Summary
The effect of a phospho-glycoprotein (HMW-GP), obtained from human platelet plasma
membranes, on the aggregation and secretion of human platelets was studied. Incubation
of PRP with 4 to 16 μg/ml of HMW-GP results in inhibition of ADP-, Epinephrine-, Collagen-,
and Thrombin-induced platelet aggregation. The effect is mainly reflected on the secondary
wave of aggregation. The inhibitory effect is partially overcome by higher concentration
of the inducers, however, even under these conditions, a clear tendency towards disaggregation
is observed. 5HT release (Col-induced) is strongly decreased from 50% to 4.5. The
inhibitory effect on Thrombin-induced aggregation is markedly dependent on external
calcium, being maximal at 5 mM calcium. The HMW-GP does not bind ADP or Thrombin.
Membrane conformation is markedly affected, as evidenced by the effect of HMW-GP on
the iodination of surface polypeptides of intact platelets. It is suggested that interaction
of HMW-GP with the platelet membrane blocks the signal(s) transmission that links
stimulus to activation. The inhibition observed might just represent an experimental
amplification of the endogenous modulatory function that has been proposed for this
high molecular weight phosphoglycoprotein.
Keywords
Platelet inhibition - Membrane-bound glycoproteins
