Survival of rabbit platelets treated in vitro with chymotrypsin, plasmin, trypsin, or neuraminidase

Abstract Platelets from which surface sialic acid has been removed by neuraminidase are rapidly cleared from the circulation. In the present study the role of platelet membrane glycoproteins in platelet survival was further investigated with 51Cr-labeled rabbit platelets from which glycoproteins were cleaved by treatment with chymotrypsin, plasmin, or trypsin. SDS-PAGE of platelet membranes followed by PAS staining showed that under the conditions used all of these enzymes cleaved major amounts of glycoprotein I and lesser amounts of glycoproteins II and III. Large percentages of the platelets subjected to any of these proteolytic enzyme treatments or to neuraminidase treatment were removed from the circulation during the hour after their injection (mean values in circulation at 1 hr: control 80.6 %, chymotrypsin-treated 18.3 %, rabbit plasmin-treated 48.4%, pig plasmin-treated 9%, trypsin-treated 2.8%, neuraminidase-treated 4.5%). Some of the platelets treated with chymotrypsin, plasmin, or trypsin were sequestered at 1 hr and had reappeared in the circulation by 4 hr. The platelets were removed by the liver, not by the spleen. Platelet survival was calculated by Murphy's gamma function. Mean platelet survivals (± SEM) for platelets in the circulation at 4 hr were control 77.2 ± 4.2 hr, chymotrypsin-treated 15.9 ± 2.2 hr, plasmin-treated 54.6 ± 6.0 hr. So few of the platelets treated with pig plasmin, trypsin, or neuraminidase remained in the circulation that it was not feasible to calculate platelet survival values. We have shown previously that thrombin treatment of platelets to cause extensive release of granule contents does not alter their survival, in keeping with the finding that it does not affect the major glycoproteins. In the present experiments, plasmin and trypsin caused the platelet release reaction, but chymotrypsin and neuraminidase did not. Thus it appears that loss of granule contents does not influence platelet survival, whereas platelets lacking sialic acid on their membranes (whether it has been removed as sialic acid or as glycopeptides) are recognized as “foreign” and are removed. It may be that the shortened platelet survival seen in thromboembolic complications of arterial disease can at least partly be attributed to the effect of plasmin on platelet glycoproteins, particularly glycoprotein I.