets. Each subsequent increase in negative pressure up to 35 X 10(-2) dynes/cm (-7.5 cm H2O) resulted in significantly longer aspirated segments on activated cells compared to control cells. Greater negative pressures did not cause further increases in lengths of membrane segments drawn into the pipette. Thus, activation, which results in constriction of the circumferential microtubule, makes more membrane available for aspiration as negative pressure is increased. In both control and activated platelets, the microtubule coils served as a barrier to further lengthening of aspirated membrane segments as negative pressure was increased beyond 35 X 10(-2) dynes (-7.5 cm H2O).

Previous reports have suggested that platelets from patients with Bernard-Soulier's syndrome (BSS) are not giant cells. Rather, they are normal-sized in suspension, but spread out on glass slides more readily than control cells, yielding the impression of being giant. The present study has used cell sizing techniques, electron microscopy, and micropipette aspiration to evaluate platelets from three patients with BSS. Cell sizing techniques revealed that BSS platelets were considerably larger than normal. The increased size was confirmed in electron microscopic studies of BSS platelets fixed in suspension. However, the BSS platelets did not contain increased amounts of internalized surface membrane considered to be the source of membrane necessary for excessive spreading. A possible explanation for increased spreading of BSS platelets was found in studies of their resistance to deformation in micropipettes. BSS platelets were much less resistant to deformation than normal cells or other abnormal platelets when aspirated under the same negative pressure. Their unusual deformability may explain the tendency of BSS platelets to spread more readily than normal cells on glass slides.

The present investigation has evaluated the influence of aspirin, Its constituents, and other nonsteroldal anti-Inflammatory agents on the resistance of human platelets to aspiration into mlcropipettes. Aspirin Increased the length of platelet extensions Into the micropipette over the entire negative tension range of 0.04 to 0.40 dynes/cm after exposure to the drug in vitro or after ingestlon of the agent. Other cyclooxygenase inhibitors, Ibuprofen and Indomethacln, did not increase platelet doformablllty. The Influence of aspirin was mimicked to some degree by high concentrations of salicylic acid, but acetylation of platelets with acetic anhydride had little influence on platelet deformablllty. Incubation of platelets with both salicylic acid and acetic anhydride had no more effect than salicylic acid alone. Benzole acid, chemically similar to salicylic acid, had a minimal effect. The studies demonstrate that aspirin makes platelets more deformable, while components of the drug or other nonsteroidal antiInflammatory agents and cyclooxygenase Inhibitors do not have the same influence on resistance to deformation. (Arteriosclerosis 7:385-388, July/August 1987) R ecent studies in our laboratory have used the technique of micropipette elastimetry to evaluate the resistance of platelets to deformation.12 Investigations into the effects of chilling and of antimitotic agents demonstrated that intact microtubule coils are critical for platelet resistance to aspiration into micropipettes.12 Exposure of platelets to cytochalasin B before aspiration showed that actin microfilament assembly was also important in platelet stability.

Human platelets undergo agglutination when stirred with bovine plasma (BP), but bovine platelets do not. The present study has shown that exposure of washed bovine platelets to subthreshold concentrations of adenosine diphosphate or thrombin before stirring restores their sensitivity to BP, and the cells undergo rapid agglutination. This agglutination was prevented by a monoclonal antibody, to glycoprotein GPIb. Flow cytometry studies revealed that exposure of bovine platelets to thrombin caused an increase in their ability to bind antibodies known to react with human GPIb or GPIIb-IIIa receptors. Interaction of bovine and human platelets with vascular subendothelium revealed additional differences in reactivity. Bovine platelets in citrate anticoagulant reacted poorly with subendothelium under flow conditions compared with human platelets. In contrast, bovine platelets in blood with low molecular weight heparin as anticoagulant adhered more readily than human cells. These findings suggest that different mechanisms are involved in hemostasis in human and bovine species.