"Shear induced platelet activation"--a critical reappraisal.

Platelets were found to be stimulated by high shear exposure for 5 minutes. Using a new technique, shear stresses up to 255 N/m2, acting for pathophysiological relevant intervals of milliseconds did not elicit active release of beta-TG, beyond the amount explained by LDH-monitored passive lysis. Neither the plasma level of ionized calcium (citrate vs heparin anticoagulation), nor a potent platelet inhibiting agent like PgI2 (20 nM) did significantly alter platelet responses to short acting high shear stress. Ultrastructural signs of activation could largely be suppressed by adding ADP-scavengers. Direct "shear induced platelet activation" appears rather unlikely and mostly reducible to secondary biochemical activation by mediators, probably adenine nucleotides, from a small percentage of passively shear-destroyed platelets. The extent of this secondary activation is largely a matter of experimental conditions.