Inhibition of platelet adherence to damaged surface of rabbit aorta

A method has been developed for quantitative measurement of adherence of rabbit platelets to the damaged intimal surface of everted segments of rabbit thoracic aorta. Platelets were labeled with 51Cr, washed, and resuspended in Tyrode solution containing 0.35 per cent albumin and apyrase. This suspending medium contains physiologic concentrations of calcium and magnesium; apyrase degrades any ADP lost from the platelets or from the damaged wall. Everted aorta segments were rotated in the platelet suspensions. Neither platelet aggregation nor lysis occurred and the platelets adhered to the subendothelium either as individual platelets or as a single layer. Damage caused by scraping the everted segments with a scalpel blade increased adherence 50-fold. Acetylsalicylic acid (ASA) in vitro, or administered orally to the rabbits from which platelet suspensions were prepared, significantly reduced the number of platelets adherent to the damaged aorta wall. ASA affected only the platelets, and did not affect the damaged wall. Platelet adherence to the damaged wall was also reduced by the use of 4 per cent albumin in the suspending medium, or by the addition of citrate. Adherence of platelets resuspended in citrated plasma was low and further inhibition by ASA was not demonstrable. ASA may affect two aspects of thrombus formation: platelet adherence to subendothelial structures and the platelet release reaction induced by collagen (and possibly by other subendothelial structures). These studies show that ASA has a marked effect on adherence of platelets to subendothelium under conditions in which aggregation and thrombus formation are prevented.