Loss of sulfated proteoglycan from the surface of rabbit platelets during adenosine 5'-diphosphate-induced aggregation.

Rabbit platelets were labeled in vivo by intravenous injection of 35SO4=60 hours before the animals were exsanguinated. The labeled platelets were washed and resuspended in Tyrode's solution containing 0.35 per cent albumin. The aggregated normally upon the addition of ADP and during aggregation 14.0 +/- 2.7 per cent (mean +/- standard deviation) of the labeled material was lost into the suspending medium. The extent of loss was dependent on ADP concentration and was inhibited by inhibitors of ADP-induced aggregation (AMP, ATP, adenosine, prostaglandin E1, parachloromercuribenzensulfonate and lack of calcium). Since neither release of granule contents nor lysis occurs when rabbit platelets are aggregated by ADP, it was concluded that the labeled material that was lost had been part of the surface coat of the platelets. The labeled material has been identified in other studies as a proteoglycan in which the mucopolysaccharide is principally chondroitin sulfate A. Loss of this material did not make the platelets unresponsive to ADP. The platelets did not reincorporate labeled proteoglycan from plasma. Thrombin caused the release of 50 to 80 per cent of the total platelet-labeled material, presumably from both the granules and the surface. The functional significance of the loss of surface proteoglycan during platelet aggregation is unknown.