The platelets of a patient with congenital thrombasthenia were not aggregated by ADP, thrombin, connective tissue particles, Polybrene, or phospholipase C, and did not adhere to glass as measured either on a glass slide or by retention in a glass-bead column. Clot retraction was markedly diminished. Raising the magnesium level partially corrected clot retraction but did not restore ADP-induced clumping. The platelets were less able to promote prothrombin consumption. Fibrinogen concentration in the supernatant of frozen and thawed platelets was low, but surface fibrinogen appeared to be normal.

The thrombasthenic platelets were normal in the following respects: concentration of ATP and glyceraldehyde-3-phosphate-dehydrogenase; adhesion to connective tissue fibers; aggregation by antiplatelet serum; microelectrophoretic mobility; isoelectric point; disc shape of platelets at 37 C.; ability of platelets to change shape with ADP or cold; decrease in ATP concentration and auramine staining of granules by thrombin; release of serotonin, ADP, and other materials absorbing at 260 mµ. by thrombin or connective tissue particles; liberation of acid phosphatase during blood clotting; and platelet Factor 5 activity.

It is concluded that responses of thrombasthenic platelets to thrombin and connective tissue particles are normal except that the liberated ADP fails to cause aggregation. The first stage of the reaction to ADP, transformation from disc to spiny sphere, is normal. Still to be determined at the molecular level is the cause(s) of failure of clot retraction and ADP-induced aggregation and the relationship of these defects to the low fibrinogen concentration of platelet extracts.

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